Research On Key Technology Of Homodyne Coherent Optical Communication

Since free space homodyne coherent optical communication has the advantages of high speed, low power, security, long communication distance, confidentiality, small size and light weight, it has became a new hotspot in space communication. Compared with the traditional intensity modulation/direct detection(IM/DD), free space homodyne coherent optical communication has the most prominent advantage of receiving sensitivity. But there are still many problems and key technologies to be solved before free space homodyne coherent optical communication is applied to engineering application.In this thesis homodyne coherent optical communication’s principles and some key technologies have been introduced in detail thesis. Homodyne coherent optical communication has outstanding advantages in compare with IM/DD and heterodyne coherent detection. The core module of optical communication system—optical phase locked loop(OPLL) is theoretically analyzed, focusing on two kind of OPLL: Balanced phase locked loop and Costas phase locked loop. Noise analysis of the two OPLL is made and the sensitivity of theory limit is calculated. When the bit error rate of the system is 10-10, we obtain the relationship between the bit rate of the system and the linewidth of the laser. The structure of the balanced phase locked loop is relatively simple, but it needs high requirement of the linewidth of the laser which is evaluated and found to be 65.9 10?? times the system bit rate. For comparison, the Costas phase locked loop requires only 45.02 10?? times, but it has more complex structure.This thesis analyzes the principle of the balanced detector and the 90 degree optical hybrid in homodyne coherent optical communication system theoretically, and derivate the effects which are caused by the error of components. According to the theory, we designed balanced detection circuit and 90 degree optical hybrid parts. The bandwidth of the balanced detector is 750 MHz and the photoelectric conversion rate is 240V/W. Besides, a 90 degree optical hybrid with coupling efficiency of 49.9% is assembled successfully and its average relative phase error differs by 5° from the theoretical value.An experimental platform is established based on the above theoretical analysis. And the frequency of the local oscillator laser is tuned by controlling the temperature of the laser crystal and the piezoelectric ceramic which is pasted on the laser crystal. The experiment is carried out based on balanced phase locked loop and Costas phase locked loop. When the balanced phase locked loop is locked stably and communication rate is 1.25 Gbps, the system bit error rate is 5.31×10-7 with signal light power is-54 dBm, then change communication rate to 2.5Gbps, signal light power to-50 d Bm, the system bit error rate is 3.12×10-6. Complete the homodyne coherent optical communication on the Costas loop.