Phase estimation for coherent optical detection

Abstract

The present invention is a method and apparatus to make an estimate of the phase of a signal relative to the local oscillator in an optical coherent detection subsystem that employs a digital signal processor having a parallel architecture. The phase estimation method comprises operations that do not use feedback of recent results. The method includes a cycle count function so that the phase estimate leads to few cycle slips. The phase estimate of the present invention is approximately the same as the optimal phase estimate.

1 . A coherent optical detection system receiving an input optical signal containing digital information, the input optical signal having signal phase noise and additive noise, the coherent optical detection system comprising: 
 a local oscillator laser, the local oscillator later having local oscillator phase noise; and    a digital signal processor, the digital signal processor having a parallel architecture; wherein:    the digital signal processor applies a nonlinear function to values of the complex electric field of the input optical signal to produce nonlinear function output values; and    the digital signal processor applies a filter to values derived from the nonlinear function output values, said filter being close to a Wiener filter appropriate to the additive noise, signal phase noise and local oscillator phase noise;    whereby said digital signal processor estimates the phase of the input optical signal compared to light from the local oscillator.    
   
   
       2 . The coherent optical detection system of  claim 1  wherein the filter is close to a finite-lag Wiener filter.  
   
   
       3 . The coherent optical detection system of  claim 1  wherein the filter is close to a zero-lag Wiener filter.  
   
   
       4 . The coherent optical detection system of  claim 1  wherein the Wiener filter is appropriate to Lorentzian lineshape signal source and local oscillator laser and appropriate to Gaussian additive noise.  
   
   
       5 . The coherent optical detection system of  claim 1  wherein the filter has an impulse response, and the non-zero values of the impulse response have a feature that the trend of the values of the impulse response is to decay from zero time in the direction of negative time, and to decay from zero time in the direction of positive time.  
   
   
       6 . A coherent optical detection system receiving an input optical signal containing digital information, the input optical signal having signal phase noise and additive noise, the coherent optical detection system comprising: 
 a local oscillator laser, the local oscillator laser having local oscillator phase noise; and    a digital signal processor, the digital signal processor having a parallel architecture; wherein:    the digital signal processor applies a nonlinear function to values of the complex electric field of the input optical signal to produce nonlinear function output values; and    the digital signal processor applies a cycle count function to values derived from the nonlinear function output values;    whereby said digital signal processor estimates the phase of the input optical signal compared to light from the local oscillator.    
   
   
       7 . The coherent optical detection system of  claim 6  wherein: 
 said digital signal processor applies a filter to values derived from the nonlinear function output values to produce filtered output values, said filter being close to a Wiener filter appropriate to the additive noise, signal phase noise and local oscillator phase noise; and    the digital signal processor applies said cycle count function to values derived from said filtered output values.    
   
   
       8 . The coherent optical detection system of  claim 6  wherein: 
 said digital signal processor calculates the phase angle of the nonlinear function output values to produce wrapped phase angle values; and    the digital signal processor applies the cycle count function to the wrapped phase angle values to produce unwrapped phase angle values; and    the digital signal processor applies a filter to values derived from the unwrapped phase angle values, said filter being close to a Wiener filter appropriate to the additive noise, signal phase noise and local oscillator phase noise.    
   
   
       9 . A coherent optical detection system receiving an input optical signal containing digital information, the input optical signal having signal phase noise and additive noise, the coherent optical system comprising: 
 a local oscillator laser, the local oscillator laser having local oscillator phase noise; and    a digital signal processor, the digital signal processor having a parallel architecture; wherein:    the digital signal processor makes an estimate of the phase of the input optical signal compared to light from the local oscillator; and    the digital signal processor includes a differential logical detection operation; and    the data values produced by the differential logical detection operation are communicated to a forward error correction decoder; and    said forward error correction decoder decodes a forward error correction code, the forward error correction code being of the type that inherently corrects short bursts of errors;    whereby cycle slip events caused by an imperfect estimate of the phase do not cause a large number of bit errors to be produced by the coherent optical detection system.    
   
   
       10 . The coherent optical detection system of  claim 9  wherein: 
 the forward error correction decoder decodes a first forward error correction code, the first forward error correction code inherently correcting short bursts of errors;    the forward error correction decoder performs an interleaving operation;    the forward error correction decoder decodes a second forward error correction code.    
   
   
       11 . The coherent optical detection system of  claim 9  wherein: 
 the differential logical decoder compares each symbol with another symbol which is not an immediately preceding symbol; and    the forward error correction decoder performs an interleaving operation; and    the forward error correction decoder decodes the forward error correction code, the forward error correction code inherently correcting short bursts of errors.    
   
   
       12 . A method of estimating the phase of an input optical signal to a coherent optical receiver compared to light from a local oscillator laser, said method comprising the steps of: 
 applying a nonlinear function to values of the complex electric field of the input optical signal to produce nonlinear function output values; and    applying a digital filter to values derived from the nonlinear function output values, said digital filter being close to a Wiener filter appropriate to the additive noise and the phase noise on the input optical signal and the phase noise of the local oscillator, and said digital filter not employing feedback of immediately preceding results.    
   
   
       13 . The method of  claim 12  wherein the digital filter is close to a zero-lag Wiener filter appropriate to Lorentzian lineshape signal source and Lorentzian lineshape local oscillator laser and appropriate to Gaussian additive noise.  
   
   
       14 . The method of  claim 12  wherein the digital filter is close to a finite-lag Wiener filter appropriate to Lorentzian lineshape signal source and Lorentzian lineshape local oscillator laser and appropriate to Gaussian additive noise.  
   
   
       15 . A method of estimating the phase of an input optical signal to a coherent optical receiver compared to light from a local oscillator laser, said method comprising the steps of: 
 applying a nonlinear function to values of the complex electric field of the input optical signal to produce nonlinear function output values; and    applying a cycle count function to values derived from the nonlinear function output values, the cycle count function not employing feedback of immediately preceding results.    
   
   
       16 . The method of  claim 15  further comprising the steps of: 
 applying a digital filter to values derived from the nonlinear function output values to produce digital filter output values, said digital filter being close to a Wiener filter appropriate to the additive noise and the phase noise on the input optical signal and the phase noise of the local oscillator, said digital filter not employing feedback of immediately preceding results; and    applying the cycle count function to values derived from the digital filter output values.    
   
   
       17 . A method of obtaining information carried on an input optical signal received by an optical coherent detection system, the method comprising the steps of: 
 estimating the phase of the input optical signal compared to a local oscillator; and    applying the phase estimate to produce symbol output values;    applying a differential logical detection operation to the symbol output values to produce differential symbol values; and    decoding a forward error correction code on the differential symbol values, the forward error correction code inherently correcting short bursts of errors.    
   
   
       18 . The method of  claim 17  wherein the following steps are performed subsequent to the step of decoding the forward error correction code that inherently corrects short bursts of errors: 
 performing an interleaving operation; and    decoding a second forward error correction code.    
   
   
       19 . The method of  claim 17  further comprising the following step of: 
 performing an interleaving operation on the differential symbol values; and    wherein the differential logical detection includes comparing a symbol output value with another symbol output value which is not the immediately preceding symbol output value.