Abstract:
An optical depolarizer includes a birefringent block, a reflector coupled to the block, a first input port for providing polarized light to the block and an output port configured to receive polarized light from the block. The depolarizer optionally includes a non-reciprocal combination-devicehaving a principal direction and including a first birefringent wedge, a second birefringent wedge, and a non-reciprocal rotating element. The non-reciprocal rotating element can be a Faraday rotator. The birefringent block can be optically coupled to the non-reciprocal combinatoin device. The optical depolarizer can include a lens that is optically coupled to the first wedge. The optical depolarizer can include a capillary for holding at least a PM optical fiber and an output optical fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation application of and claims priority to U.S. application Ser. No. 10/119,570, filed on Apr. 9, 2002.  
         [0002]     The present invention relates generally to optical technology. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     Optical depolarizers, optical combiners, and optical isolators are commonly used in optical communication systems and optical measurement systems. An optical depolarizer is generally designed to change a beam of completely polarized light or a beam of partially polarized light into a beam of depolarized light. An optical combiner is a device generally designed to combine two beams of light into one beam of light. An optical isolator is a device generally designed to allow a beam of light to pass through the device in a chosen direction and to prevent the beam of light from passing through the device in the opposite of that chosen direction.  
       SUMMARY OF THE INVENTION  
       [0004]     In one aspect, the invention provides an optical depolarizer. The optical depolarizer includes a birefringent block, a reflector coupled to the block, an input port for supplying polarized light to the block and an output port for receiving unpolarized light from the block. The reflector is located on one side of the block and the input and output ports are located on another side of the bloack.  
         [0005]     In another aspect, the optical depolarizer includes a non-reciprocal combination-device, a birefringent block, and a reflector. The non-reciprocal combination-device has a principal direction and includes a first birefringent wedge, a second birefringent wedge, and a non-reciprocal rotating element. The first birefringent wedge has a first optical axis perpendicular to the principal direction. The second birefringent wedge has a second optical axis perpendicular to the principal direction, and the second optical axis forms a first angle with respect to the first optical axis. The non-reciprocal rotating element is optically coupled between the first and the second birefringent wedge. The non-reciprocal rotating element is designed to rotate the polarization of light passing through the non-reciprocal rotating element by a second angle. The non-reciprocal rotating element can be a Faraday rotator. The birefringent block is optically coupled to the second birefringent wedge. The birefringent block has a third optical axis perpendicular to the principal direction, and the third optical axis forms a third angle with respect to the second optical axis. The reflector is optically coupled to the birefringent block. The optical depolarizer can include a lens that is optically coupled to the first wedge. The optical depolarizer can include a capillary for holding at least a PM optical fiber and an output optical fiber.  
         [0006]     In another aspect, the invention provides an optical depolarizer. The optical depolarizer includes a non-reciprocal combination-device, a birefringent block, and a reflector. The non-reciprocal combination-device has a principal direction and includes a first birefringent wedge having a first optical axis, a second birefringent wedge having a second optical axis, and a non-reciprocal rotating element. The non-reciprocal rotating element can be a Faraday rotator. The birefringent block is optically coupled to the second birefringent wedge. The birefringent block has a third optical axis perpendicular to the principal direction, and the third optical axis forms an angle with respect to the second optical axis. The reflector is optically coupled to the birefringent block. The optical depolarizer can include a lens that is optically coupled to the first wedge. The optical depolarizer can include a capillary for holding at least a PM optical fiber and an output optical fiber. The non-reciprocal combination-device is configured for enabling at least the following functions: (1) light entering the second birefringent wedge as an e-ray in a first input direction exits from the second birefringent wedge as an o-ray in the principal direction; (2) light entering the first birefringent wedge as an o-ray in a second input direction exits from the second birefringent wedge as an e-ray in the principal direction; (3) light entering the second birefringent wedge as an e-ray in a reverse principal direction exits from the first birefringent wedge as an e-ray in the reverse principal direction; and (4) light entering the second birefringent wedge as an o-ray in the reverse principal direction exits from the first birefringent wedge as an o-ray in the reverse principal direction.  
         [0007]     In another aspect, the invention provides a method of combing first and second polarized light to form depolarized light in an output port. The method includes the step of providing a birefringent block and a non-reciprocal combination-device having a principal direction and a reverse principal direction. The method includes the step of directing the first polarized light to enter the non-reciprocal combination-device in a first input direction and to exit from the non-reciprocal combination-device in the principal direction as first intermediate light. The method includes the step of directing the second polarized light to enter the non-reciprocal combination-device in a second input direction and to exit from the non-reciprocal combination-device in the principal direction as second intermediate light. The method includes the step of passing the first and the second intermediate light through the birefringent block in the principal direction. The method includes the step of reflecting the first and the second intermediate light back through the birefringent block in the reverse principal direction. The method includes the step of directing the first and the second intermediate light to pass through the non-reciprocal combination-device in the reverse principal direction and enter the output port as depolarized light.  
         [0008]     In another aspect, the invention provides a method of depolarizing a polarized light to form depolarized light in an output port. The method includes the step of providing a birefringent block and a non-reciprocal combination-device having a principal direction and a reverse principal direction. The method includes the step of directing the polarized light to enter the non-reciprocal combination-device in an input direction and to exit from the non-reciprocal combination-device in the principal direction as intermediate light. The method includes the step of passing the intermediate light through the birefringent block in the principal direction. The method includes the step of reflecting the intermediate light back through the birefringent block in the reverse principal direction. The method includes the step of directing the intermediate light to pass through the non-reciprocal combination-device in the reverse principal direction and enter the output port as depolarized light.  
         [0009]     Aspects of the invention can include one or more of the following advantages. Implementations of the invention provide an optical depolarizer and an optical depolarizing combiner that may also function as an optical isolator. Implementations of the invention provides an optical depolarizer and an optical depolarizing combiner that may have small insertion loss, compact size, and reduced manufacturing cost. Other advantages will be readily apparent from the attached figures and the description below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1   a  illustrates an implementation of a non-reciprocal combination-device.  
         [0011]      FIG. 1   b  illustrates a specific configuration of birefringent wedges and a Faraday rotator of  FIG. 1   a.    
         [0012]      FIGS. 1   c - 1   e  illustrate alternative configurations of the birefringent wedges and the Faraday rotator of  FIG. 1   a.    
         [0013]      FIG. 2   a  illustrates the paths traveled by light that enters the non-reciprocal combination-device of  FIG. 1   a  in the principal direction.  
         [0014]      FIG. 2   b  illustrates that light entering the first birefringent wedge as an e-ray in the principal direction exits from the second birefringent wedge as an o-ray in the first output direction.  
         [0015]      FIG. 2   c  illustrates that light entering the first birefringent wedge as an o-ray in the principal direction exits from the second birefringent wedge as an e-ray in the second output direction.  
         [0016]      FIG. 3   a  illustrates the paths traveled by light that enters the non-reciprocal combination-device of  FIG. 1   a  in the first and the second input direction.  
         [0017]      FIG. 3   b  illustrates that light entering the second birefringent wedge as an e-ray in the first input direction exits from the second birefringent wedge as an o-ray in the principal direction.  
         [0018]      FIG. 3   c  illustrates that light entering the first birefringent wedge as an o-ray in the second input direction exits from the second birefringent wedge as an e-ray in the principal direction.  
         [0019]      FIG. 4   a  illustrates the paths traveled by the light that enters the non-reciprocal combination-device of  FIG. 1   a  in the reverse principal direction.  
         [0020]      FIG. 4   b  illustrates that light entering the second birefringent wedge as an e-ray in the reverse principal direction exits from the first birefringent wedge as an e-ray in the reverse principal direction.  
         [0021]      FIG. 4   c  illustrates that light entering the second birefringent wedge as an o-ray in the reverse principal direction exits from the first birefringent wedge as an o-ray in the reverse principal direction.  
         [0022]      FIGS. 5   a - 5   d  illustrate an implementation of an optical depolarizer  500 .  
         [0023]      FIGS. 6   a - 6   d  illustrate an implementation of an optical depolarizing combiner  600 .  
         [0024]      FIGS. 7   a - 7   c  shows that an optical depolarizing combiner  600  can also function as an optical isolator.  
         [0025]      FIGS. 8   a  and  8   b  illustrate an implementation of an optical combiner  800 .  
         [0026]      FIGS. 9   a  and  9   b  illustrate an implementation of a PM isolator  900 .  
         [0027]      FIG. 10   a  illustrates an implementation of non-reciprocal combination-device  10  constructed using birefringent crystal materials with indexes n e  larger than n o .  
         [0028]      FIG. 10   b  illustrates an implementation of non-reciprocal combination-device  10  constructed using birefringent crystal materials with indexes n e  smaller than n o . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     The present invention relates to an improvement in optical technology. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the invention will be readily apparent to those skilled in the art and the generic principals wherein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principals and features described herein.  
         [0030]     The present invention will be described in terms of a non-reciprocal combination-device, an optical depolarizer, an optical depolarizing combiner, an optical combiner, and a Polarization Maintenance (“PM”) isolator each having specific components having specific configurations. Similarly, the present invention will be described in terms of components having specific relationships, such as distances or angles between components. However, one of ordinary skill in the art will readily recognize that the devices and systems described can include other components having similar properties, other configurations, and other relationships between components.  
         [0031]     In the instant application, implementations of optical depolarizers, optical depolarizing combiners, optical combiners, and PM isolators using non-reciprocal combination-devices are described. The configuration and operation of the non-reciprocal combination device is described in greater detail below. The non-reciprocal combination-device generally includes two birefringent wedges and a non-reciprocal rotating element such as a Faraday rotator.  
         [0032]      FIGS. 1   a  and  1   b  illustrate an implementation of a non-reciprocal combination-device  10  that includes a birefringent wedge  15 , a birefringent wedge  17 , and a non-reciprocal rotating element such as Faraday rotator  16 . Birefringent wedges  15  and  17  are in the form of tapered plates. Surface  11  of birefringent wedge  15  faces surface  12  of birefringent wedge  17 . In one implementation of non-reciprocal combination-device  10 , surface  11  of birefringent wedge  15  substantially parallels surface  12  of birefringent wedge  17 .  
         [0033]     A coordinate system is illustrated including the x-direction, the y-direction and the z-direction. The optical axis of birefringent wedge  15  is in the x-direction. The optical axis of birefringent wedge  17  is in the x-y direction. Faraday rotator  16  is designed in such a way that, when light passes through the Faraday rotator  16  either in the positive or the negative z-direction, the polarization of the light will be rotated 45 degrees with respect to the positive z-axis. Non-reciprocal combination-device  10  has a principal direction that is in the positive z-direction. Non-reciprocal combination-device  10  also has a first input direction that is in the z−αy direction, a second input direction that is in the z+βy direction, a first output direction that is in the z+γy direction, and a second output direction that is in the z−δy direction. Where α, β, γ and δ are positive numbers.  
         [0034]     In general, non-reciprocal combination-device  10 , including a first and a second birefringent wedge, is constructed to perform one or more of the following six functions:  
         [0035]     (1) light entering the first birefringent wedge as an e-ray in the principal direction exits from the second birefringent wedge as an o-ray in the first output direction;  
         [0036]     (2) light entering the first birefringent wedge as an o-ray in the principal direction exits from the second birefringent wedge as an e-ray in the second output direction;  
         [0037]     (3) light entering the first birefringent wedge as an e-ray in the first input direction exits from the second birefringent wedge as an o-ray in the principal direction;  
         [0038]     (4) light entering the first birefringent wedge as an o-ray in the second input direction exits from the second birefringent wedge as an e-ray in the principal direction;  
         [0039]     (5) light entering the second birefringent wedge as an e-ray in the reverse principal direction exits from the first birefringent wedge as an e-ray in the reverse principal direction; and  
         [0040]     (6) light entering the second birefringent wedge as an o-ray in the reverse principal direction exits from the first birefringent wedge as an o-ray in the reverse principal direction.  
         [0041]      FIGS. 2   a  and  2   b  illustrate the first function in detail.  FIGS. 2   a  and  2   c  illustrate the second function in detail.  FIGS. 3   a  and  3   b  illustrate the third function in detail.  FIGS. 3   a  and  3   c  illustrate the fourth function in detail.  FIGS. 4   a  and  4   b  illustrate the fifth function in detail.  FIGS. 4   a  and  4   c  illustrate the sixth function in detail.  
         [0042]     As shown in  FIGS. 2   a  and  2   b , light  220 ( e ) traveling in the principal direction (i.e., the positive z-direction) enters birefringent wedge  15  as an e-ray with the x polarization. Light  220 ( e ) is refracted at surface  11 . After passing through Faraday rotator  16 , the polarization of light  220 ( e ) is rotated positive 45 degrees with respect to the positive z-axis, and light  220 ( e ) becomes light  221 ( o ) with the x+y polarization. Light  221 ( o ) is refracted again at surface  12  and enters birefringent wedge  17  as an o-ray. Light  221 ( o ) exits from birefringent wedge  17  traveling generally in the first output direction (i.e., the z+γy direction).  
         [0043]     As shown in  FIGS. 2   a  and  2   c , light  210 ( o ) traveling in the principal direction (i.e., the positive z-direction) enters birefringent wedge  15  as an o-ray with the y polarization. Light  210 ( o ) is refracted at surface  11 . After passing through Faraday rotator  16 , the polarization of light  210 ( o ) is rotated positive 45 degrees with respect to the positive z-axis, and light  210 ( o ) becomes light  211 ( e ) with the x−y polarization. Light  211 ( e ) is refracted again at surface  12  and enters birefringent wedge  17  as an e-ray. Light  211  ( e ) exits from birefringent wedge  17  traveling generally in the second output direction (i.e., the z−δy direction).  
         [0044]     As shown in  FIGS. 3   a  and  3   b , light  320 ( e ) traveling in the first input direction (i.e., the z−αy direction) enters birefringent wedge  15  as an e-ray with the x polarization. Light  320 ( e ) is refracted at surface  11 . After passing through Faraday rotator  16 , the polarization of light  320 ( e ) is rotated 45 positive degrees with respect to the positive z-axis, and light  320 ( e ) becomes light  321 ( o ) with the x+y polarization. Light  321 ( o ) is refracted again at surface  12  and enters birefringent wedge  17  as an o-ray. Light  321 ( o ) exits from birefringent wedge  17  traveling in the principal direction (i.e., the positive z-direction).  
         [0045]     As shown in  FIGS. 3   a  and  3   c , light  310 ( o ) traveling in the second input direction (i.e., the z+βy direction) enters birefringent wedge  15  as an o-ray with the y polarization. Light  310 ( o ) is refracted at surface  11 . After passing through Faraday rotator  16 , the polarization of light  310 ( o ) is rotated positive 45 degrees with respect to the positive z-axis, and light  310 ( o ) becomes light  311 ( e ) with the x−y polarization. Light  311 ( e ) is refracted again at surface  12  and enters birefringent wedge  17  as an e-ray. Light  311 ( e ) exits from birefringent wedge  17  traveling in the principal direction (i.e., the positive z-direction).  
         [0046]     As shown in  FIGS. 4   a  and  4   b , light  420 ( e ) traveling in the reverse principal direction (i.e., the negative z-direction) enters birefringent wedge  17  as an e-ray with the x−y polarization. Light  420 ( e ) is refracted at surface  12 . After passing through Faraday rotator  16 , the polarization of light  420 ( e ) is rotated positive 45 degrees with respect to the positive z-axis, and light  420 ( e ) becomes light  421 ( e ) with the x polarization. Light  421 ( e ) is refracted again at surface  11  and enters birefringent wedge  15  as an e-ray. Light  421  ( e ) exits from birefringent wedge  15  traveling in the reverse principal direction (i.e., the negative z-direction).  
         [0047]     As shown in  FIGS. 4   a  and  4   c , light  410 ( o ) traveling in the reverse principal direction (i.e., the negative z-direction) enters birefringent wedge  17  as an o-ray with the x+y polarization. Light  410 ( o ) is refracted at surface  12 . After passing through Faraday rotator  16 , the polarization of light  410 ( o ) is rotated positive 45 degrees with respect to the positive z-axis, and light  410 ( o ) becomes light  411 ( o ) with the y polarization. Light  410 ( o ) is refracted again at surface  11  and enters birefringent wedge  15  as an o-ray. Light  411 ( o ) exits from birefringent wedge  15  traveling in the reverse principal direction (i.e., the negative z-direction).  
         [0048]     Due to the differences in the refractive index between the o-ray and the e-ray, light  421 ( e ) and  411 ( o ) can exit from birefringent wedge  15  with different paths. However, when the paths of  421 ( e ) and  411 ( o ) are substantially parallel, light  421 ( e ) and  411 ( o ) can be coupled to an optical fiber using a collimator.  
         [0049]     In the implementation of non-reciprocal combination-device  10  shown in  FIG. 1   b , the optical axes of birefringent wedges  15  and  17  are, respectively, in the x direction and the x−y direction. Faraday rotator  16  is designed in such a way that the polarization of light passing through the Faraday rotator  16  will be rotated a positive 45 degrees with respect to the positive z-axis.  
         [0050]     In another implementation of non-reciprocal combination-device  10 , as shown  FIG. 1   c , the optical axes of birefringent wedges  15  and  17  are, respectively, in the x direction and the x+y direction. Faraday rotator  16  is designed in such a way that the polarization of light passing through the Faraday rotator  16  will be rotated a negative 45 degrees with respect to the positive z-axis.  
         [0051]     In a third implementation of non-reciprocal combination-device  10 , as shown in  FIG. 1   d , the optical axes of birefringent wedges  15  and  17  are, respectively, in the y direction and the x+y direction. Faraday rotator  16  is designed in such a way that the polarization of light passing through the Faraday rotator  16  will be rotated a positive 45 degrees with respect to the positive z-axis.  
         [0052]     In a fourth implementation of non-reciprocal combination-device  10 , as shown in  FIG. 1   e , the optical axes of birefringent wedges  15  and  17  are, respectively, in the cos(φ) x+sin(φ) y direction and the cos(φ−45) x+sin(φ−45) y direction. Faraday rotator  16  is designed in such a way that the polarization of light passing through the Faraday rotator  16  will be rotated positive 45 degrees with respect to the positive z-axis.  
         [0053]     In the implementation of non-reciprocal combination-device  10 , as shown in  FIG. 1   a , birefringent wedges  15  and  17  are essentially in contact with Faraday rotator  16 . In other implementations, other optical media (including air) can be inserted between birefringent wedge and Faraday rotator  16 , and between birefringent wedge  17  and Faraday rotator  16 .  
         [0054]      FIG. 5   a  illustrates an implementation of an optical depolarizer  500  that includes a non-reciprocal combination-device  10 . Depolarizer  500  also includes a lens  540 , a birefringent block  580 , and a reflector  590 . A single mode fiber  510  and a Polarization Maintenance (“PM”) fiber  520  are coupled to lens  540 . The positions of single mode fiber  510  and PM fiber  520  can be fixed with a capillary  530 . The optical axis of birefringent block  580  can be in the y-direction. Birefringent block  580  includes surface  585  of facing wedge  17 .  
         [0055]     As shown in  FIGS. 5   a  and  5   b , light with the x-polarization exiting from PM fiber  520  is coupled to non-reciprocal combination-device  10  through lens  540 , and enters non-reciprocal combination-device  10  in the first input direction (i.e., the z−αy direction) as e-ray  320 ( e ). After passing through non-reciprocal combination-device  10 , e-ray  320 ( e ) becomes o-ray  321 ( o ) traveling in the principal direction (i.e., the positive z-direction) with the x+y polarization. O-ray  321 ( o ) enters surface  585  of birefringent block  580  as light  381 .  
         [0056]     Light  381  can be decomposed as light  381 (x) with the x-polarization and  381 (y) with the y-polarization. Because the optical axis of birefringent block  580  is in the y-direction, light  381 ( x ) and  381 ( y ) are, respectively, the o-ray and the e-ray in birefringent block  580 . Light  381 ( x ) travels in the positive z-direction with the phase velocity of an o-ray. Light  381 ( y ) travels in the positive z-direction with the phase velocity of an e-ray. Light  381  ( x ) and  381  ( y ) are reflected by reflector  590 , and become, respectively, light  382 ( x ) and  382 ( y ). Light  381 ( x ) travels in the negative z-direction with the phase velocity of an o-ray. Light  381 ( y ) travels in the negative z-direction with the phase velocity of an e-ray. Light  382 ( x ) and  382 ( y ) are recombined at surface  585  as light  382 .  
         [0057]     When light  381  traveling in the positive z-direction enters surface  585 , the phase difference between the decomposed light  381 ( x ) and  381 ( y ) is zero. The polarization of light  381  is x+exp (jθ i )y, with θ i =0. When light  382 ( x ) and  382 ( y ) are recombined at surface  585  as light  382  traveling in the negative z-direction, the phase difference between the decomposed light  382 ( x ) and  382 ( y ) is θ f . Phase difference θ f  is given by θ f =4π(n e -n o ) L/λ, where L is the length of the birefringent block  580 , λ is the wavelength of light  382  (and light  381 ), n e  and n o  are respectively the refractive indexes of the e-ray and the o-ray. The polarization of light  382  is x+exp(jθ f )y.  
         [0058]     For a selected wavelength λ 1 , the phase difference θ f  can be zero, and the polarization of light  382  can be in the x+y direction. For another selected wavelength λ 2 , the phase difference θ f  can be equal to π, and the polarization of light  382  can be in the x−y direction. For a third selected wavelength λ 3 , the phase difference θ f  can be equal to π/2, and the polarization of light  382  can be in the x+j y direction (i.e., light  382  is circularly polarized).  
         [0059]     When light  382  enters non-reciprocal combination-device  10  with the x+exp (jθ f )y polarization, light  382  can be decomposed as light  420 ( e ) with the x−y polarization and light  410 ( o ) with the x+y polarization and given by equation
 
[ x +exp(jθ f ) y ]/2 1/2 =[cos(θ f /2)  o - j sin(θ f /2) e]exp(jθ   f /2),
 
 where o=[x+y]/2 1/2  and e=[x−y]/2 1/2 . The intensity of light  410 ( o ) is proportional to [sin(θ f /2)] 2 . The intensity of light  410 ( o ) is proportional to [cos(θ f /2)] 2 . 
 
         [0060]     As shown in  FIG. 5   a  and  FIG. 5   c , light  420 ( e ) passes through non-reciprocal combination-device  10  as light  421 ( e ) with the x-polarization. Light  421 ( e ) passes through lens  540 , and enters single mode fiber  510  with the x-polarization.  
         [0061]     As shown in  FIG. 5   a  and  FIG. 5   d , light  410 ( o ) passes through non-reciprocal combination-device  10  as light  411 ( o ) with the y-polarization. Light  411 ( o ) passes through lens  540 , and enters single mode fiber  510  with the y-polarization.  
         [0062]     Therefore, light  320 ( e ) with the x-polarization exiting from PM fiber  520  can be directed into single mode fiber  510  as light  511  that in general has both the x-polarization component and the y-polarization component. If light  320 ( e ) has wavelength λ 1  and θ f =0, then, light  511  has mostly the y-polarization component. If light  320 ( e ) has wavelength λ 2 , and of θ f =π, then, light  511  has mostly the x-polarization component. If light  320 ( e ) has wavelength between λ 2  and λ 1 , then, light  511  in general has both the x-polarization component and the y-polarization component.  
         [0063]     When light  320 ( e ) has a certain bandwidth, with wavelengths ranging from λ 2  to λ 1 , light  511  entering single mode fiber  510  can become depolarized.  
         [0064]      FIG. 6   a  illustrates an implementation of an optical depolarizing combiner  600  that includes non-reciprocal combination-device  10 . Depolarizing combiner  600  also includes a lens  540 , a birefringent block  580 , and a reflector  590 . A single mode fiber  510 , a first PM fiber  520 , and a second PM fiber  520 ′ are coupled to lens  540 . The positions of single mode fiber  510 , the first PM fiber  520 , and the second PM fiber  520 ′ can be fixed with a capillary  530 . The optical axis of birefringent block  580  can be in the y-direction. Surface  585  of birefringent block  580  faces wedge  17 .  
         [0065]      FIG. 6   a  illustrates that light  320 ( e ) with the x-polarization exiting from PM fiber  520  can be directed into single mode fiber  510  as light  511  that in general has both the x-polarization component and the y-polarization component.  
         [0066]      FIG. 6   a  also illustrates that light  310 ( o ) with the y-polarization exiting from PM fiber  520 ′ can be directed into single mode fiber  510  as light  511 ′ that in general has both the x-polarization component and the y-polarization component.  FIGS. 6   b - 6   d  show in detail the processing of light  310 ( o ).  
         [0067]     As shown  FIGS.6   a  and  6   b , light  310 ( o ) with the y-polarization exiting from PM fiber  520 ′ is coupled to non-reciprocal combination-device  10  through lens  540 . Light  310 ( o ) enters non-reciprocal combination-device  10  in the second input direction (i.e., the z+βy direction) as an o-ray. After passing through non-reciprocal combination-device  10 , o-ray  310 ( o ) becomes e-ray  311  ( e ) in the principal direction (i.e., the positive z-direction) with the x−y polarization. E-ray  311 ( e ) enters surface  585  of birefringent block  580  as light  381 ′.  
         [0068]     Light  381 ′ can be decomposed as light  381 ′( x ) with the x-polarization and  381 ′( y ) with the y-polarization. Light  381 ′( x ) and  381 ′( y ) travels in the positive z-direction with the phase velocity of the o-ray and the e-ray respectively. Light  381 ′( x ) and  381 ′( y ) are reflected by reflector  590 , and become, respectively, Light  382 ′( x ) and  382 ′( y ). Light  382 ′( x ) and  382 ′( y ) travel in the negative z-direction with the phase velocity of the o-ray and the e-ray respectively. Light  382 ′( x ) and  382 ′( y ) are recombined at surface  585  as light  382 ′.  
         [0069]     As shown  FIG. 6   c  and  FIG. 6   d , light  382 ′ entering non-reciprocal combination-device  10  can be decomposed as light  410 ′( o ) with x+y polarization and as light ray  420 ′( e ) with x−y polarization. Light  410 ′( o ) and  420 ′( e ) exit from non-reciprocal combination-device  10 , respectively, as light  411 ′( o ) with the y-polarization and as light  421 ′( e ) with the x-polarization. Light  411 ′( o ) and  421 ′( e ) are combined and enter polarization single mode fiber  510  as light  511 ′. Light  511 ′ in general has both the x-polarization component and the y-polarization component.  
         [0070]     When light  310 ( o ) has a certain bandwidth, with wavelengths ranging from λ 2  to λ 1 , light  511 ′ entering single mode fiber  510  can become depolarized.  
         [0071]      FIG. 6   a  illustrates that optical depolarizing combiner  600  functions as both a depolarizer and a combiner. Light exiting from PM fiber  520  with the x-polarization and light exiting from PM fiber  520 ′ with the y-polarization are directed into single mode fiber  510 , and combined as depolarized light.  
         [0072]      FIG. 7   a  illustrates that optical depolarizing combiner  600  can also function as an optical isolator. Light exiting from single mode fiber  510  can be decomposed as light  220 ( e ) with the x-polarization and light  210 ( o ) with the y-polarization.  
         [0073]     As shown in  FIG. 7   b , light  220 ( e ) passes through non-reciprocal combination-device  10  as light  221 ( o ) traveling in the first output direction (i.e., the z+γy direction) with the x+y polarization. Light  221 ( o ) travels though birefringent block  580  and is deflected by reflector  590 . After deflected by reflector  590 , light  221 ( o ) does not travel back to single mode fiber  510 , first PM fiber  520 , or second PM fiber  520 ′.  
         [0074]     As shown in  FIG. 7   c , light  210 ( o ) passes through non-reciprocal combination-device  10  as light  211 ( e ) traveling in the second output direction (i.e., the z−δy direction) with the x−y polarization. Light  211 ( e ) travels though birefringent block  580  and is deflected by reflector  590 . After being deflected by reflector  590 , light  211 ( e ) does not travel back to single mode fiber  510 , first PM fiber  520 , or second PM fiber  520 ′.  
         [0075]      FIGS. 8   a  and  8   b  illustrate an implementation of an optical combiner  800  that includes non-reciprocal combination-device  10 . Optical combiner  800  also includes a lens  540 , and a reflector  590 . A single mode fiber  510 , a first PM fiber  520 ,;and a second PM fiber  520 ′ are coupled to lens  540 . The positions of single mode fiber  510 , first PM fiber  520 , and second PM fiber  520 ′ can be fixed with a capillary  530 .  
         [0076]      FIG. 8   a  illustrates that light  320 ( e ) with the x-polarization exiting from first PM fiber  520  and light  310 ( o ) with the y-polarization exiting from second PM fiber  520 ′ are coupled to non-reciprocal combination-device  10 . Light  320 ( e ) and light  310 ( o ) pass through non-reciprocal combination-device  10  as light  321 ( o ) and light  311 ( e ) respectively. Light  321 ( o ) and light  311 ( e ) are reflected by reflector  590 , and enter non-reciprocal combination-device  10  as light  410 ( o ) and light  420 ( e ) respectively. Light  410 ( o ) and light  420 ( e ) pass back through non-reciprocal combination-device  10  as light  411 ( o ) and light  421 ( e ) respectively. Light  411 ( o ) and light  421 ( e ) are directed into single mode fiber  510 , and are combined.  
         [0077]      FIG. 8   b  illustrates that light exiting from single mode fiber  510  can be decomposed as light  220 ( e ) and  210 ( o ). Light  220 ( e ) passes through non-reciprocal combination-device  10  as light  221 ( o ) traveling in the first output direction (i.e., z+γy). Light  210 ( o ) passes through non-reciprocal combination-device  10  as light  211  ( e ) traveling in the second output direction (i.e., z−δy). Light  221 ( o ) and light  211 ( e ) are deflected by reflector  590 . After being deflected by reflector  590 , light  211 ( e ) and light  221 ( o ) do not travel back to single mode fiber  510 , first PM fiber  520 , or second PM fiber  520 ′.  
         [0078]      FIGS. 9   a  and  9   b  illustrate an implementation of a PM isolator  900  that includes non-reciprocal combination-device  10 . PM isolator  900  also includes a lens  540 , and a reflector  590 . An output PM fiber  910 , and an input PM fiber  920  are coupled to lens  540 . The positions of output PM fiber  910 , and input PM fiber  920  can be fixed with a capillary  530 .  
         [0079]      FIG. 9   a  illustrates that light  320 ( e ) with the x-polarization exiting from input PM fiber  920  is coupled to non-reciprocal combination-device  10  as e-ray. Light  320 ( e ) passes through non-reciprocal combination-device  10  as light  321 ( o ). Light  321 ( o ) is reflected by reflector  590 , and enters non-reciprocal combination-device  10  as light  410 ( o ). Light  410 ( o ) pass back through non-reciprocal combination-device  10  as light  411 ( o ) and is directed into output PM fiber  910 .  
         [0080]      FIG. 9   b  illustrates that light  210 ( o ) exiting from input PM fiber  920  enters non-reciprocal combination-device  10  as o-ray. Light  210 ( o ) passes through non-reciprocal combination-device as light  211 ( e ) traveling in the second output direction (i.e., z−δy). Light  211 ( e ) is deflected by reflector  590 . After being deflected by reflector  590 , light  211 ( e ) does not travel back to output PM fiber  910  or input PM fiber  920 .  
         [0081]     In the implementation of  FIGS. 9   a  and  9   b , output PM fiber  910  and input PM fiber  920  are aligned in such a way that light exits from input PM fiber  920  as an e-ray and enters output PM fiber  910  from non-reciprocal combination-device  10  as an o-ray. In an alternative implementation, output PM fiber  910  and input PM fiber  920  can be aligned in such a way that light exits from input PM fiber  920  as an o-ray and enters output PM fiber  910  from non-reciprocal combination-device  10  as an e-ray.  
         [0082]     The optical depolarizer of  FIG. 5   a - 5   d  and the optical depolarizing combiner of  FIGS. 6   a - 6   e  include birefringent block  580  with an optical axis in the y-direction that forms a 45 degree angle with the optical axis of birefringent wedge  17 . In alternative implementations, other angles between the optical axis of birefringent block  580  and the optical axis of birefringent wedge  17  can be selected.  
         [0083]     In the implementations of  FIGS. 5   a ,  6   a  and  7   a , reflector  590  can be a mirror. In alternative implementations, reflective materials can be coated at the end of birefringent block  580  to function as reflector  590 .  
         [0084]     In the implementations of  FIGS.8   a  and  9   a , reflector  590  can be a mirror. In alternative implementations, reflective materials can be coated on surface  19  of birefringent wedge  17  to function as reflector  590 .  
         [0085]     Birefringent block  580 , birefringent wedge  15 , and birefringent wedge  17  can be constructed from birefringent crystal materials, such as, calcite, rutile, lithium niobate or yttrium orthvanadate.  
         [0086]     A birefringent crystal material in general has refractive indexes n, for e-ray and n, for o-ray. Non-reciprocal combination-device  10  can be constructed using birefringent crystal materials with indexes n e  larger than n o , or birefringent crystal materials with indexes n e  smaller than n o    
         [0087]      FIGS. 10   a  and  10   b  illustrate implementations of non-reciprocal combination-device  10  including birefringent wedges  15  and  17  in the form of tapered plate. Surface  11  of birefringent wedge  15  substantially parallels surface  12  of birefringent wedge  17 . The tapering angle of birefringent wedges  15  and  17  is χ.  
         [0088]      FIG. 10   a  illustrates an implementation of non-reciprocal combination-device  10  constructed using birefringent crystal materials with indexes n e  larger than n o .  FIG. 10   a  also illustrates the paths traveled by e-ray  320 ( e ) and o-ray  310 ( o ). E-ray  320 ( e ) is incident upon surface  11  of birefringent wedge  15  in the cos(θ e ) z−sin(θ e ) y direction and exits from birefringent wedge  17  in the positive z-direction. Here θ e  satisfies equation n e  sin(χ−θ e )=n o  sin(χ). O-ray  310 ( o ) is incident upon surface  11  of birefringent wedge  15  in the cos(θ o ) z+sin(θ o ) y direction and exits from birefringent wedge  17  in the positive z-direction. Here θ 0  satisfies equation n o  sin(χ+θ o )=n e  sin(χ).  
         [0089]      FIG. 10   b  illustrates an implementation of non-reciprocal combination-device  10  constructed using birefringent crystal materials with indexes n e  smaller than n o .  FIG. 10   b  also illustrates the paths traveled by e-ray  320 ( e ) and o-ray  310 ( o ). E-ray  320 ( e ) is incident upon surface  11  of birefringent wedge  15  in the cos(θ e ) z−sin(θ e ) y direction and exits from birefringent wedge  17  in the positive z-direction. Here θ e  satisfies equation n e  sin(χ+θ e )=n o  sin(χ). O-ray  310 ( o ) is incident upon surface  11  of birefringent wedge  15  in the cos(θ o ) z+sin(θ o ) y direction and exits from birefringent wedge  17  in the positive z-direction. Here θ o  satisfies equation n o  sin(χ−θ o )=n e  sin(χ).  
         [0090]     A method and system has been disclosed for providing optical depolarizers, optical depolarizing combiners, optical combiners, and PM isolators. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.