Frequency-Domain Equalization of the Fiber Optic Channel

Abstract

Systems and methods for frequency-domain compensation in optical communication systems. In pre-equalization embodiments, the transmitter transforms the data stream into a frequency domain signal and applies a compensation filter before transforming it back into a pre-distorted time domain signal. As the pre-distorted time domain signal propagates through the optical channel, optical dispersion effects counter the pre-distortion, producing an equalized signal at the channel output. In post-equalization embodiments, the receiver transforms the received signal into a frequency domain signal and applies a compensation filter before transforming it back into an equalized time domain signal. Pre-equalization may prove less expensive due to the square-law characteristic of photodetectors employed by most receivers.

1 . A method for countering dispersion caused by a fiber optic medium, the method comprising:
 transforming a time-domain data block into a frequency-domain data block;   acting on the frequency-domain data block with an inverse dispersion function to create a corrected frequency-domain data block;   converting the corrected frequency-domain data block into a corrected time-domain signal; and   modulating an optical beam with the corrected time-domain signal.   
     
     
         2 . The method of  claim 1 , wherein said converting includes:
 transforming the corrected frequency-domain data block into a corrected time-domain data block;   forming a serial data stream from the corrected time-domain data block; and   performing digital to analog conversion to convert the serial data stream into the corrected time-domain signal.   
     
     
         3 . The method of  claim 2 , wherein said forming includes overlapping each of the corrected time-domain data blocks by L−1 samples, so that the last L−1 samples of each block is added to the first L−1 samples of a subsequent block, where L is a predetermined integer. 
     
     
         4 . The method of  claim 1 , wherein the inverse dispersion function is expressible with a frequency dependence in the form of exp(−jβ 2 ω 2 L/2), where exp( ) is the exponential function, j is the square root of (−1), β 2  is a group velocity dispersion parameter, ω is frequency, and L is an effective length of a fiber channel that carries said optical beam. 
     
     
         5 .- 9 . (canceled) 
     
     
         10 . An optical modulator that comprises:
 a complex multiplier that applies a compensation filter to frequency-domain data blocks to produce compensated frequency-domain data blocks;   an inverse frequency transform module that transforms the compensated frequency-domain data blocks into compensated time-domain data blocks;   a conversion module that forms a compensated transmit signal from the compensated time-domain data blocks; and   an electrical-to-optical converter that produces an optical beam modulated with the transmit signal.   
     
     
         11 . The modulator of  claim 10 , further comprising:
 a frequency transform module that converts time-domain data blocks into the frequency-domain data blocks.   
     
     
         12 . The modulator of  claim 11 , wherein the time-domain data blocks are zero-padded with a predetermined number of zeros. 
     
     
         13 . The modulator of  claim 10 , wherein the conversion module overlaps compensated time-domain data blocks by a predetermined number of samples, and serializes the summed compensated time-domain data blocks to produce a transmit data stream, wherein the transmit signal is the analog form of the transmit data stream. 
     
     
         14 . The modulator of  claim 10 , wherein the inverse frequency transform module implements an inverse fast Fourier transform. 
     
     
         15 . The modulator of  claim 10 , wherein the compensation filter compensates for optical dispersion effects of the channel. 
     
     
         16 .- 19 . (canceled)