Study On Modulation Instability And Its Suppression Method In The Remote Interferometric Optical Fiber Sensing Systems

Interferometric optical fiber sensing system has been widely applied in the fields of underwater acoustic detection,oil exploration and seismic monitoring,etc.While the interferometric optical fiber sensing system is developing in the direction of longer distance and larger scale,the effect of various nonlinear effects is becoming more and more serious.Up to now,SBS in the remote interferometric optical fiber sensing system has been suppressed effectively.MI gradually becomes the most dominant nonlinear effect in the remote interferometric optical fiber sensing system.When MI occurs,the phase noise increases dramatically and the sensing performance decreases seriously.MI is the main limitation factor of the maximum input power and transmission distance.Therefore,the suppression of MI in the interferometric optical fiber sensing system is an important problem to be solved urgently.The thesis conducts theoretical and experimental study on MI and its suppression method in the remote interferometric optical fiber sensing system.Comprehensive analysis is offered to four key sections in the remote interferometric optical fiber sensing system including(1): the physical mechanism and characteristic of MI,(2): the mechanism and characteristic of MI induced noise,(3): the suppression of MI and the phase noise,(4)the relationship between MI and other nonlinear effects.Firstly,the fundamental characteristic of MI is studied.The physical mechanism of MI in the interferometric fiber sensing systems is introduced.Based on the linear stability analysis method,the MI gain in the condition of weak pump depletion is analyzed.Numerical simulation is conducted with split-step Fourier algorithm to study the evolution of MI in optical fiber.The effect of the input power and ASE noise is analyzed.Experimental observation is conducted at the end of the optical fiber to study the spectra and coherence induced by MI.A distributed measurement method based on Rayleigh scattering is proposed to measure the distributed evolution of MI in remote single mode optical fiber.Based on the distributed measurement method,the power evolution of both spontaneous MI and induced MI are measured.The experimental results are compared with the results obtained by cutting-back method and numerical simulation.The results agree well,which confirms the feasibility and accuracy of the distributed measurement method.By exciting induced MI in optical fiber,controllable FPU recurrence is observed.11 sidebands are measured in the experiment,which presents a good signal to noise in the system.As far as we know,this is the first experiment that achieves the quantitative and distributed measurement of FPU recurrence in 10 km single mode fiber.The method has great potential in the one-shot and real-time measurement of rare nonlinear events such as rogue wave and probabilitic soliton.The MI induced intensity and phase noise in the remote interferometric optical fiber sensing system are studied.When MI occurs,the intensity of the overall output light does not increases,but the fluctuation of the central frequency light power increases with the input power.This phenomenon indicates the instability of the power conversion between the central frequency light and MI sidebands,which is attributed to the fluctuation of ASE noise.The phase noise of the remote interferometric optical fiber sensing system increases seriously when MI occurs.The analysis shows that the power fluctuation of the central frequency light is the main source of the MI induced phase noise.A method based on coherent seed is proposed to suppress spontaneous MI and phase noise in the remote interferometric fiber sensing system.By exciting induced MI in remote optical fiber,the spontaneous MI and the phase noise are suppressed significantly.The effect of modulation frequency and seed power on the spontaneous MI suppression is studied.The effect of induced MI on the phase noise of the system is analyzed.By applying the method,the phase noise can be low as 89 dB/ Hz when the input power is as high as 1 W.The result presents the highest record of the avalible input power in the remote interferometric optical fiber sensing systems.An experimental observation of the competition between MI,SBS and stimulated Raman scattering(SRS)is reported.The physical mechanism is analyzed in detail.A theoretical model is established to study the effect of MI on SBS threshold of the optical pulse in long single mode fiber.The effects of ASE noise and repetition rate are analyzed.The experimental results agree well with the theoretical model.