Research On Key Technologies Of Coherent Optical Receiver Based On Digital Signal Processing

Coherent optical communication has many advantages,which can retain all optical field information to the electrical domain,so that the loss in the optical transmission process can be compensated in the electrical domain,thereby reducing the cost and improving the transmission quality.And coherent optical communication can perfectly match a variety of high-order modulation formats,greatly improving the transmission rate.These advantages make coherent optical communication reach the transmission speed that can not be achieved by traditional direct detection,so it will become an important communication method in the future.This paper introduces the principle of coherent optical communication system based on digital signal processing.The overall structure of the 100Gbps-PM-QPSK coherent optical communication system is introduced,including the transmitting end and the receiving end.The working principle of the Mach-Zehnder modulator and the modulation of the QPSK optical signal are described in detail at the transmitting end.At the receiving end,the structure and working principle of the balance detector and the optical mixer are mainly introduced in the hardware.For the DSP,the four modules of IQ orthogonalization,clock recovery,frequency offset estimation and phase compensation are functionally implemented.The introduction and detailed description of the principle of one or several algorithms for each module.The simulations of clock recovery,frequency offset estimation and phase compensation are carried out,and the feasibility of the above algorithm is proved.At the same time,it is found that for the clock recovery module,a higher loop filter bandwidth will accelerate the convergence of the system,but the accuracy after stabilization is poor,and the lower bandwidth will increase the accuracy of the system after stabilization but the convergence speed decreases.For the carrier recovery module,the premise that the phase and frequency drift are ramped relative to the symbol rate is critical.If this premise is not met,the algorithm will be invalidated.The effects of the optical performance of the laser on the coherent receiver are analyzed and discussed.For coherent optical communication systems,distributed feedback lasers are superior to other lasers when considered comprehensively.The broadening of the linewidth of the laser causes the frequency distribution of the beat signal to increase,the power of the center frequency component to decrease,and the beat signal to broaden.The above results will eventually lead to a decrease in the contrast between the signal and the noise,and the system performance will be affected.The relative intensity noise of the laser will make the system signal-to-noise ratio no longer increase with the local oscillator power,but increase and then decrease.The drift of the laser center frequency must not be greater than the ADC bandwidth.Otherwise,The IF signal is lost.The laser center frequency jitter should not be too sharp,otherwise the subsequent DSP algorithm will be affected.The influence of quantization noise caused by ADC resolution on the coherent optical receiver is analyzed.Usually,the 5-bit ADC can make the quantization error within an acceptable range,and the improvement caused by the continued increase of the number of bits is not large..For PSK-based coherent optical receivers,when the ADC bandwidth is 0.3-0.4 times the symbol rate,the inter-code crosstalk and noise band filtering caused by the bandwidth achieve an optimal balance.Based on the above conclusions,through analyzing the current market situation of ADC,it is found that for high-speed coherent optical communication systems,the most urgent need to break through is the sampling rate of the ADC.The combination of successive approximation ADC and time interleaving technology is the most suitable architecture for high speed ADCs,and some technical difficulties under this architecture are analyzed.The challenges of high-speed coherent optical communication for digital signal processing are analyzed,mainly reflected in the delay and complexity of the algorithm,the processing rate and delay of the hardware,the bandwidth of the hardware interface,and the high computation.The current mainstream digital signal processing implementation is analyzed.For research,FPGA is more suitable for the implementation of digital signal processing of coherent optical communication,while ASIC is suitable for large-scale commercial production.