Optical orthogonal frequency division multiplexed communications with nonlinearity compensation

Abstract

The present invention discloses a transmitter and receiver for optical communications system, which provide compensation of the optical link nonlinearity. M-PSK modulating is used for data embedding in an optical signal in each WDM channel using orthogonal frequency division multiplexing (OFDM) technique. At the receiver side electrical output signals from a coherent optical receiver are processed digitally with the link nonlinearity compensation. It is followed by the signal conversion into frequency domain and information recovery from each subcarrier of the OFDM signal. At the transmitter side an OFDM encoder provides a correction of I and Q components of a M-PSK modulator driving signal to compensate the link nonlinearity prior to sending the optical signal to the receiver.

1 . An optical receiver to receive a data modulated beam from an optical link, comprising:
 at least a first coherent receiver for receiving the data modulated beam and a local oscillator beam; the data modulated beam being one channel of orthogonal frequency division multiplexed (OFDM) communication system, the OFD multiplexing having N subcarrier in each channel,   a set of photodetectors receiving output beams from the coherent receiver and producing electrical signals I and Q;   a digital signal processing unit receiving the electrical signals I and Q, converting them into digital signals,   calculating an expected phase shift caused by the signal distortion for each sampling interval, performing the distortion compensation by multiplying the instant signals on a distortion coefficient depending on the expected distortion phase shift,   transforming the digital signals into frequency domain, forming a set of spectral signals each having its own digital frequency, demodulating the data encoded in each spectral signal, and outputting a received information.   
     
     
         2 . The receiver of  claim 1 , wherein the distortion is associated with linear and nonlinear transmission impairments. 
     
     
         3 . The receiver of  claim 1 , wherein the transmission is in optical fiber. 
     
     
         4 . The receiver of  claim 1 , wherein the distortion phase shift is expected nonlinear phase shift caused by the fiber nonlinearity φ 1 (t i ) i =G 1  P 1  (t i ) and G 1  is a parameter being proportional to a link nonlinearity γ, G 1 =M γEL 1 , wherein L 1  is a length of a compensating part of the optical link, E is an average optical beam power and M is a power coefficient in unit of Watt indicating the launch power in the optical link at the front end per voltage level corresponding to one digitization bit; P 1 (t i )=Q(t i ) 2 +I(t i ) 2 is an instant power of the signal. 
     
     
         5 . The receiver of  claim 4 , wherein L 1  is a second half-length of the optical link. 
     
     
         6 . The receiver of  claim 1 , wherein the coherent receiver is based on a 90-degrees optical hybrid. 
     
     
         7 . The receiver of  claim 1 , wherein the optical receiver is adapted for operation with the optical signal of two polarization states. 
     
     
         8 . The receiver of  claim 1 , wherein the modulation format is selected from QPSK or QAM or M-QAM. 
     
     
         9 . The receiver of  claim 1 , wherein the distortion is caused by the nonlinearity of the electro-optical modulation of the beam in the channel. 
     
     
         10 . An optical transmitter to transmit a data encoded beam over an optical link, comprising:
 a digital data stream entering an orthogonal frequency division multiplexed (OFDM) encoder, the encoder outputting I and Q analog signals driving an optical modulator, the modulator modulating separately each OFDM subcarrier of each channel of an initial optical beam from a light source, the modulator outputting a modulated optical beam to be transmitted in the optical link, wherein the OFDM encoder performs a compensation of the optical link transmission distortions by multiplying each subcarrier on a compensation coefficient.   
     
     
         11 . The transmitter of  claim 10 , wherein the distortion is associated with linear and nonlinear transmission impairments. 
     
     
         12 . The transmitter of  claim 10 , wherein the distortion the compensation is performed by estimating instant power P 2 (t i )=Q(t i ) 2 +I(t i ) 2 , where a sampling interval Δt i =t i+1 −t i  is equal or less than a symbol interval, calculating an expected distortion phase shift φ 2  (t i ) i  for the i-th sampling interval, performing a distortion compensation by multiplying each subcarrier on a distortion compensation coefficient depending on the expected distortion phase shift. 
     
     
         13 . The transmitter of  claim 12 , wherein the expected distortion phase shift is φ 2 (t i ) i =G 2  P 2  (t i ) and G 2  is a parameter being proportional to a link nonlinearity γ, G 2 =M γEL 2 , wherein L 2  is a length of a compensating part of the optical link, E is an average optical beam power and M is a power coefficient in unit of Watt indicating the launch power in the optical link at the front end per voltage level corresponding to one digitization bit. 
     
     
         14 . The transmitter of  claim 13 , wherein L 2  is a first half-length of the optical link. 
     
     
         15 . A system for a data transmission via an optical communication link, comprising:
 a digital data stream entering an orthogonal frequency division multiplexed (OFDM) encoder, the encoder outputting I and Q analog signals driving an optical modulator, the modulator modulating separately each OFDM subcarrier of each channel of an initial optical beam from a light source, the modulator outputting a modulated optical beam to be transmitted in the optical link, wherein the OFDM encoder performs a compensation of the optical link transmission distortions by multiplying each subcarrier on a compensation coefficient;   the modulated beam being received by an optical receiver, comprising:   at least a first coherent receiver for receiving the data modulated beam and a local oscillator beam;   a set of photodetectors receiving output beams from the optical hybrid and producing electrical signals I and Q;   a digital signal processing unit receiving the electrical signals I an d Q, converting them into digital signals,   an expected distortion phase shift for the i-th sampling interval, performing a distortion compensation by multiplying the signals I(t i ) and Q(t i ) on a coefficient depending on the expected distortion phase shift,   transforming the digital signals into frequency domain, forming a set of spectral signals each having its own digital frequency, demodulating the data encoded in each spectral signal, and outputting a received information.   
     
     
         16 . The system of  claim 15 , wherein the OFDM encoder compensates transmission distortions of a first half of the optical link, and the digital signal processing unit compensates transmission of a second half of the optical link. 
     
     
         17 . The system of  claim 15  adapted to operate with data transmission using an optical beam having two polarization states. 
     
     
         18 . The system of  claim 15 , wherein a data transmission rate is 100 Gb/s. 
     
     
         19 . The system of  claim 15 , wherein the modulation format is selected from QPSK or QAM or M-QAM. 
     
     
         20 . The system of  claim 15 , wherein the channel is one of many WDM channels of the transmission system.