CA2357428A1 - Method and apparatus for compensating for polarizatin mode dispersion in optical devices - Google Patents

Abstract

An optical device for manipulating an optical signal is disclosed. The optical device has a signal beam path passing through at least one birefringent material. The signal beam path has at least a first signal path portion having a first birefringence and at least a second signal path portion having a second birefringence, wherein said second birefringence is substantially equal to and opposite to said first birefringence. Thus, polarization mode desperation (PMD) occurring in said signal as said signal passes through said first path portion is substantially reversed as said signal passes through said second path portion. In a further aspect of the present invention, methods of compensating for polarization mode dispersion are also provided.

Description

CANADA
PATENT APPLICATION
PIASETZKI & NENNIGER
File No.: AN1002 Title: METHOD AND APPARATUS FOR COMPENSATING FOR
POLARIZATION MODE DISPERSION IN OPTICAL DEVICES
Inventor:
Jingyun Zhang IN1~ENTIONTITLE: Method and Apparatus for Compensating for Polarization Mode Dispersion in Optical Devices.
1. Field of the Invention This invention relates generally the filed of optical signal processing and more particularly to optical signal processing in which optical signals undergo polarization mode dispersion or PMD during signal processing and manipulation. Most particularly this invention relates to methods and apparatuses to compensate for such PMD.

2. Description of Prior Art PMD is a well-known phenomenon that occurs in optical signals of the type used, for example, in telecommunications. Many signal-propagating materials exhibit a property of birefringence, which means that the refractive index of the material differs for optical signals or light of different polarization. Because of the birefringence, the two polarization IS components TE and TM of the incoming light suffer dispersion, since they propagate through the signal-propagating materials at different speeds, resulting in their separation in time as they exit the material. In the earlier period of optical fiber telecommunication, the data rate of the optical signals, was not very high ( typically a few hundred Mbit/sec). At low data rates, any birefringent impact caused by such materials used in optical devices 2o and networks, for example, is limited. Further, thin components such as beam displacers, beam splitter/combiners and waveplates define a short signal path. In such cases the amount of PMD arising is small enough that it is not a problem in further signal processing.
More recently however other uses have been developed for such signal-propagating 25 birefringent materials which involve longer signal paths. Also, in high-speed digital optical telecommunications networks the temporal separation between two successive impulses (bits) of a signal can be of the order of a few tens of psec. For example, an optical signal of 12.5 Gbit/sec data rate has a temporal separation of 80 psec., while for a 40 Gbit/sec data rate the temporal separation will be 25 psec. In such new materials and at 3o such speeds PMD can be a limiting design factor. To control the quality of data transmission, it is desired that the degradation of temporal separation between two successive impulses should not be larger than 10% for the receiver to detect the incoming signal with acceptable bit error rate (BER).
In practice implementing an optical network system with devices and components made from materials having birefringence will always introduce polarization mode dispersion (PMD) into the signal. More specifically such PMD is a phase displacement of the two polarization components of the same bit. 'This situation is worse when a number of such devices or components are arranged in cascade, which causes a superimposition between successive bits, resulting in a higher BER. PMD is calculated by the following equation:
PMD= ~x(nt-no) where L is the path length of birefringence crystal, C is the speed of light, ne and no are the refractive indices of crystal for the extra-ordinary beam and ordinary beam, respectively.
If such PMD limitations can be overcome, the data quality of optical signals can be improved significantly. Thus, there have been a number of attempts in the past to try to overcome unacceptable PMD arising in optical devices and Garners. Some of the prior art attempts to deal with the problems associated with PMD include the teachings of the following references:
"Polarization mode dispersion compensation via an automatic tracking of a principal state of polarization", Cao; Xiang-Dong, US patent No. 6,130,766, issued on January 7, 1999. Compensation of PMD is through an active control process implemented by polarization beam sputter and polarization controller. This is costly, complex and difficult to reliably implement.
"Method and apparatus for providing high-order polarization mode dispersion compensation using temporal imaging", Cao; Xiang-Dong, US patent No.
6,104,515, issued on February 1, 1999. Compensation of PMD is through an active control process implemented by phase modulator. This suffers from the same limitations as the previous one.
"Method and apparatus for automatic compensation of first-order polarization mode dispersion (PMD)", Fishman; Daniel A., US patent No. 5,930,414, issued on September 16, 1997. Compensation of PMD is achieved using a birefringent compensator, in which the compensator automatically and adaptively generates a level of differential time delay that substantially equals the differential time delay that the optical signal experiences, but of different sign, and, therefore, essentially cancels out the undesired delay. It is very difficult and expensive to develop the automatic, adaptive compensation taught in this reference.
"Acousto-optical waveguide device with compensation of polarization mode dispersion", Morasca; Salvatore, US patent No. 5,850,492, issued on December 15, 1998.
An acousto-optical waveguide device with compensation of polarization mode dispersion (PMD) comprises at least one first and one second optical path in an optical wave-guide to and at least one compensation optical path connected to the first and second optical path.
The compensation optical path has a prefixed PMD such that the passage time of a first polarization component of an optical signal in the first optical path and in the compensation optical path is substantially equal to the passage time of a second polarization component in the second optical path and in the compensation optical path.
This teaches setting up a parallel but separate compensation path which is awkward and expensive and introduces additional alignment issues.
"Fiber-optic transmission polarization-dependent distortion compensation", Haas;
Zygmunt. US patent No. 5,311,346, issued on May 10, 1994. The polarization-dependent distortion of an optical signal transmitted through an optical fiber is reduced by aligning 2o the polarization of the optical signal to minimize the received signal distortion. A
polarization controller (a device which can change the polarization of light in an optical fiber) may be located at either the input or output end of a long haul optical fiber system and is used to align the polarization of the signal to minimize the received signal distortion.
Automatic operation of the polarization controller can be obtained by using a steepest-descent method based on a distortion measure of the received signal for the optical signal transmitted through the optical fiber to generate control signals which are used to control the polarization controller. While such alignment may work in some circumstances, such as long haul, it is not really an appropriate solution for short hops such as within a metro area network.

' -4-Most of the foregoing attempts involve complex adaptive systems, which are expensive, difficult to control, and unwieldy. What is needed is a simple yet effective solution to overcome PMD induced limitations, which solution is also reliable and inexpensive.

3. Summary of the Invention According to the present invention compensation of Polarization Mode Dispersion (PMD) can be achieved in an optical device, such as, a Polarization Independent (PI) ultra-fast optical switch. Such an optical switch may be, for example, comprised of a to birefringent electro-optic crystal, such as LiNb03. When a block of a different crystal of an opposite birefringence with proper length, or a block of the same type of crystal with equal length but finished at orthogonal cut, is incorporated into the signal path between the input and output of the device, the PMD introduced by the switching crystal will be reduced significantly. An object of the present invention therefore is to compensate for the PMD
introduced in the switching crystal by substantially reducing the PMD in a compensating crystal. A fiarther object is to achieve this PMD compensation for at least a specified wavelength as well as over a range of wavelengths such as the C-band telecommunications spectrum.
Thus, an aspect of the present invention is to provide techniques and methods for 2o compensating for such PMD occurring, for example, in such optical switches made from electro-optic crystal of strong birefi-ingence, like LiNbO3. The usage of electro-optic crystal makes it possible to build electro optic devices (EOD) such as ultra-fast switches in bulk optic form. A typical crystal in a working EOD is usually z-cut and has a three dimensional structure, which includes dimensions which are in x and y directions. An external electric field can be applied for example in a z direction for controlling the variation of refractive indices through electro-optic co-efficiencies. Optical signal propagating inside EOD through a certain length in x-y plane will suffer no PMD when the wave has only either TE or TM component, but will suffer PMD when the wave has both TE and TM components.
3o Therefore according to a general aspect of the invention, there is provided an optic device for manipulating an optical signal, said optic device comprising:

A signal beam path passing through at least one birefringent material;
said signal beam path having at least a first signal path portion having a first birefringence; and at least a second signal path portion having a second birefi~ingence, s wherein said second birefringence is substantially equal to and opposite to said first birefi-ingence;
whereby polarization mode desperation occurring in said signal as said signal passes through said first path portion is substantially reversed as said signal passes through said second path portion.
to Optical fiber carriers typically have indeterminate signal path lengths, since the signal may be routed in any one of a number of ways. Thus, PMD will vary greatly from one data stream to the next. For polarization independent (PI) wave guide devices which have a complicated structure any PMD introduced is generally unpredictable. In contrast, any PMD introduced by the EOD in a bulk optical switch can be determined from the EOD
is design and thus can be compensated for. The present invention comprehends a number of specific approaches, all comprehended by the general aspect of the invention noted above, by which such PMD compensation can be achieved, including:
1. In a first more specific aspect of the present invention, when the sign and magnitude of PMD introduced by the EOD is known, a PMD compensator made 2o from another type of crystal can be designed and used, as long as:
- 'The compensating crystal has the opposite sign of that of EOD crystal - The compensating crystal has the same cut of that of EOD crystal - The length L ' of the compensating crystal (with ne' and no ~ is related to that L of EOD crystal (with ne and no) by: L x (ne - no ) + L'x(ne - no ) = 0 2s - The installation of the said compensator in the optical path will cancel out the PMD
2. In a second aspect of the present invention when the sign and magnitude of PMD
introduced by the EOD is known, a compensator made from the same type of crystal can be designed and used as long as:
30 - The compensating crystal has an orthogonal cut of that of EOD crystal - The length of the compensating crystal is the same as that of EOD crystal - The installation of the said compensator in the optical path will cancel out the PMD
3. In a third aspect of the present invention when the sign and magnitude of PMD
introduced by the EOD is known, its compensation can be achieved by use of a compensator and a'/~ ~. wave plate:
- The compensating crystal of the same material as the EOD has the same cut of that of EOD crystal - The length of the compensating crystal is the same as that of EOD crystal - A'h ~, wave plate is placed behind the EOD
to - The installation of the said compensator after'/Z ~, wave plate in the optical path will cancel out the PMD

4. In a forth aspect of the present invention when the sign and magnitude of PMD
introduced by the EOD is known, its compensation can be achieved by EOD itself with the use of a '/ ~. wave plate with mirror coating:
- The % ~. wave plate is placed behind the EOD
- Optical wave propagates through EOD first, then the % ~, wave plate, and is reflected back to the % ~, wave plate and the EOD
- The PMD suffered by the wave on its first half way into EOD is cancelled on its second half way back through EOD
2o With respect to the foregoing aspects of the invention, the sign and magnitude of the PMD do not need to be calculated or even measured in some cases. In most, knowing the length of the signal path will be enough, and in the fourth aspect the length of the signal path does not even need to be known. It will be appreciated by those skilled in the art that an optical device which is polarization independent (PI) is desirable and one which is both PI and PMD free is even more desirable.
4. Brief Description of the Drawings) Reference will now be made, by way of example only, to various figures, which illustrate preferred embodiments of the present invention, and in which:
3o Figure 1 is schematic of a switch having a YV04 crystal block PMD
compensator;

Figure 2 is a schematic of a switch having a LiNb03 crystal block as PMD
compensator Figure 3 is a schematic of a switch having a PMD compensated via %2 ~, wave plate Figure 4 is a schematic of a switch having a. PMD compensated via '/ ~. wave plate with reflection Figure 5 is a schematic of a switch having PMD compensated by crystal of PMD
opposite to that of EOD;
Figure 6 is a schematic of PMD compensation employing a %Z ~, plate at the optimum wavelength; and to Figure 7 is a schematic of PMD compensation employing a 'h ~, wave plate at a wavelength slightly different from the optimum wavelength.

5. Description of the Preferred Embodiment Output collimator 2 is Figure 1:1 x 2 switch with YV04 block as PMD compensator Figure 1 is a schematic drawing of 1 x 2 switch composed of LiNb03 EOD and PMD
compensator, which is a piece of YV04 crystal. The white and gray areas of LiNb43 have opposite domain. When there is no external switching electric field, optical signal exits from output collimator 1; when there is an external switching electric field, the domain 20 inversion interface functions as a mirror through the Total Internal Reflection (TIR) and optical signal exits from output collimator 2. The PMD compensation principle is same as Approach 1 described above.
It will be understood that a PMD compensator according to this aspect of the present invention provides a first signal path portion through the LiNb03 and a second 25 signal path portion through the YV04 crystal. For both switch configurations, namely when the signal path is from input collimator 1 to output collimator 1 or from input input commator Output collimator 1 _8_ collimator 1 to output collimator 2 the first signal path length does not change nor does the second signal path length. Thus, if PMD is compensated for in one switch orientation it will also be compensated for in the other. In this way no adjustment to PMD
compensation is required if the switch changes from one connection configuration to the other. Thus, the present invention provides a simple and effective way to compensate for PMD that would otherwise arise in such a switch. Further such PMD compensation remains effective throughout all switch modes or settings. It should be pointed out that the compensating crystal can be made from other types of materials, of high birefringence like rutile (same cut) and calcite (orthogonal cut).
to T.iNhOz_ z-cut Output collimator 2 Figure 2: 1 x 2 switch with LiNb03 block as PMD compensator Figure 2 is similar in configuration to Figure 1, except that both the switch element and the PMD compensator are made from the same crystal namely, LiNb03 . In this aspect of the present invention the 1 x 2 switch is composed of LiNb03 EOD and PMD
compensator, which is a piece of LiNb03 crystal of the same length, but with orthogonal cut as compared to the EOD. The PMD compensation principle is same as second specific aspect of the invention described above. Thus, since the first and second signal path lengths are the same, and since they are through the same crystal material, and since they are different 2o only in the cut of one being orthogonal to the other, any PMD introduced by the first signal path portion will be removed by the second signal path portion. Again it will be noted that this result applies no matter what the switch connection configuration is and is constant regardless of switching operations occurring in the EOD. For both the embodiments shown in Figures 1 and 2, the compensating crystal can be cut into any shape, such as the shape of a prism, to direct the optical path through a more compact configuration, as long as the total length of optical path within the shaped crystal remains the desired length for PMD compensation.
~..r... ....______~_.._ LiNb03, x or y-cut Ou~ut collimator 1 _g_ 'h wave plate Figure 3: 4 x 4 switch with PMD compensated via'/ ~, wave plate Figure 3 is a schematic drawing of 4 x 4 switch composed of two pieces of LiNb03 EODs with same cut and '/2 ~, wave plate. The 2°d piece EOD functions as PMD
compensator as well. The PMD compensation principle is same as Approach 3 described above.
There are five regions inside LiNb03 where gray and white areas have double TIR
structures, allowing basic 2 x 2 switching.
It will be appreciated by those skilled in the art that the foregoing embodiment 1o relies on the'/z ~, wave plate, to rotate the two orthogonal polarization components TE and TM by 90° around the axis of beam propagation or the beam path. Such a wave plate functions precisely at a single wavelength of its design (or central wavelength). Thus, at the design wavelength, where the majority of the signal intensity is located, a$er compensation, the signal is substantially free of PMD (see Fig. 6). At wavelengths slightly greater or slightly less other than the design wavelength the TE and TM
components are rotated by angles slightly larger or slightly smaller than 90°, resulting PMD free signal after compensation, but with some residual noise (see Fig. 7). Such noise is carried forward and displaced by PMD, but usually has very low intensity. In general, the acceptable level of bandwidth noise is application dependent. For fiber devices and components working over a spectral window of less than 30nm, such residual noise is negligible.
Figure 4 is a schematic drawing of 2 x 2 switch composed of one piece of LiNb03 EODs and % ~, wave plate. The PMD compensation principle is same as Approach 4 described LiNb03 of same length and same cut -1~-above. Gray and white areas inside LiNb03 have double TIR structures, allowing basic 2 x '/4 D wave plate 2 switching.
LiNb03, z-cut Mirror coating Figure 4: 2 x 2 switch with PMD compensated via'/ ~. wave plate and double pass through reflection s One example of a lireferred embodiment will be to construct a 2 x 2 switch, which is PMD free using this invention, as shown in Figure 5. Light is launched into the switch via Port 1 and Port 2. The double TIR structure built up by the external switching electric field will switch the outputs of Port 3 and Port4 from each other.
1o As a light wave propagates through the switching EOD, it will be degraded by PMD since LiNb03 is a negative crystal of strong birefringence. Inside LiNbOj the extra-ordinary beam propagates faster than the ordinary beam and PMD is negative.
For a length of 40 mm, the PMD will be around -11 psec. at wavelength of 1310 nm.
When the same light wave propagates through the YV04 compensator, its PMD
1s will be cancelled, this is because YV04 is a positive crystal of even stronger birefringence, and inside YV04 the extra-ordinary beam propagates slower than the ordinary beam and PMD is positive. For a length of 15 mm, its PMD will be around +11 psec. at wavelength 1310 nm.
The use of crystal with opposite birefringence also brings the lateral walk-off 2o between TE and TM components to zero.

Port 1 Port 4 Figure 5: 2 x 2 switch with PMD compensated by crystal of PMD opposite to that of EOD
This invention describes techniques for compensating PMD occurring in electro-optic crystal switches. The usage of electro-optic crystal makes it possible to build ultra-fast switches, which are desirable essential devices in optical fiber telecommunication systems with high & optimum data handling capacity, but must be PMD free.
'This invention provides a solution for achieving that goal.
It will be understood by those skilled in the art that while reference is made to a 1o first signal path portion and a second signal path portion, the order of what happens is not that relevant. The present invention comprehends that the signal switching could happen in either the first signal path portion or the second signal path portion. The switch might thus start with a PMD compensating crystal, and then finish with switch element in reverse to that shown in the drawings without departing from the broad spirit of the invention.
15 It will be further understood by those skilled in the art that while the examples of EOD used were forms of switches, the present invention can also be used on all kinds of crystal devices which might otherwise induce PMD into a signal to the detriment of the BER. Thus, the present invention comprehends all forms of crystal devices that as a result of birefringence introduce PMD into a signal. Switches are merely one form of crystal 2o devices that can benefit from such PMD compensation.
It will be further understood that although reference has been made to a few specific materials having birefringence (LiNb03, YVOa) and many other materials exhibiting birefringence are also comprehended by the present invention.
Essentially as long as the crystal device introduces PMD because of the signal propagating materials used 25 to form the crystal devices the present invention will have application.
While the foregoing description has been provided in relation to specific examples of preferred embodiments of the invention it will be understood that various alterations and Port 2 LiNb03 ~Oa Port 3 modifications can be made without departing from the broad scope of the invention as defined by the appended claims. Certain of these variations have been discussed above and others will be apparent to those skilled in the art.

Claims (10)

1. An optic device for manipulating an optical signal, said optic device comprising:
a signal beam path passing through at least one birefringent material;
said signal beam path having at least a first signal path portion having a first birefringence; and at least a second signal path portion having a second birefringence, wherein said second birefringence is substantially equal to and opposite to said first birefringence;
whereby polarization mode desperation occurring in said signal as said signal passes through said first path portion is substantially reversed as said signal passes through said second path portion.

2. An optical device for manipulating an optical signal as claimed in claim 1 comprising at least two birefringent materials and wherein said first birefringent material comprises a signal manipulating device and said second birefringent material comprises a polarization mode dispersion compensator.

3. An optical device for manipulating an optical signal as claimed in claim 2 wherein said signal manipulating device and said compensator are both crystals.

4. An optical device for manipulating an optical signal as claimed in claim 3 wherein said signal-manipulating device is an electro-optic crystal.

5. An optical device for manipulating an optical signal as claimed in claim 4 or 3 wherein both said manipulating device and said compensator are electro-optic crystals.

6. An optical device for manipulating an optical signal as claimed in claims 3 to 5 wherein said compensator crystal has one sign and said signal manipulating crystal has an opposite sign, said crystals both have the same cut and the length of said second signal path portion is sufficient to substantially reduce any polarization mode dispersion arising in a signal passing through said first signal path portion.

7. An optical device for manipulating an optical signal as claimed in claims 3 to 5 wherein said compensator crystal and said signal manipulating crystal are made from the same crystal, said first signal path portion and said second signal path portion are the same length and said compensator crystal has an orthogonal cut as compared to a cut of said signal manipulating crystal.

8. An optical device for manipulating an optical signal as claimed in claims 3 to 5 wherein said compensator crystal and said signal manipulating crystal are made from the same crystal, said first signal path portion and said second signal path portion are the same length and the same cut, and a 1/2 .lambda. wave plate is positioned in said beam path between said crystals.

9. An optical device for manipulating an optical signal as claimed in claims 3 to 5 wherein said compensator crystal and said signal manipulating crystal are the same crystal, and said device further includes a 1/4 .lambda. wave plate, then a mirror in said beam path, so that said first beam path portion comprises passing through said crystal in one direction and said second beam path portion comprises passing through said crystal in an opposite direction.

10. A method of compensating for PMD arising in an optical device, the method comprising the steps of;
A) directing an optical signal along a first signal path portion having a first birefringence;

B) directing said optical signal along a second signal path having a second birefringence; and C) selecting said first or second birefringence to be equal and opposite to the other of said first or second birefringence whereby any PMD arising in said first signal path portion is substantially reduced over said second signal path portion.