Research On The Modulation And Demodulation Of Homodyne Coherent Optical Communication

One of the trends in the development of inter-satellite communication homodynecoherent optical communication. Compared with the traditional microwavecommunication systems, space optical communication has the advantages ofhigh-speed, long-distance, small system size, low power requirements and high safetyperformance. Homodyne coherent optical communication has a higher receiversensitivity than IM/DD optical transmission, average increase of about10²0dB,which has great development prospects in the inter-satellite communications field. Thekey technologies of Homodyne coherent optical communication are phase-locked loopand modulation and demodulation techniques, optical phase-locked loop synchronizesfrequency and phase of the local oscillator and signal light, which is a guarantee ofhigh sensitivity of homodyne coherent optical communication.First a homodyne coherent optical communication modulation and demodulationprogram is selected in this thesis. The most simple optical phase modulation-BPSK（Binary Phase Shift Keying） is selected for modulator program, this modulationprogram can maximize the optical communication advantage of high sensitivity. Thecore of coherent demodulation is optical phase-locked loop, the performance of severalcommonly optical phase-locked loops are introduced, then in accordance with theexisting conditions of the laboratory choose balanced loop and Costas loop. Thetheories of the two phase-locked loops are analysed in the thesis, and with the samenoise the demodulation sensitivity limiting of two phase-locked loops are21photons/bit,18photons/bit in a bit error rate of10-9.According to the theoretical analysis results, a homodyne coherent opticalcommunication system is built with software-Optisystem. The simulation results showthat with the narrower laser linewidth, the higher the received optical power and thelower transmission rate, the coherent receiver will have a higher sensitivity. Then amore comprehensive simulation of optical communication systems are built with software-simulink. The performance of balanced phase-locked loop and Costasphase-locked loop are tested by simulation systems, and the demodulation maximumsensitivities of two phase-locked loops are measured, which are in line with theoreticalcalculation results with the same shot noise. Further the demodulation error rates oftwo loops are8.27×10^-6and2.67×10^-6with-60dBm received optical power.Finally, with the theoretical analysis and simulation results, a homodyne coherentmodulation and demodulation experiment are built, in this experiment opticalphase-locked loop lock in a short time, and the locked status sustain about10s,eventually achieve100Mbit/s coherent communication.