Study On Key Technologies Of Subcarrier Modulated Non-optical Heterodyne Detection System And Relative Experiments

In recent years,atmospheric laser communication technology has gradually attracted people's attention,but the biggest obstacle of the applications of this technology is atmospheric channel attenuation.Improving the detection sensitivity is a way to suppress atmospheric turbulence.It is well known that coherent detection is a considerable technology to improve sensitivity of the wireless optical detection.In this paper,a novel non-optical heterodyne detection method is proposed,which is able to inhibit atmospheric turbulence.This method possesses same high detection sensitivity as the classic method and is much easier to implement.In order to improve the detecting sensitivity,the main innovations and contents arc as follows:1.A new wiireless optical heterodyne detection(NOHD)method based on subcarrier modulation(SCM)is proposed that generates cross term through square process of the local oscillator(LO)intensity signal and the signal light intensity signal,which improves the detecting sensitivity by using LO intensity signal to amplify the signal light intensity signal.A wireless laser communication NOHD model is constructed and heterodyne detection gain is discussed and verified through simulation experiment,which lays a foundation for applications of the method.2.Received SNR,bit error rate and the detection sensitivity of NOHD under additive noise are theoretically derived.For atmospheric turbulence,system average SNR and the average BER are analyzed,system simulation experiments are produced under additive noise,multiplicative noise and mixed noise,respectively,which provides a experimental basis for SCM/NOHD applications.3.Orthogonal frequency division multiplexed optical signals is detected by utilizing NOHD method,the mathematical model is established,the closed expressions of average carrier noise ratio and average bit error rate of subcarrier in turbulent channel is theoretically researched,and receiving performance of the SCM-OFDM/NOHD system is numerically analyzed.Doppler frequency shift of the atmospheric turbulence channel and nonlinear modulation effect of the impact to the system are also analyzed.The feasibility of this method is validated by result of the experiments.4.NOHD experimental system is built,NOHD detected signals of different subcarrier modulated signal lights are collected.The power spectrum of the beat signal,eye graph and constellation of the demodulated signal are analyzed,which proves feasibility of the NOHD method.In this paper,the communication experiment of 1 km atmospheric channel link is operated.The experimental results show that this system obtains a higher heterodyne gain around 20dB than the direct detection system,which verified NOHD model and provides experimental evidence for engineering applications of this method.Results of the study in this paper provides a new way of suppressing turbulence and improving the detection sensitivity of the receiver for the wireless optical communication system,lays theoretical foundation and provides experimental evidence for the popularization and application of SCM/NOHD.