Studies On The Influence Of Backscattering On Reflective Single Wavelength Single Fiber Transmission System

The traditional optical fiber communication system has the risk of information theft.Especially in the application of secure communication,it is more difficult to meet the security requirements.Aiming at the technical problem of safe information transmission,here an improved high-security reflective coherent optical communication system is studied in this thesis.The signal light is homologous with the local oscillator light,which can ensure the high frequency stability of the system while achieving homodyne detection.In addition,the problem of loop instability caused by the use of optical phase-locked loop or closed-loop frequency tracking control system is avoided.At the same time,the structure of the transmitter is improved to avoid re-modulation noise caused by repeated modulation during the modulation of the signal and modulate as in previous reflection type modulator.Due to the system structure of single-fiber bidirectional transmission,backscattering,mainly Rayleigh backscattering and stimulated Brillouin scattering,has a negative impact on system performance.Therefore,the study of such problems is of great significance for evaluating the performance of reflective systems.First,a brief introduction to the three kinds of scattering common in optical fibers-Rayleigh scattering,Raman scattering,and Brillouin scattering.Then the physical mechanism of Rayleigh backscattering and stimulated Brillouin scattering is expounded.Secondly,the structure and working principle of the improved reflective system are introduced in detail.Based on the structural characteristics of the reflective system,the source of coherent Rayleigh noise in the system is analyzed,and the transmission characteristics of the system and the characteristics of coherent Rayleigh noise are analyzed in linear transmission mode.From the perspective of fiber parameters and light source parameters,the threshold characteristics of stimulated Brillouin scattering are analyzed.Based on the coupled wave equation of stimulated Brillouin scattering,a high-precision 4^th Runge-Kutta method+shooting method are used in MATLAB to analyze the downlink transmission characteristics of the reflective system.At the same time,the change of the signal optical power at the receiving end of the reflective system with the incident optical power is analyzed,and the characteristics of coherent Rayleigh noise based on stimulated Brillouin scattering are studied.Finally,based on the working principle of balanced detection,the design scheme of balanced detection for reflective systems is introduced.The noise composition of single detection and balanced detection of reflective system is analyzed.When the balance mismatch is considered,the detection performance of balance detection and single tube detection is compared and analyzed.When Coherent Rayleigh noise is introduced into the signal-to-noise ratio formula of balanced detection and single detection,the sensitivity of the two detections are compared.The results show that balanced detection has a significant improvement effect on sensitivity and is suitable for the detection of weak signals.According to the presence or absence of stimulated Brillouin scattering in the optical fiber,the system performance under two operating modes of low incident optical power and high incident optical power were analyzed and evaluated by using BPSK modulation balance detection method.Analysis and comparison of the data results show that the improved reflective system exhibits good communication performance in both low-incident optical power and high-incident optical power operating modes.In addition,the reflective system can be used in secure communications or high-speed coherent optical communications in some special fields.