Monitoring and in-line compensation of polarization dependent loss for lightwave systems

Abstract

A multi-wavelength optical signal copropagates through a fiber-optic communication link with a continuous-wave ancillary wavelength having an unknown state of polarization (SOP), which is scrambled periodically in time. The instantaneous value of polarization dependent loss (PDL) in the ancillary wavelength is monitored in real time, and is used as an error signal to adjust at least one polarization controller. Polarization scrambling is performed by periodically changing the SOP with time, such that the polarization-scrambled optical signal covers approximately an entire Poincaré sphere surface, preferably uniformly, during each time period. At an optical node between fibers, an adjustable PDL compensator contains two ordered pairs each consisting of one polarization controller and one optical element introducing fixed PDL.

1 . A method of polarization-scrambling an incoming optical signal, comprising the steps of: 
 causing a variation of the state of polarization (SOP) as a function of time for an incoming optical signal that has an unknown SOP to produce a polarization-scrambled optical signal; and    periodically changing said SOP of said polarization-scrambled optical signal with time, such that said periodically changing polarization-scrambled optical signal covers approximately an entire Poincaré sphere surface during each time period of said periodic changing, over a plurality of periods.    
   
   
       2 . The method of  claim 1  wherein said SOP of said periodically changing polarization-scrambled optical signal is distributed substantially uniformly over said entire Poincaré sphere surface during each said time period.  
   
   
       3 - 6 . (canceled)  
   
   
       7 . An optical apparatus, comprising: 
 a first optical polarization controller having an input port operable to receive an input optical signal having a polarization state, said first optical polarization controller being operable to adjust the polarization state of said input optical signal to produce a first intermediate optical signal;    a first optical element coupled to said first polarization controller and operable to receive and to cause a fixed polarization dependent loss (PDL) in said first intermediate optical signal to produce a second intermediate optical signal;    a second optical polarization controller coupled to said first optical element, said second optical polarization controller being operable to adjust the polarization state of said second intermediate optical signal to produce a third intermediate optical signal; and    a second optical element substantially identical to said first optical element, said second optical element being operable to receive and to cause a fixed polarization dependent loss (PDL) in said third intermediate optical signal to produce an output optical signal.    
   
   
       8 . The optical apparatus of  claim 7  having adjustable polarization dependent loss (PDL).  
   
   
       9 . The optical apparatus of  claim 8  operable as a PDL compensator.  
   
   
       10 - 19 . (canceled)  
   
   
       20 . A system for real-time compensation of the performance degrading effect induced by polarization dependent loss (PDL) in a multi-wavelength fiber-optic communication system, said system comprising: 
 a first optical polarization controller having an input port operable to receive an input optical signal having a polarization state, said first optical polarization controller being operable to adjust the polarization state of said input optical signal to produce a first intermediate optical signal;    a first optical element coupled to said first polarization controller and operable to receive and to cause a fixed polarization dependent loss (PDL) in said first intermediate optical signal to produce a second intermediate optical signal;    a second optical polarization controller coupled to said first optical element, said second optical polarization controller being operable to adjust the polarization state of said second intermediate optical signal to produce a third intermediate optical signal; and    a second optical element substantially identical to said first optical element, said second optical element being operable to receive and to cause a fixed polarization dependent loss (PDL) in said third intermediate optical signal to produce an output optical signal.    
   
   
       21 . The system of  claim 20  having adjustable PDL.  
   
   
       22 . The system of  claim 20  further comprising a recirculating optical loop.