Time-Domain Characterization And Regulation Of Photonic Systems Based On Rayleigh Scattering In Optical Fibers

The Rayleigh scattering in optical fiber has transient response,high randomness,and other attractive characteristics,making it suitable for adoption in both the non-oscillatory Rayleigh scattering system,such as phase-sensitive optical time-domain reflectometry(Φ-OTDR),and the oscillatory Rayleigh scattering system,such as the random fiber laser system.These two types of Rayleigh scattering photonic systems show excellent application value in seismic wave detection,borderline monitoring,optical amplification,optical imaging,high-energy physics research,and other applications.However,they both suffer from time domain fluctuations caused by the randomness of Rayleigh scattering,which leads to distortion of the sensing signal and damage to the optical components.However,such time-domain fluctuations are not completely harmful.It has become a bridge to connect photonic systems with other complex physical systems,and related works have been presented on the official website of the Nobel Prize in 2021.Therefore,the time-domain properties of fiber Rayleigh scattering systems deserve to be further explored,and how to characterize,modulate and even exploit such time-domain fluctuations is the focus of this work.In this dissertation,the optical fiber Rayleigh scattering photonic systems are taken as the research object,and its time domain properties are investigated in terms of characteristics,modulations and applications.For non-oscillatory Rayleigh scattering photonic system,a multi-Rayleigh-channel superposition model of Φ-OTDR is established to characterize the time domain properties and system performance? for oscillatory Rayleigh scattering photonic system,its quasi-static time domain properties,full-bandwidth dynamic time domain properties and transient dynamic features are analyzed based on the random fiber laser.The long-distance large-bandwidth sensing is realized by utilizing the modulation of spectral fluctuations on the time domain,and the high-power pulsed random fiber laser is also realized based on the passive and active modulation.The main research contents and results of this dissertation are as follows.(1)The effects of multi-Rayleigh-channel superposition on the Φ-OTDR time domain characteristics and system performance are elucidated through theory,simulation and experiment.First,to investigate the effect of time-domain intensity fluctuations on the Φ-OTDR phase demodulation,a mapping method based on Jacobi determinant is proposed to combine the intensity noise distribution and phase demodulation process.It is deduced that the intensity noise of Φ-OTDR will be linearly transferred to the phase component after the demodulation process,and can be found that the intensity minima points(interference fading points)will seriously deteriorate the demodulation quality of the sensing signal.In addition,the quantitative relationship between the number of superimposed channels and the time domain characteristics of Φ-OTDR is further analyzed,and the distribution of phase signal-to-noise ratio of Φ-OTDR under different superimposed channels is deduced based on mathematical statistics.This work provides a clear theoretical map of the relationship between the degree of interference fading suppression and the number of superimposed channels.The related results can also be extended to applications in optical imaging,wireless communications,and other two-dimensional or three-dimensional Rayleigh channel systems.(2)The time-domain properties of the random fiber laser for oscillatory Rayleigh scattering photonic system are characterized in three aspects: quasi-static,full-bandwidth dynamic,and transient state.For the quasi-static time-domain characteristics,a semianalytical power steady-state model is proposed for rapidly characterizing the quasi-static time-domain characteristics of the random fiber laser.The simulation speed is increased by two orders of magnitude,and the model can further reveal the physical nature of the high optical-optical conversion efficiency of the short-cavity structure.For dynamic timedomain characteristics,a forward-pumped ultra-narrow-band Raman random fiber laser is built,and the full-bandwidth time-domain characteristics are measured for the first time.The actual time-domain statistical properties of random fiber laser are different from the previous simulation results by foreign scholars [Optics Express 21(18),21236,2013].It exhibits an exponential probability density distribution at the fiber end,and deviate from the exponential distribution at the pump end.The results provide an experimental basis for optimizing random fiber laser simulation model? For the transient state dynamics,based on nonlinear Schr?dinger equations and the time-slicing method,it can be found that random fiber laser exhibits a completely different transient state from the resonant cavity laser in the laser build-up process,which shows a typical Verhulst logistic growth curve described in biology.The results are expected to open up new ways to understand complex biological phenomena.The study of the time-domain characteristics in this part will be of great help to the comprehensive understanding of the underlying working mechanism of random fiber laser,the establishment process of complex physical systems,etc.(3)The time-domain modulation of random fiber laser for oscillatory Rayleigh scattering photonic system is carried out based on two types of schemes: active and passive modulation schemes,and it is extended to the field of ultra-long-distance wide-band dynamic sensing and laser inertial confinement fusion(ICF).For the active time-domain modulation,a Raman-gain-modulated power-balance model is proposed to quantitatively reveal how the spectral variation of the random fiber laser induces dynamic modulation of the time-domain characteristics,which can be used to demonstrate the principle for its utilization in long-distance dynamic sensing.In the proof-of-concept experiment,a sensing bandwidth of 65 k Hz over 100 km fiber is achieved showing a good linearity to the disturbance? For the passive time-domain modulation,a pulsed random fiber laser with repetition frequency tunable,nanosecond pulse width,kilowatt-scale output power,and passive Q-switching is realized based on the self-pulsing processes.The scheme is further used to study the nonlinear optical dynamics processes of cascaded stimulated Raman scattering,four-wave mixing and self Q-switching process? To meet the requirements of ICF for low coherent seed sources,a two-level nonlinear Schr?dinger model for ytterbium random fiber laser is proposed,based on which a broadband random fiber laser with specific wavelength is designed and the pulsed random fiber laser with MW-level peak power is realized for the first time.