WO2001094992A1

WO2001094992A1 - Birefringent devices and filters

Info

Publication number: WO2001094992A1
Application number: PCT/US2001/018377
Authority: WO
Inventors: Bin Zhao
Original assignee: Cirvine Corporation
Priority date: 2000-06-07
Filing date: 2001-06-07
Publication date: 2001-12-13
Also published as: WO2001094992A9

Abstract

Birefringent devices and filters which provide enhanced performance in transmission and dispersion. Such birefringent devices and filters may optionally utilize a difference in optical path lengths so as to eliminate the use of birefringent crystals (15).

Description

BIREFRINGENT DEVICES AND FILTERS

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of the filing date of United States Provisional Patent Application Serial No. 60/210,049, filed on June 7, 2000, and entitled LOW CROSSTALK FLAT BAND FILTER, United States Provisional Patent Application Serial No. 60/210,052 filed on June 7, 2000, and entitled METHOD AND APPARATUS FOR LOW DISPERSION IN HIGH BIT RATE COMMUNICATIONS, United States Provisional Patent Application Serial No. 60/213,369, filed on June 23, 2000, and entitled METHOD AND APPARATUS FOR DISPERSION COMPENSATION IN HIGH BIT RATE COMMUNICATIONS, United States Provisional Patent Application Serial No. 60/210,051, filed on June 1, 2000, and entitled SPATIAL BIREFRINGENT DEVICES, United States Provisional Patent Application Serial No. 60/210,033, filed on June 7, 2000, and entitled HIGH PERFORMANCE INTERLEAVER FOR OPTICAL COMMUNICATIONS, United States Provisional Patent Application Serial No. 60/210,046, filed on June 7, 2000, and entitled APPARATUS FOR BIREFRINGENT DEVICES, and United States Provisional Patent Application Serial No. 60/210,050, filed on June 7, 2000, and entitled APPARATUS FOR SIGNAL INTERLEAVERING, the entire contents of * w .h.. i .ch are hereby expressly incorporated by reference.

This patent application is related to co-pending patent application serial number , filed on June , 2001 entitled LOW CROSSTALK FLAT BAND FILTER

(Attorney Docket No. 12569-01); co-pending patent application serial number , filed on June , 2001 entitled APPARATUS AND METHOD FOR LOW

DISPERSION IN COMMUNICATIONS (Attorney Docket No. 12569-13); co-pending patent application serial number , filed on June , 2001 entitled BIREFRINGENT DEVICES (Attorney Docket No. 12569-02); co-pending patent application serial number , filed on June , 2001 entitled COMB FILTER FOR

DEFENSE WAVELENGTH DIVISION MULTIPLEXING (Attorney Docket No. 12569-

11); and co-pending patent application serial number , filed on June , 2001 entitled INTERLEAVER USING SPATIAL BIREFRINGENT ELEMENTS (Attorney Docket No. 12569-03), all filed on the instant date herewith and commonly owned by the Assignee of this patent application, the entire contents of all which are hereby expressly incorporated by reference. FIELD OF THE INVENTION Low Crosstalk Flat Band Filter

The present invention relates generally to optical devices. The present invention relates more particularly to an apparatus and method for filtering electromagnetic radiation, such as optical communication signals used in dense wavelength-division multiplexing optical communication systems and the like.

Apparatus and Method for Low Dispersion in Communications

The present invention relates generally to optical communications devices and systems and relates more particularly to a low dispersion filter or interleaver for use in wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) optical communication systems and the like

Birefringent Devices

The present invention relates generally to optical devices and relates more particularly to a birefringent device which provides a birefringent effect without using a birefringent crystal.

Comb Filter for Defense Wavelength Division Multiplexing

The present invention relates generally to optical devices and relates more particularly to a high performance filter or interleaver for optical communications and the like.

Interleaver Using Spatial Birefringent Elements The present invention relates generally to optical devices and relates more particularly to a high performance filter or interleaver for optical communications and the like.

BACKGROUND OF THE INVENTION Low Crosstalk Flat Band Filter

Optical communication systems which utilize wavelength-division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, typically infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information may be transmitted than is possible utilizing a single wavelength optical communication system.

The individual channels of a wavelength-division multiplexed or dense wavelength- division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed by an interleaver. A similar device facilitates multiplexing of the individual channels by a transmitter.

It is important that the interleaver separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Typically, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.

However, as channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to reduce communication speed in order to compensate for the effects of crosstalk. Moreover, it is generally desirable to reduce channel spacing so as to facilitate the communication of a greater number of channels.

Filters are typically used within interleavers (and are also used in various other optical devices), so as to facilitate the separation of channels from one another in a wavelength- division multiplexing or dense wavelength division multiplexing system. Various characteristics of such filters contribute to the mitigation of crosstalk and thus to contribute reliable communications. For example, the ability of a filter to separate one optical channel from another or one set of channels from another set of channels is dependent substantially upon width and depth of the filter's stopband. Generally, the wider and deeper the stopband, the more effectively the filter rejects unwanted adjacent channels and thus the more effectively the filter mitigates crosstalk.

Further, the flatness and width of the filter's passband is important. The flatness of the filter's passband determines how much the signal is undesirably altered during the filtering process. A substantially flat passband is desired, so as to assure that minimal undesirable alteration of the signal occurs. The width of the passband determines how far from the ideal or nominal channel center frequency a signal can be and still be effectively selected. A wide passband is desirable because the nominal center frequency of a carrier which is utilized to define a communication channel is not perfectly stable, and therefore tends to drift over time. Further, the nominal center frequency of a filter passband likewise tends to drift over time. Although it is possible to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is generally impractical and undesirably expensive to do so.

In order to construct a system wherein the center frequency of the communication channel and the, center frequency of the filter are comparatively stable, it is necessary to provide precise control of the manufacturing processes involved. Since it is generally impractical and undesirably expensive to provide such precise control during manufacturing, the center frequency of communication channels and the center frequency of filters generally tend to mismatch with each other. Precise control of manufacturing processes is difficult because it involves the use of more stringent tolerances which inherently require more accurate manufacturing equipment and more time consuming procedures. The center frequency of the communication channel and the center frequency of the filter also tend to drift over time due to inevitable material and device degradation over time and also due to changes in the optical characteristics of optical components due to temperature changes. Therefore, it is important that the passband be wide enough so as to include a selected signal, even when both the carrier frequency of the selected signal and the center frequency of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.

Birefringent filters for use in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems are well known. Such birefringent filters are used to select or deselect optical signals according to the channel wavelengths thereof. However, contemporary birefringent filters tend to suffer from deficiencies caused by inherent carrier and passband instability due to manufacturing difficulties and due to drifting over time, as discussed above. That is, the passband of a contemporary birefringent filter is not as flat or as wide as is necessary for optimal performance. Further, the stopbands of such contemporary birefringent filters are not as deep or as wide as is necessary for optimal performance. Therefore, it is desirable to optimize such birefringent filters in a manner which enhances the width of the passband, makes the passband more flat, and which also widens and deepens the stopband. Further, it is desirable to provide a birefringent filter whereby the width of the stopband is roughly equal to the width of the passband, so as to facilitate the efficient separation of equally spaced channels in a wavelength-division multiplexing or dense wavelength-division multiplexing communication system. Such birefringent filters typically comprise a plurality of birefringent elements placed end-to-end between two polarization selection devices, so as to define a contemporary Sole- type optical filter.

Referring now to Figure 1, a typical layout of a Sole-type filter is shown. This layout is common to both contemporary Sole-type filters and the present invention. This filter comprises an input polarization selection device (e.g., polarizer) 11, an output polarization selection device 12, and a birefringent element assembly disposed generally intermediate the input polarization selection device 11 and the output polarization selection device 12. The polarization axis of the input polarization selection device 11 and the output polarization selection device 12 are typically parallel to one another.

According to contemporary practice, the birefringent element assembly 13 of such a Sole-type filter comprises three birefringent elements or crystals. A first birefringent crystal 15 has a length of L. A second birefringent crystal 16 has a length of 2L. A third birefringent crystal 17 has a length of 2L. Further, according to contemporary practice the orientations of the fast axes of the birefringent crystals 15, 16 and 17 with respect to the polarization axis of the input polarizer 11 (and typically with respect to the polarization axis of the output polarization selection device 12, as well), are 45° for the first birefringent crystal 15, -15° for the second birefringent crystal 16, and 10° for the third birefringent crystal 17. Although such contemporary Sole-type filters are generally suitable for some applications in optical communications, such contemporary Sole-type filters suffer from inherent deficiencies which detract from their overall effectiveness. Such contemporary Sole- type filters are birefringent filters which suffer from an insufficiently flat and undesirably narrow passband, as well as an insufficiently deep and undesirably narrow stopband, as discussed above.

Referring now to Figures 2 and 3, transmission vs. wavelength curves for both the present invention and contemporary filters are shown.

With particular reference to Figure 2, the stopband of the contemporary filter has peaks which are 20dB down from the OdB level of the passband. Thus, the illustrated contemporary Sole-type filter provides only approximately 20dB of cutoff in the stopband thereof. Further, the contemporary filter has a comparatively narrow -3 OdB stopband.

With particular reference to Figure 3 (which shows the two curves of Figure 2 with increased resolution), it can be seen that the passband of the contemporary Sole-type filter contains an undesirable amount of ripple, and therefore is not as flat as desirable. Thus, such a contemporary Sole-type filter undesirably alters a signal which is transmitted therethrough. The comparatively large amount of ripple in the passband of the contemporary Sole- type filter, in combination with the insufficiently deep stopband thereof, substantially degrades the performance of the filter such that the contemporary Sole-type filter frequently cannot meet the desired performance requirement therefor. Such poor performance all too frequently facilitates undesirable crosstalk between adjacent channels in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems, particularly in those systems wherein the carrier wavelengths and/or the passband/stopband positions of the filter are insufficiently stable and not well controlled.

In view of the foregoing, it is desirable to provide a filter which has a comparatively flat transmission vs. wavelength characteristic curve at that portion of the curve defining the passband and which also has a comparatively deep stopband, so as to substantially mitigate crosstalk and so as to enhance filter performance in wavelength-division multiplexing, dense wavelength-division multiplexing, and similar communication systems.

Apparatus and Method for Low Dispersion in Communications Optical communication systems which utilize wavelength-division multiplexing

(WDM) and dense wavelength division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, typically infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information may be transmitted than is possible utilizing a single wavelength optical communication system.

However, as channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to reduce communication speed in order to compensate for the effects of crosstalk. Moreover, it is generally desirable to reduce channel spacing so as to facilitate the communication of a greater number of channels .

Filters are typically used within interleavers (and are also used in various other optical devices), so as to facilitate the separation of channels from one another in a wavelength- division multiplexing or dense wavelength division multiplexing system. Various characteristics of such filters contribute to the mitigation of crosstalk and thus to contribute reliable communications. For example, the ability of a filter to separate one optical channel from another or to separate one set of channels from another set of channels is dependent substantially upon width and depth of the filter's stopband. Generally, the wider and deeper the stopband, the more effectively the filter rejects unwanted adjacent channels and thus the more effectively the filter mitigates crosstalk. Further, the flatness and width of the filter's passband is important. The flatness of the filter's passband determines how much the signal is undesirably altered during the filtering process. A substantially flat passband is desired, so as to assure that minimal undesirable alteration of the signal occurs. The width of the passband determines how far from the ideal or nominal channel center frequency a signal can be and still be effectively selected. A wide passband is desirable because the nominal center frequency of a carrier which is utilized to define a communication channel is not perfectly stable, and therefore tends to drift over time. Further, the nominal center frequency of a filter passband likewise tends to drift over time. Although it is possible to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is generally impractical and undesirably expensive to do so.

Although having a wider filter passband is generally desirable, so as to facilitate the filtering of signals which have drifted somewhat from their nominal center wavelength, the use of such wider pass bands and the consequent accommodation of channel center wavelength drift does introduce the possibility for undesirably large dispersion being introduced into a filtered channel. Typically, the dispersion introduced by a birefringent filter or interleaver increases rapidly as the channel spacing is reduced and as a channel moves away from its nominal center wavelength, as discussed in detail below. Thus, as more channel wavelength error is tolerated in a birefringent filter or interleaver, greater dispersion valves are likely to be introduced. In order to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is necessary to provide precise control of the manufacturing processes involved. Since it is generally impractical and undesirably expensive to provide such precise control during manufacturing, the center frequency of communication channels and the center frequency of filters generally tend to mismatch with each other. Precise control of manufacturing processes is difficult because it involves the use of more stringent tolerances which inherently require more accurate manufacturing equipment and more time consuming procedures. The center frequency of the communication channel and the center frequency of the filter also tend to drift over time due to inevitable material and device degradation over time and also due to changes in the optical characteristics of optical components due to temperature changes.

Therefore, it is important that the passband be wide enough so as to include a selected signal, even when both the carrier frequency of the selected signal and the center frequency of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time. Birefringent filters for use in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems are well known. Such birefringent filters are used to select or deselect optical signals according to the channel wavelengths thereof. However, contemporary birefringent filters tend to suffer from deficiencies caused by inherent carrier and passband instability due to manufacturing difficulties and due to drifting over time, as discussed above. That is, the passband of a contemporary birefringent filter is not as flat or as wide as is necessary for optimal performance. Further, the stopbands of such contemporary birefringent filters are not as deep or as wide as is necessary for optimal performance. Third, it typically has large dispersion which would introduce significant signal distortion. Therefore, it is desirable to optimize such birefringent filters in a manner which enhances the width of the passband, makes the passband more flat, and which also widens and deepens the stopband. It is desirable to provide a birefringent filter whereby the width of the stopband is roughly equal to the width of the passband, so as to facilitate the efficient separation of equally spaced channels in a wavelength-division multiplexing or dense wavelength-division multiplexing communication system. Further, it is desirable to provide a birefringent filter which possess zero or extremely low dispersion. Such birefringent filters typically comprise a plurality of birefringent elements placed end-to-end between two polarization selection devices, so as to define a contemporary Sole- type optical filter.

Referring now to Figure 1, a typical layout of a Sole-type filter is shown. This layout is common to Sole-type filters. This filter comprises an input polarization selection device (e.g., polarizer) 11, an output polarization selection device 12, and a birefringent element assembly disposed generally intermediate the input polarization selection device 11 and the output polarization selection device 12. The polarization axis of the input polarization selection device 11 and the output polarization selection device 12 are typically parallel to one another.

According to contemporary practice, the birefringent element assembly 13 of such a Sole-type filter comprises three birefringent elements or crystals. A first birefringent crystal 15 has a length of L. A second birefringent crystal 16 has a length of 2L. A third birefringent crystal 17 has a length of 2L. Although such contemporary Sole-type filters are generally suitable for some applications in optical communications, such contemporary Sole-type filters suffer from inherent deficiencies which detract from their overall effectiveness. Such contemporary Sole- type filters are birefringent filters which suffer from high dispersion when the actual channel wavelength is not at the nominal channel center wavelength. As those skilled in the art will appreciate, dispersion is the non-linear phase response of an optical device or system wherein light of different wavelengths is spread or dispersed, such that the phase relationship among the different wavelengths varies undesirably as the light passes through the device or system. Such dispersion undesirably distorts optical signals, such as those used in optical communication systems. The nonlinear phase response or dispersion of WDM and DWDM devices is responsible for signal distortion which results in undesired limitations on channel capability. That is, such dispersion undesirably limits the useable bandwidth of a channel, such as that of a fiber optic communication system. Such undesirable limitation of the bandwidth of a channel in a fiber optic communication system inherently reduces the bit rate of digital data transmitted thereby.

Contemporary interleavers have dispersion versus wavelength curves which have zero dispersion value at a particular wavelength, such as at nominal channel center wavelength. The dispersion versus wavelength curve of such contemporary interleavers departs drastically from this zero dispersion value as the wavelength moves away from the nominal channel center wavelength. Thus, small deviations in channel center wavelength can result in undesirably large dispersion values being realized.

Since, as discussed in detail above, it is extremely difficult, if not impossible, to maintain a channel center wavelength precisely at its nominal value, such channel center wavelengths do vary, thereby resulting in undesirably large dispersion values.

The problem of comparatively small differences between actual channel center wavelength and the nominal value thereof causing undesirably large dispersion values can be mitigated by constructing an interleaver having either a dispersion versus wavelength curve which has a value of approximately zero for all wavelengths, or alternatively, by constructing an interleaver having a dispersion versus wavelength curve which does not deviate substantially from a zero dispersion value at least for those wavelengths to which the actual channel center wavelength is likely to drift.

An optical interleaver is one type of comb filter which is commonly used in optical communications systems. Such interleavers have the potential for substantially enhancing performance in future optical communications networks by substantially enhancing bandwidth thereof. Common contemporary interleavers provide channel spacings of 200 GHz and 100 GHz. 50 GHz interleavers are just beginning to emerge in the marketplace. Further reduction of optical channel spacing to 25 GHz, 12.5 GHz and beyond presents substantial technical challenges. As channel spacing is decreased below 50 GHz, significant and undesirable dispersion appears and can dramatically degrade optical signal quality, particularly in high bit rate optical communication systems. Thus, there is substantial need for techniques and apparatus which mitigate or suppress the dispersion introduced by an interleaver in an optical communication system. More generally, there also exists a similar need for techniques and apparatus which compensate for dispersion in various other devices, such as those commonly used in WDM/DWDM communication systems.

Birefringement Devices

Birefringent devices which comprise birefringent crystals are commonly used in optical applications and are well known. For example, birefringent crystals are commonly used in Sole filters for separating multiplexed optical channels in dense wavelength division multiplexing (DWDM) communication systems. Thus, birefringent devices are important device elements in optical signal processing applications and the like.

Birefringent crystals are materials in which the phase velocity of an optical beam propagating therein depends upon the polarization direction of the optical beam. As mentioned above, birefringent devices are important elements in optical signal processing applications and the like. However, birefringent devices which comprise birefringent crystals suffer from inherent limitations which seriously degrade their performance, limit their application and reduce their desirability. Contemporary crystal birefringent devices suffer from limitations imposed by the crystal's physical, mechanical and optical properties, as well as by problems associated with temperature instability. Further, such contemporary crystal birefringent devices have comparatively small birefringent values. The crystals utilized in such contemporary crystal birefringent devices are comparatively high in cost, both with regard to the synthesis thereof and with regard to their use in fabrication of optical devices, e.g., birefringent devices.

Further, such contemporary crystal birefringent devices have a fixed birefringent value (not taking into consideration undesirable variations due to their temperature instability) and are thus not tunable.

It is desirable to provide a birefringent device which does not utilize a birefringent crystal and thus does not suffer from the inherent shortcomings thereof. More particularly, it is desirable to provide a birefringent device which has comparatively good temperature stability, provides a comparatively wide range of birefringent values, is tunable, and is comparatively less expensive to produce and utilize.

Comb Filter for Defense Wavelength Division Multiplexing Optical communication systems which utilize wavelength-division multiplexing

(WDM) and dense wavelength-division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, preferably infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.

The individual channels of a wavelength-division multiplexed or dense wavelength- division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed or assisted by an interleaver. A similar device facilitates multiplexing of the individual channels by a transmitter.

It is important that the interleaver separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Generally, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal .

However, as channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to increase channel spacing and/or to reduce communication speed in order to compensate for the effects of crosstalk. Moreover, it is generally desirable to decrease channel spacing and to increase communication speed so as to facilitate the communication of a greater quantity of information utilizing a given bandwidth.

Modern dense wavelength-division multiplexed (DWDM) optical communications and the like require that network systems offer an ever-increasing number of channel counts, thus mandating the use of a narrower channel spacing in order to accommodate the increasing number of channel counts. The optical interleaver, which multiplexes and demultiplexes optical channels with respect to the physical media, i.e., optical fiber, offers a potential upgrade path, so as to facilitate scalability in both channel spacing and number of channel counts in a manner which enhances the performance of optical communication networks.

As a multiplexer, an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream. As a demultiplexer, an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels. Thus, the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwidth efficient, channel spacings.

There are four basic types of interleavers suitable for multiplexing and demultiplexing optical signals. These include birefringent filters, thin-film dielectric devices, planar waveguides, and fiber-based devices. All of these contemporary interleaving technologies suffer from substantial limitations with respect to channel spacing, dispersion, insertion loss, channel isolation, temperature stability, cost, reliability and flexibility. For example, most commercially available interleavers provide only 100 GHz and 50 GHz channel spacings. Reduction of channel spacing to 25 GHz, 12.5 GHz and beyond appears to be difficult and challenging.

Thus, there is a need to provide an optical interleaver which can overcome or mitigate at least some of the above-mentioned limitations.

Interleaver Using Spatial Birefringent Elements

Optical communication systems which utilize wavelength-division multiplexing (WDM) and dense wavelength-division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, preferably infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.

It is important that the interleaver separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Generally, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.

As a multiplexer, an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream. As a demultiplexer, an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels. Thus, the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwith efficient, channel spacings.

SUMMARY OF THE INVENTION Low Crosstalk Flat Band Filter The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarizing elements and a birefringent element assembly disposed generally intermediate the polarizing elements. The birefringent element assembly is preferably configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances transmission vs. wavelength curve flatness for a passband thereof. The birefringent element assembly is also preferably configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances the depth of the stopband thereof.

Apparatus and Method for Low Dispersion in Communications

The present invention comprises techniques and apparatus which mitigate undesirable interleaver dispersion. The present invention also provides techniques and apparatus which compensate for dispersion from various different optical devices in an optical communication system.

More particularly, the present invention comprises a zero or low dispersion birefringent filter or interleaver assembly having a first interleaver and a second interleaver. The second interleaver is configured so as to provide a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for the first interleaver, so as to mitigate dispersion in the interleaver assembly. In this manner, the dispersion of an interleaver substantially cancels out the dispersion of the other interleaver. In a similar manner, a single interleaver may be utilized to substantially mitigate dispersion in various other optical components in an optical communication system or the like. These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.

Birefringent Devices The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a birefringent device comprising a polarization separating device configured to separate a first composite light beam into first and second components thereof. The first and second components are orthogonally polarized with respect to one another. A first path is configured to transmit the first component and has a first optical path length. A second path is configured to transmit the second component and has a second optical path length. The second optical path length is different from the first optical path length. A polarization combining device is configured to recombine the first and second components, so as to form a second composite light beam. The second composite light beam is birefringent with respect to the first composite light beam.

These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.

Comb Filter for Defense Wavelength Division Multiplexing

The present inventions specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, one embodiment of the present invention comprises an interleaver comprising an input polarization beam separation element, a birefringent filter assembly in optical communication with the input polarization beam separation element, and an output polarization beam separation/combination element assembly in optical communication with the birefringent filter assembly. The birefringent filter assembly comprises at least one birefringent filter stage. According to the present invention birefringent crystals, such as those commonly used in contemporary birefringent filters, are eliminated, so as to mitigate at least some of the problems associated with prior art interleavers. Rather than using birefringent crystals, the interleaver of the present invention utilizes a device which provides optical paths having different optical path lengths for two orthogonally polarized light beams, so as to provide a birefringent effect.

Interleaver Using Spatial Birefringent Elements

The present inventions specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises an interleaver comprising an input polarization beam separation element, a birefringent filter assembly in optical communication with the input polarization beam separation element, and an output polarization beam separation/combination element assembly in optical communication with the birefringent filter assembly. The birefringent filter assembly comprises at least one birefringent filter stage. The input polarization beam separation element, the birefringent filter assembly, and the output polarization beam separation/combination element assembly are configured to minimize feedback to input source and transmission losses.

According to the present invention birefringent crystals, such as those commonly used in contemporary birefringent filters, are eliminated so as to mitigate at least some of the problems associated with prior art interleavers. Rather than using birefringent crystals, the interleaver of the present invention utilizes a device which provides optical paths having different optical path lengths for two orthogonally polarized light beams so as to provide a birefringent effect.

BRIEF DESCRIPTION OF THE DRAWINGS

These, and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings wherein:

Low Crosstalk Flat Band Filter

Figure 1 is a schematic representation showing generally the relative positions of the input polarizing element, the birefringent element assembly (which comprises the first, second and third birefringent elements or crystals) and the output polarizing element, with respect to one another;

Figure 2 is a transmission vs. wavelength chart showing a characteristic curve for a contemporary three element filter and also showing a characteristic curve for the three element filter of the present invention; Figure 3 is enlarged view of a passband of the contemporary and present invention response curves of Figure 2;

Figure 4 is a transmission vs. wavelength chart showing a characteristic curve for a contemporary two element filter and also showing a characteristic curve for the two element filter of the present invention; and Figure 5 is enlarged view of a passband of the contemporary and present invention response curves of Figure 4. Apparatus and Method for Low Dispersion in Communications

Figure 2 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 10° and having phase delays of T, 2T and 2T respectively, for the first, second and third birefringent crystals, respectively;

Figure 3 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 10° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively;

Figure 4 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 10° and having phase delays of T, 2r and 2T, for the first, second and third birefringent crystals, respectively; Figure 5 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -75°, and 80° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively;

Figure 6 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -75°, and 80° and having phase delays of Y, 2T and 2T, for the first, second and third birefringent crystals, respectively;

Figure 7 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -75°, and 80° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively;

Figure 8 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -21°, and 7° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively;

Figure 9 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -21°, and 7° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively; Figure 10 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -21°, and 7° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively; Figure 11 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -69°, and 83° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively;

Figure 12 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -69°, and 83° and having phase delays of T, 2Y and 2r, for the first, second and third birefringent crystals, respectively;

Figure 13 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -69°, and 83° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively; Figure 14 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -65°, and 15° and having phase delays of T, 2T and 2r, for the first, second and third birefringent crystals, respectively;

Figure 15 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -65°, and 15° and having phase delays of T, 2Y and 2r, for the first, second and third birefringent crystals, respectively;

Figure 16 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -65°, and 15° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively;

Figure 17 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 0° and having phase delays of T, 2Y and T, for the first, second and third birefringent crystals, respectively;

Figure 18 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 0° and having phase delays of T, 2T and T, for the first, second and third birefringent crystals, respectively; Figure 19 is a transmission vs. wavelength chart for a three element filter having angular orientations of 45°, -15°, and 0° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively;

Figure 20 is a dispersion vs. wavelength chart for a three element filter having angular orientations of 45°, -75°, and 90° and having phase delays of T, 2T and T, for the first, second and third birefringent crystals, respectively;

Figure 21 is a phase vs. wavelength chart for a three element filter having angular orientations of 45°, -75°, and 90° and having phase delays of T, 2T and 2T, for the first, second and third birefringent crystals, respectively; Figure 22 is a transmission vs. wavelength chart for three element filter having angular orientations of 45°, -75°, and 90° and having phase delays of T, 2T and T, for the first, second and third birefringent crystals, respectively;

Figure 23 is a dispersion vs. wavelength chart for a two element filter having angular orientations of 45° and -15° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively;

Figure 24 is a phase vs. wavelength chart for a two element filter having angular orientations of 45° and -15° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively; Figure 25 is a transmission vs. wavelength chart for a two element filter having angular orientations of 45° and -15° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively;

Figure 26 is a dispersion vs. wavelength chart for a two element filter having angular orientations of the birefringent crystals thereof of 45° and -75° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively;

Figure 27 is a phase vs. wavelength chart for a two element filter having angular orientations of the birefringent crystals thereof of 45° and -75° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively; and

Figure 28 is a transmission vs. wavelength chart for a two element filter having angular orientations of the birefringent crystals thereof of 45° and -75° and having phase delays of T and 2r, for the first and second birefringent crystals, respectively.

Birefringent Devices

These, and other features, aspects and advantages of the present invention, will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings, wherein:

Figure 1 is a schematic diagram showing a top view of an exemplary birefringent device which utilizes two polarization beam displacers and two prisms according to the present invention;

Figure 2a is a schematic diagram showing a cross-sectional view of the first beam displacer and the prisms of Figure 1 ;

Figure 2b is a schematic diagram showing a cross-sectional view of the second beam displacer and the prisms of Figure 1; Figure 3 is a series of frames, wherein each frame shows the state of the optical beam at the indicated locations of the birefringent device of Figures 1, 2a and 2b and also shows the half-wave waveplate orientations thereof;

Figure 4 is a top view of an alternative configuration of a birefringent device which utilizes a polarization beam splitter and mirrors according to the present invention;

Figure 5 is a series of frames wherein each frame shows the state of the optical beam at the indicated locations of the birefringent device of Figure 4 and also shows the half-wave waveplate orientations thereof;

Figure 6 is a schematic diagram of a top view of an alternative configuration of a birefringent device which utilizes a polarization beam splitter and prisms according to the present invention; and

Figure 7 is a series of frames wherein each frame shows the state of the optical beam at the indicated locations of the birefringent device of Figure 6 and also shows the half-wave waveplate orientations thereof.

Comb Filter for Defense Wavelength Division Multiplexing

FIG. 1 is a schematic diagram of a one-stage birefringent filter or interleaver utilizing mirrors according to the present invention;

FIG. 2 is a series of frames showing the optical beam states and the orientations of half-wave waveplates and quarter-wave waveplates at different locations in the one-stage interleaver of FIG. 1, wherein the underlined number associated with each frame shows the location where the optical beam state or waveplate orientation occurs in FIG. 1 ;

FIG. 3 is a schematic diagram of a two-stage birefringent filter or interleaver utilizing mirrors according to the present invention;

FIG. 4 is a series of frames showing the optical beam states and the orientations of half-wave waveplates and quarter-wave waveplates at different locations in the two-stage interleaver of FIG. 3, wherein the underlined number associated with each frame shows the location where the optical beam state or waveplate orientation occurs in FIG. 3;

FIG. 5 is a schematic diagram of a three-stage birefringent filter or interleaver utilizing mirrors according to the present invention; FIG. 6 is a series of frames showing the optical beam states and the orientations of half-wave waveplates and quarter-wave waveplates at different locations in the three-stage interleaver of FIG. 5, wherein the underlined number associated with each frame shows the location where the optical beam state or waveplate orientation occurs in FIG. 5; FIG. 7 is a schematic diagram of a three-stage birefringent filter or interleaver utilizing mirrors, showing an array of input and output optical beams;

FIG. 8 is a schematic diagram of a five-stage birefringent filter or interleaver utilizing mirrors, showing an array of input and output optical beams; FIG. 9a is a schematic diagram showing an alternative layout or configuration for a three-stage birefringent filter or interleaver utilizing mirrors;

FIG. 9b is a schematic diagram showing an alternative layout or configuration for a five-stage birefringent filter or interleaver utilizing mirrors;

FIG. 10 is a schematic diagram of a one-stage birefringent filter or interleaver utilizing prisms according to the present invention;

FIG. 11 is a schematic diagram of a two-stage birefringent filter or interleaver utilizing prisms according to the present invention;

FIG. 12 is a schematic diagram of a three-stage birefringent filter or interleaver utilizing prisms according to the present invention; FIG. 13 is a schematic diagram of a five-stage birefringent filter or interleaver utilizing prisms, showing an array of input and output optical beams;

FIG. 14a is a schematic diagram showing an alternative layout or configuration for a three-stage birefringent filter or interleaver utilizing prisms; and

FIG. 14b is a schematic diagram showing an alternative layout or configuration for a five- stage birefringent filter or interleaver utilizing prisms.

Interleaver Using Spatial Birefringent Elements

These, and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings, wherein: FIG. 1 is a schematic diagram of a one-stage birefringent filter or interleaver according to the present invention;

FIG. 2 is a series of frames showing the optical beam states and the half-wave waveplate orientations at different locations for the interleaver shown in FIG. 1 ;

FIG. 3 is a schematic diagram of a two-stage birefringent filter or interleaver according to the present invention;

FIG. 4 is a series of frames showing the optical beam states and the half-wave waveplate orientations at different locations for the birefringent filter shown in FIG. 3;

FIG. 5 is a schematic diagram of a three-stage birefringent filter interleaver according to the present invention; and FIG. 6 is a series of frames showing the optical beam states and the half-wave waveplate orientations at different locations for the birefringent filter shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Low Crosstalk Flat Band Filter The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions of the invention and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

Generally, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarization selection elements (such as polarizers) and a birefringent element assembly (such as an assembly of birefringent crystals) disposed intermediate the two polarization selection elements and configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances transmission vs. wavelength curve flatness for a passband thereof and also in a manner which makes the stopband thereof deeper. It has been found that an element of a birefringent element assembly, such as an element comprised of a birefringent crystal, can be used to alter a contribution of a fundamental and a plurality of odd harmonics to a transmission vs. wavelength curve, wherein the alteration depends upon the parameters selected for the birefringent element. For example, a birefringent element may be selected so as to have a phase delay and a fast axis orientation (such as with respect to a polarization selection element) wherein the values of these parameters determine how much of a fundamental and a plurality of odd harmonics are present in the transmission vs. wavelength characteristic curve of a filter assembly. Changing these parameters tends to change the amount of the fundamental and the amount of each odd harmonic present in the transmission vs. wavelength curve. By carefully selecting the parameters for each birefringent element, a transmission vs. wavelength curve can be defined having desired characteristics, such as flatness of the passband, width of the passband, depth of the stopband, and width of the stopband.

Thus, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarization selection elements and a birefringent element assembly disposed intermediate the two polarization selection elements. The birefringent element assembly comprises a first birefringent element which provides an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave; a second birefringent element which cooperates with the first birefringent element to provide an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave plus a third harmonic of the fundamental sine wave; and a third birefringent element which cooperates with the first and second birefringent elements to provide an output vs. transmission curve which is approximately defined by a fundamental sine wave plus a third harmonic of the fundamental sine wave, plus a fifth harmonic of the fundamental sine wave. The parameters of the first, second and third birefringent elements are selected so as to enhance transmission vs. wavelength curve flatness for a filter passband and so as to deepen the stopband, by causing the fundamental sine wave and its third and fifth harmonics to sum in an advantageous manner.

According to the present invention, the birefringent elements preferably have parameters which are specifically selected so as to provide generally optimized curve flatness for the passband and so as to provide enhanced depth for the stopband, thus enhancing performance of the filter and also substantially mitigating undesirable cross-talk.

Those skilled in the art will appreciate that various different types of polarization selection elements and birefringent elements may be utilized in such a birefringent element assembly. Thus, various different types of birefringent elements are suitable for use in the present invention. Also, various different types of polarization selection elements may similarly be utilized.

Generally, such a birefringent element must provide paths having different optical path lengths for two orthogonally polarized (with respect to one another) optical signals. Thus, according to the present invention, a birefringent element is defined as any optical device suitable for providing different optical path lengths for generally orthogonal optical signals, so as to substantially mimic the effect provided by birefringent crystals.

Generally, such polarization selection elements must select or favor the transmission of light having one particular polarization direction and substantially reject, i.e., either absorb, reflect or deflect light having all other polarization directions. Thus, according to the present invention, a polarization selection element or polarizing element is defined as any optical device suitable for facilitating the transmission of light having one polarization direction, while substantially mitigating the transmission of light having other polarization directions.

Thus, a birefringent filter for filtering electromagnetic radiation may be provided according the present invention by disposing a birefringent element assembly between two polarization selection elements, wherein the birefringent element assembly provides an effect approximate to an effect provided by a first birefringent crystal providing a phase delay and having an angular orientation of a fast axis thereof of approximately 45° with respect to the polarization direction of the first polarization selection element, a second birefringent element providing a phase delay of approximately twice that of the first birefringent element and having an angular orientation of a fast axis thereof of approximately -21° with respect to the polarization axis of the same polarization selection element, and a third birefringent element providing a phase delay of approximately twice that of the first birefringent element and having an angular orientation of a fast axis thereof of approximately 7° with respect to the polarization axis of the same polarization selection element.

As defined herein, angular orientations are positive when they are clockwise as viewed looking into oncoming light and are negative when they are counterclockwise as viewed looking into oncoming light.

It is important to understand that, although the second polarization selection element will typically have a polarization direction which is parallel to the polarization direction of the first polarization selection element, the second polarization selection element may alternatively have a polarization direction which is orthogonal to the polarization direction of the first polarization selection element. Indeed, the second polarization selection element may have any desired angular orientation with respect to the first polarization selection. Varying the angular orientation of the second polarization selection element with respect to the first polarization selection element merely shifts the transmission vs. wavelength curve of the filter in frequency. Thus, a filter having passbands and stopbands at desired wavelengths may be constructed by orienting the polarization direction of the second polarization selection element with respect to the polarization direction of the first polarization selection element appropriately.

The filter of the present invention is suitable for use in an interleaver for separating channels from one another and/or for combining separate channels together in an optical communication system, such as a wavelength-division multiplexing system or a dense wavelength-division multiplexing system. That is, as those skilled in the art will appreciate, the filter of the present invention is suitable for use in both multiplexing and demultiplexing applications. Thus, the filter of the present invention may find application in such interleavers as those commonly used in optical communication system receivers and transmitters.

Referring again to Figure 1, the general position of components according to the present invention is the same as according to the contemporary filter discussed above. The lengths of the birefringent crystals of the present invention are also the same as in contemporary Sole-type filters (i.e., L, 2L and 2L for the first 15, second 16, and third 17 birefringent crystals, respectively). However, the angular orientation of each birefringent element or crystal has been varied, so as to optimize the resulting transmission vs. wavelength curve in a manner which enhances performance and mitigates undesirable crosstalk.

According to one preferred embodiment of the present invention, the polarization axis of the input polarization selection element 11 and the output polarization selection element 12 are approximately parallel with respect to one another. Again, the polarization axis of the input polarization selection element 11 and the output polarization selection element 12 may alternatively be orthogonal to one another, or at any other desired angle with respect to one another so as to provide the desired transmission vs. wavelength curve. The fast axis of the first birefringent crystal 15 is oriented at an angle of between approximately 43° and approximately 47°, preferably approximately 45°, with respect to the polarization axis of the input polarization selection element 11. The fast axis of the second birefringent crystal 16 is oriented at an angle of between approximately -19° and approximately -23°, preferably approximately -21°, with respect to the polarization axis of the input polarization selection element 11. The fast axis of the third birefringent crystal 17 is oriented at an angle of between approximately 5° and approximately 9°, preferably approximately 7°, with respect to the polarization axis of the input polarization selection element 11. It has been found that use of the above-listed angular orientations of the first 15, second 16, and third 17 birefringent crystals enhances the flatness of the passband of the filter, while simultaneously significantly deepening the stopband thereof. Thus, according to the present invention, performance is enhanced. The enhanced flatness of the passband and the deeper stopband cooperate to mitigate undesirable modification of the passed signal and to substantially mitigate undesirable cross-talk.

Referring again to Figures 2 and 3, it can be seen that the filter response curve of the present invention is substantially flatter (best shown in Figure 3) than the response curve of the contemporary filter for the passband thereof. More importantly, it can be seen that the stopband according to the present invention is approximately -40dB down with respect to the passband, whereas the stopband of the contemporary filter is only approximately -20dB down with respect to the past band. Further, the stopband is substantially wider, e.g., such as at the -30 dB points, according to the present invention than according to the contemporary filter.

Those skilled in the art will appreciate that such filter characteristics of the present invention provide substantially enhanced channel separation, as compared to the channel separation provided by a contemporary filter. Therefore, undesirable crosstalk will be substantially mitigated when utilizing the filter of the present invention. The present invention's birefringent element orientations of approximately 45° (first crystal 15), approximately -21° (second crystal 16), and approximately 7° (third crystal 17) provide enhanced performance as compared to the contemporary filter birefringent element orientations of 45° (first crystal 15), -15° (second crystal 16) and 10° (third crystal 17).

It has been found that the filter of the present invention provides crosstalk of less than -25dB and can be as low as -38dB over the flat passband, while having a ripple (and consequently loss) of less than O.OOldB.

The first 15 and third 17 birefringent crystals may optionally be swapped with one another without altering the performance of the improved filter of the present invention. The angular orientation of the first 15 and third 17 birefringent crystals remain the same after swapping. Thus, the third birefringent crystal 17 may be located next to the input polarization selection element (while keeping its angular orientation of approximately 7°), while the first birefringent crystal 15 may be located next to the output polarization selection element 12 (while keeping its angular orientation of approximately 45°), without altering the performance of the filter. This flexibility in arranging the birefringent elements provides convenience in system assembly, particularly when birefringent techniques other than birefringent crystals are utilized.

It is important to appreciate that, although the improved filter of the present invention has been described herein as utilizing birefringent crystals, other birefringent elements may be utilized to achieve similar effects. For example, polarization beam splitters (PBSs) or polarization beam displacers (PBDs) may be utilized as the polarization selection elements to separate the incoming optical signal into two orthogonally polarized optical beams and to cause the two beams to travel over different optical paths before being recombined. In this manner, the optically path length of each birefringent element may be varied for each polarization.

When birefringent elements other than birefringent crystals are utilized, then the angular orientations thereof necessary to provide the transmission vs. wavelength characteristic curve enhancement of the present invention can be converted into relative angles between the optical beam polarization direction and the equivalent fast axes of such devices.

Polarization selection elements other than polarizers may be utilized to effect desired polarization of the signal prior to encountering the birefringent element assembly and upon exiting the birefringent element assembly. For example, polarization beam splitters, (PBSs) or polarization beam displacers (PBDs) may be utilized to effect the desired polarization of the optical signals which are input to and output from the birefringent element assembly. Those skilled in the art will appreciate that various other optical devices are likewise suitable for facilitating such polarization selection functionality.

Further, angles other than 45°, -21° and 7° may be utilized for the first-15, second-16, and third- 17 crystals, respectively. That is, other sets of angles can give the same filter passband/stopband performance. If φi, φ₂, Φ₃ are the crystal orientations for crystals or other equivalent birefringent elements 1, 2 and 3, respectively, the same passband/stopband performance can be obtained at corresponding crystal orientations at 90°-φ_l5 90°-φ₂, 90°-φ₃ and 90°+φ₁, 90°+φ₂, 90°+φ₃, respectively. Examples of such angle sets are (45°, 111⁰ (-69°), 83°) and (135°, 69°, 97°) for the case of (45°, -21°, 7°) and (45°, 105° (-75°), 80°) and (135°, 75°, 100°) for the case of (45°, -15°, 10°). It is clear that other angles that are incremental of +/-180⁰ with respect to any one of these corresponding angles are possible solutions too.

The crystal orientation angles recited herein are defined as the angle between the fast axis of the birefringent device or crystal and the input light polarization direction of the light which has passed through the input polarization selection device or input polarizer 11. The sign convention for such angles is such that an angle is positive when resulting from clockwise rotation when facing the oncoming light and an angle is negative resulting from counterclockwise rotation when facing the oncoming light is negative. This convention is standard for the description of orientation angles in Sole-type filters. Further, according to the present invention, the phase delay of the first 15, second 16, and third 17 birefringent elements is provided by the relationship: 2Tι = T₂ = T₃, where T_\> r_2> Γ₃ are the phase delays for the first birefringent element 15, second birefringent element 16, and the third birefringent element 17, respectively.

The input polarization selection element 11 and the output polarization selection device 12 need not be parallel with respect to one another, but rather may have any other desired orientation. Generally, changing the orientation of the polarization axis of the output polarization selection device 12 with respect to the polarization axis of the input polarization selection element 11 results in a frequency shift (left or right movement of the transmission vs. wavelength curve) in the filter response. Optionally, the third birefringent crystal 17 may be omitted. In this instance, the transmission vs. wavelength curves of Figures 4 and 5 result. The angular orientations of the first birefringent element 15 and second birefringent element 16 remain the same, i.e., 45° (first crystal 15) and -21° (second element 16). Although the performance utilizing only the first birefringent crystal 15 and the second birefringent element 16 is reduced when compared with the performance of the three element filter of the present invention, applications for such a low cost filter exist.

The orientation of the polarization direction of the output polarization selection element with respect to the polarization direction of the input polarization selection element can be any desired angle. Changing the angle between the polarization direction of the output polarization selection element with respect to the polarization direction of the input polarization selection element merely shifts the transmission vs. wavelength curve in wavelength. That is, changing this angle merely changes the positions of the passbands and stopbands, so as to facilitate the selection of different desired channels in a wavelength division multiplexing or dense wavelength division multiplexing communication system. Thus, any desired or predetermined angle between the polarization direction of the output polarization selection element and the input polarization selection element may be utilized.

When a sequence of birefringent elements or crystals is recited herein, such as a sequence denoted as first, second and third, that sequence may either be from the input polarization element or from the output polarization element. For example, the sequence of first birefringent element, second birefringent element and third birefringent element can either define a series of birefringent elements wherein the first birefringent element is closest to the input polarization element, the second birefringent element is next, and the third birefringent element is closest to the output polarization element, or can alternatively define such a series wherein the first birefringent element is closest to the output polarization element, the second birefringent element is next, and the third birefringent element is closest to the input polarization element. Thus, the sequence can run either from input polarization element to the output polarization element or vice versa. This is possible since reversing the first and third polarization elements in a birefringent element assembly does not affect the transmissions characteristics thereof.

It is understood that the exemplary low cross-talk flat band filters described herein and shown in the drawings represent only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. For example, various means for separating an input signal into orthogonal signals which then are caused to follow different optical paths so as to achieve a birefringent effect are contemplated. Also, various means for polarizing signals provided to and emitted from the birefringent element assembly are known. Generally, any device which allows the selective transmission of light having a predetermined polarization direction (which does not substantially transmit light having other polarization directions) is suitable. Thus, these modifications and additions may be obvious to those skilled in the art and may be implemented adapt the present invention for use in a variety of different applications.

Apparatus and Method for Low Dispersion in Communications

The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions of the invention and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

Generally, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarization selection elements (such as polarizers) and a birefringent element assembly (such as an assembly of birefringent crystals) disposed intermediate the two polarization selection elements and configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances transmission vs. wavelength curve flatness for a passband thereof and also in a manner which makes the stopband thereof deeper and provides low dispersion, as well. It has been found that an element of a birefringent element assembly, such as an element comprised of a birefringent crystal, can be used to alter a contribution of a fundamental and a plurality of odd harmonics to a transmission vs. wavelength curve, wherein the alteration depends upon the parameters selected for the birefringent element. For example, a birefringent element may be selected so as to have an optical path length and a fast axis orientation (such as with respect to a polarization selection element) wherein the values of these parameters determine how much of a fundamental and a plurality of odd harmonics are present in the transmission vs. wavelength characteristic curve of a filter assembly. Changing these parameters tends to change the amount of the fundamental and the amount of each odd harmonic present in the transmission vs. wavelength curve. By carefully selecting the parameters for each birefringent element, a transmission vs. wavelength curve can be defined having desired characteristics, such as flatness of the passband, width of the passband, depth of the stopband, and width of the stopband. In this invention, it is shown that the dispersion characteristics can be controlled by carefully selecting the parameters for each birefringent element. Thus, the present invention comprises a filter for filtering electromagnetic radiation, wherein the filter comprises two polarization selection elements and a birefringent element assembly disposed intermediate the two polarization selection elements. The birefringent element assembly comprises a first birefringent element which provides an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave; a second birefringent element which cooperates with the first birefringent element to provide an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave plus a third harmonic of the fundamental sine wave; and a third birefringent element which cooperates with the first and second birefringent elements to provide an output vs. transmission curve which is approximately defined by a fundamental sine wave plus a third harmonic of the fundamental sine wave, plus a fifth harmonic of the fundamental sine wave. The parameters of the first, second and third birefringent elements are selected so as to enhance transmission vs. wavelength curve flatness for a filter passband and so as to deepen the stopband, and in addition, to also get desired dispersion behavior by causing the fundamental sine wave and its third and fifth harmonics to sum in an advantageous manner.

According to the present invention, the birefringent elements preferably have parameters which are specifically selected so as to provide generally optimized curve flatness for the passband and so as to provide enhanced depth for the stopband and to get desired dispersion behavior, thus enhancing performance of the filter and also substantially mitigating undesirable cross-talk and dispersion.

Those skilled in the art will appreciate that various different types of polarizing elements and birefringent elements may be utilized in such a birefringent element assembly. Thus, various different types of birefringent elements are suitable for use in the present invention. Also, various different types of polarization selection elements may similarly be utilized.

Generally, such polarization selection elements must select or favor the transmission of light having one particular polarization direction and substantially reject, i.e., either absorb, reflect or deflect light having all other polarization directions. Thus, according to the present invention, a polarization selection element or polarizing element is defined as any optical device suitable for facilitating the transmission of light having one polarization direction, while substantially mitigating (such as via reflection, deflection or absorption) the transmission of light having other polarization directions.

Thus, a birefringent filter for filtering electromagnetic radiation may be provided according to the present invention by disposing a birefringent element assembly between two polarization selection elements, wherein the birefringent element assembly provides an effect approximate to an effect provided by a first birefringent crystal providing a phase delay and having an angular orientation of a fast axis thereof of such as at approximately φ ι with respect to the polarization direction of the first polarization selection element, a second birefringent element providing a phase delay of approximately twice that of the first birefringent element and having an angular orientation of a fast axis thereof of such as at approximately φ ₂ with respect to the polarization axis of the same polarization selection element, and a third birefringent element providing a phase delay of approximately twice that of the first birefringent element and having an angular orientation of a fast axis thereof of such as at approximately φ ₃ with respect to the polarization axis of the same polarization selection element.

As defined herein, angular orientations (φ i, φ and φ ₃) are positive when they are clockwise as viewed looking into oncoming light and are negative when they are counterclockwise as viewed looking into oncoming light. It is important to understand that, although the second polarization selection element will typically have a polarization direction which is parallel to the polarization direction of the first polarization selection element, the second polarization selection element may alternatively have a polarization direction which is orthogonal to the polarization direction of the first polarization selection element. Indeed, the second polarization selection element may have any desired angular orientation with respect to the first polarization selection. Varying the angular orientation of the second polarization selection element with respect to the first polarization selection element merely shifts the transmission vs. wavelength curve of the filter in frequency. Thus, a filter having passbands and stopbands at desired wavelengths may be constructed by orienting the polarization direction of the second polarization selection element with respect to the polarization direction of the first polarization selection element appropriately.

The filter is suitable for use in an interleaver for separating channels from one another and/or for combining separate channels together in an optical communication system, such as a wavelength-division multiplexing system or a dense wavelength-division multiplexing system. That is, as those skilled in the art will appreciate, the filter is suitable for use in both multiplexing and demultiplexing applications. Thus, the filter of the present invention may find application in such interleavers as those commonly used in optical communication system receivers and transmitters.

Referring again to Figure 1, the lengths of the birefringent crystals of the Sole-type filters are L, 2L and 2L for the first 15, second 16, and third 17 birefringent crystals, respectively. However, the angular orientation of each birefringent element or crystal has been varied, so as to optimize the resulting transmission vs. wavelength curve in a manner which enhances performance and mitigates undesirable crosstalk. More importantly, according to the present invention an interleaver is configured in a manner to get desired dispersion behavior.

The crystal orientation angles recited herein are defined as the angle between the fast axis of the birefringent element or crystal and the input light polarization direction of the light which has passed through the input polarization selection device or input polarizer 11. The sign convention for such angles is such that an angle is positive when resulting from clockwise rotation when facing the oncoming light and an angle is negative resulting from counterclockwise rotation when facing the oncoming light is negative. This convention is standard for the description of orientation angles in Sole-type filters.

Further, according to the one embodiment of the present invention, the phase delay of the first 15, second 16, and third 17 birefringent elements is provided by the relationship: 2F! = r₂ = r₃, where r_1; T_2; r₃ are the phase delays for the first birefringent element 15, second birefringent element 16, and the third birefringent element 17, respectively. As those skilled in the art will appreciate, phase delay is proportional to a difference in optical path length.

The input polarization selection element 11 and the output polarization selection device 12 need not be parallel with respect to one another, but rather may have any other desired orientation. Generally, changing the orientation of the polarization axis of the output polarization selection device 12 with respect to the polarization axis of the input polarization selection element 11 results in a frequency shift (left or right movement of the transmission vs. wavelength curve) in the filter response.

It is understood that the exemplary zero or extremely low dispersion filters described herein and shown in the drawings represent only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. For example, various means for separating an input signal into orthogonal signals which then are caused to follow different optical paths so as to achieve a birefringent effect are contemplated. Also, various means for polarizing signals provided to and emitted from the birefringent element assembly are known. Generally, any device which allows the selective transmission of light having a predetermined polarization direction (which does not substantially transmit light having other polarization directions) is suitable. Thus, these modifications and additions may be obvious to those skilled in the art and may be implemented adapt the present invention for use in a variety of different applications.

Sole birefringent filters are commonly used as interleavers. Such Sole birefringent filters are discussed in detail above. When a single Sole birefringent filter is utilized alone, the Sole birefringent filter contributes some amount of dispersion to an optical signal transmitted therethrough. However, for a given Sole birefringent filter the quantity and the sign of the dispersion can be controlled by carefully selecting the birefringent phase delays (the optical path lengths) and the birefringent element orientations. In this manner, one Sole birefringent filter or interleaver may be constructed so as to substantially cancel the dispersion introduced by another Sole birefringent filter or interleaver. More generally, a Sole birefringent filter or interleaver can be constructed so as to substantially mitigate the dispersion caused by itself or any other device or combination of devices so as to minimize the total dispersion.

Referring now to FIG. 1, as discussed above, according to contemporary Sole filter construction, several, typically three, birefringent elements, such as birefringent crystal 15, birefringent crystal 16 and birefringent crystal 17 are disposed between two polarization selection elements, such as input polarizer 11 and output polarizer 12. Birefringent crystal 15, birefringent crystal 16 and birefringent crystal 17 define a birefringent element assembly 13. Typically, each birefringent crystal 15, 16, and 17 is comprised of a material having the same indices of refraction as each other birefringent crystal and the physical lengths of the three birefringent crystals are L, 2L and 2L, for each of the birefringent crystals 15, 16 and 17, respectively. However, as those in the field will appreciate, crystals comprised of different materials (and therefore having different indices of refraction) may alternatively be utilized and the physical lengths thereof may be adjusted to provide the equivalent phase delay, F, 2Σ, 2Y, for each of the birefringent crystals 15, 16 and 17, respectively. An angle between the fast axis of each birefringent crystal 15, 16 and 17 and the polarization direction of the input polarizer 11 is 45° for the first birefringent crystal 15; - 15° for the second birefringent crystal 16; and 10° for the third birefringent crystal 17. Each of the angular orientations is defined as positive if rotation is clockwise while viewing oncoming light from the input polarizer 11 and is negative if rotation is counterclockwise while viewing oncoming light from the input polarizer 11. This sign convention is the same sign convention that is commonly used by those skilled in the art of Sole filter construction.

Referring now to FIGs. 2-4, the dispersion (FIG. 2), phase distortion (FIG. 3), and transmission (FIG. 4) for a Sole birefringent filter having orientations of 45°, -15°, and 10° for the first 15, second 16 and third 17 birefringent crystals thereof and having phase delays of r, 2T, and 2T for the first 15, second 16 and third 17 birefringent crystals, respectively, are provided.

With particular reference to FIG. 2, the filter dispersion as a function of wavelength for a 50 GHz interleaver using the contemporary crystal orientations of 45°, -15° and 10° is shown. The dispersion increased very rapidly as wavelength moves away from the center wavelength of the pass band. The channel wavelength cannot always be well controlled at the pass band center due to various limitations in devices and in the communication system, as described in detail above. Therefore, channel wavelength deviation can lead to undesirably large dispersion and thereby substantially degrade the signal quality.

According to the present invention, dispersion can be substantially compensated for interleavers as well as in other devices as discussed in detail below. If φ _\, φ ₂ and φ ₃ are the crystal orientations for the first 15, second 16 and third 17 crystals, respectively, then the same transmission performance can be obtain at crystal orientations of 90°-φ ₁, 90°-φ ₂ and 90°-φ ₃, as well 90°+φ ₁, 90°+φ ₂ and 90°+φ ₃, respectively. However, for each of these two new sets of crystal orientations, the dispersion curve is flipped about the zero dispersion axis. That is, for each of the two new sets of crystal orientations, the dispersion for each wavelength has an opposite value to that of the original set of orientations (φ _1, φ ₂ and φ ₃). By matching one set of crystal orientations with another set of crystal orientations having opposite dispersion valves, approximately zero dispersion is obtained. Thus, one set of birefringent crystals can be made to substantially cancel the dispersion introduced by another set of birefringent crystals (or by any other component or system) when letting optical signals pass through them sequentially.

Thus, for example, the two angle sets of 45°, -75°, and 80° (corresponding to 90°-φ _\, 90°-φ ₂ and 90°-φ ₃) and 135°, 75° and 100° (corresponding to 90°+φ i, 90°+φ ₂ and 90°+φ ₃) provide the same transmission performance as the original angles of 45°, -15° and 10°, but provide dispersion curves which are the opposite of, i.e. flipped with respect to, these original angles. It is important to appreciate that adding or subtracting 180° to any of the birefringent element angular orientations provides an equivalent angular orientation therefor, and thus does not alter the characteristics the filter. Thus, two separate birefringent filters, wherein one birefringent filter has angles of φ _\, φ and φ ₃ while the other birefringent filter has birefringent crystals of either 90°-φ ₁, 90°-φ ₂ and 90°-φ ₃ or 90°+φ _h 90°+φ ₂ and 90°+φ ₃ define a birefringent filter assembly wherein the dispersion of one of the birefringent filters thereof substantially cancels out the dispersion of the other birefringent filter thereof.

FIGs. 5-7 show the dispersion, phase distortion, and transmission versus wavelength characteristic charts for a birefringent filter having angles of 45°, -75°, and 80° for the first 15, second 16, and third 17 birefringent crystals thereof and having phase delay of T, 2T, and 2T therefore.

With particular reference to FIG. 5, it should be appreciated that dispersion curve is flipped, with respect to the dispersion curve of FIG 2, such that each dispersion value of FIG. 5 is opposite that of FIG 2. Therefore, combining the birefringent filter which provides the dispersion curve of FIG. 2 with the birefringent filter which provides the dispersion curve of FIG. 5 will result in a substantially flat dispersion curve. It is worthwhile to note that the substantially flat dispersion curve resulting from the cooperation of two such birefringent filters (wherein each birefringent filter has a dispersion curve which is flipped with respect to the other birefringent filter) extends well beyond the pass band of the transmission versus wavelength curve for each of the birefringent filters and that the dispersion obtained by such a dual birefringent filter device is not dependent upon maintaining a channel center wavelength near the center of the pass band.

Various different sets of angles for the birefringent crystals may be utilized. Thus, the set of angles of 45°, -21°, and 7°, which provides enhanced pass band/stop band characteristics, may be utilized so as to facilitate mitigation of undesirable crosstalk. For the angles 45°, -21° and 7° the angle sets which provide such canceling or flipped dispersion characteristics are 45°, -69° and 83°, as well as 135°, 69° and 97°. Thus, by utilizing the set of angles of 45°, -21°, and 7° in a first birefringent filter or interleaver, along with a set of angels of either 45°, -69° and 83° or 135°, 69° and 97° in a second birefringent filter or interleaver, both enhanced pass band/stop band characteristics and enhanced dispersion may be achieved.

FIGs. 8-10 show the dispersion, phase distortion and transmission versus wavelength curves for a Sole birefringent filter having orientation angles of 45°, -21°, and 7° for the first 15, second 16 and third 17 crystals thereof and having phase delays of T, 2r, and 2T therefor. Similarly, FIGs. 11-13 show the dispersion, phase distortion and transmission versus wavelength curves for a birefringent filter having crystal angles of 45°, -69°, and 83° for the first 15, second 16 and third 17 crystals respectively and also having phase delays of T, 2T, and 2T therefor. It is clear from an inspection of FIGs. 8 and 11 that the dispersions provided by the birefringent filter having 45°, -21°, and 7° is opposite to the values of dispersion provided by the birefringent filter having 45°, -69°, and 83°. Thus, by combining these two birefringent filters, a single birefringent filter assembly or interleaver can be provided wherein this dispersion of one birefringent filter cancels out the dispersion of the other birefringent filter. In this manner, a birefringent filter assembly having approximately zero dispersion for the pass band portion of the transmission curve (as well as for other portions of the transmission curve) is provided.

Thus, for example, if optical communication beams are transmitted through two separate interleavers sequentially, wherein the two separate interleavers have been designed such that they have flipped dispersion curves with respect to one another, then the dispersion of one filter substantially cancels the dispersion of the other filter, such that approximately zero dispersion is obtained.

Two interleavers may be utilized in a cascaded configuration so as to enhance channel capacity. For example a 50 GHz interleaver and a 25 GHz interleaver can be used together so as to either multiplex or demultiplex optical signals from a 25 GHz space into a 100 GHz spacing and vice versa. According to the present invention, such a 50 GHz interleaver and such a 25 GHz interleaver can be configured in such a manner that the dispersion of each interleaver is generally opposite with respect to the dispersion of the other interleaver and the two interleavers substantially cancel the dispersion of each other. More generally, the first interleaver comprises an N GHz interleaver and the second interleaver comprises an N/2 GHz interleaver. That is, the first interleaver preferably has a channel spacing which is twice as large as that of the second interleaver. Those skilled in the art will appreciate that various other multiples or ratios of the channel spacing of the first interleaver to the channel spacing of the second interleaver are likewise suitable. Further, according to the present invention, a birefringent filter can be formed in a manner which provides a desired dispersion curve so as to substantially cancel dispersion caused by another optical device (such as by an optical device other than another interleaver). Thus, according to the present invention, dispersion versus wavelength curves having various different shapes can be obtained by varying the angular orientation of the crystals or birefringent elements of a birefringent filter. In order to compensate for the dispersion of optical device other than birefringent filter, the angular orientations of the birefringent crystals of a birefringent filter are varied in a manner which provides a dispersion versus wavelength curve having values which are opposite to those of the other device over the desired range of wavelengths.

Further, according to the present invention, dispersion can be mitigated in a single interleaver assembly, i.e. an interleaver assembly having only a single Sole filter, by carefully selecting the crystal orientations thereof.

FIGs. 14-16 show the dispersion, phase distortion and transmission versus wavelength curves for a single Sole filter assembly, i.e. a birefringent filter assembly, having only a single Sole filter, wherein the angular orientations of the first birefringent crystal 15, second birefringent crystal 16 and third birefringent crystal 17, having phase delays of T, 2Σ and 2T, respectively, have been carefully selected so as to minimize dispersion over the range of wavelengths defined by the pass band (as shown in FIG. 16). Similar and small dispersion can be obtained at (45, -65, 15), (45°, -25°, 75°) or (135°, 25°, 105°). These angles are selected by choosing φ _\ to be close to 45° or at 135° and then selecting φ and φ ₃ such that φ ₃ - φ ₂ is approximately plus or minus 90°. In this manner, a birefringent filter or interleaver can be made having only three birefringent elements or crystals (rather than having six birefringent elements or crystals as in the paired filters described above), wherein dispersion is mitigated.

Further angles which are + or - 180° with respect to any of the above discussed angles is equivalent, thus will provide the same results. Very similar results may be obtained if the angles deviate from the ideal angles by only a few degrees.

Further, for three crystal birefringent filters, the first and third crystals can be swapped in their physical locations and the interleaver will provide the same performance, i.e., will have the same transmission vs. wavelength curve and the same dispersion vs. wavelength curve. Thus, as shown in FIG. 1, the positions of the first birefringent crystal 15 and third birefringent crystal 17 may be swapped without altering the performance of the birefringent filter. As discussed above, it is possible to utilize devices other than birefringent crystals in order to obtain a birefringent effect. In such instances, the angles discussed above with respect to birefringent crystals may be converted to relative angles between the polarization direction of the input polarization selection element and the fast axis of the birefringent elements.

Further, note that 2 T i = T₂ = r₃, where r _1; T ₂ and T ₃ are the phase delays for birefringent element 1, birefringent element 2 and birefringent element 3, respectively. For example, a polarization beam splitter (PBS) or a polarization beam displacer (PBD) may be utilized to separate an incoming unpolarized optical beam into 2 orthogonally polarized optical beams, wherein the two beams experience different optical paths before being recombined to realize the birefringent effect.

Those skilled in the art will appreciate that the present invention may be utilized to provide an interleaver having any desired channel spacing.

Referring now to FIGs. 17-22, the dispersion versus wavelength, phase distortion versus wavelength and transmission versus wavelength curves for two different birefringent filters or interleavers are shown, wherein the two birefringent filters or interleavers have dispersion versus wavelength curves which are flipped with respect to one another. Thus, use of the two birefringent filters or interleavers configured according to the present invention results in approximately zero dispersion. The birefringent filters or interleavers which provide the curve shown in Figs. 17-22 both have phase delays of T , 217 , T for birefringent elements 1, 2, 3, respectively. The birefringent filter or interleaver which provides the curves of FIGs. 17-19 uses birefringent element orientations of 45°, -15° and 0° for birefringent elements 1, 2, 3. The birefringent filter or interleaver which provides the curves of Figs. 20-22 utilizes birefringent element orientations of 45°, -75°, and 90° for birefringent elements 1, 2, 3.

It may be beneficial, at least in some instances, to provide a birefringent filter or interleaver having phase delays of T , 2 T , T , since the birefringent elements used in such a device may be constructed so as to have an overall length which is shorter than that of a device having phase delays of T , 2T and 2T . Thus, by constructing such a device to have phase delays or birefringent element lengths of T , 2 T , T , a device which is shorter and less expensive maybe constructed. The device may be less expensive since a smaller third birefringent element is utilized, thereby reducing costs.

The exemplary low dispersion birefringent filter assemblies discussed above utilizes two birefringent filters, wherein each birefringent filter is comprised of three birefringent elements. It is also possible to construct a birefringent filter assembly, wherein each birefringent filter thereof comprises only two birefringent elements. Such a birefringent filter assembly can be constructed in a manner wherein each birefringent filter substantially cancels out the dispersion caused by the other birefringent filter. However, since only two birefringent elements are used in each such birefringent filter, the pass bands and stop bands thereof are not optimized as in the three birefringent element filters described above. That is, the pass bands of the two birefringent filters tend to cover a narrower range of wavelengths and the stop bands thereof tend to be more shallow. However, it is anticipated that in some instances it will be beneficial to provide a low dispersion filter assembly utilizing only two birefringent elements in each filter thereof, such as to maintain low cost of the birefringent filter assembly. As with the three birefringent element Sole birefringent filters described above, two element birefringent filters are constructed such that the dispersion of one birefringent filter substantially cancels the dispersion of the other birefringent filter.

Referring now to FIGs. 23-25, angles of 45° and -15° are utilized for a first birefringent crystal 15 and a second birefringent crystal 16, respectively and the phase delays are T and 2T, respectively. This is one example of a two element birefringent filter which may be utilized in a birefringent filter assembly, wherein each two element birefringent filter thereof tends to cancel out the dispersion introduced by the other two element birefringent filter.

With particular reference to FIG. 23, these crystal orientations provide a characteristic dispersion curve, as shown. As discussed above, a second birefringent filter, having a flipped dispersion curve with respect to this birefringent filter, can be utilized so as to substantially mitigate dispersion in a birefringent filter assembly comprised of both such birefringent filters.

Referring now to FIGs. 26-28, angular orientations of 45° and -75° for the first birefringent crystal 15 and the second birefringent crystal 16 are provided and the first and second birefringent crystals have phase delays of T and 2T, respectively.

With particular reference to FIG. 26 it is clear that the dispersion curve shown therein is flipped with respect to the dispersion curve of FIG. 23. Thus, as with the three element birefringent filters discussed above, two element birefringent filters may be utilized in a single birefringent element assembly so as to substantially cancel dispersion particularly over a desired range of wavelengths.

Also, such two element birefringent filters may be utilized to substantially cancel dispersion from any other (non-interleaver) optical device by defining a dispersion curve for such canceling, as discussed above. Further, according to the present invention a birefringent filter having any desired number of elements may be formed so as to provide variable dispersion, such as by facilitating the rotation of one or more of the birefringent elements thereof. Thus, a tunable birefringent filter may be provided wherein adjustments to the dispersion versus wavelength curve thereof may be effected either in a realtime or non-realtime mode.

It is important to appreciate that the technique for mitigating dispersion of the present invention is applicable to optical devices such as birefringent filters and interleavers regardless of the angles of the birefringent elements thereof. That is, for any given set of angles of birefringent elements, dispersion may typically be substantially mitigated by transmitting the light through another, typically similar, device having angular orientations of 90° - φ i, 90° - φ ₂ and 90° - φ ₃, or 90° + φ _b 90° + φ ₂ and 90° + φ ₃. Thus, the technique of the present invention may be utilized to mitigate dispersion whether the birefringent angular orientations of 45°, -21° and 7° (which provide enhanced transmission characteristics) or the angular orientations of 45°, -15°, and 10° (which provide a less desirable transmission characteristics of the prior art) or any other angles are utilized.

When birefringent elements other than birefringent crystals are utilized, then the angular orientations thereof necessary to provide the desired dispersion vs. wavelength characteristic curve of the present invention can be converted into relative angles between the optical beam polarization direction and the equivalent fast axes of such devices.

Polarization selection elements other than polarizers may be utilized to effect desired polarization of the signal prior to encountering the birefringent element assembly and upon exiting the birefringent element assembly. For example, polarization beam splitters, (PBSs) or polarization beam displacers (PBDs) may be utilized to effect the desired polarization of the optical signals which are input to and output from the birefringent element assembly. Those skilled in the art will appreciate that various other optical devices are likewise suitable for facilitating such polarization selection functionality. It is understood that the exemplary dispersion compensating birefringent filter described herein and shown in the drawings represents only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. For example, those skilled in the art will appreciate the various different configurations of birefringent filters may be utilized according to the present invention. For example, birefringent filters having four, five, six or more elements may similarly be configured so as to mitigate dispersion from either a similar birefringent filter or from any other component. Indeed, two birefringent filters, each having a different number of elements, may be utilized so as to tend to mitigate dispersion from one another. That is, one of the two differently configured birefringent filters may be constructed so to cancel at least a portion of the dispersion contributed by the other birefringent filter.

Thus, these and other modifications and additions may be obvious to those skilled in the art and may implemented to adapt the present invention for use in a variety of different applications.

Birefringent Devices

The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. The description contained herein is directed primarily to the configuration of an interleaver as a demultiplexer. However, as those skilled in the art will appreciate, the present invention may be used in both demultiplexers and multiplexers. The difference between demultiplexers and multiplexers is small and the configuration of the present invention as either desired device is well within the ability of one of the ordinary skill in the art.

Two different reference systems are used in this patent application for the determination of angular orientations. One reference system is used for the determination of the angular orientations of birefringent elements, such as birefringent crystals, with respect to the polarization direction of input light. Another reference system is used for the determination of the angular orientations of birefringent elements and the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system. Thus, for the birefringent element angular orientations, two separate reference systems are utilized. Thus, when reading the detailed description below, it will be very helpful to understand these two reference systems.

When angular orientation of birefringent element is discussed, the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counterclockwise with respect to the polarization direction of the oncoming light.

If there is a series of birefringent elements, such as in a birefringent filter, the angular orientations of each of the elements of the filter are measured by their fast axes with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the angular orientations are determined separately for each birefringent filter (the angular orientations are measured with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of each different filter). Thus, each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.

By the way of contract, the angular orientation of birefringent elements and angular orientations of waveplates are also measure by the fast axes of birefringent elements and the optic axes of waveplates with respect to the +χ axis. However, it is very important to appreciate that the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system. The use of such a moving coordinate system allows the optical beam states, the birefringent elements, and the waveplates to be viewed in a consistent manner at various locations in the devices, i.e., always looking into the light, and therefore substantially simplifies viewing and analysis of the devices.

Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +χ axis (which is consistent the conventional use of (x, y, z) coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements with respect to the input polarization direction, as discussed above). As those skilled in the art will appreciate, an interleaver is an optical device which typically includes at least one birefringent filter. Further, a birefringent filter is one example of a comb filter.

The present invention comprises a method and apparatus for providing birefringence without the use of a birefringent crystal. The method comprises separating a first composite light beam into first and second components thereof, wherein the first and second components are orthogonally polarized with respect to one another. The first component is transmitted along a first path and the second component is transmitted along a second path. The first and second paths have different optical path lengths. The first and second components are recombined, so as to form a second composite light beam. The second composite light beam is birefringent with respect to the first composite light beam, because the first and second paths have different optical path lengths and the different optical path lengths cause the light traveled along the first and second paths to be recombined with a relative phase shift therebetween.

Thus, according to the present invention, different optical paths simulate the effect of a birefringent crystal for components of a composite light beam which have been separated and which are subsequently recombined. The different optical path lengths may be provided either by having different physical path lengths, i.e., wherein each component travels a different physical distance in the same type of medium (for example, one component may travel four centimeters, while the other component travels five centimeters), or, alternatively, the different optical path lengths may be provided by transmitting the light through materials having different indices of refraction. Any desired combination of physical path lengths and indices of refraction may be utilized so as to provide the desired different optical path lengths, according to well known principles.

According to the present invention, a birefringent device comprises a polarization separating device which is configured to separate a first composite light beam into first and second components thereof, where the first and second components are orthogonally polarized with respect to one another. As used herein, the term "polarization separating device" is defined to include any device which will separate a composite light beam (a light beam containing components having different polarizations) into differently (such as orthogonally) polarized components thereof. Thus, for example, the polarization separating device may comprise a polarization beam displacer (PBD) or a polarization beam splitter (PBS).

A first path is configured to transmit the first component and a second path is configured to transmit the second component. The first path has a first optical path length and the second path has a second optical path length. The first optical path length is different with respect to the second optical path length. Thus, as described above, a birefringent effect is provided.

A polarization combining device is configured to recombine the first and second components, so as to form a second composite light beam. The second composite light beam is birefringent with respect to the first composite light beam. As used herein, the term "polarization combining device" is defined to include any device which will combine light components having different polarizations into a composite light beam containing those components.

According to the present invention, the polarization separating device and the polarization combining device may each comprise a polarization beam displacer, a polarization beam splitter, or any desired combination thereof. Thus, for example, both the polarization separating device and the polarization combining device may comprise a polarization beam displacer, or the polarization separating device may comprise a polarization beam displacer while the polarization combining device comprises a polarization beam splitter.

When both the polarization separating device and the polarization combining device comprise polarization beam displacers, then both the polarization separating device and the polarization combining device may comprise a common, i.e., the same, polarization beam displacer. That is, the first composite light beam is separated by a polarization beam displacer into first and second components thereof, and then (after having traveled different paths) the first and second components are subsequently recombined by the same polarization beam displacer so as to form the second composite beam. Thus, the birefringent device may comprise either one or two polarization beam displacers, as desired.

If two polarization beam displacers are utilized, then the two polarization beam displacers may be disposed side-by-side with respect to one another and mirrors and/or prisms may be utilized so as to direct the two components from the first beam displacer to the second beam displacer, in a manner which facilitates the definition of different optical path lengths therefor.

Alternatively, when two separate polarization beam displacers are utilized, then the two separate polarization beam displacers may be oriented linearly, i.e., in-line with and along a common axis with respect to, one another. In this instance, it is not necessary to provide prisms or mirrors to define the two different optical path lengths for the two components. Rather, the two optical path lengths may be defined by the insertion of materials having differing indices of refraction, intermediate the first and second beam displacer. Two such polarization beam displacers may have any desired position with respect to one another according to the present invention. Thus, the two polarization beam displacers may be orthogonal with respect to one another, or may be at any other desired angle with respect to one another.

According to one configuration (such as that shown in Figure 6) of the present invention, a polarization beam splitter is used to define two optical paths, each optical path having a different optical path length. When a polarization beam splitter is used to define two optical paths in a manner wherein beam splitting occurs twice (such as shown in Figure 6) within the polarization beam splitter, each time at a different location, then two separate polarization beam splitters may be utilized in place of the single (as shown in Figure 6) polarization beam splitter. That is, rather than having a single polarization beam splitter which splits light twice at two different locations therein, two separate polarization beam splitters may alternatively be utilized, wherein one polarization beam splitter is located at each of the two beam splitting locations.

According to the present invention, the first and second paths may comprise paths of air or vacuum or any other medium of desired properties, wherein the first and second components travel through materials having substantially similar indices of refraction. Alternatively, materials having different indices of refraction may be utilized in the first and second paths to provide different optical path lengths. Further, any desired combination of variation in physical path length and index of refraction between the first and second paths may be utilized, so as to provide the desired degree of birefringence.

Optionally, at least one of the first and second optical path lengths is variable, so as to facilitate changing of the amount of birefringence in the second composite light beam. Varying the optical path length of at least one of the first and second paths may be accomplished by varying the physical length of at least one of the first and second paths or by varying an index of refraction of a material disposed along at least one of the first and second paths. The physical length of the first and/or second path may be varied by moving a mirror or prism which defines the first and/or second path or by applying an electrical field, a magnetic field, or mechanical force to the material to change the refractive index thereof. Varying the index of refraction of a material disposed in the first and/or second path may be accomplished by selectively removing and inserting different materials into at least one of the first and second paths. For example, a filter wheel wherein each filter is configured to pass the first and/or second component and wherein each filter has a different index of refraction, may be utilized to selectively insert a desired filter (material having a desired index of refraction) into at least one of the first and second paths. Such a filter wheel may, for example, have a plurality of such filters disposed proximate a periphery thereof and would be rotatable so as to move a desired filter into at least one of the first and second paths.

Optionally, a half-wave waveplate is disposed along the first and second paths. The half-wave waveplate is preferably positioned where the first and second paths are co-located, such that a single half-wave waveplate affects both paths. Alternatively, two half-wave waveplates could be utilized, wherein each half-wave waveplate is only in one of the two paths.

In those configurations of the polarization beam displacer based birefringent device, wherein two polarization beam displacers are utilized and the two polarization beam displacers are in a linear or co-axial configuration with respect to one another, the half-wave waveplate may be omitted altogether. If the half- wave waveplate is omitted, then the second polarization beam displacer should be rotated 180° around the light propagation direction, so that the two beams recombine.

Optionally, a second half-wave waveplate is provided at the output of the birefringent device, so as to orient the second composite output beam the same as the first composite or input light beam.

In those configurations of the birefringent device of the present invention which utilize the polarization beam splitter and mirrors, a quarter-wave waveplate is placed in each path such that each component passes through the quarter-wave waveplate twice (once on its way to a mirror and once on its return from the mirror). In the birefringent device utilizing a polarization beam splitter and prisms, a half- wave waveplate is placed in each path such that each component passes through the quarter- wave waveplate once as it traverses a path.

The present invention thus utilizes a polarization separating device and a polarization combining device so as to effect splitting of an input light beam into components thereof and subsequent recombining of the components into an output light beam. According to one configuration of the present invention, the polarization separating device and the polarization combining device each comprise polarization beam splitters. According to another configuration of the invention, the polarization separating device and the polarization combining device each comprise polarization beam displacers. As those skilled in the art will appreciate, a physical path length, as used herein, is the distance which a beam of light travels and the optical path length is the distance that the beam of light travels multiplied by the index of refraction of any materials that the beam of light is transmitted through along the path. Referring now to Figures 1, 2a and 2b, an exemplary embodiment of the present invention having two polarization beam displacers 101 and 102 is shown. It should be appreciated that, in the configuration shown in Figure 1, wherein at least one prism is utilized to define the first and second paths, the two separate polarization beam displacers 101 and 102 may be replaced with a single polarization beam displacer which is configured such that the light paths remain substantially the same. That is, by merely substituting a single polarization beam displacer which is large enough to perform the functions of the two separate polarization beam displacers 101 and 102, the same effect may be achieved.

Figure 1 is a top view of an exemplary birefringent device which consists of the two polarization beam displacers 101 and 102, two prisms 103 and 104, and two half- wave waveplates 105 and 106.

When a composite light beam, enters the first polarization beam displacer 101, the polarization beam displacer 101 splits the composite light input beam into two separate component beams (as best shown in Figure 2a). The composite input beam is split according to the optical field polarization direction of the components thereof. For the input optical component polarized in the x direction (within the plane of the paper), the polarization beam displacer 101 does not substantially affect the path thereof and this component travels substantially straight through the polarization beam displacer 101. For the input component polarized in the y direction (which is perpendicular to the plane of the paper), the polarization beam displacer 101 displaces this component and this component leaves the polarization beam displacer 101 in the same direction as the composite input beam, but its optical path is shifted laterally (downwardly as shown in Figure 2a) from the original path defined by the composite input beam.

According to the present invention, this shift in direction is used to facilitate the definition of two separate paths, wherein each path has a different optical path length, so as to create birefringence when the two components are recombined. The need for a birefringent crystal is eliminated and disadvantages associated with the use of such a birefirngent crystal are mitigated.

As shown in Figures 1, 2a and 2b, the arrows within the polarization beam displacers 101 and 102 show the direction in which one component of the composite beam is shifted. The direction in which the components are shifted can be seen most clearly in Figures 2a and 2b.

As shown in Figures 1, 2a and 2b, two prisms 103 and 104 are utilized. Prism 103 is a bottom prism and prism 104 is a top prism. The top prism 104 is farther away from the polarization beam displacers 101 and 102 than the bottom prism 103. Thus, two different paths, each path having a different physical length, are defined.

Optionally, a material having desired optical characteristics, e.g., temperature stability, may be disposed along that portion of the top path which defines the difference in path lengths (shown as L/2 in Figure 1). As those skilled in the art will appreciate, having such a thermally stable material disposed within this area facilitates enhanced control over the optical properties for the optical path length difference. For example, when that portion of the upper path which defines the difference in path lengths (L/2 as shown in Figure 1) is occupied by a material of extremely good thermal stability in its optical path length, then changes in temperature will not result in undesirable changes in optical path length difference between the first and second paths. Such material thereby provides good control of the birefringence value over the temperatures. By way of contrast, disposing a generally thermally stable material, i.e., a material which does not vary substantially in optical path length, in that area which defines the difference in path length (L/2 as shown in Figure 1), results in changes in temperature having substantially less effect upon the birefringence value provided by the device.

One important aspect of this invention is the ability to control the difference in optical path length between the first and second paths, so that the birefringence value provided by this difference in optical path length does not vary undesirably during operation of the invention, such as due to temperature changes. As those skilled in the art will appreciate, the birefringence values of a device determine the operational characteristics, i.e., transmission, dispersion, and phase distortion, thereof. Therefore, it is very important that the optical path length differences (and consequently the birefringence values) remain substantially fixed during operation of the devices. Portions of the first and second paths, other than the portions which contribute the optical path length differences, are less critical since these other portions do not determine birefringence values. Generally, portions of the first and second paths, other than the portions which contribute to the optical path length differences, tend to vary in physical length and/or experience changes in an index of refraction thereof in response to environment (e.g., temperature) changes by approximately the same amount, due to structural similarity and symmetry of the first and second paths, and thus do not generally tend to change the optical path length difference. Therefore, it is that portion of the first and second paths (e.g., the L/2 or L portion shown in the figures) which directly provides the difference in optical path length that must be most carefully controlled. According to the present invention, the difference in optical path length between the first and second paths may optionally be controlled by inserting a material having desired optical, thermal and/or mechanical properties into at least the longer of the two paths, so as to substantially fix the optical path length which defines the difference between the first and second paths. Thus, by inserting such a material into at least that portion of one path that defines an optical path length difference (e.g., the L/2 portion of the path shown in the figures), substantially more stable operation of the devices is achieved.

Optionally, according to the present invention, those portions of the first and second paths which do not contribute to the optical path length difference comprise air, vacuum or any other material. Of course, these portions of the first and second paths are inherently equal in physical lengths to one another (since they do not contribute to the optical path length difference).

As those skilled in the art will appreciate, a single prism may be utilized instead of the two separate prisms 103 and 104. In this instance, the physical path length would be the same for each path and a material having a desired index of refraction would be inserted into one of the two paths, so as to change the optical path length of that path and make the optical path lengths of the two paths different.

As a further alternative, the two prisms 103 and 104 may be eliminated altogether and the second polarization beam displacer 102 may be placed along the longitudinal axis of the first polarization beam displacer 101. In this co-linear configuration of the two polarization beam displacers, the first beam displacer separates the composite beam into two parallel traveled component beams. A material having a desired index of refraction is placed in the path of one of the two component beams so as to vary the optical path length thereof and so as to effect birefringence when the two component beams are recombined by the second beam displacer 102. However, the use of the prism(s) facilitate the economical use of a single beam displacer for both separating and recombining, as described above.

Those skilled in the art will appreciate that, rather than prisms 103 and 104, a set of mirrors may alternatively be utilized so as to redirect the component beams from the first polarization beam displacer 101 to the second polarization beam displacer 102, or from a single polarization beam displacer back to the same polarization beam displacers. It is important to appreciate that in all of the drawings, where a coordinate system is utilized, light always travels in the +z direction. Thus, when the light changes direction, such as via the prisms of Figure 1, then the coordinate system is transformed by the change of direction of the light such that the +z direction is still in the direction in which light propagates. Thus, in Figure 1, the +z direction for the upper light path (such as though the input beam displacer 101) has the +z direction point to the right and the lower path (such as via output beam displacer 102) has the +z direction pointing to the left. This convention is particularly useful because it allows a single frame of reference or coordinate system to be utilized in the optical beam state diagrams (such as those of Figure 3), wherein the beam states can thereby always be viewed as looking into on-coming light without a change of coordinate systems. Thus, as shown in Figure 3, all of the optical beam states are viewed by looking in the -z direction (since the light is always propagating in the +z direction, toward the viewer). ,*

Referring now to Figures 2a and 2b, the first prism 103 is shifted along the z axis with respect to the second prism 104 by a distance of L/2. As those skilled in the art will appreciate, such shifting provides a difference in path length between the first path and the second path of L. This distance may be varied, so as to facilitate corresponding variation in the amount of birefringence in the second composite light beam, by changing the distance by which the first prism 103 is shifted with respect to the second prism 104. This may be achieved either by moving the first prism 103 or by moving the second prism 104, along the z axis.

Referring now to Figure 3, the optical beam states and the half- wavelength waveplate orientations at various locations can be seen. The number for each frame shown in Figure 3 corresponds to a physical location in Figures 1, 2a and 2b. Thus, frame 0 shows a composite beam having a first polarization along the y axis and a second polarization along the x axis and corresponds to the composite input light beam shown at position 0 in Figures 1, 2a and 2b. After the beam propagates through the first polarization beam displacer 101, component 2 remains at the top beam position and component 1 shifts to the bottom beam position. Component 2 enters the top prism and is reflected twice before being transmitted through the half-waveplate 105. Similarly, component 1 enters the bottom prism 103 and is reflected twice before being transmitted through the half- wave waveplate 105. The optical axis of the half -wave waveplate is shown in frame 2 and is oriented at 45° with respect to the +x axis. The half- wave waveplate changes the polarization direction of the components 1 and 2 by 90° as shown in frame 3. After the two components pass through the second polarization beam displacer 102, component 1 remains at the bottom beam position and component 2 shifts from the top beam position to the bottom beam position.

Because of the position difference between the top prism 104 and the bottom prism

103, there is a phase difference T (T = L ^• 2π / λ , where λ is the optical wavelength) between component 1 and component 2 when component 1 and component 2 are combined at location 4. In this manner, birefringence is created between the two orthogonally polarized components of the composite light beam.

Optionally, a second half-wave waveplate 106 may be plotted at the output of the second polarization beam displacer 102 and oriented at 45° with respect to the +x axis, so as to change the polarization directions of components 1 and 2 back to their original directions as shown in frame 6.

According to the present invention, birefringence is created by providing a difference in optical path length, i.e., by providing a difference in either physical path length or index of refraction. Creating birefringent with differing optical path lengths provides many advantages of the present invention as compared to contemporary birefringement elements, which utilizes birefringent crystals. For example, the birefringence value which may be obtained according to the present invention is comparatively large. Further, by controlling the relative positions of the prisms or mirrors or by varying an index of refraction, the birefringence is tunable and either negative or positive birefringence can selectively be obtained from the same device configuration. Because the beam shift is symmetric in the apparatus, the polarization mode dispersion (PMD) is minimized.

Optionally, ultra-low expansion (ULE) or fused silica, or any other desired material having a very low thermal expansion coefficient may be utilized as a gasket, package, optical bench or mounting bracket to mount or host the device components, i.e., polarization beam displacer(s) and the prism(s) or mirrors, so as to enhance temperature stability.

According to the present invention, the cost of both materials and assembly is substantially mitigated as compared to contemporary birefringent devices.

Referring now to Figures 4-7, a birefringent device utilizes a polarization beam splitter, rather than the polarization beam displacers shown in Figures 1-3. With particular reference to Figure 4, a top view of the birefringent device has the polarization beam splitter 201, two quarter- wave waveplates 202 and 203 and two etalons or mirrors 204 and 205.

When a composite beam enters the polarization beam splitter 201, the composite beam splits into two component beams according to their optical field polarization directions.

For the input optical component polarized in the x direction (within the plane of the paper), that component leaves the polarization beam splitter 201 in a propagation direction which is parallel to the input beam propagation direction. For the input optical component which is polarized in the y direction (which is perpendicular to the plane of the paper), that component leaves the polarization beam splitter 201 in a propagation direction which is orthogonal to the input beam propagation direction. Referring now to Figure 5, the optical beam states and the quarter- wave waveplate orientations at various locations for the birefringent device shown in Figure 4 are provided. At location 0, the input polarization optical beam has two linearly polarized components, i.e., 1 (which is along the y direction) and 2 (which is along the x direction). Only component 2 travels to location 1. The optical axis of the quarter-wave waveplate at location 2 is oriented at 45° with respect to the +χ axis. Thus, the light at location 3 is circularly polarized. After light is reflected by mirror 205, the light remains circularly polarized with a reverse rotation direction at location 4. After the light passes through the quarter-wave waveplate, it becomes a linearly polarized light component with a polarization direction along the y direction at location 5. When this component enters the polarization beam splitter 201, it is reflected so as to propagate to location 11 and thereby help define the composite output light beam.

Similarly, for component 1, light travels through locations 6, 7, 8, 9, and 10. Similarly, the optical axis of the quarter-wave waveplate at location 7 is oriented at 45° with respect to the +x axis. At location 10, the linear polarized light has a polarization direction along the x direction. Thus, component 1 can propagate directly from location 10 to location 11.

Since the distance between the polarization beam splitter 201 and the first mirror 205 is Li, and the distance between the polarization beam splitter 201 and the second mirror 204 is L₂, which is different from L_ls there is a phase difference T (T = 2 • (Li - L₂) • 2π /λ =

L-2π /λ , where λ is the optical wavelength) between component 1 and component 2 when component 1 and component 2 are combined at location 11. Thus, birefringence is created between the two orthogonally polarized components. Optionally, a half-wave waveplate (not shown) oriented at 45 degrees with respect to the +χ axis may be positioned beyond location 11, so as to change the polarization directions of components 1 and 2 to their original directions. Alternatively, birefringence can be obtained by inserting a material having a desired index of refraction into one of the two optical paths, rather than by forming the two optical paths so as to have different physical lengths. The material is configured so as to provide a phase delay in the path into which it is inserted and with respect to the other path, such that the desired birefringence value is obtained. That is, the length of the material inserted and the index refraction thereof is such that the desired phase delay, and consequently the desired value of birefringence is obtained.

As mentioned above, the portion of one path which contributes to the difference in path lengths (e.g., an L/2 portion between location 1 and location 2 in Fig. 4) may be filled with a material having desired optical, thermal or mechanical properties. One advantage of inserting such a material into this area is control of the birefringence value over various temperatures, as discussed above. Another advantage of inserting such a material is that the optical path difference can be realized with smaller space. Indeed, any of the path lengths of any of the configuration of the patent invention may be shortened if desired, via the insertion of such material.

Referring now to Figure 6, an alternative configuration of the birefringent device comprises polarizing beam splitter 201, first right-angle prism 209 and second right-angle prism 210. A first half- wave waveplate 211 is disposed intermediate the first prism 209 and the polarization beam splitter 201 and a second half- wave waveplate 212 is disposed intermediate the second prism 210 and the polarization beam splitter 201. It is important to appreciate that in the birefringent device shown in Figure 6, the component light beams entering and exiting the first 209 and second 210 prisms are parallel to one another, but are also offset with respect to one another. By way of contrast, the light beams instant upon and reflected from the mirrors 204 and 205 of the birefringent device shown in Figure 4 are coincident with one another, although they travel in different directions.

As those skilled in the art will appreciate, the use of prisms, 209 and 210, as shown in Figure 6, rather than mirrors, 204 and 205, as shown in Figure 4, provides an important advantage with respect to the undesirable feedback of light to the input light source. When mirrors are shown in Figure 4, then some portion of the light returning to the polarization beam splitter 201 from the mirror 205 will be transmitted from the polarization beam splitter 201 back to the light source, where undesirable feedback will occur. However, when the prism 209 in Figure 6 returns light to the polarization beam splitter 201, that light is returned as a different position with respect to which the light was originally split. Thus, none of the light returned to the polarization beam splitter 201 by the prism 209 is transmitted back to the light source and such undesirable feedback is thus avoided.

The optical beam states and the half-wave waveplate orientations at various locations are schematically shown in Figure 7. The input light component polarized along the x direction propagates through locations 1, 2, and 3 and the light component polarized along the y direction propagates through locations 4, 5 and 6. The two components join each other at location 7. Similarly, the phase difference T between the two orthogonally polarized light components is controlled by the position of the prisms with respect to the PBS (T = L-2π /λ ).

As in the previously discussed embodiments of the present invention, the difference in path length may be provided by making the LI between the first prism 209 and the polarization beam splitter 207 different from the distance L2 between the second prism 210 and the polarization beam splitter 201. Alternatively, a material having a desired index refraction may be positioned along either the first or second path so as to cause the optical path lengths to differ. Negative or positive birefringence can be obtained from the same device configuration. As before, using an ultralow expansion (ULE) or fused silica or any other material having a very low thermal expansion coefficient as a gasket to mount or host the device components, i.e., the polarization beam splitter 201 and the prisms 209 and 210 will provide enhanced temperature stability.

As mentioned above, the single polarization beam splitter 201 may, optionally, be replaced with two separate beam splitters, if desired.

The description contained herein is directed primarily to the configuration of an interleaver as a demultiplexer. However, as those skilled in the art will appreciate, the present invention may be used in both demultiplexers and multiplexers. The difference between demultiplexers and multiplexers is small and the configuration of the present invention as either desired device is well within the ability of one of the ordinary skill in the art.

Two different reference systems are used in this patent application for the determination of angular orientations. One reference system is used for the determination of the angular orientations of birefringent elements, such as birefringent crystals with respect to the polarization direction of input light. Another reference system is used for the determination of the angular orientations of birefringent elements and the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system. Thus, for the birefringent crystal angular orientations, two separate reference systems are utilized. Thus, when reading the detailed description below, it will be very helpful to understand these two reference systems.

When angular orientations of birefringent elements are discussed, the angular orientations are typically the fast axes of the birefringent elements with respect to the polarization direction of incoming light just prior to the incoming light reaching a birefringent element. Determination of these angular orientations is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counter-clockwise with respect to the polarization direction of the oncoming light.

If there is a series of birefringent elements, such as in a birefringent filter, the angular orientations of each of the elements of the filter are measured by their fast axes with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the angular orientations are determined separately for each birefringent filter (the angular orientations are measured with respect to the polarization direction if incoming light just prior to the incoming light reaching the first birefringent element of each different filter). Thus, each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.

By the way of contract, the angular orientation of birefringent elements and angular orientations of waveplates are also measure by the fast axes of birefringent elements and optic axes of waveplates with respect to the +x axis. However, it is very important to appreciate that the +x axis is part of the moving coordinate frame. This coordinate frame travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate frame rotates with the +y axes thereof so as to provide a new coordinate frame. The use of such a moving coordinate frame allows the optical beam states, the birefringent elements, and the waveplates to be viewed in a consistent manner at various locations in the devices, i.e., always looking into the light, and therefore substantially simplifies viewing and analysis of the devices.

Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +χ axis (which is consistent the conventional use of x, y, z coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements, as discussed above).

As used herein, the term gasket is defined to include any bracket, mount, optical bench, host, enclosure or any other structure which is used to maintain components of the present invention in desired positions relative to one another. Preferably, such gasket is comprised of an ultra low expansion (ULE) material, fused silica or any other material having a very low thermal expansion coefficient.

It is understood that the exemplary birefringent devices described herein and shown in the drawings represent only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. For example, those skilled in the art will appreciate that mirrors and prisms are generally interchangeable as light reflecting devices for defining the first and second physical path lengths. Also, various different layouts were configurations of the various different components of the present invention are contemplated. Generally, layouts are suitable which provide for different optical path lengths for the first and second paths. Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.

Comb Filter for Defense Wavelength Division Multiplexing

The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

The description contained herein is directed primarily to the configuration of an interleaver as a demultiplexer. However, as those skilled in the art will appreciate, the present invention may be used in both demultiplexers and multiplexers. The difference between demultiplexers and multiplexers is small and the configuration of the present invention as either desired device is well within the ability of one of the ordinary skill in the art. Two different reference systems are used in this patent application for the determination of angular orientations. One reference system is used for the determination of the angular orientations of birefringent elements, such as birefringent crystals, with respect to the polarization direction of input light. Another reference system is used for the determination of the angular orientations of birefringent elements and the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system. Thus, for the birefringent element angular orientations, two separate reference systems are utilized. Thus, when reading the detailed description below, it will be very helpful to understand these two reference systems. When the angular orientation of a birefringent element is discussed, the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counterclockwise with respect to the polarization direction of the oncoming light.

By the way of contract, the angular orientation of birefringent elements and angular orientations of waveplates are also measured by the fast axes of birefringent elements and the optic axes of waveplates with respect to the +x axis. However, it is very important to appreciate that the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system. The use of such a moving coordinate system allows the optical beam states, the birefringent elements, and the waveplates to be viewed in a consistent manner at various locations in the devices, i.e., always looking into the light, and therefore substantially simplifies viewing and analysis of the devices.

Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +x axis (which is consistent the conventional use of (x, y, z) coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements with respect to the input polarization direction, as discussed above). As those skilled in the art will appreciate, an interleaver is an optical device which typically includes at least one birefringent filter. Further, a birefringent filter is one example of a comb filter.

More particularly, the present invention comprises an interleaver comprising an input polarization beam displacer, a birefringent filter assembly in optical communication with the input polarization beam displacer, a first output polarization beam displacer in optical communication with the birefringent filter assembly and a second output polarization beam displacer in optical communication with the first output polarization beam displacer. The birefringent filter assembly comprises at least one birefringent filter stage. Each birefringent filter stage comprises a polarization beam splitter and two reflectors.

The two reflectors may comprise either mirrors or etalons.

Each birefringent filter stage preferably further comprises a quarter-wave waveplate disposed intermediate each reflector and the polarization beam splitter. Preferably, the quarter-wave waveplate has an optical axis thereof oriented at 45° with respect to a +χ axis at that location.

The interleaver further comprises a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly which is configured so as to transmit a non-displaced beam therethrough. The first input half-wave waveplate preferably has an optical axis thereof oriented at 22.5° with respect to the +χ axis at that location. A second input half-wave waveplate is similarly disposed intermediate the input polarization beam displacer and the birefringent filter assembly and is configured so as to transmit a displaced beam therethrough. The second input half-wave waveplate preferably has an optical axis thereof oriented at -22.5° with respect to the +x axis at that location.

A half-wave waveplate is configured to receive an output of each polarization beam splitter.

A first output half-wave waveplate is disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer. A second half-wave waveplate is disposed intermediate the first output polarization beam displacer. A third half-wave waveplate is disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer. A fourth half- wave waveplate is disposed intermediate the first output polarization beam displacer. The positioning and operation of the first, second, third and fourth output half-wave waveplates is discussed in detail below.

The birefringent filter assembly may comprise any desired number of birefringent filter stages. As those skilled in the art will appreciate, the use of additional birefringent filter stages enhances the transmission versus wavelength curve, such that a more flat and wider passband is defined and such that a deeper and wider stopband is defined. Thus, for example, the birefringent filter assembly may comprise one, two, three, four, five or more stages, as desired. Preferably, the input polarization beam displacer, the birefringent filter assembly, the first output polarization beam displacer and the second output polarization beam displacer are configured so as to facilitate interleaving of a plurality of beams simultaneously, preferably so as to facilitate interleaving of a plurality of arrayed beams simultaneously. Those skilled in the art will appreciate that various different configurations of arrays, e.g., two dimensional, square, rectangular, circular, oval, etc., may similarly be utilized.

Thus, the interleaver of the present invention comprises a birefringent filter assembly coupled to receive at least two beams of polarized light (such as two beams which are orthogonally polarized with respect to one another at the input). The birefringent filter assembly is configured so as to provide a birefringent effect with respect to the two beams, without the use of birefringent crystals. Rather, the birefringent filter assembly provides a birefringent effect with respect to the two beams by causing the two beams to travel along two different paths, wherein each path has a difference optical path length.

Referring now to FIG. 1, a one-stage optical interleaver comprises an input polarization beam displacer 10, a birefringent filter assembly 11 in optical communication with the input polarization beam displacer 10, a first output polarization beam displacer 12 in optical communication with the birefringent filter assembly 11, and a second output polarization beam displacer 13 in optical communication with the first output polarization beam displacer 12.

According to a first embodiment of the present invention, each stage of the birefringent filter assembly comprises a polarization beam splitter 14 and two reflectors, such as first 16 and second 17 mirrors.

FIGS. 1, 3, 5, 7, 8, 9a and 9b show the first embodiment of the interleaver having various different numbers of birefringent filter stages. Similarly, Figs. 10, 11, 12, 13, 14a and 14b show the second embodiment of the interleaver having various different numbers of birefringent filter stages. In each of FIGS. 1, 3, 5, 7, 8, 9a, 9b, 11, 12, 13, 14a and 14b, the components of the first birefringent filter stage have an "a" following the component number thereof, the components of the second birefringent filter stage have the letter "b" following the number of the component thereof and so on. Thus, like components within each different stage have the same number, but have a different letter which indicates which stage they are part of. For example, the polarization beam splitter is always number 14, regardless of which stage it is in, and is followed by the letter "a" (to form the reference number "14a") when in the first stage and is followed by the letter "b" (to form the reference number "14b") when in the second stage, and so on. When a component is referred to generically, i.e., without regard as to which specific stage the component is part of, then the letter may be omitted. Preferably, each birefringent filter stage 11 further comprises a first quarter-wave waveplate 18 intermediate each first mirror 16 and the polarization splitter 14 and similarly comprises a second quarter- wave plate 19 between each second mirror 17 and the polarization beam splitter 14 thereof. Preferably, the optical axis of both the first 18 and the second 19 quarter- wave waveplates are oriented at approximately 45° with respect to the +x axis at that location.

Preferably, the interleaver comprises a first input half- wave waveplate 21 disposed intermediate the input polarization beam displacer 10 and the birefringent filter assembly 11 and configured so as to transmit the non-displaced beam therethrough and also further comprises a second input half-wave waveplate 22 disposed intermediate the input polarization beam displacer 10 and the birefringent filter assembly 11 and configured so as to transmit the displaced beam therethrough. The first input half- wave waveplate 21 preferably has an optical axis thereof oriented at approximately 22.5° with respect to the plane within which the +x axis at that location. The second input half-wave waveplate 22 has an optical axis thereof oriented at approximately -22.5° with respect to the +x axis at that location. A half-wave waveplate 23 is configured to receive an output of each polarization of beam splitter 14. The half- wave waveplate 23 preferably has an optical axis oriented at an angle of -22.5° with respect to a +x axis at that location.

Four half-wave waveplates 26 are disposed intermediate the first output polarization beam displacer 12 and the second output polarization beam displacer 13. More particularly, a first half- wave waveplate, preferably having an optic axis orientation of approximately 45°; a second half-wave waveplate, preferably having an optic axis orientation of approximately 90°; a third half-wave waveplate, preferably having an optic axis orientation of 0°; and a fourth half- wave waveplate, preferably having an optic axis orientation of 45° are all disposed, preferably within a common plane, intermediate the first output beam displacer 12 and a second output beam displacer beam 13. The positions and the orientations of each of these half- wave waveplates 26 are shown in frame 14 of FIG. 2.

As discussed in detail below, the birefringent filter assembly 11 may comprise one birefringent filter stage, two birefringent filter stages, three birefringent filter stages, four birefringent filter stages, five birefringent filter stages or any other desired number birefringent filter stages. FIG. 1 schematically shows an interleaver comprising a one stage birefringent filter assembly. A right-hand coordinate system of axes is used to characterize the optical beam propagation in the system at various locations utilizing the convention that light is always propagating in the +z direction and that the +y direction is out of the plane of the paper in FIG. 1. This convention applies to all of the figures discussed herein.

Referring now to FIG. 2, the optical beam states, the quarter-wave waveplate and the half-wave waveplate orientations at various locations are shown in a plurality of frames, wherein the underlined number associated with each frame corresponds to the location where the wave state or waveplate orientation occurs in FIG. 1. Each of the 4 boxes of a frame corresponds to a physical beam position at various locations. As those skilled in the art will appreciate, the beam displacers provide both horizontal and vertical displacement of the beam, resulting in the formation of four separate beams. Each box of a frame of FIG. 2 corresponds to one of these four beams when viewed as looking into oncoming light. This applies to the frames of Figs. 4 and 6, as well. At location 0, an input or composite optical beam has two linearly polarized components 1 along the y direction and 2 along the x direction, at the top-right box or beam position. After the beam propagates through the first polarization beam displacer at location 1, the component 2 shifts to the top-left beam position and component 1 remains at the top- right beam position. The arrows shown on the polarization beam displacers on FIG. 1 indicate the beam shift direction for the polarization beam displacers of FIG. 1. After components 1 and 2, respectively, pass through the half- wave waveplates 21 at location 2, the linearly polarized components 1 and 2 are polarized along the same direction, i.e., -45° with respect to the +χ axis at location 3. At location 2, the optical axis of the half- wave waveplate for component 1 is oriented at 22.5° with respect to the +χ axis and the optical axis of the half-wave waveplate for component 2 is oriented at -22.5° with respect to the +x axis.

When component 1 enters the polarization beam splitter, component 1 splits into two beams according to the optical field polarization direction of each. The input optical component polarized in the x direction (la) propagates along its original propagation to location 4. At location 5, the quarter-wave waveplate 18a is oriented at 45° with respect to +x axis. After component la passes through the quarter-wave waveplate 18a, it is reflected by etalon or mirror 16a and passes through the quarter-wave waveplate 18a again. Its polarization direction is changed from the x direction to the y direction at location 6.

The input optical component polarized in the y direction (lb) is deflected by the polarization beam splitter and propagates in a direction orthogonal to the input beam propagation direction to location 7. The quarter- wave waveplate 19a at location 8 is oriented at 45° with respect to the +x axis. The polarization direction of the component lb is changed from the y direction to the x direction when it travels back to location 9. Components 1 a and lb are combined at location 10. It is worthwhile to understand that the distance L_x between the polarization beam splitter 14a and the first mirror 16a is different from the distance L₂ between the polarization splitter 14a and the second mirror 17a. Thus, there is a phase difference T in component la and component lb at location 10, as represented by the equation: r = 2 - (L_rL₂ )- 2π /λ =L- 2π /λ where λ is the optical wavelength. Component 2 (which becomes split into components 2a and 2b) propagates in a similar manner. The beam positions for components 1 and 2 are exchanged at location 10 due to deflection by the polarization beam splitter 14a.

Components and 1 and 2 pass through half- wave waveplate 23 a at location 11. Half- wave waveplate 23a is oriented at -22.5° with respect to +χ axis. Half-wave waveplate 23a changes the polarization direction of components 1 and 2 before they enter the second output polarization beam displacer 12. The new x and y components are shown in the frame for location 12. After these components pass through the first output polarization beam displacer 12, the components are polarized along the y direction are moved to the bottom beam location and the components polarized along the x direction remain at the top beam positions as shown in the frame for location 13.

Four half-wave waveplates 26 are disposed intermediate the first output polarization beam displacer 12 and the second output polarization beam displacer 13, such that each of the four beams from the first polarization beam displacer 12 passes through one of the four half- wave waveplates 26. The orientations of the four half-wave waveplates 26 are shown in the frame 14. After the beams pass through the four half- wave waveplates 26, their polarization directions are shown in the frame 15. After these beams pass through the second output polarization displacer 13, component la' moves to the top-right beam position to combine with component 2a' and component lb' moves to the bottom-right beam position to combine with component 2b'. Thus, light from the four half-wave plates 26 passes through the second output polarization displacer 13 to form two composite output beams, as shown in frame 16. One of the two composite output beams may be considered to contain the even channels, while the other of the two composite output beams may be considered to contain the odd channels of the communication signals. The interleaver shown in FIG. 1 is thus equivalent a conventional one-stage Sole-type interleaver having the fast axis of the birefringent crystal thereof oriented at 45° with respect to the polarization direction of light input thereto. The equivalent birefringent crystal orientation provided by the one-stage interleaver of FIG. 1 is determined by the orientation of the half- wave waveplates 21 and 22. Thus, various different birefringent crystal orientations can similarly be simulated by varying the orientation of the half- wave waveplates 21 and 22.

Further, a plurality of stages, wherein each stage corresponds to and simulates to a separate birefringent crystal having a unique angular orientation of a fast axis thereof, can be provided by providing a plurality of birefringent filter stages 11, wherein birefringent filter stage has a half-wave waveplate or the like at an input thereto so as to define the equivalent or simulated angular orientation corresponding to the angular orientation of a birefringent crystal. In this manner a plurality of stages, each stage having a unique angular orientation, can be provided so as to simulate a multi-crystal Sole birefringent filter (a multi-stage filter) having desired transmission characteristics. Thus, according to the present invention, two output beams (la', 2a') and (lb', 2b') are the two series of interleave channels and the phase delay T (T = L 2π / λ ), determines the channel spacing.

One important aspect of this invention is the ability to control the difference in optical path length between the first and second paths, so that the birefringence value provided by this difference in path length does not vary undesirably during operation of the invention, such as due to temperature changes.

As those skilled in the art will appreciate, the birefringence values of a device determine the operational characteristics, i.e., transmission, dispersion, phase distortion, thereof. Therefore, it is very important that the optical path length differences (and consequently the birefringence values) remain substantially fixed during operation of the devices.

Portions of the first and second paths, other than the portions which contribute the optical path length differences, are less critical since these other portions do not determined birefringence values. Generally, portions of the first and second paths, other than the portions which contribute to the optical path length differences, tend to vary (changes in physical length and/or changes in an index of refraction thereof) in response to environment (e.g., temperature) changes by approximately the same amount, due to structural similarity and symmetry of the first and second paths, and thus do not generally tend to change the optical path length difference. Therefore, it is that portion of the first and second paths (e.g., the L/2 or L portion shown in the figures) which directly provides the difference in optical path length that must be most carefully controlled.

According to the present invention, the difference in optical path length between the first and second paths may optionally be controlled by inserting a material having desired optical, thermal and/or mechanical properties into at least the longer of the two paths, so as to substantially fix the optical path length which defines the difference between the first and second paths. Thus, by inserting such a material into at least that portion of one path that defines optical path length difference (e.g., the L/2 portion or the L portion of the path shown in the figures), substantially more stable operation of the devices is achieved. Optionally, according to the present invention, those portions of the first and second paths which do not contribute to the optical path length difference comprise air, vacuum or any other material. Of course, these portions of the first and second paths are inherently equal in physical lengths to one another (since they do not contribute to the optical path length difference). According to the present invention, birefringence is obtained by optical path length differences, which may occur in free space, e.g., air or vacuum. A material of desired optical, thermal, and/or mechanical properties and having a desired index of refraction may be inserted along desired portion of the light paths of the present invention. For example, such a material may be utilized to shorten any desired path lengths and/or to provide a difference in optical path lengths to achieve a birefringent effect. For example, both paths can have the same physical dimensions, and birefringence may be obtained by inserting material having desired optical properties, e.g., an index of refraction greater than one, so as to cause the two paths to have different optical paths lengths. There are many advantages to the present invention as compared to conventional interleavers which utilize birefringent crystals. For example, the difference in optical path length can be manipulated so as to provided desired, comparatively high, birefringence values. An ultra low expansion (ULE) or fused silica may be utilized as a gasket in device construction, so as to obtain excellent temperature stability for the interleaver. Those skilled in the art will appreciate the various other materials having a very low thermal expansion coefficient are likewise suitable for use as such a gasket. Further, the optical path lengths may be made so as to be variable, thus providing adjustability of the birefringence value and a tunable interleaver. The interleaver of the present invention is simple in construction and low in cost. Thus, the present invention overcomes many of the limitations associated with contemporary birefringent crystal interleavers, such as those limitations associated with the optical, physical, mechanical and thermal properties of birefringent crystals. Because the beam shift is symmetric in the apparatus, the polarization mode dispersion (PMD) is minimized.

As mentioned above, interleavers having multiple stages of birefringent effect may be used. As those skilled in the art will appreciate, such multi-stage interleavers provide enhanced passband and stopband characteristics.

Referring now to FIG. 3, an interleaver having two stages of birefringent effect is shown schematically.

Referring now to FIG. 4, the optical beam states, the quarter-wave waveplate and the half-wave waveplate orientations at various locations are shown schematically. This interleaver corresponds to a Sole filter having birefringent crystal orientations of 45° and -21° and phase delays of T and 2T , respectively for an exemplary interleaver shown in FIG. 3.

Referring now to FIG. 5, a multi-stage interleaver having three stages of birefringent effect is shown schematically. The phase delay in the second and third stages is twice as large as that for the first stage (T_j = L - 2π /λ,T₂ = T₃ = 2L - 2π /λ). The interleaver channel spacing is determined by the phase delay in the first stage T_j

Referring now to FIG. 6, the optical beam states, the quarter-wave waveplate and the half-wave waveplate orientations at various locations of an exemplary three-stage interleaver of FIG. 5 are shown, the interleaver being equivalent to a Sole filter having birefringent crystal orientations of 45°, -21°, and 7° and having phase delays of T , 2T , and 2T , respectively.

It is important to appreciate that in each configuration of the present invention, some of the half-wave waveplates may be eliminated by rotating the beam displacer orientation accordingly.

Referring now to FIG. 7, a parallel array of input beams (e.g., 2, 4, 8, 16, ..., 256 or more channels may be configured so as to utilize the same interleaver. The use of such a parallel array of input beams is shown schematically in FIG. 7, wherein the wider beams indicate such a parallel array thereof. These input beams can be configured to a two- dimensional array, so as to facilitate high packaging density and low cost per channel.

It will therefore be appreciated that it is comparatively easy to expand the multi-stage interleaver of the present invention to have any desired number of stages so as to facilitate and enhance interleaver performance.

Referring now to FIG. 8, a five-stage interleaver configured to demultiplex a parallel array of beams simultaneously is shown schematically. Referring now to FIG. 9a, an alternative layout for a three-stage interleaver is shown. This three-stage interleaver is configured to demultiplex an array of beams simultaneously.

Referring now to FIG. 9b, an alternative layout for a five-stage interleaver is shown. This five-stage interleaver is configured to demultiplex an array of beams simultaneously. Referring now to FIGS. 10 - 12, an alternative embodiment of the interleaver of the present invention is shown schematically, wherein the quarter-wave waveplates and the etalons or mirrors are replaced by half- wave plates 31 and 32 and right-angle prisms 33 and 34. FIG. 10 shows a one-stage interleaver, FIG. 11 shows a two-stage interleaver, and FIG. 12 shows a three-stage interleaver. One advantage of the birefringent filter configuration of FIGS. 10-12 is that feedback is minimized. Feedback occurs when the optical signal is transmitted back to the source, where the optical signal may undesirably interfere with operation of the source. Feedback can occur in the embodiments of the present invention depicted in FIGS. 1-9 (which utilized mirrors rather than prisms), since the light is reflected back to the same point within the polarization beam splitter where the light was originally split. This provides an opportunity for some portion of the light which should be reflected away from the split point to be undesirably transmitted back to the source. Thus, isolation apparatus should be implemented between the interleaver and the input source if the feedback causes undesirable interference to the input source. According to the embodiments of the present invention shown in FIGS. 10- 12, which utilize prisms rather than mirrors, light from the prisms is directed back to a different point within each polarization beam splitter from where the light was originally split, such that any light which is transmitted through the polarization beam splitter, rather than reflected thereby, is not directed back to the source and undesirable feedback is thus mitigated. The corresponding optical beam states, waveplate orientations are the same as shown in Fig. 2 for an exemplary one-stage (45°, T ) interleaver, Fig. 4 for an exemplary two- stage (45°, r ; -21°, 2T) interleaver, and Fig. 6 for an exemplary three-stage (45°, T ; -21°, 2T ; 7°, 2T ) interleaver, respectively.

Referring now to FIG. 13, a five-stage interleaver utilizing half-wave waveplates and right angle prisms is shown. This five stage interleaver is configured to demultiplex an array of beams simultaneously .

Referring now to FIG. 14a, an alternative layout for a three-stage interleaver utilizing half-wave waveplates and right angle prisms is shown. This three-stage interleaver is configured to demultiplex an array o beams simultaneously. Referring now to FIG. 14b, an alternative layout for a five-stage interleaver utilizing half-wave waveplates and right angle prisms is shown. This five-stage interleaver is configured to demultiplex an array of beams simultaneously.

It is important to appreciate that, as mentioned above, the phase delay necessary for providing a birefringent effect may be obtained by inserting a material having desired optical, thermal, and/or mechanical properties into at least a portion of either the first or second path.

Although examples discussed above utilize equivalent birefringent filter element angles of 45°, -21° and -7° and utilize phase delays of T , 2Y and 2T , those skilled in the art will appreciate that various other angles and phase delays are likewise suitable. For example, phase delays of T , 2T and Y may alternatively be utilized.

The interleavers described herein are suitable for demultiplexing optical signals. Those skilled in the art will appreciate similar structures may be utilized to multiplex optical signals.

As those skilled in the art will appreciate, the waveplates which are utilized in the present invention can optionally be omitted in some instances by rotating subsequent components appropriately. Further, various devices and/or materials may alternatively be utilized to orient the polarization direction of light beams. For example, devices and/or materials which are responsive to applied voltages, currents, magnetic fields and/or electrical fields may be used to orient the polarization direction of light beams. Thus, the use of waveplates herein is by way of example only, and not by way of limitations.

Further, when waveplates having identical orientations are dispose next to one another, then a common waveplate may be substituted therefor.

It is understood that the exemplary interleavers described herein and shown in the drawings represent only presently preferred embodiments of the present invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. Those skilled in the art will appreciate that various different means for defining the first and second paths, wherein each of the first and second paths have a different optical path length, are contemplated. Further, various different devices for separating the beams at each stage into separate components, such that each component can travel along a different path, are likewise contemplated. For example, rather than using polarization beam displacers to separate composite light beams into components thereof and/or to combine component light beams into corporate light beams, those skilled in the art will appreciate that polarization beam splitters, typically in cooperation with mirrors, may alternatively be utilized and are therefore considered equivalent to polarization beam displacer for the purpose of separating and recombining light beams. Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for the use of a variety of different applications.

Interleaver Using Spatial Birefringent Elements

When angular orientation of birefringent element is discussed, the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counterclockwise with respect to the polarization direction of the oncoming light. If there is a series of birefringent elements, such as in a birefringent filter, the angular orientations of each of the elements of the filter are measured by their fast axes with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the angular orientations are determined separately for each birefringent filter (the angular orientations are measured with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of each different filter). Thus, each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.

By the way of contract, the angular orientation of birefringent elements and angular orientations of waveplates are also measure by the fast axes of birefringent elements and the optic axes of waveplates with respect to the +χ axis. However, it is very important to appreciate that the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system. The use of such a moving coordinate system allows the optical beam states, the birefringent elements, and the waveplates to be viewed in a consistent manner at various locations in the devices, i.e., always looking into the light, and therefore substantially simplifies viewing and analysis of the devices. Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +x axis (which is consistent the conventional use of (x, y, z) coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements with respect to the input polarization direction, as discussed above).

As those skilled in the art will appreciate, an interleaver is an optical device which typically includes at least one birefringent filter. Further, a birefringent filter is one example of a comb filter. More particularly, the present invention comprises an interleaver comprising an input polarization beam displacer, a birefringent filter assembly in optical communication with the input polarization beam displacer, a first output polarization beam displacer in optical communication with the birefringent filter assembly and a second output polarization beam displacer in optical communication with the first output polarization beam displacer. The birefringent filter assembly comprises at least one birefringent filter stage. Each birefringent filter stage comprises a first filter polarization beam displacer, a second filter polarization beam displacer and at least one reflector configured to direct light from the first filter polarization beam displacer to the second filter polarization beam displacer. The whole device is configured so as to minimize feedback to the input source as well as to minimize the signal transmission losses.

Preferably, the reflectors comprise prisms. The reflectors may alternatively comprise mirrors or etalons.

According to one aspect of the present invention, two prisms or two sets of reflectors or mirrors are utilized so as to define two paths, wherein each path has a different physical length. Alternatively, the two paths may have the same physical length and may have different optical path lengths, such as by utilizing materials having different indices of refraction in the two paths. Further, any desired combination of variation in physical path length and index of refraction between the first and second paths may be utilized. Thus, each of the birefringent filter stages define first and second paths and a single prism may optionally be utilized to direct light from the first polarization beam displacer to the second polarization beam displacer of each stage, wherein a material is inserted into at least one of the first and second paths such that the material causes the first and second paths to have different optical path lengths. Each birefringent filter stage preferably further comprises a half-wave waveplate intermediate each reflector and the first filter polarization beam displacer thereof. Alternatively, the half-wave waveplate may be disposed intermediate each reflector and the second filter polarization beam displacer. The half- wave waveplate disposed intermediate the reflector and the first filter polarization beam displacer preferably has an optical axis thereof oriented at approximately 45° with respect to the +χ axis at that location.

The interleaver preferably further comprises a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly. The first input half-wave waveplate is configured so as to transmit a non-displaced beam (a beam which was not displaced by the input polarization beam displacer) therethrough. Similarly, the interleaver preferably further comprises a second input half- wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly. The second input half-wave waveplate is preferably configured so as to transmit a displaced beam (a beam which was displaced by the input polarization beam displacer) therethrough. The first input half-wave waveplate preferably has an optical axis thereof oriented at 22.5° with respect to the +x axis at that location. The second input half-wave waveplate preferably has an optical axis thereof oriented at -22.5° with respect to the +x axis at that location.

Preferably, a first half-wave waveplate is configured to receive an output of each second filter polarization beam displacer. The interleaver preferably further comprises a first half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

Similarly, the interleaver preferably further comprises a second half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

Similarly, the interleaver preferably further comprises a third half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

Similarly, the interleaver preferably further comprises a fourth half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

The interleaver may comprise one, two, three, four, five or more birefringent filter stages, as desired. As those skilled is the art will appreciation, additional birefringement filter stages provide enhanced transmission filter characteristics. The input polarization beam displacer, the birefringent filter assembly, the first output polarization beam displacer and the second output polarization beam displacer are preferably all configured so as to facilitate interleaving of a plurality of beams simultaneously, preferably so as to facilitate the interleaving of an arrayed plurality of beams simultaneously.

Those skilled in the art will appreciate that various other devices, other than an input polarization beam displacer, may similarly be utilized so as to separate a composite input beam into two orthogonally polarized component beams. For example, a polarization beam splitter and a mirror may similarly be utilized.

In a like manner, devices other than output polarization beam displacers may be utilized so as to separate and recombine the beams from the birefringent filter assembly into a pair of composite beams. For example, polarization beam splitters may be so utilized. Referring now to FIG. 1, a one-stage interleaver constructed according to the present invention comprises an input polarization beam displacer 10, a birefringent filter assembly 11 in optical communication with the input polarization beam displacer 10, a first output polarization beam displacer 21 in optical communication with the birefringent filter assembly 11 and a second output polarization beam displacer 23 in optical communication with the first output polarization beam displacer 21. The birefringent filter assembly 11 comprises a first filter polarization beam displacer 12a, a second filter polarization beam displacer 13a, and at least one reflector configured to direct light from the first filter polarization beam displacer 12a to the second filter polarization beam displacer 13a. According to one configuration of the present invention, the reflectors comprise two prisms 14a and 15a, which are configured so as to define two different paths between the first filter polarization beam displacer 12a and the second filter polarization beam displacer 13a, wherein each of the two paths have a different optical path length. Alternatively, mirrors may similarly be utilized so as to define the first and second paths. As shown in FIG. 1, the two prisms 14a and 15a have different distances between themselves and the first filter polarization beam displacer 12a and the second filter polarization beam displacer 13a. This difference in distances is L/2, which provides a difference in physical path lengths of the two paths of L.

Rather than having two prisms (or sets of mirrors) which define two paths having different physical path lengths, a single prism may alternatively be utilized so as to define two paths having the same physical path length and materials having different indices of refraction may be inserted into one or both of the two paths so as to define paths having different optical path lengths.

Preferably, each birefringent filter stage further comprises a half-wave waveplate 17 disposed intermediate each reflector and the first filter polarization beam displacer 12. Each quarter- wave waveplate 17 preferably has an optical axis thereof oriented at approximately 45° with respect to the +χ axis at that location.

The interleaver preferably further comprises a first input half- wave waveplate 18 disposed intermediate the input polarization beam displacer 10 and the birefringent filter assembly 11 and configured so as to transmit a non-displaced beam therethrough. Similarly, the interleaver preferably further comprises a second input half- wave waveplate 19 disposed intermediate the input polarization beam displacer 10 and the birefringent filter assembly 11 and configured so as to transmit a displaced beam therethrough. The first half-wave waveplate preferably has an optical axis thereof oriented at 22.5° with respect to the +x axis at that location and the second input half- wave waveplate 19 preferably has an optical axis thereof oriented at -22.5° with respect to the +x axis at that location.

Each birefringent filter stage of the interleaver preferably further comprises a half- wave waveplate 20 configured to receive an output thereof. The interleaver preferably further comprises four half-wave waveplates 22 disposed intermediate a first output polarization beam displacer 21 and a second output polarization beam displacer 23. The first half-wave waveplate 22 preferably has optic axis orientation of approximately 0° with respect to the +x axis at that location. The second half-wave waveplate 22 preferably has an optic axis orientation of approximately 45° with respect to the +x axis at that location. The third half- wave waveplate 22 preferably has an optic axis orientation of approximately 45° with respect to the +x axis. The fourth half- wave waveplate 22 preferably has an optic axis orientation of approximately 90° with respect to the +χ axis at that location.

The birefringent filter assembly may comprise one birefringent filter stage (as shown in FIG. 1), two birefringent filter stages (as shown in FIG. 3), three birefringent filter stages (as shown in FIG. 5), four birefringent filter stages, five birefringent filter stages or more birefringent filter stages, as desired.

FIGS. 1, 3 and 5 show the interleaver having various different numbers of birefringent filter stages. In each of FIGS. 1, 3 and 5, the components of the first birefringent filter stage have an "a" following the component number thereof, the components of the second birefringent filter stage have the letter "b" following the number of the component thereof and so on. Thus, like components within each different stage have the same number, but have a different letter which indicates which stage they are part of. For example, the first filter polarization beam displacer is always number 12, regardless of which stage it is in, and is followed by the letter "a" (to form the reference number "12a") when in the first stage and is followed by the letter "b" (to form the reference number "12b") when in the second stage, and so on. When a component is referred to generically, i.e., without regard as to which specific stage the component is part of, then the letter may be omitted.

FIG. 1 shows a schematic of a one-stage interleaver. A right-hand coordinate system of axes is used to characterize the optical beam propagation and the system at various locations with a convention that the light is always propagating in the +z direction and the +y direction is always out of the paper for FIG. 1.

Referring now to FIG. 2, the optical beam states and the half-wave waveplate orientations at various locations are schematically shown. In FIG. 2, each of the four boxes of a frame corresponds to a beam position at various locations. The four boxes are oriented as the beams would appear looking into oncoming light. The polarization beam displacers shift the optical beams to various positions or boxes, according to the orientation of the polarization beam displacer and the optical beam polarization. As those skilled in the art will appreciate, polarization beam splitters and/or mirrors may alternatively be use to shift the optical beams to various positions or boxes.

At location 0, an input composite optical beam has two linearly polarized components 1 (along the y direction and z along the x direction) at the top-right beam position. After the beam propagates through the first polarization beam displacer at location 1, component 2 shifts to the top-left beam position and component 1 remains at the top-right beam position. The arrows shown on the beam displacers indicate the beam shift direction for the polarization beam displacers. After components 1 and 2 pass through two half-wave waveplates at location 2, the linearly polarized components 1 and 2, respectively, polarize along the same direction, i.e., -45° with respect to the +χ axis, at location 3. At location 2, the optical axis of the half-wave waveplate for component 1 is oriented at 22.5° with respect to the +x axis and the optical axis of the half-wave waveplate for component 2 is oriented at - 22.5° with respect to the +x axis.

When component 1 enters the second beam displacer, the light components polarized along the y direction move to the bottom beam locations and the light components polarized along the x direction remain at their corresponding top beam positions at location 4. A half- wave waveplate 17a or four half- wave waveplates are used at location 5 and have orientations of 45° with respect to the +χ axis. After the optical beams pass through this half-wave waveplate 17a, the polarization directions are changed by 90°.

There are two right-angle prisms, 14a and 15a. Prism 14a is a top prism as looking down at FIG 1 and prism 15a is a bottom prism as looking down on FIG 1. As discussed above, there is a position difference between the two prisms of L/2. Therefore, the optical beams which follow the two optical paths travel different lengths. The optical beams at the top beam positions are deflected twice by the first prism 14a before arriving at location 6. The optical beams at the bottom beam positions are deflected twice by the second prism 15a before arriving at location 6. The optical beam states are shown in FIG 2 at location 6. Because of the position difference between the top prism 14a and the bottom prism 15a, there is a phase difference T between the top beams and the bottom beams at location 6, according to the equation: T = L • 2π / , where λ is the optical wavelength.

After light passes through the second filter polarization beam displacer, component la moves to the bottom-right beam position to combine with component lb and component 2a moves to the bottom-left beam position to combine with component 2b at location 7. Then, the light passes through another half-wave waveplate 20a at location 8, where the half-wave waveplate 20a is oriented at -22.5°. This changes the polarization directions of the optical beams and the new x and y components are shown in FIG 2 at location 9. After the optical beams pass through the first output polarization beam displacer 21, component la' moves to the top-right beam position and component 2a' moves to the top-left beam position at location 10. Four half- wave waveplates 22 are used at location 11, with their corresponding orientations as shown in frame 11 of FIG 2. After the optical beams pass through the waveplates 22, their polarization directions are shown in FIG 2 at location 12. When the light passes through the second output polarization beam displacer 23, component la' moves to the top-left beam position to combine with component 2a' and component lb' moves to the bottom-left beam position to combine with component 2b'. Thus, at location 13, two composite output beams are formed. One composite output beam may be considered to contain the even channels, while the other may be considered to contain the odd channels of the communication signals. The interleaver shown in FIG. 1 is thus equivalent a conventional one-stage Sole-type interleaver having the fast axis of the birefringent crystal thereof oriented at 45° with respect to the polarization direction of light input thereto. The equivalent birefringent crystal orientation provided by the one-stage interleaver of FIG. 1 is determined by the orientation of the half- wave waveplates 18 and 19. Thus, various different birefringent crystal orientations can similarly be simulated by varying the orientation of the half- wave waveplates 18 and 19.

Further, a plurality of stages, wherein each stage corresponds to and simulates to a separate birefringent crystal having a unique angular orientation of a fast axis thereof, can be provided by providing a plurality of birefringent filter stages 11, wherein birefringent filter stage has half-wave waveplates or the like thereof oriented so as to define the equivalent or simulated angular orientation corresponding to the desired angular orientation of a birefringent crystal. In this manner a plurality of stages, each stage having a unique angular orientation, can be provided so as to simulate a multi-crystal Sole-type birefringent filter (a multi-stage filter) having desired transmission characteristics. Thus, the interleaving function is obtained in a similar fashion to when a birefringent crystal Sole filter is utilized. The two output beams (la', 2a' and lb', 2b') are the two series of interleaved channels and the phase delay T = L ■ 2π / λ, which determines the channel spacing.

According to one embodiment of the present invention, birefringence is obtained by optical path length differences, which may occur in free space. There are many advantages to the present invention as compared to conventional interleavers which utilize birefringent crystals. For example, the difference in optical path length can be manipulated so as to provided desired, comparatively high, birefringence values. An ultra low expansion (ULE) or fused silica may be utilized as a gasket in device construction, so as to obtain excellent temperature stability for the interleaver. Those skilled in the art will appreciate the various other materials having a very low thermal expansion coefficient are likewise suitable for use as such a gasket.

Because the beam shift is symmetric according to the present invention, the polarization mode dispersion mode is minimized.

Referring now to FIG 3, interleavers having more than one birefringent filter stage may be utilized so as to enhance passband and stopband characteristics. As shown in FIG 3, a two-stage interleaver comprises two birefringent filter stages in series with one another.

Referring now to FIG 4, the optical beam states and half-wave waveplate orientations for an exemplary two-stage birefringent filter or interleaver of FIG 3 are shown. This interleaver corresponds to a Sole-type birefringent filter having birefringent crystal orientations of 45°, -21° and phase delay of T and 2T , respectively. Referring now to FIG 5, an interleaver having a three-stage birefringent filter is shown. The phase delay in the second stage and the third stage is twice as large as that for the first stage (Tι = L ■ 2π / λ, T₂ = T₃ = 2L • 2π / λ). The interleaver channel spacing is determined by the phase delay in the first stage (Ti).

Referring now to FIG 6, the optical beam states and half-wave waveplate orientations for an exemplary three-stage birefringent filter of FIG 5 are shown. This interleaver is equivalent to a Sole-type filter having birefringent crystal orientations of 45°, -21°, and 7° and having phase delay of T , 2Y and 2T , respectively.

In each configuration of the present invention, some of the half- wave waveplates may be eliminated by rotating the beam displacer orientation accordingly. Thus, the interleaver of the present invention comprises a birefringent filter assembly coupled to receive at least two beams of polarized light. The birefringent filter assembly is configured so as to provide a birefringent effect with respect to the beams, without the use of birefringent crystals. Rather, the birefringent filter assembly provides a birefringent effect with respect to the beams by causing the beams to travel along two paths, wherein each path has a difference in optical path length.

One important aspect of this invention is the ability to control the difference in optical path length between the first and second paths, so that the birefringence value provided by this difference in optical path length does not vary undesirably during operation of the invention, such as due to temperature changes. As those skilled in the art will appreciate, the birefringence values of a device determine the operational characteristics, i.e., transmission, dispersion, phase distortion, thereof. Therefore, it is very important that the optical path length differences (and consequently the birefringence values) remain substantially fixed during operation of the devices.

Portions of the first and second paths, other than the portions which contribute the optical path length differences, are less critical since these other portions do not determined birefringence values. Generally, portions of the first and second paths, other than the portions which contribute to the optical path length differences, tend to vary (changes in physical length and/or changes in index of refraction thereof) in response to environment (e.g., temperature) changes by approximately the same amount, due to structural similarity and symmetry of the first and second paths, and thus do not generally tend to change the path length difference. Therefore, it is that portion of the first and second paths (e.g., the L/2 or L portion shown in the Figs) which directly provides the difference in optical path length that must be most carefully controlled.

According to the present invention, the difference in optical path length between the first and second paths may optionally be controlled by inserting a material having desired optical, thermal and/or mechanical properties into at least the longer of the two paths, so as to substantially fix the optical path length which defines the difference between the first and second paths. Thus, by inserting such a material into at least that portion of one path that defines optical path length difference (e.g., the L/2 portion or the L portion of the path show in the figs.), substantially more stable operation of the devices is achieved.

Further, the optical path lengths may be made so as to be variable, thus providing adjustability of the birefringence values and a tunable interleaver. As used herein, the term gasket is defined to include any bracket, mount, optical bench, host, enclosure or any other structure which is used to maintain components of the present invention in desired positions relative to one another. Preferably, such gasket is comprised of an ultra low expansion (ULE) material, fused silica or any other material having a very low thermal expansion coefficient. It will therefore be appreciated that it is comparatively easy to expand the multi-stage interleaver of the present invention to have any desired number of stages so as to facilitate and enhance interleaver performance.

Since no polarization beam splitter is used according to this configuration of the invention, both feedback to the input light source and transmission loss are mitigated.

Although examples discussed above utilize equivalent birefringent filter element angles of 45°, -21° and -7° and utilize phase delays of T , 2Y and 2T , those skilled in the art will appreciate that various other angles and phase delays are likewise suitable. For example, phase delays of r , 2T and Y may alternatively be utilized. The interleavers described herein are suitable for demultiplexing optical signals.

Those skilled in the art will appreciate similar structures may be utilized to multiplex optical signals.

As those skilled in the art will appreciate, the waveplates which are utilized in the present invention can optionally be omitted in some instances by rotating subsequent components appropriately. Further, various devices and/or materials may alternatively be utilized to orient the polarization direction of light beams. For example, devices and/or materials which are responsive to applied voltages, currents, magnetic fields and/or electrical fields may be used to orient the polarization direction of light beams. Thus, the use of waveplates herein is by way of example only, and not by way of limitations. Further, when waveplates having identical orientations are dispose next to one another, then a common waveplate may be substituted therefor.

It is understood that the exemplary interleaver described herein and shown in the drawings represent only a presently preferred embodiment of the present invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Those skilled in the art will appreciate that various different means for the defining the first and second paths, wherein each of the first and second paths have a different optical path length, are contemplated. Further, various different devices for separating the beams at each stage into separate components such that each component can travel along a different path, are likewise contemplated. Various means for recombining such separated components into composite beams are also contemplated. Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for the use of a variety of different applications.

Claims

1. A filter for filtering electromagnetic radiation, the filter comprising: two polarization selection elements; and a birefringent element assembly disposed intermediate the two polarization selection elements and configured so as to optimize contributions of a fundamental and at least one odd harmonic of a transmission vs. wavelength curve in a manner which enhances transmission vs. wavelength curve stopband depth and flatness for a passband thereof.

2. A filter for filtering electromagnetic radiation, the filter comprising: two polarization selection elements; a birefringent element assembly disposed intermediate the two polarization selection elements, the birefringent element assembly comprising: a first birefringent element having an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave; a second birefringent element which cooperates with the first birefringent element to provide an output transmission vs. wavelength curve which is approximately defined a fundamental sine wave plus a third harmonic of the fundamental sine wave; and a third birefringent element which cooperates with the first and second birefringent element to provide an output transmission vs. wavelength curve which is approximately defined by a fundamental sine wave plus a third harmonic of the fundamental sine wave and plus a fifth harmonic of the fundamental sine wave; and wherein parameters of the first, second and third birefringent elements are selected so as to provide transmission vs. wavelength curve stopband depth greater than -25dB.

3. The filter as recited in claim 2, wherein the first birefringent element is closer to the input polarization selection element than the third birefringent element and the second birefringent element is disposed intermediate the first birefringent element and the third birefringent element.

4. The filter as recited in claim 2, wherein the third birefringent element is closer to the input polarization selection element than the first birefringent element and the second birefringent element is disposed intermediate the first birefringent element and the third birefringent element.

5. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element having a polarization direction; an output polarization selection element having a polarization direction; a birefringent element assembly disposed intermediate the input polarization selection element, and the output polarization selection element, the birefringent element assembly comprising: a first birefringent element providing a phase delay and the fast axis thereof having an effective angle of approximately 45° with respect to the polarization direction of the input polarization selection element; a second birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately -21° with respect to the polarization direction of the input polarization selection element; and a third birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately 7° with respect to the polarization direction of the input polarization selection element.

6. The filter as recited in claim 5, wherein the input polarization selection element, the output polarization selection element, and the birefringent element assembly are configured to transmit infrared light.

7. The filter as recited in claim 5, wherein: the first birefringent element comprises a first birefringent crystal and the effective angle of the first birefringent element is defined by an angular orientation of a fast axis of the first birefringent crystal with respect to the polarization direction of the input polarization selection element; the second birefringent element comprises a second birefringent crystal and the effective angle of the second birefringent element is defined by an angular orientation of a fast axis of the second birefringent crystal with respect to the polarization direction of the input polarization selection element; and the third birefringent element comprises a third birefringent crystal and the effective angle of the third birefringent element is defined by an angular orientation of a fast axis of the third birefringent crystal with respect to the polarization direction of the input polarization selection element.

8. The filter as recited in claim 5, wherein: the input polarization selection element comprises a polarizer; and the output polarization selection element comprises a polarizer.

9. The filter as recited in claim 5, wherein the polarization selection elements comprise polarization beam splitters.

10. The filter as recited in claim 5, wherein the polarization selection elements comprise a plurality of polarization beam displacers.

11. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element having a polarization direction; an output polarization selection element having a polarization direction; a birefringent element assembly disposed intermediate the input polarization selection element and the output polarization selection element, the birefringent element assembly comprising: a first birefringent element providing a birefringent phase delay and having a fast axis oriented at an effective angle of approximately 45° with respect to the polarization direction of the input polarization selection element; and a second birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and having a fast axis oriented at an effective angle of approximately -21° with respect to the polarization direction of the input polarization selection element.

12. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element; an output polarization selection element; a first birefringent element disposed intermediate the input polarization selection element and the output polarization selection element and having a fast axis thereof oriented at an angle of approximately 7° with respect to a polarization direction of the input polarization selection element; a second birefringent element disposed intermediate the first birefringent element and the output polarization selection element and having a fast axis thereof oriented at an angle of approximately -21° with respect to the polarization direction of the input polarization selection element; and a third birefringent element disposed intermediate the second birefringent element and the output polarization selection element and having a fast axis thereof oriented at an angle of approximately 45° with respect to the polarization direction of the input polarization selection element.

13. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element having a polarization direction; an output polarization selection element having a polarization direction; a birefringent element assembly disposed intermediate the input polarization selection element, and the output polarization selection element, the birefringent element assembly comprising: a first birefringent element providing a phase delay and the fast axis thereof having an effective angle of approximately 45° with respect to the polarization direction of the input polarization selection element; a second birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately -69° with respect to the polarization direction of the input polarization selection element; and a third birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately 83° with respect to the polarization direction of the input polarization selection element.

14. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element having a polarization direction; an output polarization selection element having a polarization direction; a birefringent element assembly disposed intermediate the input polarization selection element, and the output polarization selection element, the birefringent element assembly comprising: a first birefringent element providing a phase delay and the fast axis thereof having an effective angle of approximately 135° with respect to the polarization direction of the input polarization selection element; a second birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately 69° with respect to the polarization direction of the input polarization selection element; and a third birefringent element providing a birefringent phase delay of approximately twice that of the first birefringent element and the fast axis having an effective angle of approximately 97° with respect to the polarization direction of the input polarization selection element.

15. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element; an output polarization selection element; a birefringent element assembly disposed intermediate the input polarization element and the output polarization element, the birefringent element assembly comprising one set of birefringent elements selected from the group of sets of birefringent elements consisting of: a first set of birefringent elements comprising a first birefringent element having a phase delay of Y and having a fast axis thereof oriented at an angle of approximately 45° with respect to a polarization direction of the input polarization selection element, a second birefringent element having a phase delay of approximately 2r and having a fast axis thereof oriented at an angle of approximately -21° with respect to the polarization direction of the input polarization selection element and a third birefringent element having a phase delay of approximately 217 and having a fast axis thereof oriented at an angle of approximately 7° with respect to the polarization direction of the input polarization selection element; a second set of birefringent elements comprising a first birefringent element having a phase delay of Y and having a fast axis thereof oriented at an angle of approximately 45° with respect to the polarization direction of the input polarization selection element, a second birefringent element having a phase delay of approximately 21" and having a fast axis thereof oriented at an angle of approximately -69° with respect to the polarization direction of the input polarization selection element and a third birefringent element having a phase delay of approximately 217 and having a fast axis thereof oriented at an angle of approximately 83° with respect to the polarization direction of the input polarization selection element; a third set of birefringent elements comprising a first birefringent element having a phase delay of Y and having a fast axis thereof oriented at an angle of approximately 135° with respect to the polarization direction of the input polarization selection element, a second birefringent element having a phase delay of approximately 2Y and having a fast axis thereof oriented at an angle of approximately 69° with respect to the polarization direction of the input polarization selection element and a third birefringent element having a phase delay of approximately 217 and having a fast axis thereof oriented at an angle of approximately 97° with respect to the polarization direction of the input polarization selection element; wherein the order of the birefringent elements, from the input polarization element to the output polarization element, is selected from the group of orders consisting of: first birefringent element, second birefringent element and third birefringent element; and third birefringent element, second birefringent element and first birefringent element.

16. A filter for filtering electromagnetic radiation, the filter comprising: an input polarization selection element; an output polarization selection element; a birefringent element assembly disposed intermediate the input polarization element and the output polarization element, the birefringent element assembly comprising: a first birefringent element having a phase delay of Y, a second birefringent element having a phase delay of approximately 2Y and a third birefringent element having a phase delay of approximately 21"; wherein the order of the birefringent elements, from the input polarization element to the output polarization element, is selected from the group of orders consisting of: first birefringent element, second birefringent element and third birefringent element; and third birefringent element, second birefringent element and first birefringent element.

17. A method for forming a filter for filtering electromagnetic radiation, the method comprising: providing an input polarization selection element; providing an output polarization selection element; providing a birefringent element assembly disposed intermediate the input polarization element and the output polarization element, the birefringent element assembly comprising one set of three birefringent elements, wherein the set of three birefringent elements has been selected according to the method comprising: determining a first set of angular orientations of the three birefringent elements with respect to a polarization direction of the input polarization selection element so as to define a first set of three birefringent elements, the first set of angular orientations being represented by φ_ls φ , φ₃, the first set of angular orientations being determined so as to provide a desired wavelength vs. transmission curve; determining a second set of angular orientations of the three birefringent elements with respect to the polarization direction of the input polarization selection element so as to define a second set of three birefringent elements, the second set of angular orientations being 90°-φ_l3 90°-φ₂, 90°-φ₃; determining a third set of angular orientations of the three birefringent elements with respect to the polarization direction of the input polarization selection element so as to define a third set of three birefringent elements, the third set of angular orientations being 90°+φ_l5 90°+φ₂, 90°+φ₃; selecting a desired one of the first, second and third sets of three birefringent elements, wherein each of the first, second and third sets of three birefringent elements provide substantially the same wavelength vs. transmission curve and selection from among the three sets of three birefringent elements facilitates manufacturing flexibility; selecting an order of the birefringent elements of the selected set thereof, the order being from the input polarization element to the output polarization element, the desired order being selected according to the method comprising: selecting a order of first birefringent element, second birefringent element and third birefringent element; selecting an order of third birefringent element, second birefringent element and first birefringent element; and wherein each of the orders provide substantially the same wavelength vs. transmission curve and wherein selection of the order facilitates manufacturing flexibility.

18. A dispersion compensating birefringent filter comprising: a pair of polarization selections elements; a birefringent element assembly disposed intermediate the pair of polarization selections elements; wherein the birefringent assembly is configured so as to provide a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for at least one other component of an optical system, so as to mitigate dispersion in the optical system.

19. The dispersion compensating birefringent filter as recited in claim 18, wherein the birefringent assembly comprises three birefringent elements, each birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of an input polarization selection element of the birefringent filter such that the birefringent filter provides a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for at least one other component of an optical system.

20. The dispersion compensating birefringent filter as recited in claim 18, wherein the birefringent assembly comprises two birefringent elements, each birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of an input polarization selection element of the birefringent filter such that the birefringent filter provides a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for at least one other component of an optical system.

21. The dispersion compensating birefringent filter as recited in claim 18, wherein the dispersion vs. wavelength curve is tunable.

22. The dispersion compensating birefringent filter as recited in claim 18, wherein the dispersion vs. wavelength curve is tunable by varying angular orientations of a fast axis of at least one birefringent element with respect to a polarization direction of an input polarization selector.

23. A low dispersion interleaver assembly comprising: a first interleaver; a second interleaver; and wherein the first interleaver is configured so as to provide a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for the second interleaver, so as to mitigate dispersion in the interleaver assembly.

24. The low dispersion interleaver assembly as recited in claim 23, wherein: each interleaver comprises a plurality of birefringent elements; and angular orientations of the birefringent elements of the first interleaver are related to angular orientations of the birefringent elements of the second interleaver in a manner which mitigate dispersion in the interleaver assembly.

25. The low dispersion interleaver assembly as recited in claim 23, wherein: each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are φ i, φ ₂ and φ ₃, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 90° - φ _ls 90° - φ ₂ and 90° - φ ₃, respectively.

26. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are φ ₁, φ ₂ and φ ₃, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 90° + φ ₁, 90° + φ ₂ and 90° + 9₃, respectively.

27. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are approximately 45°, approximately -21° and approximately 7°, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 45°, -69° and 83°, respectively.

28. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are approximately 45°, approximately -21° and approximately 7°, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 135°, 69° and 97°, respectively.

29. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are approximately 7°, approximately -21° and approximately 45°, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 83°, -69° and 45°, respectively.

30. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first, second and third birefringent elements; angular orientations of the first, second and third birefringent elements of the first interleaver are approximately 7°, approximately -21° and approximately 45°, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 97°, 69° and 135°, respectively.

31. The low dispersion interleaver assembly as recited in claim 23, wherein: each interleaver comprises first and second birefringent elements; angular orientations of the first and second birefringent elements of the first interleaver are φ 1 and φ 2, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 90° - φ i and 90° - φ ₂, respectively.

32. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first and second birefringent elements; angular orientations of the first and second birefringent elements of the first interleaver are φ i and φ ₂, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 90° + φ i and 90° + φ ₂, respectively.

33. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first and second birefringent elements; angular orientations of the first and second birefringent elements of the first interleaver are approximately 45° and approximately -21°, respectively; and angular orientations of the first, second and third birefringent elements of the second interleaver are 45° and -69°, respectively.

34. The low dispersion interleaver assembly as recited in claim 23, wherein each interleaver comprises first and second birefringent elements; angular orientations of the first and second birefringent elements of the first interleaver are approximately 45° and approximately -21°, respectively; and angular orientations of the first and second birefringent elements of the second interleaver are 135° and 69°, respectively.

35. The low dispersion interleaver assembly as recited in claim 23, wherein: the first interleaver comprises a N GHz interleaver; and the second interleaver comprises a N/2 GHz interleaver.

36. A low dispersion interleaver comprising: an input polarization selection element; an output polarization selection element; a birefringent element assembly disposed intermediate the input and output polarization selection elements, the birefringent element assembly comprising: a first birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 45° or 135°; and second and third birefringent elements having a difference in angular orientation therebetween of a fast axis thereof with respect to one another of approximately +/- 90°.

37. The low dispersion interleaver as recited in claim 36, wherein: the first birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 45°; the second birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately -65°; and the third birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 15°.

38. The low dispersion interleaver as recited in claim 36, wherein: the first birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 45°; the second birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately -25°; and the third birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 75°.

39. The low dispersion interleaver as recited in claim 36, wherein: the first birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 135°; the second birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 25°; and the third birefringent element having an angular orientation of a fast axis thereof with respect to a polarization direction of the input polarization element of approximately 105°.

40. A low dispersion interleaver assembly comprising two interleavers configured such that light passes sequentially therethrough, each interleaver comprising: an input polarization selection element; an output polarization selection element; a birefringent element assembly disposed intermediate the input polarization element and the output polarization element, the birefringent element assembly comprising: a first birefringent element having a phase delay of Y, a second birefringent element having a phase delay of approximately 217 and a third birefringent element having a phase delay of approximately 217; wherein the order of the birefringent elements for each interleaver, from the input polarization element to the output polarization element, is selected independently from the group of orders consisting of: first birefringent element, second birefringent element and third birefringent element; and third birefringent element, second birefringent element and first birefringent element.

41. A method for forming a low dispersion interleaver assembly, the method comprising forming two interleavers configured such that light passes sequentially therethrough, each interleaver being formed by a method comprising: providing an input polarization selection element; providing an output polarization selection element; providing a birefringent element assembly disposed intermediate the input polarization element and the output polarization element, the birefringent element assembly comprising one set of three birefringent elements, wherein the set of three birefringent elements has been selected according to the method comprising: determining a first set of angular orientations of the three birefringent elements with respect to a polarization direction of the input polarization selection element so as to define a first set of three birefringent elements, the first set of angular orientations being represented by φi, φ₂, φ₃, the first set of angular orientations being determined so as to provide a desired wavelength vs. transmission curve; determining a second set of angular orientations of the three birefringent elements with respect to the polarization direction of the input polarization selection element so as to define a second set of three birefringent elements, the second set of angular orientations being 90°-φ₁, 90°-φ₂, 90°-φ₃; determining a third set of angular orientations of the three birefringent elements with respect to the polarization direction of the input polarization selection element so as to define a third set of three birefringent elements, the third set of angular orientations being 90°+φ₁, 90°+φ , 90°+φ₃; forming a first one of the two interleavers using the first set of angular orientations and forming a second one of the two interleavers using either the second or third set of angular orientations; selecting an order of the birefringent elements for the first and second interleavers, the order being from the input polarization element to the output polarization element, the order being selected independently according to one of the methods comprising: selecting a order of first birefringent element, second birefringent element and third birefringent element; and selecting an order of third birefringent element, second birefringent element and first birefringent element.

42. A low dispersion interleaver assembly comprising: a first interleaver comprising: a first birefringent element having an angular orientation of 45° and a phase delay of Y; a second birefringent element having an angular orientation of -15° and a phase delay of 2Y; a third birefringent element having an angular orientation of 0° and a phase delay of Y; a second interleaver comprising: a first birefringent element having an angular orientation of 45° and a phase delay of Y; a second birefringent element having an angular orientation of -75° and a phase delay of 2Y; a third birefringent element having an angular orientation of 90° and a phase delay of Y.

43. A method for providing birefringence, the method comprising: separating a first composite light beam into first and second components thereof, the first and second components being orthogonally polarized with respect to one another; transmitting the first component along a first path and transmitting the second component along a second path, the first and second paths having different optical path lengths; recombining the first and second components so as to form a second composite light beam; and wherein the second composite light beam is birefringent with respect to the first composite light beam.

44. A birefringent device comprising: a polarization separating device configured to separate a first composite light beam into first and second components thereof, the first and second components being orthogonally polarized with respect to one another; a first path configured to transmit the first component and having a first optical path length; a second path configured to transmit the second component and having a second optical path length, the second optical path length being different from the first optical path length; a polarization combining device configured to recombine the first and second components so as to form a second composite light beam; and wherein the second composite light beam is birefringent with respect to the first composite light beam.

45. The birefringent device as recited in claim 44, wherein the polarization separating device and the polarization combining device each comprises a polarization beam displacer.

46. The birefringent device as recited in claim 44, wherein the polarization separating device and the polarization combining device both comprise a common polarization beam displacer.

47. The birefringent device as recited in claim 44, wherein the polarization separating device and the polarization combining device each comprises a polarization beam splitter.

48. The birefringent device as recited in claim 44, wherein the polarization separating device and the polarization combining device both comprise a common polarization beam splitter.

49. The birefringent device as recited in claim 44, wherein the first and second paths comprise air or vacuum paths.

50. The birefringent device as recited in claim 44, wherein the first and second paths comprise only non-air or non-vacuum paths.

51. The birefringent device as recited in claim 44, wherein the first and second physical path lengths are approximately equal and material having an index of refraction greater than one is disposed within one of the first and second paths to cause the first and second paths to have different optical path lengths.

52. The birefringent device as recited in claim 44, further comprising a material having an index of retraction greater than one disposed in a portion of one of the first and second paths, the portion being that portion that solely provides the difference in optical path length.

53. The birefringent device as recited in claim 44, further comprising at least one mirror configured to define at least one of the first and second paths.

54. The birefringent device as recited in claim 44, further comprising at least one prism configured to define at least one of the first and second paths.

55. The birefringent device as recited in claim 44, wherein second physical path length is different from the first physical path length, so as to make the second optical path length different from the first optical path length.

56. The birefringent device as recited in claim 44, wherein an index of refraction of a portion of the second path is different from an index of refraction of a portion of the first path, so as to make the second optical path length different from the first optical path length.

57. The birefringent device as recited in claim 44, wherein at least one of the first optical path length and the second optical path length is variable, so as to facilitate changing of the amount of birefringence of the second composite light beam.

58. The birefringent device as recited in claim 44, wherein at least one of the first physical path length and the second physical path length is variable, so as to facilitate changing of the amount of birefringence of the second composite light beam.

59. The birefringent device as recited in claim 44, wherein at least one of an index of refraction of the first path and an index of refraction of the second path is variable, so as to facilitate changing of the amount of birefringence of the second composite light beam.

60. The birefringent device as recited in claim 44, further comprising: at least one mirror configured to define at least one of the first and second paths; and at least one quarter-wave waveplate disposed along the defined path(s).

61. The birefringent device as recited in claim 44, further comprising: at least one prism configured to define at least one of the first and second paths; and at least one half- wave waveplate disposed along the defined path(s).

62. The birefringent device as recited in claim 44, further comprising a half-wave plate disposed along a path of the second composite light beam.

63. A polarization beam displacer birefringent device comprising: a first polarization beam displacer configured to separate a first composite light beam into first and second components thereof, the first and second components being orthogonally polarized with respect to one another; at least one prism configured to: at least partially define a first path which is configured to transmit the first component, the first path having a first optical path length; at least partially define a second path which is configured to transmit the second component, the second path having a second optical path length, the second optical path length being different with respect to the first optical path length; and a second polarization beam displacer configured to recombine the first and second components so as to form a second composite light beam.

64. The polarization beam displacer birefringent device as recited in claim 63, wherein the first polarization beam displacer and the second polarization beam displacer comprise separate first and second polarization beam displacers.

65. The polarization beam displacer birefringent device as recited in claim 63, wherein the first polarization beam displacer and the second polarization beam displacer comprise a common polarization beam displacer.

66. The polarization beam displacer birefringent device as recited in claim 63, wherein: the prism(s) comprise a common prism configured to at least partially define the first path and at least partially define the second path; and further comprising a material in at least one of the first and second paths such that an index of refraction is different for at least a portion of the first and second paths, so as to cause the first and second paths to have different optical path lengths.

67. The polarization beam displacer birefringent device as recited in claim 63, wherein the prism(s) comprise a first prism configured to at least partially define the first path and a second prism configure to at least partially define the second path.

68. The polarization beam displacer birefringent device as recited in claim 63, wherein: the prism(s) comprise a first prism configured to at least partially define the first path and a second prism configure to at least partially define the second path; the first prism is a first distance from the first polarization beam displacer and the second prism is a second distance from the first polarization beam displacer; and the first and second distances are different from one another.

69. The polarization beam displacer birefringent device as recited in claim 63, wherein: the prism(s) comprise a first prism configured to at least partially define the first path and a second prism configure to at least partially define the second path; the first prism is a first distance from the first polarization beam displacer and the second prism is a second distance from the first polarization beam displacer; and at least one of the first and second distances is variable, so as to facilitate changing of the birefringence of the second composite light beam.

70. The polarization beam displacer birefringent device as recited in claim 63, wherein: the prism(s) comprise a common prism configured to at least partially define the first path and at least partially define the second path; and an index of refraction is different for at least a portion of the first and second paths, so as to cause the first and second paths to have different optical path lengths.

71. The polarization beam displacer birefringent device as recited in claim 63, further comprising a first half- wave waveplate disposed in the first and second paths so as to transmit light reflected by the prism(s) prior to the light being recombined.

72. The polarization beam displacer birefringent device as recited in claim 63, further comprising a second half-wave waveplate disposed in the first and second paths so as to transmit light reflected by the prism(s) after the light has been recombined.

73. The polarization beam displacer birefringent device as recited in claim 63, further comprising a gasket configured to host the first and second polarization beam displacers and the prism(s), the gasket comprising an ultra low expansion material to enhance temperature stability.

74. The polarization beam displacer birefringent device as recited in claims 63, wherein a difference in the first and second optical path lengths is provided by a material having an index of refraction greater than one which is disposed within at least a portion of one of the first and second paths.

75. A polarization beam splitter birefringent device comprising: a polarization beam splitter configured to split a first composite light beam into first and second components thereof, the first and second components being orthogonally polarized with respect to one another; a first reflector configured to at least partially define a first path which is configured to transmit the first component, the first path having a first optical length; a second reflector configured to at least partially define a second path which is configured to transmit the second component, the second path having a second optical length, the second optical path length being different with respect to the first optical path length; and wherein the polarization beam splitter is further configured to recombine the first and second components so as to form a second composite light beam.

76. The polarization beam splitter birefringent device as recited in claim 75, wherein the first and second reflectors comprise mirrors.

77. The polarization beam splitter birefringent device as recited in claim 75, wherein: the first and second reflectors comprise mirrors; further comprising a first quarter-wave waveplate in the first path intermediate the first reflector and the polarization beam splitter, the first quarter-wave waveplate being configured so as to transmit the first component both before and after the first component is reflected by the first reflector; and further comprising a second quarter-wave waveplate in the second path intermediate the second reflector and the polarization beam splitter, the second quarter-wave waveplate being configured so as to transmit the second component both before and after the second component is reflected by the second reflector.

78. The polarization beam splitter birefringent device as recited in claim 75, wherein the first and second reflectors comprise prisms.

79. The polarization beam splitter birefringent device as recited in claim 75, wherein: the first and second reflectors comprise prisms; further comprising a first half-wave waveplate in the first path intermediate the first reflector and the polarization beam splitter, the first half-wave waveplate being configured so as to transmit the first component either before or after the first component has been reflected by the first reflector; and further comprising a second half-wave waveplate in the second path intermediate the second reflector and the polarization beam splitter, the second half-wave waveplate being configured so as to transmit the second component either before or after the second component has been reflected by the second reflector.

80. The polarization beam splitter birefringent device as recited in claim 75, wherein the first path has a physical path length that is different from a physical path length of the second path.

81. The polarization beam splitter birefringent device as recited in claim 75, wherein an index of refraction is different for at least a portion of the first and second paths, so as to cause the first and second paths to have different optical lengths.

82. The polarization beam splitter birefringent device as recited in claim 75, wherein at least one of the physical first path length and the physical second path length is variable so as to facilitate changing of the amount of birefringence in the second composite light beam.

83. The polarization beam splitter birefringent device as recited in claim 75, wherein at least one of the optical first path length and the optical second path length is variable by varying an index of refraction of at least a portion thereof, so as to facilitate changing of the amount of birefringence in the second composite light beam.

84. The polarization beam splitter birefringent device as recited in claim 75, wherein a difference in path lengths between the first and second paths is provided by a material having an index of refraction greater than one disposed within at least a portion of one of the first and second paths.

85. The polarization beam splitter birefringent device as recited in claim 75, further comprising gasket configured to host the polarization beam splitter and each reflector, the gasket comprising an ultra low expansion material to enhance temperature stability.

86. A method for controlling birefrigence, the method comprising: forming a beam of birefringent light by combining light from two paths having different optical path lengths; controlling a portion of at least one of the two paths such that the path length difference does not change substantially during operation; and canceling changes in the remaining portions of the two paths.

87. A method for controlling birefringence, the method comprising: transmitting light along two paths, at least one of the two paths having a portion where the optical path length thereof is controlled, the controlled portion defining an optical path length difference; and wherein light not transmitted along the controlled portion is transmitted along another portion where changes in optical path length cancel one another for the two paths.

88. An interleaver comprising: an input polarization beam displacer; a birefringent filter assembly in optical communication with the input polarization beam displacer, the birefringent filter assembly comprising at least one birefringent filter stage, each birefringent filter stage comprising: a polarization beam splitter; two reflectors; a first output polarization beam displacer in optical communication with the birefringent filter assembly; and a second output polarization beam displacer in optical communication with the first output polarization beam displacer.

89. The interleaver as recited in claim 88, wherein the two reflectors comprise mirrors.

90. The interleaver as recited in claim 88, wherein each birefringent filter stage further comprises a quarter-wave waveplate intermediate each reflector and the polarization beam splitter.

91. The interleaver as recited in claim 88, wherein each birefringent filter stage further comprises a quarter-wave waveplate disposed intermediate each reflector and the polarization beam splitter.

92. The interleaver as recited in claim 88, further comprising: a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a non-displaced beam therethrough; and a second input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a displaced beam therethrough.

93. The interleaver as recited in claim 88, further comprising: a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a non-displaced beam therethrough; and a second input half- wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a displaced beam therethrough.

94. The interleaver as recited in claim 88, further comprising a half-wave waveplate configured to receive an output of each polarization beam splitter.

95. The interleaver as recited in claim 88, further comprising a half-wave waveplate configured to receive an output of a polarization beam splitter, the half-wave waveplate having an optical axis angle of approximately -22.5° with respect to the +χ axis at that location.

96. The interleaver as recited in claim 88, further comprising: a half-wave waveplate of a first stage thereof configured to receive an output of a polarization beam splitter, the half-wave waveplate having a optical axis angle of approximately -33° with respect to the +χ axis at that location; and a half-wave waveplate of a second stage thereof configured to receive an output of a polarization beam splitter, the half-wave waveplate having a optical axis angle of approximately 10.5° with respect to the +χ axis at that location.

97. The interleaver as recited in claim 88, further comprising: a half- wave waveplate of a first stage thereof configured to receive an output of a polarization beam splitter, the half-wave waveplate having a optical axis angle of approximately -33° with respect to the +χ axis at that location; a half-wave waveplate of a second stage thereof configured to receive an output of a polarization beam splitter, the half-wave waveplate having a optical axis angle of approximately 14° with respect to the +x axis at that location; and a half-wave waveplate of a third stage thereof configured to receive an output of a polarization beam splitter, the half-wave waveplate having a optical axis angle of approximately -3.5° with respect to the +x axis at that location.

98. The interleaver as recited in claim 88, further comprising: a first half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; a second half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; a third half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; and a fourth half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

99. The interleaver as recited in claim 88, further comprising: a first half-wave waveplate disposed intermediate the first output polarization beam displacer and the third output polarization beam displacer, the first half-wave waveplate having an optic axis orientation of approximately 45°; a second half-wave waveplate disposed intermediate the second output polarization beam displacer and the second output polarization beam displacer, the first half-wave waveplate having an optic axis orientation of approximately 90°; a third half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the third half-wave waveplate having an optic axis orientation of approximately 0°; and a fourth half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the fourth half-wave waveplate having an optic axis orientation of approximately 45°.

100. The interleaver as recited in claim 88, wherein the birefringent filter assembly comprises one birefringent filter stage.

101. The interleaver as recited in claim 88, wherein the birefringent filter assembly comprises a plurality of birefringent filter stages.

102. The interleaver as recited in claim 88, wherein the birefringent filter assembly comprises two birefringent filter stages.

103. The interleaver as recited in claim 88, wherein the birefringent filter assembly comprises three birefringent filter stages.

104. The interleaver as recited in claim 88, wherein the input polarization beam displacer, the birefringent filter assembly, the first output polarization beam displacer and the second output polarization beam displacer are configured so as to facilitate interleaving of a plurality of input beams simultaneously.

105. The interleaver as recited in claim 88, wherein the polarization beam splitter and the two reflectors for each birefringent filter stage define two light paths wherein a difference in the first and second optical path lengths is provided by a material having an index of refraction greater than one which is disposed within at least a portion of one of the first and second paths.

106. The interleaver as recited in claim 88, wherein the polarization beam splitter and the two reflectors for each birefringent filter stage define two light paths wherein an index of refraction is different for at least a portion of the first and second paths, so as to cause the first and second paths to have different optical lengths.

107. The interleaver as recited in claim 88, wherein the interleaved channels have spacing which is tunable.

108. An interleaver comprising: a birefringent filter assembly coupled so as to receive at least two beams of polarized light; and wherein the birefringent filter assembly is configured so as to provide a birefringent effect with respect to the beams without the use of birefringent crystals.

109. An interleaver comprising: a birefringent filter assembly coupled so as to receive at least two beams of polarized light; and wherein the birefringent filter assembly is configured so as to provide a birefringent effect with respect to the beams by causing the beams to travel along two paths, each path having a different optical path length.

110. The interleaver as recited in claim 109, wherein the difference in the first and second optical path lengths is provided by a material having an index of refraction greater than one which is disposed within at least a portion of one of the first and second paths.

111. An interleaver comprising: an input beam separator; a birefringent filter assembly in optical communication with the input beam separator, the birefringent filter assembly comprising at least one birefringent filter stage, each birefringent filter stage comprising: a polarization beam splitter; two reflectors; and a polarization beam recombiner.

112. The interleaver as recited in claim 111, wherein; the beam separator comprises a polarization beam displacer; and the beam recombiner comprises two polarization beam displacers.

113. The interleaver as recited in claim 111, wherein: the beam separator comprises a polarization beam splitter; and the recombiner comprises at least one polarization beam splitter.

114. A method for interleaving, the method comprising: separating a composite beam into two components thereof; separating each of the two components into two sub-components and transmitting the two sub-components along two different paths, each of the two paths having a different optical path lengths with respect to one another; recombining the sub-components of each component so as to achieve a birefringent effect; separating the two components into sub-components thereof according to a polarization of each; and recombining the sub-components so as to form two new composite beams, wherein each new composite beam contains substantially different channels with respect to the other new composite beam.

115. A method for interleaving, the method comprising: separating a composite light beam into first and second orthogonally polarized components thereof; separating the first component into first and second sub-components thereof and transmitting the first and second sub-components of the first component along two different paths, wherein each path has a different optical path length and recombining the first and second sub-components with one another so as to form a first component having a birefringent effect; separating the second component into first and second sub-components thereof and transmitting the second and second sub-components of the second component along two different paths, wherein each path has a different optical path length and recombining the second and first sub-components with one another so as to form a second component having a birefringent effect; separating the first component into orthogonally polarized first and second sub- components thereof; separating the second component into orthogonally polarized first and second components thereof; combining the first sub-component of the first component with the first subcomponent of the second component, so as to form a first composite output beam; and combining the second sub-component of the first component with the second subcomponent of the second component, so as to form a second composite output beam.

116. An interleaver comprising: an input polarization beam displacer; a birefringent filter assembly in optical communication with the input polarization beam displacer, the birefringent filter assembly comprising at least one birefringent filter stage, each birefringent filter stage comprising: a first filter polarization beam displacer; a second filter polarization beam displacer; at least one reflector configured direct light from the first filter polarization beam displacer to the second filter polarization beam displacer; a first output polarization beam displacer in optical communication with the birefringent filter assembly; and a second output polarization beam displacer in optical communication with the first output polarization beam displacer.

117. The interleaver as recited in claim 116, wherein the reflector(s) comprise prisms.

118. The interleaver as recited in claim 116, wherein the reflector(s) comprise mirrors.

119. The interleaver as recited in claim 116, wherein the reflector(s) comprise two reflectors.

120. The interleaver as recited in claim 116, wherein: the birefringent filter stage(s) define first and second paths; the reflector(s) comprise a single prism; and further comprising a material disposed in at least one of the first and second paths, the material having an index of refraction which causes the first and second paths to have different optical path lengths.

121. The interleaver as recited in claim 116, wherein each birefringent filter stage further comprises at least a half-wave waveplate intermediate each reflector and the first or the second filter polarization beam displacer.

122. The interleaver as recited in claim 116, wherein each birefringent filter stage further comprises at least a half-wave waveplate disposed intermediate each reflector and the first or the second filter polarization beam displacer, each half-wave waveplate having an optical axis thereof oriented at approximately 45° with respect to a +x axis at that location.

123. The interleaver as recited in claim 116, further comprising : a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a non-displaced beam therethrough; and a second input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a displaced beam therethrough.

124. The interleaver as recited in claim 116, further comprising: a first input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a non-displaced beam therethrough, the first input half- wave waveplate having an optic axis thereof oriented at approximately 22.5° with respect to a +x axis at that location; and a second input half-wave waveplate disposed intermediate the input polarization beam displacer and the birefringent filter assembly and configured so as to transmit a displaced beam therethrough, the second input half-wave waveplate having an optic axis thereof oriented at approximately -22.5° with respect to a +x axis at that location.

125. The interleaver as recited in claim 116, further comprising at least a half- wave waveplate configured to receive an output of each birefringent filter assembly.

126. The interleaver as recited in claim 116, further comprising a half-wave waveplate configured to receive an output of a birefringent filter assembly, the half-wave waveplate having an optical axis angle of approximately -22.5° with respect to the +χ axis at that location.

127. The interleaver as recited in claim 116, further comprising: a half-wave waveplate of a first stage thereof configured to receive an output of a birefringent filter assembly, the half-wave waveplate having a optical axis angle of approximately -33° with respect to the +χ axis at that location; and a half-wave waveplate of a second stage thereof configured to receive an output of a birefringent filter assembly, the half-wave waveplate having a optical axis angle of approximately 10.5° with respect to the +x axis at that location.

128. The interleaver as recited in claim 116, further comprising: a half-wave waveplate of a first stage thereof configured to receive an output of a birefringent filter assembly, the half-wave waveplate having a optical axis angle of approximately -33° with respect to the +x axis at that location; a half-wave waveplate of a second stage thereof configured to receive an output of a birefringent filter assembly, the half-wave waveplate having a optical axis angle of approximately 14° with respect to the +x axis at that location; and a half-wave waveplate of a third stage thereof configured to receive an output of a birefringent filter assembly, the half-wave waveplate having a optical axis angle of approximately -3.5° with respect to the +x axis at that location.

129. The interleaver as recited in claim 116, further comprising: a first half- wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; a second half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; a third half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer; and a fourth half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer.

130. The interleaver as recited in claim 116, further comprising: a first half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the first half-wave waveplate having an optic axis orientation of approximately 0° with respect to the +χ axis at that location; a second half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the second half-wave waveplate having an optic axis orientation of approximately 45° with respect to the +χ axis of that location; a third half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the third half-wave waveplate having an optic axis orientation of approximately 45° with respect to the +x axis at that point; and a fourth half-wave waveplate disposed intermediate the first output polarization beam displacer and the second output polarization beam displacer, the fourth half-wave waveplate having an optic axis orientation of approximately 90°.

131. The interleaver as recited in claim 116, wherein the birefringent filter assembly comprises one birefringent filter stage.

132. The interleaver as recited in claim 116, wherein the birefringent filter assembly comprises a plurality of birefringent filter stages.

133. The interleaver as recited in claim 116, wherein the birefringent filter assembly comprises two birefringent filter stages.

134. The interleaver as recited in claim 116, wherein the birefringent filter assembly comprises three birefringent filter stages.

135. The interleaver as recited in claim 116, wherein the filter polarization beam displacers and the reflector of each birefringent filter stage define two light paths wherein a difference in the first and second optical path lengths is provided by a material having an index of refraction greater than one which is disposed within at least a portion of one of the first and second paths.

136. The interleaver as recited in claim 116, wherein the filter polarization beam displacers and the reflector for each birefringent filter stage define two light paths wherein an index of refraction is different for at least a portion of the first and second paths, so as to cause the first and second paths to have different optical lengths.

137. The interleaver as recited in claim 116, wherein the input polarization beam displacer, the birefringent filter assembly, the first output polarization beam displacer and the second output polarization beam displacer are configured so as to facilitate interleaving of a plurality of beams simultaneously.

138. The interleaver as recited in claim 116, wherein the input polarization beam displacer, the birefringent filter assembly, the first output polarization beam displacer and the second output polarization beam displacer are configured so as to facilitate interleaving of a plurality of linearly arrayed beams simultaneously.

139. The interleaver as recited in claim 116, wherein the interleaver channels have spacing which is tunable.