Method and Apparatus for Detecting Localized Polarization Dependent Anomalies on Optical Transmission Lines

Abstract

The position and amount of localized polarization dependent anomalies such as polarization mode dispersion and/or polarization dependent loss may be measured by applying a polarization modulated probe signal to an optical transmission line. The polarization modulated probe signal is returned via optical feedback paths positioned along the line, and is detected by a probe signal receiver to identify the position and strength of the anomaly.

1 . A polarization anomaly locator system for an optical communication network, said system comprising: 
 a transmitter configured to transmit a probe signal on a network having first and second optical transmission paths and a loop-back path, said first and second optical transmission paths supporting communication of optical signals in opposite directions on said network, said loop-back path coupling at least a portion of said probe signal from said first transmission path to said second transmission path as a returned probe signal; said transmitter having a modulation section optically communicating with said probe signal source, said modulation section configured to impart a depth of modulation on said probe signal at a particular frequency; and    a receiver coupled to said second transmission path and configured to receive said returned probe signal and to provide an output indicating an approximate position of a polarization anomaly in said optical communication network in response to said returned probe signal, said polarization anomaly comprising polarization dependent loss or polarization mode dispersion.    
     
     
         2 . A system according to  claim 1 , wherein said modulation section further comprises: 
 an amplitude modulator configured to modulate the amplitude of said probe laser output signal at a carrier frequency; and    a polarization modulator configured to modulate the polarization of said probe laser output signal at a polarization frequency,    said amplitude and polarization modulators thereby providing a modulated probe laser output signal.    
     
     
         3 . A system according to  claim 2 , wherein said amplitude modulator is configured to modulate the amplitude of said probe laser output signal to provide sinusoidal intensity variation of said probe laser output signal at said carrier frequency.  
     
     
         4 . A system according to  claim 2 , wherein said polarization modulator is configured to polarization modulate said probe laser output signal to provide switching of the polarization state of said probe laser output signal between substantially orthogonal states of polarization at said polarization frequency.  
     
     
         5 . A system according to  claim 2 , wherein said transmitter further comprises an optical switch configured to selectively switch said modulated probe laser output signal onto an optical path in response to a control signal.  
     
     
         6 . A system according to  claim 1 , wherein said transmitter further comprises an optical reference element, and wherein said probe signal includes a reference anomaly level imparted by said optical reference element.  
     
     
         7 . A system according to  claim 6 , wherein said reference anomaly level comprises a predetermined level of polarization dependent loss.  
     
     
         8 . A system according to  claim 6 , wherein said reference anomaly level comprises a predetermined level of polarization mode dispersion.  
     
     
         9 . A system according to  claim 1 , wherein said polarization anomaly comprises polarization dependant loss and wherein said receiver comprises an amplitude detector configured to detect said polarization dependent loss by detecting an amplitude of a signal representative of said returned probe signal.  
     
     
         10 . A system according to  claim 1 , wherein said polarization anomaly comprises polarization mode dispersion and wherein said receiver comprises a phase detector configured to detect said polarization mode dispersion by detecting phase modulation of a signal representative of said returned probe signal.  
     
     
         11 . A system according to  claim 1 , further comprising an inbound loading laser optically communicating with said second transmission path, said loading laser configured to reduce a noise level associated with the second transmission path.  
     
     
         12 . A method of detecting a polarization dependent anomaly comprising polarization dependent loss or polarization mode dispersion in an optical network, said method comprising: 
 transmitting a probe signal on said network;    receiving said probe signal from said network as a returned probe signal; and    providing an output indicating an approximate position of said polarization anomaly in response to said returned probe signal,    wherein said network comprises first and second optical transmission paths and a loop-back path, said first and second optical transmission paths supporting communication of optical signals in opposite directions on said network, and wherein at least a portion of said probe signal is transmitted on said first transmission path, through said loop-back path, and received as said returned probe signal from said second transmission path.    
     
     
         13 . A method according to  claim 12 , said method further comprising calculating a location of said polarization anomaly in response to a distance of travel for said probe signal.  
     
     
         14 . A method according to  claim 12 , wherein said probe signal is amplitude modulated.  
     
     
         15 . A method according to  claim 12 , wherein probe signal is polarization modulated.  
     
     
         16 . A method of approximating the level of a polarization dependent anomaly comprising polarization dependent loss or polarization mode dispersion in an optical network, said method comprising: 
 transmitting a probe signal on said network;    receiving said probe signal from said network as a returned probe signal;    transmitting a reference probe signal on said network with a known reference anomaly;    receiving said reference probe signal from said network as a returned reference probe signal; and    approximating said level of said polarization dependent anomaly by comparing said returned reference probe signal with said returned probe signal,    wherein said network comprise first and second optical transmission paths and a loop-back path, said first and second optical transmission paths supporting communication of optical signals in opposite directions on said network, and wherein at least a portion of said probe signal is transmitted on said first transmission path, through said loop-back path, and received as said returned probe signal from said second transmission path.    
     
     
         17 . A method according to  claim 16 , wherein said probe signal and said reference probe signal are amplitude modulated.  
     
     
         18 . A method according to  claim 16 , wherein said probe signal and said reference probe signal are polarization modulated.  
     
     
         19 . A method according to  claim 16 , wherein said known reference anomaly comprises a known amount of polarization mode dispersion.  
     
     
         20 . A method according to  claim 16 , wherein said known reference anomaly comprises a known amount of polarization mode dispersion.