System and method for coherent detection with digital signal procession

Abstract

Methods and apparatus to realize high spectral efficiency in optical signals transmitted over long distances.

What is claimed is: 
     
         1 . A method of generating a wide bandwidth multiplexed optical signal and transmitting it over an optical fiber, the method comprising:
 generating, by a laser, a continuous lightwave;   generating, from the continuous lightwave, a multicarrier signal having a fixed channel spacing off GHz;   separating the multicarrier signal into a plurality of optical subcarriers;   routing each of the optical subcarriers to a respective I/Q modulator and modulating each of the subcarriers to carry f Gbaud of data as a QPSK signal by the corresponding I/Q modulator;   combining, in an optical multiplexer, the modulated optical subcarriers into a multiplexed optical signal; and   transmitting the multiplexed optical signal with no synchronization information over an optical fiber, so that blind data detection is required at the receiving end of the communication.   
     
     
         2 . The method of  claim 1 , wherein the continuous lightwave has a line width of about 2 MHz. 
     
     
         3 . The method of  claim 1 , wherein the laser is a tunable external laser with a line width narrower than 2 MHz and low phase noise. 
     
     
         4 . The method of  claim 1  wherein the multicarrier generator comprises cascaded phase and intensity modulators driven by a sinusoidal wave source. 
     
     
         5 . The method of  claim 1 , wherein at least ten subcarriers are generated. 
     
     
         6 . The method of  claim 1 , wherein the QPSK signals are polarization multiplexed together. 
     
     
         7 . The method of  claim 1 , wherein the optical I/Q modulator is driven by four data signals, including in phase (I) and quadrature phase (Q) for X polarization and I and Q for Y polarization. 
     
     
         8 . The method of  claim 1 , wherein the I signals and Q signals are each modulated to carry f Gbaud/s. 
     
     
         9 . The method of  claim 1 , wherein the multiplexer is one of a regular WDM filter, a WDM coupler, an array waveguide grating, and an optical fiber Bragg grating. 
     
     
         10 . The method of  claim 1 , wherein the optical multiplexer has a 3 dB bandwidth of −f GHz. 
     
     
         11 . The method of  claim 1 , wherein the modulated signals are polarization multiplexed. 
     
     
         12 . The method of  claim 1 , further comprising amplifying the multiplexed signal before it is transmitted to compensate for transmission loss in the optical fiber. 
     
     
         13 . A method of blind detecting of data contained in a wide bandwidth multiplexed optical signal transmitted over optical fiber, the method comprising:
 receiving, by a coherent optical receiver, a wide bandwidth multiplexed optical signal from an optical fiber;   applying a local oscillator having a frequency substantially equal to a subchannel spacing f of the received multiplexed optical signal to the received multiplexed optical signal to obtain a plurality of subcarriers;   polarization demultiplexing an X signal and a Y signal from each of the subcarriers;   demultiplexing an in phase (I) signal and a quadrature phase (Q) signal from each of the X and Y signals; and   coherently detecting data carried by each of the I and Q signals using:
 an ADC with a bandwidth of about 0.5 f; 
 a signal sampler with a sampling frequency of about 1.5 f, and 
 a digital signal processor DSP configured to:
 condition the sampled signals; and 
 apply maximum likelihood sequence estimation (MLSE) to the conditioned signals to estimate data carried on each of the I and Q signals. 
 
   
     
     
         14 . The method of  claim 13 , further comprising correcting an I/Q imbalance of the received signal. 
     
     
         15 . The method of  claim 13 , further comprising compensating for chromatic dispersion in the received signal. 
     
     
         16 . The method of  claim 13 , further comprising compensating for a frequency offset of the demultiplexed signals. 
     
     
         17 . The method of  claim 13 , further comprising phase compensating the demultiplexed signal. 
     
     
         18 . The method of  claim 17 , further comprising post filtering the phase compensated signal. 
     
     
         19 . The method of  claim 13 , further comprising calculating a bit error rate of the estimated data. 
     
     
         20 . An apparatus for detecting data contained in an optical signal received from an optical transmission fiber, comprising:
 a local oscillator having a frequency substantially equal to a subchannel spacing f separating a plurality of subchannels containing in the received optical signal, to obtain a plurality of subcarrier signals;   a polarization demultiplexer to obtain an X signal and a Y signal from each of the subcarrier signals;   an OFDM demultiplexer to obtain an in phase (I) signal and a quadrature phase (Q) signal from each of the X and Y signals;   an ADC with a bandwidth of about 0.5 f;   a signal sampler with a sampling frequency of about 1.5 f; and   a digital signal processor DSP configured to condition the sampled signals and apply maximum likelihood sequence estimation (MLSE) to the conditioned signals to estimate data carried on each of the I and Q signals.