Adaptive optical equalization for chromatic and/or polarization mode dispersion compensation

Abstract

An adaptive optical parallel equalizer architecture is based on a controllable optical modulator device to realize an optical FIR (finite-impulse-response) filter including a plurality of coefficient taps in order to have independent control of each optical FIR filter coefficient. A unique adaptive opto-electronic LMS (least mean squares) process is utilized to generate an electronic error signal utilized to control the plurality of parallel tap coefficients of the optical parallel equalizer. The electronic error signal is used as the optimization criterion because the electronic signal after photo-detection is needed to achieve any measurable performance in terms of bit error rate (BER). In a specific embodiment, the controllable optical parallel FIR filter is realized by employing an optical vector modulator. The optical vector modulator is realized by splitting a supplied input optical signal into a plurality of parallel similar optical signals, controllably adjusting the phase and/or amplitude of each of the plurality of optical signals and delaying the resulting optical signals in a prescribed manner relative to one another. Then, the “delayed” signals are combined to yield the optical signal comprising the vector modulated input optical signal to be transmitted as an output. In one particular embodiment, both the phase and amplitude is adjusted of each of the plurality of parallel optical signals, and the error control signals for effecting the adjustments are generated in response to the optical modulator optical output signal utilizing the unique Opto-Electronic LMS process.

1 . Apparatus for use in an adaptive optical equalizer comprising: 
 a controllable optical modulator having an input and an output, and being coupled to receive an incoming optical signal and configured to generate an output optical signal by phase modulation and/or amplitude modulation of the received optical signal, said controllable optical modulator including a plurality of similar optical signals in a corresponding plurality of optical paths, each of said parallel optical paths including an opto-electronic controller responsive to electronic control signals for effecting said phase modulation and/or amplitude modulation of said optical signal being transported in said optical path; and    a control signal generator responsive to an optical output signal from said output of said controllable modulator for generating said electronic control signals in accordance with predetermined criteria.    
   
   
       2 . The apparatus as defined in  claim 1  wherein said controllable optical modulator comprises arrayed waveguide gratings.  
   
   
       3 . The apparatus as defined in  claim 1  wherein said control signal generator is configured to update said adjustable control signals at a predetermined sampling rate.  
   
   
       4 . The apparatus as defined in  claim 1  wherein said controllable optical modulator is configured to operate as a controllable optical finite impulse response (FIR) filter.  
   
   
       5 . The apparatus as defined in  claim 4  wherein said FIR filter includes a plurality of parallel adjustable taps.  
   
   
       6 . The apparatus as defined in  claim 1  wherein said controllable optical modulator is configured to operate as a controllable optical vector modulator.  
   
   
       7 . The apparatus as defined in  claim 6  wherein said controllable optical vector modulator includes an optical splitter having an input and a plurality of outputs for splitting said received optical signal into a plurality of similar optical signals which are supplied on a one-to-one basis to said plurality of optical vector modulator output ports, an optical combiner having a plurality of inputs and an output and a plurality of controllable waveguides coupling predetermined ones of said optical splitter outputs to said optical combiner input ports, and wherein said opto-electronic control signals are supplied to control at least one of said plurality of controllable waveguides.  
   
   
       8 . The apparatus as defined in  claim 1  wherein said predetermined criteria includes an opto-electronic least means square (LMS) process.  
   
   
       9 . The apparatus as defined in  claim 8  wherein said control signal generator includes an optical detector having prescribed characteristics and being responsive to said received optical signal for generating a version of said received optical signal and apparatus utilizing said version of said received optical signal in accordance with said opto-electronic LMS process to generate amplitude and/or phase adjustment values for each tap of said controllable optical modulator.  
   
   
       10 . The apparatus as defined in  claim 9  further including an interferometer supplied with said optical input signal to and said optical output signal from said optical vector modulator, a differential amplifier supplied with electronic versions of outputs from said interferometer for generating a difference signal, said difference signal being supplied to said utilization apparatus to be employed in said opto-electronic LMS process to generate said amplitude and/or phase adjustment values.  
   
   
       11 . The apparatus as defined in  claim 9  wherein said optical detector includes a photodiode for generating a current version of said received optical, and said signal utilization apparatus includes an amplifier for generating a voltage version of said current signal, a slicer for generating a sliced version of said voltage signal in accordance with a supplied adjustable threshold level, an algebraic combiner supplied with said voltage signal and an output from said slicer for generating an error signal and a control signal generator responsive to said error signal for generating said amplitude and/or phase adjustment signals in accordance with said opto-electronic LMS process.  
   
   
       12 . A method for use in an adaptive optical equalizer including a controllable optical modulator comprising the steps of: 
 adaptively controlling said controllable optical modulator to modulate a supplied optical signal to generate an equalized optical output signal;    converting, in accordance with predetermined first criteria, said equalized optical output signal to an electronic signal version;    utilizing said electronic signal version to generate, in accordance with second predetermined criteria, amplitude and/or phase control signals;    feeding back said control signals to adaptively control said controllable optical modulator; and    employing each control signal to adjust the amplitude and/or phase of a corresponding optical signal propagating on a corresponding optical waveguide of a parallel array of waveguides of said controllable optical modulator.    
   
   
       13 . The method as defined in.claim  12  wherein said controllable optical modulator is configured to operate as a controllable optical finite impulse response (FIR) filter, and wherein said parallel array of waveguides form parallel optical taps of said controllable optical FIR filter.  
   
   
       14 . The method as defined in  claim 12  wherein said controllable optical modulator is configured to operate as a controllable optical vector modulator.  
   
   
       15 . The method as defined in  claim 14  wherein in operation of said controllable optical vector modulator a supplied optical signal is split into a plurality of similar optical signals corresponding in number to a number of said parallel waveguides in said array, said plurality of similar optical signals being supplied on a one-to-one basis to said plurality of optical waveguides, in response to said control signals adjusting the amplitude and/or phase of said optical signals being transported in said optical waveguides, optically combining said adjusted optical signals from said plurality of optical waveguides to generate said equalized optical output signal.  
   
   
       16 . The method as defined in  claim 1  wherein said second predetermine criteria includes use of an opto-electronic least means square (LMS) process.  
   
   
       17 . The method as defined in  claim 16  wherein said control signals are generated by optical detecting said output optical signal from said controllable optical modulator to generate an electronic signal version, and processing said electronic signal version in accordance with said opto-electronic LMS process to generate said amplitude and/or phase control signals.  
   
   
       18 . The method as defined in  claim 17  further including the steps of generating sum and difference optical signals of an optical signal supplied to the input of said controllable optical modulator and said optical output signal generated by said controllable optical modulator, converting said sum and difference optical signals into electronic versions, differentially amplifying said sum and difference signal versions to generate a difference signal, utilizing said difference signal in said opto-electronic LMS process for generating said amplitude and/or phase adjustment control signals.  
   
   
       19 . The method as defined in  claim 17  wherein said optical detecting includes utilizing a photodiode to convert said output signal from said controllable optical modulator to an electronic signal having predetermined characteristics, converting said photo diode output signal into a voltage electronic signal, slicing said voltage signal in accordance with a supplied threshold level, algebraically combining said voltage signal and a sliced version of said voltage signal to generate an error signal and utilizing said error signal in said opto-electronic LMS process for generating said amplitude and/or phase adjustment control signals.  
   
   
       20 . The method as defined in  claim 12  wherein said controllable optical modulator comprises arrayed waveguide gratings.