Polarization mode dispersion compensator, polarization mode dispersion compensating method, and its application to optical communication system

Abstract

The present invention provides a polarization mode dispersion compensator comprising: a compensating portion which includes a first polarization controller which performs polarization conversion on light propagating along an optical transmission line and a DGD (Differential Group Delay) emulator which adds a DGD to the light which is polarization-converted by said first polarization controller, said compensating portion being for compensating polarization mode dispersion which occurs in the light while the light propagates along the optical transmission line; a second polarization controller for performing polarization conversion on the light, of which the polarization mode dispersion is compensated, so that a state of polarization of the light can be one linear polarization; a polarization beam splitting portion for splitting the light, which is polarization-converted by said second polarization controller, into the one linear polarization and an other linear polarization which is orthogonal to the one linear polarization; an optical-intensity measuring portion for measuring intensity of the other linear polarization; and a controlling portion for controlling said compensating portion and said second polarization controller so that the intensity of the other linear polarization measured by said optical-intensity measuring portion becomes minimized.

1 . A polarization mode dispersion compensator comprising: 
 a compensating portion which includes a first polarization controller which performs polarization conversion on light propagating along an optical transmission line and a DGD (Differential Group Delay) emulator which adds a DGD to the light which is polarization-converted by said first polarization controller, said compensating portion being for compensating polarization mode dispersion which occurs in the light while the light propagates along the optical transmission line;    a second polarization controller for performing polarization conversion on the light, of which the polarization mode dispersion is compensated, so that a state of polarization of the light can be one linear polarization;    a polarization beam splitting portion for splitting the light, which is polarization-converted by said second polarization controller, into the one linear polarization and an other linear polarization which is orthogonal to the one linear polarization;    an optical-intensity measuring portion for measuring intensity of the other linear polarization; and    a controlling portion for controlling said compensating portion and said second polarization controller so that the intensity of the other linear polarization measured by said optical-intensity measuring portion becomes minimized.    
   
   
       2 . The polarization mode dispersion compensator as claimed in  claim 1 , wherein the DGD emulator of said compensating portion selects an optimal DGD value which statistically exhibits optimal behavior based on a probability density distribution of the polarization mode dispersion of the optical transmission line and adds the optimal DGD value as the DGD.  
   
   
       3 . The polarization mode dispersion compensator as claimed in  claim 2 , wherein the optimal DGD value is 75% of a maximum in the probability density distribution.  
   
   
       4 . A polarization mode dispersion compensating method comprising the steps of: 
 compensating for polarization mode dispersion which occurs in light which propagates along an optical transmission path by performing polarization conversion on the light and adding a DGD to the light;    performing polarization conversion on the light of which the polarization mode dispersion is compensated, so that a state of polarization of the light can be one linear polarization;    splitting the light, which is subjected to polarization conversion, into the one linear polarization and an other linear polarization which is orthogonal to the one linear polarization;    measuring intensity of the other linear polarization; and    controlling said step of compensating and said step of performing polarization conversion so that the intensity of the other linear polarization measured at the step of measuring becomes minimized.    
   
   
       5 . The polarization mode dispersion compensating method as claimed in  claim 4 , wherein addition of the DGD is performed by selecting an optimal DGD value which statistically exhibits optimal behavior based on a probability density distribution of the polarization mode dispersion of the optical transmission line to add the optimal DGD value as the DGD.  
   
   
       6 . The polarization mode dispersion compensating method as claimed in  claim 5 , wherein the optimal DGD value is 75% of a maximum in the probability density distribution.  
   
   
       7 . A polarization mode dispersion compensator for compensating for polarization mode dispersion that occurs in an optical signal propagating along a transmission line, comprising: 
 a polarization controller for performing polarization conversion on the optical signal input via the transmission line;    a fixed PMD (Polarization Mode Dispersion) emulator for adding a fixed PMD to the optical signal which is polarization-converted by said polarization controller;    monitoring means for monitoring a state of the optical signal output from said fixed PMD emulator; and    controlling means for controlling said polarization controller based on a feedback signal from said monitoring means,    wherein the fixed PMD added by said fixed PMD emulator consists of a fixed first-order PMD and a fixed second-order PMD.    
   
   
       8 . The polarization mode dispersion compensator as claimed in  claim 7 , wherein said fixed PMD emulator is configured by a plurality of polarization maintaining fibers or uniaxial birefringent crystals coupled with a relative angle formed between axes of eigen states of polarization of the polarization maintaining fibers or the uniaxial birefringent crystals.  
   
   
       9 . A polarization mode dispersion compensator for compensating for polarization mode dispersion that occurs in an optical signal propagating along a transmission line, comprising: 
 a first polarization controller for performing polarization conversion on the optical signal input via the transmission line;    a first fixed PMD emulator for adding only a fixed first-order PMD to the optical signal which is polarization-converted by said first polarization controller;    a second polarization controller for performing polarization conversion on the optical signal output from said first fixed PMD emulator;    a second fixed PMD emulator for adding only a fixed second-order PMD to the optical signal which is polarization-converted by said second polarization controller;    monitoring means for monitoring a state of the optical signal output from said second fixed PMD emulator; and    controlling means for controlling said first polarization controller and said second polarization controller based on a feedback signal from said monitoring means.    
   
   
       10 . The polarization mode dispersion compensator as claimed in  claim 9 , wherein said first fixed PMD emulator is one polarization maintaining fiber or one uniaxial birefringent crystal.  
   
   
       11 . The polarization mode dispersion compensator as claimed in  claim 9 , wherein said second fixed PMD emulator is configured by three or more polarization maintaining fibers or uniaxial birefringent crystals coupled with a relative angle formed between axes of eigen states of polarization of the polarization maintaining fibers or the uniaxial birefringent crystals.  
   
   
       12 . The polarization mode dispersion compensator as claimed in  claim 11 , wherein a fixed polarization converter is arranged between any two components of said second fixed PMD emulator, said polarization converter having a function of polarization conversion such that a first-order PMD of said second fixed PMD emulator is zero.  
   
   
       13 . The polarization mode dispersion compensator as claimed in  claim 7  or  9 , further comprising temperature adjusting means for adjusting temperature of said first fixed PMD emulator or said second fixed PMD emulator.  
   
   
       14 . The polarization mode dispersion compensator as claimed in  claim 7  or  9 , wherein a second PMD canceller including one polarization controller and one polarizer or polarization beam splitter is arranged at a latter part of the polarization mode dispersion compensator.  
   
   
       15 . The polarization mode dispersion compensator as claimed in any one of claims  7 ,  9  and  14 , further comprising a fixed PMD emulator having an optimal fixed first-order PMD and an optimal fixed second-order PMD which statistically exhibit optimal behavior based on a probability density distribution of the polarization mode dispersion of the transmission line.  
   
   
       16 . An optical communication system comprising the polarization mode dispersion compensator according to  claim 2 ,  5  or  15 .