Measurements of polarization-dependent loss (PDL) and degree of polarization (DOP) using optical polarization controllers

Abstract

Devices and techniques that use a polarization controller and a feedback control to the polarization controller to systematically control the polarization of light output from the polarization controller in measuring the polarization dependent loss (PDL) of an optical device or material that receives the light from the polarization controller or the degree of polarization (DOP) of a light beam directed into the polarization controller.

1 . A device, comprising:
 a polarization unit to control a polarization of light received by the polarization unit in response to a control signal, wherein the light from the polarization unit passes through an optical medium;   an optical detector to receive light transmitted through the optical medium and to produce a detector output; and   a feedback unit that receives the detector output and, in response to the received detector output, produces the control signal to control the polarization unit to adjust the polarization of light to the optical medium to measure a maximum transmission through the optical medium and a minimum transmission through the optical medium at the optical detector.   
   
   
       2 . The device as in  claim 1 , wherein the feedback unit comprises an electronic circuit that converts the detector output into a feedback signal, and a processor which processes the feedback signal and produces the control signal. 
   
   
       3 . The device as in  claim 1 , wherein the polarization controller comprises a plurality of adjustable polarization elements, and wherein the feedback unit is configured to adjust one adjustable polarization element at a time while keeping other adjustable polarization elements at fixed settings to adjust each of the adjustable polarization elements to search for the maximum transmission and the minimum transmission at the optical detector. 
   
   
       4 . The device as in  claim 3 , wherein the adjustable polarization elements comprise an electrically controlled electro-optic element. 
   
   
       5 . The device as in  claim 3 , wherein the adjustable polarization elements comprise a fiber and a plurality of adjustable fiber squeezers. 
   
   
       6 . The device as in  claim 3 , wherein the adjustable polarization elements comprise a rotatable waveplate. 
   
   
       7 . The device as in  claim 3 , wherein the adjustable polarization elements comprise a fiber coil which is rotatable. 
   
   
       8 . The device as in  claim 3 , wherein the polarization controller further comprises (1) a feed-forward control responsive to an input polarization to the polarization controller to change the adjustable polarization elements to produce a desired output polarization and (2) a feedback control responsive to an output polarization to control the adjustable polarization elements to reduce a deviation of the output polarization from the desired output polarization. 
   
   
       9 . A method, comprising:
 separating a plurality of WDM channels into separated signals;   directing the separated WDM channels, one at a time, into a device that measures a degree of polarization of light; and   measuring the degree of polarization of light of each separated WDM channel by using the device.   
   
   
       10 . The method as in  claim 9 , wherein the device comprises:
 a polarization unit to receive an input beam of light and to control a polarization of the received input beam in response to a control signal;   an optical polarizer placed to receive light output from the polarization unit to produce an optical output;   an optical detector to receive the optical output from the optical polarizer and to produce a detector output; and   a feedback unit that receives the detector output and, in response to the received detector output, produces the control signal to control the polarization unit to adjust the polarization of light to the optical polarizer to measure a maximum transmission and a minimum transmission at the optical detector, and   the method further comprising computing the degree of polarization using the measured maximum transmission and minimum transmission at the optical detector.   
   
   
       11 . A method, comprising:
 directing light from a light source through an optical bandpass filter to produce a reference light beam;   measuring a first degree of polarization of the reference light beam output from the optical bandpass filter;   using the first degree of polarization to compute a first optical signal to noise ratio;   directing the light from the light source through an optical amplifier first and then through the optical bandpass filter;   measuring a second degree of polarization of light output from the optical bandpass filter;   using the second degree of polarization to compute a second optical signal to noise ratio; and   using the first degree of polarization and the second degree of polarization to determine a noise figure of the optical amplifier.