Analysis Of Balanced Detection In Spatial Coherent Optical Communication

The spatial coherent optical communication has many advantages,including diversity of modulation mode,high sensitivity,strong selectivity of wavelength,high security,and so on,which make it to be an important choice to realize long haul and highly reliable optical communication in the future.However,the relative intensity noise(RIN)of the local-oscillator(LO)and the atmospheric channel which are important factors affecting the performance of the system,should be considered in the practical system.The RIN noise can be suppressed by using balanced detection,which could improve the communication performance of the system The channel characteristics of tracking error in Exponential Weibull(EW)channel are helpful to study the characteristics of the outage probability and bit error rate of the balanced detection in the atmospheric channel,which can guide the practical engineering.Firstly,depended on the basic principle of the spatial coherent optical communication and atmospheric channel,this paper presents the basic structure of spatial coherent optical communication system and the signal attenuation.model of atmospheric channel about the EW model.Focused on the analysis of the principle about the path loss,tracking error and atmospheric turbulence,the closed expression of the probability density distribution formula under the combined affection of these three is given as well,which lays the theoretical foundation for the subsequent analysis.Besides,the basic principle of the binary phase shift keying(BPSK)and the balanced detection is introduced,and the theoretical analysis and experimental research on the balanced detection are carried out.The advantage that the balanced detection can effectively suppress the RIN noise of the system of the balanced detection over the single tube detection is analyzed.The influence of the balance mismatch and the local oscillator intensity on the performance of the coherent optical communication is discussed,it's concluded that the balance mismatch will lead to the decrease of the balanced detection performance,and it is possible to improve the performance of the balanced detection by selecting the appropriate local oscillator intensity.According to the previous theoretical study,the coherent optical communication experimental platform with optical fiber is built for the test and experiment of the balanced detector developed by our laboratory.The test result shows that the common mode rejection ratio of the balanced detector is better than 29dB with a good characteristic of balance consistency.The communication eye pattern and the real-time waveform of the balanced detector are tested as well,which show the good performance of the balanced detector.The experimental results verify that the signal power required for coherent detection increases with the increase of the communication rate,and decreases firstly and increase later with the increase of the local-oscillator power.Finally,based on the nature of the balanced detection,the closed expression about outage probability and error rate of the coherent optical communication with atmospheric channel are given.Then,the influence of turbulence intensity,receiving aperture,tracking error and beam radius on the performance of the system is studied.The results show that the communication performance of the system decreases with the increase of turbulence intensity,and increase with the increase of signal noise to ratio;the pointing error will drop the communication performance of the system,the decrease of the jitter deviation could improve the communication performance of the system;when the beam radius of the system is large and the tracking precision is superior,the communication performance of the system could be improved by reducing the beam radius;increasing the receiving aperture is helpful to reduce the geometric loss and alleviate the turbulence,which could improve the communication performance of the system.With the same aperture and turbulence intensity,the communication performance of BPSK modulation is better than that of DPSK,and the communication performance of PSK modulation decreases with the increase of phase modulation order.