Transmit optical sub-assembly with local feedback

Abstract

Aspects of a method and system for feedback during optical communications are provided. In one embodiment, a system for optical communications comprises a digital-to-analog converter (DAC), a driver, and a transmit optical subsystem. The DAC is operable to receive a digital code of a plurality of digital codes and output an analog current signal having an analog current level of a plurality of analog current levels. The driver is operable to condition the analog current signal output from the digital-to-analog converter. The transmit optical subsystem is operable to generate an optical signal from the conditioned analog current signal. A digital modification of an input digital signal is dynamically controlled by a feedback path according to one or more characteristics of the optical signal. The one or more characteristics comprise a nonlinearity that may be temperature dependent.

What is claimed is: 
     
         1 - 20 . (canceled) 
     
     
         21 . A system comprising:
 a digital predistortion subsystem operable to modify an input digital signal to generate a predistorted digital signal, wherein the digital predistortion subsystem is controlled according to a plurality of Volterra series coefficients that approximate a nonlinearity of an optical channel;   a transmit optical subsystem operable to transmit an optical signal over the optical channel, wherein the optical signal is generated according to the predistorted digital signal; and   a feedback subsystem operable to adaptively generate the plurality of Volterra series coefficients according a current temperature and a comparison between the optical signal and an expected signal.   
     
     
         22 . The system of  claim 21 , wherein the plurality of Volterra series coefficients models a nonlinear relationship between the predistorted digital signal and the optical signal. 
     
     
         23 . The system of  claim 21 , wherein the feedback subsystem is operable to determine a modification of the input digital signal that linearizes a model of the transmit optical subsystem. 
     
     
         24 . The system of  claim 21 , wherein the plurality of Volterra series coefficients is temperature dependent. 
     
     
         25 . The system of  claim 21 , wherein the feedback subsystem comprises a backside monitor photodiode. 
     
     
         26 . The system of  claim 21 , wherein the transmit optical subsystem comprises a pre-equalizer circuit. 
     
     
         27 . The system of  claim 21 , wherein the optical signal is intermittently monitored. 
     
     
         28 . The system of  claim 21 , wherein the feedback subsystem comprises a photodiode. 
     
     
         29 . The system of  claim 28 , wherein the photodiode intermittently monitors the optical signal. 
     
     
         30 . The system of  claim 21 , wherein the current temperature is provided via a control channel. 
     
     
         31 . A method comprising:
 generating a predistorted digital signal by modifying an input digital signal in digital hardware, wherein the modification is controlled according to a plurality of Volterra series coefficients that approximate a nonlinearity of an optical channel;   generating, via a transmit optical subsystem, an optical signal according to the predistorted digital signal;   transmitting the optical signal over the optical channel; and   adaptively generating the plurality of Volterra series coefficients, in digital hardware, according a current temperature and a comparison between the optical signal and an expected signal.   
     
     
         32 . The method of  claim 31 , wherein the plurality of Volterra series coefficients models a nonlinear relationship between the predistorted digital signal and the optical signal. 
     
     
         33 . The method of  claim 31 , wherein a feedback subsystem is operable to determine a modification of the input digital signal that linearizes a model of the transmit optical subsystem. 
     
     
         34 . The method of  claim 31 , wherein the plurality of Volterra series coefficients is temperature dependent. 
     
     
         35 . The method of  claim 31 , wherein the method comprises monitoring the optical signal using a backside monitor photodiode. 
     
     
         36 . The method of  claim 31 , wherein the transmit optical subsystem comprises a pre-equalizer circuit. 
     
     
         37 . The method of  claim 31 , wherein the method comprises periodically monitoring the optical signal. 
     
     
         38 . The method of  claim 31 , wherein the transmit optical subsystem comprises a photodiode. 
     
     
         39 . The method of  claim 38 , wherein the photodiode intermittently monitors the optical signal. 
     
     
         40 . The method of  claim 31 , wherein the method comprises providing the current temperature via a control channel.