WO2002043296A2 - Entrelaceur biregringent destine a des telecommunications par fibre optique et a multiplexage par repartition en longueur d'onde - Google Patents

Entrelaceur biregringent destine a des telecommunications par fibre optique et a multiplexage par repartition en longueur d'onde Download PDF

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Publication number
WO2002043296A2
WO2002043296A2 PCT/US2001/051097 US0151097W WO0243296A2 WO 2002043296 A2 WO2002043296 A2 WO 2002043296A2 US 0151097 W US0151097 W US 0151097W WO 0243296 A2 WO0243296 A2 WO 0243296A2
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WIPO (PCT)
Prior art keywords
polarization
optical
frequencies
light
beams
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PCT/US2001/051097
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English (en)
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WO2002043296A3 (fr
WO2002043296A9 (fr
Inventor
Boying Barry Zhang
Zhicheng Yang
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Adc Telecommunications, Inc.
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Priority to AU2002239762A priority Critical patent/AU2002239762A1/en
Publication of WO2002043296A2 publication Critical patent/WO2002043296A2/fr
Publication of WO2002043296A9 publication Critical patent/WO2002043296A9/fr
Publication of WO2002043296A3 publication Critical patent/WO2002043296A3/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
    • G02B6/29386—Interleaving or deinterleaving, i.e. separating or mixing subsets of optical signals, e.g. combining even and odd channels into a single optical signal
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29302—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means based on birefringence or polarisation, e.g. wavelength dependent birefringence, polarisation interferometers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
    • G02B6/2713—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations
    • G02B6/272—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations comprising polarisation means for beam splitting and combining
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
    • G02B6/2766—Manipulating the plane of polarisation from one input polarisation to another output polarisation, e.g. polarisation rotators, linear to circular polarisation converters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
    • G02B6/2773—Polarisation splitting or combining
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/06—Polarisation multiplex systems

Definitions

  • the present invention is directed generally to a fiber optic device, and more particularly to an interleaver for combining and/or separating even and odd sets of optical communication channels.
  • WDM wavelength division multiplexing
  • ITU International Telecommunications Union
  • More information may be transmitted over a fixed bandwidth when the channel separation is smaller, since more channels can fit into the fixed bandwidth.
  • One approach to producing a smaller channel separation is to create two combs of channels that are separated by twice the desired channel separation, i.e. that have a spacing of 2 ⁇ v.
  • the frequencies of the channels in the first comb are selected to be intermediate the channel frequencies of the second comb.
  • the first comb of frequencies may then be interleaved with the second comb of frequencies, to produce a WDM signal having a channel spacing of ⁇ v.
  • the device that interleaves the two combs of frequencies is called an interleaver.
  • An advantage provided by an interleaver is that standard WDM components may be used to generate the different combs of channels, and the addition of the interleaver permits operation at dense multiplexing.
  • an optical interleaver that is suitable for operating with fiber optic components. It is desirable that the insertion loss of the interleaver be low and flat across the entire bandwidth of interest. It is also important that the interleaver operates over a wide frequency range and that the cross-talk between channels is low.
  • the present invention relates to a device that interleaves and de-interleaves odd and even channels by rotating the polarization of the WDM channels.
  • de-interleaving the polarization of the odd channels is rotated to a different angle than the polarization of the even channels, and the odd and even channels are then polarization mode separated.
  • interleaving the even and odd channels are polarization mode combined, and the even and odd channels are then polarization rotated are rotated so that all channels have the same polarization.
  • One embodiment of the invention is directed to an optical communications channel interleaver for operating on an optical signal comprising multiple optical channels having channel frequencies of v 0 + m ⁇ v, where v 0 is a lowest channel frequency, ⁇ v is a channel separation and m is an integer, the optical communications signal comprising at least one optical beam.
  • the interleaver includes an input and a birefringent interleaving unit disposed to receive at least one polarized light beam from the input.
  • the birefringent interleaving unit includes a birefringent polarization rotating element having a length selected to rotate polarizations of even optical channels passing therethrough to a first angle and to rotate polarizations of odd optical channels passing therethrough to a second angle differing from the first angle by approximately 90°, the even optical channels having frequencies where m has an even integer value and the odd optical channels having frequencies where m has an odd integer value, and a first polarization-sensitive splitting/combining element.
  • the interleaver also includes an output coupled to receive light from the birefringent interleaving unit.
  • Another embodiment of the invention is directed to an optical fiber communications system that includes an optical transmitter unit generating light in multiple optical channels having channel frequencies of v 0 + m ⁇ v, where v 0 is a lowest channel frequency, ⁇ v is a channel separation and m is an integer, an optical detector unit detecting signals of the multiple optical channels, and an optical communications network coupled between the optical transmitter unit and the optical detector unit, the optical communications network including at least one optical fiber. At least one of the optical transmitter unit and the optical detector unit includes an optical interleaver coupled to the optical communications network.
  • the optical interleaver includes a first optical unit disposed to receive light generated within the optical transmitter unit as at least one input beam and to transmit at least one polarized light beam, and a birefringent interleaving unit disposed to receive the at least one polarized light beam from the first optical unit, the birefringent interleaving unit including a birefringent polarization rotating element having a length selected to rotate polarizations of even optical channels passing therethrough to a first angle and to rotate polarizations of odd optical channels passing therethrough to a second angle differing from the first angle by approximately 90°, the even optical channels having frequencies where m has an even integer value and the odd optical channels having frequencies where m has an odd integer value, and a first birefringent splitting/combining element.
  • the optical interleaver also includes a second optical unit disposed to receive at least one beam from the birefringent interleaving unit and to transmit at least one output light beam for detection within the optical detection unit.
  • Another embodiment of the invention is directed to an optical device for use with a polarized light beam comprising light at a plurality of uniformly spaced optical frequencies.
  • the device includes polarization rotation means for rotating polarization of the polarized light beam, wherein the polarization rotation means light effectively rotates polarization of light at one optical frequency to a first angle and effectively rotates polarization of light at an adjacent, uniformly spaced frequency to a second angle different from the first angle by approximately 90° to produce a polarization rotated beam.
  • the device also includes optical separation means for separating light rotated to the first angle from light rotated to the second angle.
  • Another embodiment of the invention is directed to an optical device for interleaving a first polarized light beam having at least two first frequencies uniformly spaced by a frequency difference, and a second polarized light beam having at least two second frequencies uniformly spaced by the frequency difference, the second frequencies being different from the first frequencies, the first polarized light beam having a polarization orthogonal to a polarization of the second beam.
  • the optical device includes combining means for combining the first and second polarized light beams into a single beam.
  • the device also includes polarization rotation means for rotating polarization of the single beam received from the combining means, wherein the polarization rotation means light effectively rotates polarization of light at the at least two first frequencies to a first angle and effectively rotates polarization of light at the at least two second frequencies to a second angle so as to produce an output comprising light at the first and second frequencies having a same polarization.
  • Another embodiment of the invention is directed to a method for de- interleaving a light beam comprising light at a plurality optical frequencies uniformly spaced by a frequency difference.
  • the method includes directing at least one polarized light beam comprising light at the plurality of optical frequencies to a polarization rotating element, and rotating, with the polarization rotating element, polarization of light having a first set of frequencies of the plurality of uniformly spaced frequencies to a first angle, frequencies of the first set of frequencies being uniformly spaced by twice the frequency difference.
  • the method also includes rotating, with the polarization rotating element, polarization of light having a second set of frequencies of the plurality of uniformly spaced frequencies to a second angle different from the first angle by about 90°, frequencies of the second set of frequencies being uniformly spaced by twice the frequency difference and being different from the frequencies of the first set of frequencies, and separating the light having the first set of frequencies from the light having the second set of frequencies.
  • Another embodiment of the invention is directed to a method for interleaving two light beams, the first light beam comprising light at a first set of optical frequencies uniformly spaced by a frequency difference and the second light beam comprising light at a second set of optical frequencies uniformly spaced by the frequency difference, frequencies of the first set of frequencies being different from frequencies of the second set of frequencies.
  • the method includes directing at least one first polarized light beam of light at the first set of frequencies and at least one second polarized light beam of light at the second set of frequencies a polarization polarization-sensitive combining element, the at least one first light beam having a first polarization and the at least one second light beam having a second polarization orthogonal to the first polarization and respectively combining the at least one first polarized light beam with the at least one second polarized light beam to produce a combined light beam.
  • the method also includes rotating polarization of light in the combined light beam at the first set of frequencies to a first angle, and rotating polarization of light in the combined light beam at the second set of frequencies to a second angle different from the first angle by about 90 ° so as to polarize the combined light beam.
  • FIG. 1 schematically illustrates a wavelength division-multiplexed (WDM) fiber optics communications system
  • FIGs. 2A and 2C schematically illustrate one particular embodiment of a birefringent interleaver according to the present invention
  • FIGs. 2B and 2D illustrate polarization states of light propagating through the interleavers of FIGS. 2A and 2C respectively;
  • FIG. 3A schematically illustrates another embodiment of a birefringent interleaver according to the present invention
  • FIG. 3B illustrates polarization states of light passing through the interleaver of FIG. 3A
  • FIG. 3C schematically illustrates polarization rotator location in the interleaver of FIG. 3A
  • FIG. 4A schematically illustrates another embodiment of a birefringent interleaver according to the present invention
  • FIG. 4B illustrates polarization states of light passing through the interleaver of FIG. 4A
  • FIG. 5 illustrates experimental results showing the spectrum of a
  • FIG. 6A schematically illustrates an embodiment of a birefringent de- interleaver according to the present invention
  • FIG. 6B illustrates polarization states of light passing through the de- interleaver of FIG. 6A in the forward direction
  • FIG. 6C illustrates polarization states of light passing through the de- interleaver of FIG. 6A in the backward direction
  • FIG. 7A schematically illustrates another embodiment of a birefringent interleaver according to the present invention
  • FIG. 7B illustrates polarization states of light passing through the interleaver of FIG. 7A in the forward direction;
  • FIG. 7C illustrates polarization states of light passing through the interleaver of FIG. 7A in the backward direction
  • FIG. 8 schematically illustrates an embodiment of an interleaver that includes a turning prism in the optical path, according to the present invention.
  • the present invention is applicable to optical fiber systems, and is believed to be particularly suited to interleaving and de-interleaving optical communications channels in a wavelength division multiplexed (WDM) optical communications system.
  • WDM wavelength division multiplexed
  • WDM systems include several channels of light at different optical frequencies.
  • the channels are evenly spaced by frequency.
  • the mth channel has a frequency given by v 0 + m ⁇ v, where v 0 is a lowest channel frequency, ⁇ v is the channel separation and m is an integer value ranging from 0 to mo, the upper value.
  • the channel separation, ⁇ v is 100 GHz or 50 GHz.
  • Interleaving is the operation of mixing two signals, one containing the even channels with the containing the odd channels, to produce a signal containing both the even and odd channels.
  • De-interleaving is the operation of separating a signal containing odd and even channels into a first signal containing the even channels and a second signal containing the odd channels.
  • Many devices used for interleaving may also be used in reverse for de-interleaving. Consequently, the term "interleaving" is often used to denote the operations of interleaving and de-interleaving. Not all interleaving devices are able to perform both interleaving and de-interleaving functions, as is described hereinbelow.
  • a WDM transmitter 102 directs a WDM signal having mo+1 channels through a fiber communications link 104 to a WDM receiver 106.
  • This particular embodiment of WDM transmitter 102 includes a number of light sources 108a - 108c that generate light at different wavelengths, ⁇ O, ⁇ 2 and ⁇ mo-1 , corresponding to the even optical channels.
  • the light output from the light sources 108a-108c is combined in a first WDM combiner 110a, to produce a first output 112a.
  • the light in the first output 112a from the first WDM combiner 110a includes light at the wavelengths ⁇ O, ⁇ 2 and ⁇ m 0 -1.
  • the WDM transmitter 102 also includes other light sources 108d - 108f that generate light at a different set of wavelengths, ⁇ 1 , ⁇ 3 and ⁇ m 0 respectively, corresponding to the odd optical channels.
  • the light output from the light sources 108d-108f is combined in a second WDM combiner 110b to produce a second output 112b.
  • the light in the second output 112b from the second WDM combiner 110b includes light at the wavelengths ⁇ 1 , ⁇ 3 and ⁇ -o.
  • the channel spacing in each of the first and second outputs 112a and 112 b is 2 ⁇ v.
  • the light of the first and second outputs 112a and 112b is combined in the interleaver 114 to produce an interleaved output containing ⁇ O, ⁇ 1 , ⁇ 2 .... ⁇ mo, at a channel separation of ⁇ v.
  • the interleaved output is launched into the fiber communications link 104 for propagation to the WDM receiver 106.
  • Light sources 108a-108f may be modulated laser sources, or laser sources whose output is externally modulated, or the like. It will be appreciated that the WDM transmitter 102 may be configured in many different ways to produce the first and second outputs 112a and 112b that are input to the interleaver 114.
  • the WDM transmitter 102 may be equipped with any suitable number of light sources for generating the required number of optical channels. For example, there may be twenty, forty or eighty optical channels. The WDM transmitter 102 may also be redundantly equipped with additional light sources to replace failed light sources.
  • the interleaved signal is passed through a de-interleaver 116, which separates the interleaved signal into an even channel signal 118a, containing the even channels, and an odd channel signal 118b, containing the odd channels.
  • the even channel signal 118a is passed into a first wavelength division demultiplexer (WDDM) unit 120a which separates the even channels into individual channels that are directed to respective detectors 122a-122c.
  • the odd channel signal 118b is passed into a second WDDM unit 120b that separates the odd channels into individual channels that are directed to respective detectors 122d-122f.
  • the exemplary WDM transmitter and receiver architecture illustrated in FIG. 1 permits the user to employ relatively straightforward WDM components for all multiplexing and demultiplexing operations except for interleaving and de-interleaving.
  • the component costs for the transmitter 102 and receiver 106 may be kept low, since only the interleaver and de-interleaver have the requirement of operating at dense multiplexing, at the channel separation ⁇ v, while the other components in the transmitter 102 and receiver 106 typically operate with channels separated by at least 2 ⁇ v.
  • the interleaver 200 includes a birefringent polarization rotating crystal 202 and a polarization-sensitive beam splitting element 204.
  • the polarization-sensitive beam splitting element 204 may be any suitable element that splits an incoming light beam into beams of orthogonal polarizations, such as a polarizing beamsplitter or a birefringent splitting crystal.
  • a birefringent splitting crystal is particularly advantageous for maintaining small size in devices compatible with fiber optical components.
  • the interleaver 200 may be used to de-interleave a dense multiplexed signal into two less densely multiplexed signals. De-interleaving with the interleaver 200 is described with reference to FIG. 2B, which illustrates the polarization state and lateral position of the light beam passing through the interleaver 200 at various positions along the interleaver 200.
  • FIG. 2B schematically represents the cross-section of the interleaver 200 as viewed in a direction along the z-axis.
  • a first optical unit 206 delivers a polarized light beam 208, containing both the even and odd channels, to the interleaver 200, as illustrated for position z1. The even and odd channels are indicated as ⁇ even and ⁇ odd respectively.
  • the birefringent polarization rotating crystal 202 is oriented so that its optical axis lies in the x-y plane, the plane perpendicular to the direction that light propagates within the crystal 202. Furthermore, the optical axis of the birefringent polarization rotating crystal 202 lies at 45° to the y axis, the axis along which the light entering the polarization crystal 202 is polarized. As a result of the particular orientation of the polarization rotating crystal relative to the z-axis, the propagation direction, the polarization of the light beam 208 is rotated by the polarization rotating crystal 202.
  • the length and birefringence of the polarization rotating crystal 202 are selected so that, after passing through the polarization rotating crystal 202, the polarizations of the even channels are each effectively rotated to the same angle. Likewise, the polarizations of the odd channels are each effectively rotated to the same angle. However, the angle through which the even channels are rotated differs from the angle through which the odd channels are rotated by approximately 90°. Consequently, at the output of the polarization rotating crystal 202, position z2, the even channels are polarized parallel to each other and are orthogonal to the polarization of the odd channels.
  • the label ⁇ e refers to even channels and ⁇ o refers to odd channels.
  • the polarization rotating crystal 202 effectively rotates the polarization of the odd channels through 90° while effectively not rotating the polarization of the even channels
  • the polarization of the even channels might be rotated through 90°, while the polarization of the odd channels is effectively un rotated.
  • the length, I, of the polarization rotating crystal 202 that is required to effectively rotate the odd channels through an angle 90° different from the even channels is given by:
  • c is the speed of light
  • (n e -n 0 ) is the difference between the ordinary and extraordinary refractive indices for the crystal, also known as the birefringence
  • ⁇ v is the spacing between odd and even channels.
  • YVO ortho-vanadate
  • the length of the polarization rotating crystal 202 is approximately 14.7 mm.
  • any suitable birefringent material may be used, for example lithium niobate.
  • YVO 4 is particularly advantageous since its birefringence is high, which reduces the length of crystal required for the polarization rotating crystal 202, thus making the overall length of the interleaver 200 shorter.
  • the optical path length of the polarization rotating crystal 202 may be adjusted using a multiply-segmented element, where the multiple segments have faces that are not perpendicular to the passage of light therethrough.
  • the optical path length of the polarization rotating crystal may be adjusted to a precise value translating of one or more of the segments across the light beam.
  • the segments may be formed from different materials or from similar materials. A multiple segmented polarization rotating crystal is discussed in greater detail in U.S. Patent Application Serial No.
  • the optical path length of the polarization rotating crystal 202 may vary with temperature. Thermal fluctuations in path length may be reduced or avoided by using a multiply-segmented polarization rotating crystal 202 having segments formed of different materials, having different thermal expansion coefficients and different thermal variations in the birefringence. Judicious selection of the lengths of the different segments so that the net thermal variation in optical path is reduced, if not zero. This is discussed in greater detail in U.S. U.S. Patent Application Serial No. 09/694,148, titled “Method and Apparatus for Thermally Compensating a Birefringent Optical Element", filed on October 23, 2000 by Xiaofeng Han and Zhicheng Yang, incorporated herein by reference.
  • the polarization rotated beam 210 After leaving the polarization rotating crystal 202, the polarization rotated beam 210 enters the polarization-sensitive beam splitting element 204, where the two polarizations are split from each other.
  • the polarization-sensitive beam splitting element 204 is a birefringent splitting crystal, where the entering beam 210 is split into an ordinary ray 212 and an extraordinary ray 214.
  • the odd channels, propagating as the extraordinary ray 214 have been separated from the even channels, propagating as the ordinary ray 212, as shown for position z3.
  • the two beams 212 and 214 from the birefringent splitting crystal 204 may then be directed to two different output fibers 220 by the second optical unit 216.
  • birefringent splitting crystal 204 has its optical axis at -45° to the z-axis in the x-z plane.
  • the birefringent splitting crystal 204 may be formed from any suitable birefringent material, such as lithium niobate or ortho-vanadate.
  • a highly birefringent material, such as ortho-vanadate is advantageous since it reduces the length of the crystal required to obtain separation between the ordinary and extraordinary beams 212 and 214.
  • the first optical unit 206 may be coupled to receive input light from an external optical fiber 218.
  • the first optical unit 206 may also include one or more collimating lenses to collimate the light from the fiber 218 before passage through the interleaver 200.
  • the first optical unit 206 may also be provided with optical elements to produce the polarized beam 208. For example, if the output from the fiber 218 is unpolarized, then the first optical unit 206 may include a polarizer to polarize the output from the fiber 218. Furthermore, the first optical unit may produce more than one polarized beam 208 for propagation through the interleaver, as is discussed with reference to other embodiments below.
  • the output from the fiber 218 may be polarized, for example if the fiber 218 is a polarization maintaining fiber, in which case the first optical unit need not include a polarizer to produce the polarized beam 208.
  • the second optical unit 216 may be coupled to output fibers 220 and may include a light focusing system (not shown) to direct the separated beams 212 and 214 into respective fibers 220.
  • the light focusing system may include separate lenses for each beam 212 and 214, or may include a lens system that operates on both beams 212 and 214.
  • the second optical unit 216 may be provided with combining optics to combine two or more of the output beams into a single output beam before transmitting the single output beam into the respective optical fiber 220.
  • the birefringent interleaver 200 is able to perform a de-interleaving operation, as has just been described, in other words it separates the odd channels from the even channels. It will be appreciated that the interleaver may also perform an interleaving operation, in other words combining a beam that includes odd channels with a beam that includes oven channels, to produce a single beam that includes both odd and even channels. This may be achieved by passing light through the interleaver 200 in the backwards direction, as is now discussed with reference to FIGs. 2C and 2D. Two orthogonally polarized beams 230 and 232 are directed at the birefringent splitting crystal 204 from the second optical unit 216.
  • the first polarized beam 230 contains the even channels, while the second polarized beam 232 contains the odd channels.
  • the beams 230 and 232 are separate upon entering the birefringent splitting crystal 204.
  • One of the beams 230 and 232, in this case the second beam 232 enters the birefringent splitting crystal 204 as an extraordinary beam and the other beam, in this case beam 230, enters as an ordinary beam, as shown for position z3.
  • Passage through the birefringent splitting crystal 204 in the reverse direction results in the extraordinary beam and ordinary beam combining into a single beam 234 at position z2.
  • the single beam 234 contains the odd channels having one polarization and the even channels having the orthogonal polarization, as shown for position z2.
  • the single beam 234 then passes through the polarization rotating crystal 202.
  • the polarization rotating crystal 202 effectively rotates the polarization of the odd channels through a first angle and the polarization of the even channels through a second angle different from the first angle by approximately 90°. Consequently, after propagating through the polarization rotating crystal 202, the beam 236 is polarized and contains all the even and odd channels.
  • the beam 236 may then pass through the first optical unit 206 to the fiber 218.
  • the interleaver 200 may be operated to interleave odd and even channels when the light is passed therethrough in one direction and as a de-interleaver when the light passes through the interleaver 200 in the opposite direction.
  • FIG. 3A Another particular embodiment of a birefringent interleaver 300 is illustrated in FIG. 3A.
  • FIG. 3B schematically illustrates cross-sections through the interleaver 300, showing polarization states of the various light beams at different points along the interleaver 300 when performing a de- interleaving operation.
  • the interleaver 300 is based around a basic interleaver unit 301 similar to that of FIG. 2A, and includes a polarization rotating crystal 302 and a first birefringent splitting crystal 304.
  • the interleaver 300 also includes a light coupling module 306 couples light output from a fiber 308 to the interleaver unit 301.
  • the light coupling module 306 may include a single lens, a lens pair, as illustrated, or some other combination of lenses. In one particular embodiment, the light coupling module 306 may include a single lens associated with each fiber 308.
  • the light collimating module 306 may be a coupling module formed from a pair of lenses separated by a distance approximately equal to the sum of their focal lengths. Where the input fiber or fibers are placed at a distance from the first lens of approximately the focal length of the first lens, the first lens collimates the light beams from the input fiber or fibers. The second lens parallelizes the light beams.
  • a dual-lens coupling module is described in greater detail in commonly owned U.S. Patent Application Serial No. 09/181 ,145, which is incorporated herein by reference.
  • the dual lens light coupling module may also be used as the light coupling module 346 at the output of the interleaver 300.
  • the collimated beam 310 from the light collimating module 306 passes into a polarization-sensitive beam splitting element 312. It is advantageous to use a birefringent splitting crystal as the polarization-sensitive beam splitting element 312, since the birefringent splitting crystal may be compact.
  • the present description hereafter refers to the polarization-sensitive beam splitting element 312 as a second birefringent splitting crystal.
  • the polarization-sensitive beam splitting element 312 may be any suitable element that splits an unpolarized light beam into two orthogonally polarized components, such as a polarization beamsplitter or the like.
  • the collimated light beam 310 from the light collimating module 306 may be unpolarized, as illustrated for the position z1 , which shows a mixture of polarization states.
  • the second birefringent splitting crystal 312 splits the collimated light beam 310 into an ordinary beam 314 and an extraordinary beam 316 which is separated from the ordinary beam 314 due to birefringent walk-off. Accordingly, two orthogonally polarized beams 314 and 316 enter the polarization rotating crystal 302, as illustrated for position z2.
  • the second birefringent splitting crystal 312 may be formed from any suitable birefringent material, such as lithium niobate and ortho-vanadate.
  • Ortho-vanadate is advantageous because its high birefringence results in a relatively short crystal length.
  • the optical axis of the second birefringent splitting crystal 312 may be set at -45° to the z-axis in the y-z plane.
  • the beams 314 and 316 each include both even and odd channels, the polarizations of the even and odd channels are effectively rotated to different angles by the polarization rotating crystal 302. Therefore, beam 314 is converted to beam 318 at the output of the polarization rotating crystal 302, at position z3, having the polarization of the even channels oriented in one polarization and the polarization of the odd channels oriented orthogonally to the polarization of the even channels.
  • beam 316 is converted to beam 320, also having the polarization of the even channels oriented in one polarization and the polarization of the odd channels oriented orthogonally to the polarization of the even channels.
  • the polarization of the even channels in the first beam 318 is orthogonal to the polarization of the even channels in the second beam 320.
  • the polarization of the odd channels in the first beam 318 is orthogonal to the polarization of the odd channels in the second beam 320
  • the first and second beams 318 and 320 pass through a first polarization rotator 322, which re- orients the polarization direction of at least one of the first and second beams 318 and 320, so that the polarizations of the even channels of the first and second beams 318 and 320 are parallel, as shown for position z4.
  • the polarizations of the odd channels of the first and second beams 318 and 320 are concomitantly oriented so as to be parallel.
  • the first polarization rotator 322 may operate on one or both of the first and second beams.
  • the first beam 318 passes through a polarization rotating element 324 of the polarization rotator, while the polarization of the second beam 320 remains unchanged on passage through the first polarization rotator 322.
  • the position of the polarization rotating element 324 relative to the beams 318 and 320 is illustrated in FIG. 3C.
  • the polarization rotating element 324 advantageously rotates the polarization of all channels, both odd and even, through substantially the same angle.
  • the polarization rotating element 324 may be, for example a half- wave plate or a Faraday rotator. Since WDM signals typically have a large bandwidth, a zero-order retardation plate is advantageously employed as the half-wave plate in order to reduce the spread in angles through which the different channels are rotated.
  • the beams 318 and 320 After passing through the first polarization rotator 322, the beams 318 and 320 enter the first birefringent splitting crystal 304, in which the first beam 318 is split into two orthogonally polarized beams 326 and 328, containing the even and odd channels respectively. Likewise, the second beam 320 is split into two orthogonally polarized beams 330 and 332, containing the even and odd channels respectively.
  • the polarizations of the beams 326 and 330 containing the even channels are parallel and the polarizations of the beams 328 and 332 containing the odd channels are parallel, as illustrated for position z5.
  • the beams 326- 332 After exiting the first birefringent splitting crystal 304, the beams 326- 332 pass through a second polarization rotator 334, which re-orients the polarization direction of at least one of the beams 326 and 330 containing the even channels so that the polarizations of beams 326 and 330 become mutually orthogonal.
  • the second polarization rotator 334 also rotates the polarization of at least one of the beams 328 and 332 containing the odd channels, so that the polarizations of the beams 328 and 332 also become mutually orthogonal, as illustrated for position z6.
  • the second polarization rotator 334 includes a first polarization rotating element 336 that rotates the polarization of beam 330 through 90° and a second polarization rotating element 338 that rotates the polarization of beam 328 through 90°.
  • the polarization rotating elements 336 and 338 may be any suitable polarization rotating element such as a half-wave plate or Faraday rotator.
  • the position of the polarization rotating elements 336 and 338 relative to the different beams 326-332 is illustrated in FIG. 3C.
  • the mutually orthogonal even channel beams 326 and 330 are combined in a polarization-sensitive beam combining element 340 into a single even channel beam 342 and the mutually orthogonal odd channel beams 328 and 332 are combined into a single odd channel beam 344, as illustrated for position z7.
  • the polarization-sensitive beam combining element 340 is typically similar to a polarization-sensitive beam splitting element, but operated in reverse.
  • the polarization-sensitive beam combining element 340 may be a birefringent combining crystal that uses the birefringent walk-off effect to combine two beams in a manner that is reversed from the second birefringent splitting crystal 312.
  • the polarization-sensitive beam combining element 340 is referred to as a birefringent combining crystal hereafter, although it will be appreciated that the polarization-sensitive beam combining element 340 may be any other suitable element capable of combining two beams of orthogonal polarization, such as a polarization beamsplitter.
  • the birefringent combining crystal 340 may be formed from any suitable birefringent material, such as lithium niobate and ortho-vanadate. Ortho-vanadate is advantageous because its high birefringence results in a relatively short crystal length.
  • the optical axis of the birefringent combining crystal 340 may be set at -45° to the z-axis in the y-z plane.
  • the even and odd channel beams 342 and 344 may be focused using a light coupling unit 346 into respective output fibers 348.
  • the light coupling unit 346 may include a single lens for each beam 342 and 344, or may use a system of multiple lenses to focus the beams 342 and 344.
  • the light coupling unit 346 may include a dual lens light coupling module as described above.
  • Various components of the interleaver 300 including the first and second birefringent splitting crystals 304 and 312, the polarization rotating crystal 302, the birefringent combining crystal 340, and the polarization rotators 322 and 334, may be mounted on a base 350.
  • the configuration of the interleaver 300 may be varied.
  • the birefringent combining crystal 340 may be reoriented by rotation about the z-axis by 180°, in which case the beams 326 and 328 would be ordinary beams, while beams 330 and 332 would be extraordinary beams. This would also require other elements to be reconfigured.
  • the orientation of the birefringent combining crystal 340 as illustrated is advantageous since it provides compensation for polarization mode dispersion: the light that propagates as ordinary beam 314 through the second birefringent splitting crystal 312 also propagates through the birefringent combining crystal 340 as extraordinary beams 326 and 328.
  • the light that propagates as extraordinary beam 316 through the second birefringent splitting crystal 312 also propagates through the birefringent combining crystal 340 as ordinary beams 330 and 332.
  • no light propagates through both the second birefringent splitting crystal 312 and the birefringent combining crystal 340 solely as an ordinary beam or an extraordinary beam. Therefore, dispersion that results from light in different polarizations propagating along paths of different lengths is reduced.
  • the first polarization rotator 322 may be positioned between the second birefringent splitting crystal 312 and the polarization rotating crystal 302, as is illustrated in FIG. 4A.
  • FIG. 4B illustrates the polarization states of the different light beams at the same stages within the interleaver as were illustrated in FIG. 3B.
  • interleaver 300 and 400 may be used to perform both de-interleaving and interleaving operations.
  • two beams respectively containing the even and odd channels, are fed into the birefringent combining crystal 340 in the -z direction.
  • the even channel beam is positioned on the face of the birefringent combining crystal 340 at the point where the even channel beam 342 exits the birefringent combining crystal 340.
  • the odd channel beam is positioned on the face of the birefringent combining crystal 340 at the point where the odd channel beam 344 exits the birefringent combining crystal 340.
  • the light passes in the reverse direction through the interleaver 300 and 400 along the same path as the forward direction.
  • the interleavers 300 and 400 also operate in the backwards direction even if Faraday rotators, which are non- reciprocal polarization rotating elements, are employed instead of half-wave plates, since the polarization is rotated through 90° by the first and second polarization rotators 322 and 334.
  • Experimental results with the interleaver 300 of FIG. 3A are illustrated in a graph in FIG. 5.
  • the graph illustrates the output transmitted from the interleaver 300 as a function of frequency following an interleaving operation.
  • the interleaved frequencies are separated by approximately 50 GHz, and the loss across the entire bandwidth is flat, with all channels having the same intensity to within ⁇ 0.3 dB.
  • FIG. 6A Another particular embodiment of a de-interleaver 600 is schematically illustrated in FIG. 6A.
  • FIG. 6B illustrates the polarization states for the different beams propagating through the de-interleaver 600 at various points along the z-axis.
  • the de-interleaver 600 is based around an interleaver unit that includes a polarization rotating crystal 602 and a first birefringent splitting crystal 604.
  • the illustrated embodiment of de- interleaver 600 is particularly useful for de-interleaving in one direction while preventing propagation of signals in the reverse direction.
  • a WDM signal enters the de-interleaver 600 as a collimated beam
  • the collimated beam 610 typically propagates from a light collimating module (not shown) into a polarization-sensitive beam splitting element 612, such as a second birefringent splitting crystal.
  • a polarization-sensitive beam splitting element 612 such as a second birefringent splitting crystal.
  • the polarization-sensitive beam splitting element 612 may be any element that splits an unpolarized light beam into two orthogonally polarized components, such as a polarization beamsplitter or the like.
  • the second birefringent splitting crystal 612 splits the collimated light beam 610 into an ordinary beam 614 and an extraordinary beam 616 which is separated from the ordinary beam 614 due to birefringent walk-off. Accordingly, two orthogonally polarized beams 614 and 616 enter the first polarization rotator 622, as illustrated for position z2.
  • the polarization rotator 622 re-orients the polarization direction of the two orthogonally polarized beams 614 and 616.
  • the polarization of the beam 614 is rotated by -45°, whereas the polarization of beam 616 is rotated by +45°, so that the beams 614 and 616 are polarized parallel upon leaving the first polarization rotator 622, as is illustrated for position z3.
  • the convention is adopted herein that clockwise rotations are positive, while counterclockwise rotations are negative.
  • the polarization rotator 622 includes two polarization rotating elements 624 and 625 to rotate the polarization of the beams 614 and 616 respectively.
  • the polarization rotating elements 624 and 625 may be half- wave retardation plates or may be Faraday rotators.
  • the polarization rotating elements 624 and 625 rotate the polarization of the odd and even channels in their respective beams 614 and 616 by an equal amount.
  • the two beams 614 and 616 Upon exiting the first polarization rotator 622, the two beams 614 and 616 propagate through the polarization rotating crystal 602, which effectively rotates the polarizations of the odd and even channels through angles differing by 90°.
  • the first and second beams 618 and 620 each include even channels in one polarization state and odd channels in an orthogonal polarization state, as illustrated for position z4.
  • the polarization of the even channels is effectively rotated through 90°, while the polarization of the odd channels is not effectively rotated.
  • the first and second beams 618 and 620 propagate into the first birefringent splitting crystal 604, in which the first beam 618 is split into two orthogonally polarized beams 626 and 628, containing the even and odd channels respectively. Likewise, the second beam 620 is split into two orthogonally polarized beams 630 and 632, containing the even and odd channels respectively.
  • the polarizations of the beams 626 and 630 containing the even channels are parallel and the polarizations of the beams 628 and 632 containing the odd channels are parallel, as illustrated for position z5.
  • the beams 626- 632 After exiting the first birefringent splitting crystal 604, the beams 626- 632 pass through a second polarization rotator 634, which orients the polarization direction of the beams 626 and 630 containing the even channels so that the polarizations of beams 626 and 630 become mutually orthogonal.
  • the second polarization rotator 634 also rotates the polarization of the beams 628 and 632 containing the odd channels, so that the polarizations of the beams 628 and 632 also become mutually orthogonal, as illustrated for position z6.
  • the second polarization rotator 634 includes a first polarization rotating element 636 that rotates the polarization of beam 626 through +45° and a second polarization rotating element 637 that rotates the polarization of beam 630 through -45°. Furthermore, the second polarization rotator 634 includes a third polarization rotating element 638 that rotates the polarization of beam 628 through -45° and a fourth polarization rotating element 639 that rotates the polarization of beam 632 through +45°.
  • the polarization rotating elements 636 - 639 may be any suitable polarization rotating element such as a half-wave plate or Faraday rotator.
  • the position of the polarization rotating elements 636 - 639 relative to the different beams 626-632 is illustrated in FIG. 6B.
  • Each polarization rotating element 636 - 639 advantageously rotates the polarization of all channels passing therethrough by an amount that is independent of channel frequency.
  • the mutually orthogonal, even channel beams 626 and 630 are combined in a birefringent combining crystal 640 into a single even channel beam 642 and the mutually orthogonal, odd channel beams 628 and 632 are combined into a single odd channel beam 644, as illustrated for position z7.
  • the even and odd channel beams 642 and 644 may be focused using a focusing unit (not shown) into respective output fibers.
  • the de-interleaver 600 may interleave odd and even channels if operated with backward directed beams.
  • the de-interleaver 600 is not reciprocal, and operates as an isolator in the backward direction.
  • FIG. 6C shows polarization states of various beams within the de- interleaver 600 for light propagating in a backwards direction, from the birefringent combining crystal 640 to the second birefringent splitting crystal 612.
  • the two input beams 662 and 664, containing even and odd channels respectively, are incident on the birefringent combining crystal 640, as illustrated for position z7.
  • the birefringent combining crystal 640 operates in the backwards direction as a birefringent splitting crystal, and so the even channel beam 662 is split into an extraordinary beam 666 and an orthogonally polarized ordinary beam 670. Likewise, the odd channel beam 664 is split into an extraordinary beam 668 and an orthogonally polarized ordinary beam 672, as illustrated for position z6.
  • the four beams 666 - 672 then pass through the second polarization rotator 634, which employs Faraday rotators, in the reverse direction. Therefore, the handedness of the polarization rotation experienced upon propagating through the Faraday rotators is the same as in the forward direction. Accordingly, after propagating through the Faraday rotators 636, 637, 638 and 639 of the second polarization rotator 634, the polarizations of beams 666 and 672 are each rotated by +45° and the polarizations of beams 668 and 670 are each rotated through -45°, as illustrated for position z5.
  • the separation between beams 666 and 668, and also between beams 670 and 672, is increased, as illustrated at position z4.
  • the polarization rotating crystal 602 effectively rotates the polarizations of the even channel beams 666 and 670 through 90°, whereas the polarizations of the odd channel beams 668 and 672 remain effectively unrotated, as illustrated for position z3.
  • the polarizations of all four beams 666, 668, 670 and 672 are parallel.
  • the first polarization rotator 622 rotates the polarizations of beams 666 and 668 by -45°, while the polarizations of beams 670 and 672 are rotated through +45°.
  • the polarizations of beams 666 and 668 are perpendicular to the polarizations of beams 670 and 672, as shown for position z2.
  • the four beams 666 - 672 then enter the second birefringent splitting crystal 612 which combines the beams, since the light is traveling in the reverse direction.
  • even channel beam 666 an ordinary beam
  • even channel beam 670 an extraordinary beam
  • odd channel beam 668 an ordinary beam
  • odd channel beam 672 an extraordinary beam
  • odd channel beam 676 of mixed polarization
  • the locations of beams 674 and 676 on the second birefringent splitting crystal 612 may be compared with the location of beam 610 shown in FIG. 6B.
  • the de-interleaver 600 de-interleaves even and odd channels propagating in the forwards direction, it does not operate as an interleaver in the opposite direction when implemented with Faraday rotators.
  • the backwards traveling beams 662 and 664 do not return to the same position as the input beam 610, and so the de-interleaver 600 operates as an isolator in the reverse direction.
  • FIG. 7A Another embodiment of an interleaver 700 is illustrated in FIG. 7A.
  • This particular embodiment of interleaver 700 interleaves light traveling in the forward direction, but may be operated as an isolator for light propagating in the reverse direction.
  • the interleaving process in the forward direction is explained with reference to FIG. 7B, which shows the polarization states for the different beams propagating through the interleaver 700 at various points along the z-axis.
  • the interleaver is based around an interleaver unit that includes a polarization rotating crystal 702 and a first birefringent splitting crystal 704, although the primary function of the first birefringent splitting crystal 704 is to combine light beams.
  • This particular embodiment of interleaver 700 is useful for de-interleaving.
  • the beams 709 and 710 typically propagate from a light collimating module (not shown) into a polarization-sensitive beam splitting element 712, such as a second birefringent splitting crystal.
  • a polarization-sensitive beam splitting element 712 such as a second birefringent splitting crystal.
  • the polarization-sensitive beam splitting element 712 may be any element that splits an unpolarized light beam into two orthogonally polarized components, such as a polarization beamsplitter or the like.
  • the second birefringent splitting crystal 712 splits the collimated light beams 709 and 710 into respective extraordinary beams 714 and 715, and ordinary beams 716 and 717. Therefore, beam 714, containing even channels, has a polarization parallel to that of beam 715, containing odd channels. Likewise, beam 716, containing even channels, has a polarization parallel to that of beam 717, containing odd channels, while the polarizations of beams 714 and 715 are orthogonal to the polarizations of beams 716 and 717, as shown for position z2.
  • the four beams 714-717 enter the first polarization rotator 721 , which re-orients the polarization direction of each of the four beams 714-717.
  • the polarization of beam 714 is rotated by -45° and the polarization of beam 715 is rotated +45°, so that the beams 714 and 715 are polarized perpendicularly to each other upon leaving the first polarization rotator 722, as is illustrated for position z3.
  • the polarization of beam 716 is rotated by +45° and the polarization of beam 717 is rotated by -45°, so that the beams 716 and 717 are polarized perpendicularly to each other upon leaving the first polarization rotator 722.
  • the polarization rotator 721 includes four polarization rotating elements 722, 723, 724 and 725 to rotate the polarizations of the beams 714, 715, 716 and 717 respectively.
  • the polarization rotating elements 722 - 725 may be half-wave retardation plates or may be nonreciprocal rotators, such as Faraday rotators.
  • the polarization rotating elements 722-725 rotate the polarization of the odd and even channels in their respective beams 714- 717 by an equal amount.
  • the four beams 714-717 Upon exiting the first polarization rotator 722, the four beams 714-717 propagate through the first birefringent splitting crystal 704, which combines beams 714 and 715 into beam 718, and combines beams 716 and 717 into beam 720, as shown for position z4. Beams 718 and 720 each contain both odd and even channels.
  • Beams 718 and 720 then propagate through the polarization rotating crystal 702, which effectively rotates the polarizations of the odd and even channels through angles differing by 90°.
  • beams 718 and 720, each of mixed polarization respectively produce polarized beams 726 and 728, each of which contains both odd and even channels. This is illustrated for position z5.
  • the polarization of the even channels is effectively rotated through 90°, while the polarization of the odd channels is not effectively rotated.
  • the polarization of the odd channels may be effectively rotated through 90°, while the polarization of the even channels is not effectively rotated.
  • the polarizations of beams 726 and 728 are parallel.
  • the beams 726 and 728 then pass through a second polarization rotator 734, which orients the polarization direction of the beams 726 and 728 to be mutually orthogonal, as illustrated for position z6.
  • the second polarization rotator 734 includes a first polarization rotating element 736 that rotates the polarization of beam 726 through +45° and a second polarization rotating element 738 that rotates the polarization of beam 728 through -45°.
  • the polarization rotating elements 736 and 738 may be any suitable polarization rotating element such as a half-wave plate or Faraday rotator. Each polarization rotating element 736 and 738 advantageously rotates the polarization of all channels passing therethrough by an amount that is independent of channel frequency.
  • the mutually orthogonal beams 726 and 728 are combined in a birefringent combining crystal 740 into a densely multiplexed WDM beam 742 of mixed polarization that contains both the odd and even channels, as illustrated for position z7.
  • the WDM beam 742 may be focused using a focusing unit (not shown) into an output fiber.
  • the beams 709 and 710 are effectively mixed together in the interleaver 700, to produce an interleaved WDM output.
  • the polarization rotators 722 and 734 employ retardation plates to rotate polarization, then all the elements in the interleaver 700 are reciprocal, and so the interleaver may de-interleave odd and even channels if operated with a backwardly directed beam. However, if the polarization rotators 722 and 734 employ Faraday rotators, then the interleaver 700 is not reciprocal, and operates as an isolator for light propagating in the reverse direction. This is explained further with reference to FIG. 7C, which shows polarization states of various beams within the interleaver 700 for light propagating in the reverse direction, from the birefringent combining crystal 740 to the second birefringent splitting crystal 712.
  • the WDM input beam 762 is incident on the birefringent combining crystal 740, as illustrated for position z7.
  • the birefringent combining crystal 740 operates in the reverse direction as a birefringent splitting crystal, and so the input beam 762 is split into an extraordinary beam 766 and an orthogonally polarized ordinary beam 768, as illustrated for position z6.
  • the beams 766 and 768 each contain both odd and even channels.
  • the two beams 766 and 768 then propagate through the second polarization rotator 734 in the backwards direction.
  • the handedness of the polarization rotation experienced upon propagating through the second polarization rotator 734 is the same as in the forward direction. Accordingly, after propagating through the Faraday rotator 736, the polarization of beam 766 is rotated by +45°, while the polarization of beam 768 is rotated through -45° after propagating through the Faraday rotator 738.
  • beams 766 and 768 have parallel polarizations, as illustrated for position z5.
  • the polarization rotating crystal 702 effectively rotates the polarizations of the even channels in beams 766 and 768 through 90°, whereas the polarizations of the odd channels in beams 766 and 768 remain effectively unrotated.
  • polarized beams 766 and 768 each produce respective beams 770 and 772 of mixed polarization, as illustrated for position z4.
  • the even channels have a polarization that is orthogonal to the polarization of the odd channels.
  • the first birefringent splitting crystal 704 splits the beam 770 into an extraordinary beam 774 that contains the odd channels, and an ordinary beam 776 that contains the even channels.
  • the beam 772 is split into an extraordinary beam 778 that contains the odd channels, and an ordinary beam 780 that contains the even channels, as is illustrated for position z3.
  • the first polarization rotator 721 rotates the polarizations of beams 774 and 780 by -45°, while the polarizations of beams 776 and 778 are rotated through +45°.
  • the polarizations of beams 774 and 776 are perpendicular to the polarizations of beams 778 and 780, as shown for position z2.
  • the four beams then enter the second birefringent splitting crystal 712 which, since the light is traveling in the backwards direction, splits the beams further.
  • odd channel beam 778 and even channel beam 780 pass through the second birefringent splitting crystal 712 as extraordinary beams and are translated further away from beams 774 and 776, as is shown for position z1. Therefore, none of the beams 774, 776, 778 or 780 return to the same area of the entrance face of the second birefringent splitting crystal 712 as the input WDM beams 709 and 710: the locations of beams 774, 776, 778 or 780 on the second birefringent splitting crystal 712 may be compared with the locations of beams 709 and 710 shown in FIG. 7B.
  • the interleaver 700 interleaves even and odd channels propagating in the forwards direction, it does not operate as a de- interleaver in the opposite direction when implemented with Faraday rotators.
  • the backwards traveling beam 762 does not return to the same positions as the input beams 709 and 710, and so the interleaver 700 operates as an isolator in the reverse direction.
  • the interleaver 800 includes components similar to those of interleaver 400, but additionally includes a turning prism 802 between the polarization rotating crystal 302 and the first birefringent splitting crystal 304.
  • the incorporation of the turning prism advantageously places the input beam 310 and output beams 342 and 344 at the same end of the interleaver 800.
  • This arrangement permits any connecting fibers to be placed only at one end of the interleaver device.
  • the first birefringent splitting crystal 304 may not have the same length as the polarization rotating crystal 302. Where it is important, for example for ease of fabrication, for the first and second polarization rotators 322 and 334 to be spaced apart from the turning prism 802 by the same distance, then the shorter of the first birefringent splitting crystal 304 and the polarization rotating crystal 302 may be provided with an adjacent air gap or spacer in order to make up the difference in length.
  • the present invention is applicable to fiber optic systems and is believed to be particularly useful for interleaving and de- interleaving channels of a WDM signal.
  • the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims.
  • Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification.
  • the handedness of polarization rotation in a polarization rotator may be different from that shown, or the relative orientation of the birefringent splitting and combining elements may be different from what is illustrated.
  • the claims are intended to cover such modifications and devices.

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Abstract

L'invention concerne un entrelaceur qui entrelace et désentrelace des canaux à valeur impaire et paire, par rotation de la polarisation des canaux à multiplexage par répartition en longueur d'onde. Dans le désentrelacement, la polarisation des canaux à valeur impaire est tournée selon un angle différent de celui de la polarisation de canaux à valeur paire, et les canaux à valeur impaire et paire sont ainsi séparés par un mode de polarisation. Lors de l'entrelacement, les canaux à valeur paire et impaire sont combinés par un mode de polarisation, et les canaux à valeur paire et impaire sont alors tournés par polarisation, de façon que tous les canaux possèdent la même polarisation.
PCT/US2001/051097 2000-10-23 2001-10-23 Entrelaceur biregringent destine a des telecommunications par fibre optique et a multiplexage par repartition en longueur d'onde WO2002043296A2 (fr)

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Publication number Priority date Publication date Assignee Title
US6678476B1 (en) * 2000-10-30 2004-01-13 Oplink Communications, Inc. WDM with optical interleaving for increased channel capacity
WO2006124424A1 (fr) * 2005-05-19 2006-11-23 Raytheon Company Diplexeur optique avec lame d'onde accordable à cristaux liquides
US7140926B2 (en) 2002-03-11 2006-11-28 3M Innovative Properties Company Telecommunications terminal module
WO2009015241A1 (fr) * 2007-07-24 2009-01-29 Infinera Corporation Séparateur de faisceau de polarisation-rotateur de polarisation
CN108476070A (zh) * 2015-12-30 2018-08-31 脸谱公司 多通道多波束的强度调制直接检测

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WO2000011510A1 (fr) * 1998-08-21 2000-03-02 Corning Incorporated Filtre periodique accordable
WO2000057589A1 (fr) * 1999-03-22 2000-09-28 Chorum Technologies Lp Procede et appareil de multiplexage/demultiplexage en longueur d'onde
WO2001067143A1 (fr) * 2000-03-03 2001-09-13 Arroyo Optics, Inc. Filtre optique par entrelacement
EP1136857A2 (fr) * 2000-03-03 2001-09-26 E-Tek Dynamics, Inc. Entrelaceur/désentrelaceur provoquant une dispersion petite ou nulle des signaux optiques

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Publication number Priority date Publication date Assignee Title
US4566761A (en) * 1984-09-13 1986-01-28 Gte Laboratories Incorporated Birefringent optical wavelength multiplexer/demultiplexer
WO2000011510A1 (fr) * 1998-08-21 2000-03-02 Corning Incorporated Filtre periodique accordable
WO2000057589A1 (fr) * 1999-03-22 2000-09-28 Chorum Technologies Lp Procede et appareil de multiplexage/demultiplexage en longueur d'onde
WO2001067143A1 (fr) * 2000-03-03 2001-09-13 Arroyo Optics, Inc. Filtre optique par entrelacement
EP1136857A2 (fr) * 2000-03-03 2001-09-26 E-Tek Dynamics, Inc. Entrelaceur/désentrelaceur provoquant une dispersion petite ou nulle des signaux optiques

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6678476B1 (en) * 2000-10-30 2004-01-13 Oplink Communications, Inc. WDM with optical interleaving for increased channel capacity
US7140926B2 (en) 2002-03-11 2006-11-28 3M Innovative Properties Company Telecommunications terminal module
WO2006124424A1 (fr) * 2005-05-19 2006-11-23 Raytheon Company Diplexeur optique avec lame d'onde accordable à cristaux liquides
WO2009015241A1 (fr) * 2007-07-24 2009-01-29 Infinera Corporation Séparateur de faisceau de polarisation-rotateur de polarisation
CN108476070A (zh) * 2015-12-30 2018-08-31 脸谱公司 多通道多波束的强度调制直接检测
EP3353911A4 (fr) * 2015-12-30 2019-05-22 Facebook Inc. Détection directe modulée en intensité à multi-canal et multi-faisceau
CN108476070B (zh) * 2015-12-30 2021-04-20 脸谱公司 用于光通信的系统和方法

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WO2002043296A9 (fr) 2003-04-24
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