Research On The Mechanism Of Random Fiber Laser Pulses

The random fiber laser breaks the structural characteristics of the fixed resonator of the traditional laser,and realizes the output of the laser through random feedback,so it provides a good source for the research of many optical phenomena such as nonlinear effects,soliton collisions,modulation instability,photon localization,etc.a good research platform.At present,a lot of research work on the time-domain and frequency-domain output characteristics of fiber-optic random lasers has been carried out immediately,especially the generation and regulation of optical pulses in fiber-optic random lasers has become the focus of researchers.It has laid an important foundation for applications in fields such as coherent tomography and optical fiber sensing.Random fiber laser pulses can be divided into disordered light pulses and ordered light pulses according to the degree of regularity in the time domain.A typical representative of disordered optical pulses is the optical rogue wave phenomenon in supercontinuum,while ordered pulses are regular time-domain pulses obtained by orderly cutting random fiber lasers by using Q-switching or mode-locking technology.The thesis mainly studies the generation mechanism of nonlinear optical rogue wave and active mode-locked pulse in random fiber laser and its regulation method,which is divided into the following two parts.Firstly,a semi-open-cavity fiber stochastic laser composed of two different nonlinear optical fibers cascaded was built,and the process and characteristics of supercontinuum light generated by this structure were studied.The structure was pumped by continuous light and used second-order amplification The principle realizes the supercontinuum with3 dB bandwidth of 120 nm and good flatness.At the same time,by studying the timedomain characteristics of the supercontinuum,the existence of optical rogue waves was detected,and the frequency of optical rogue waves was controlled by using the method of externally injected seed photoperiodic perturbation.In this process,the seed optical power is only 1.5% of the pump power.However,when its modulation frequency is aligned with or deviates from the characteristic frequency of the system,the optical rogue wave can be suppressed by 69.2% or increased by 26.5%.This proves that the randomly generated soliton behavior in the process of supercontinuum generation can be effectively controlled.The research provides a simple and effective new method for the generation of high-quality supercontinuum.At the same time,it also provides a theoretical reference for the research and suppression of ocean rogue waves in nature.Secondly,a random fiber laser based on a randomly distributed fiber grating array was built.By analyzing the characteristics of photon localization and free running state,an active mode-locking method was introduced to realize the quasi-continuously adjustable repetition frequency in the range of 4.9990～5.1200 MHz.The pulsed laser output,and at the same time realize the 2-4th harmonic mode-locking corresponding to5.1200 MHz.In addition,by changing the shape of the modulation signal,such as pulse signal,square wave signal,etc.,the influence of different modulation signal shapes on the optical pulse output effect is studied.This work is of great significance for the timedomain active control and ultrafast optical pulse output of random fiber lasers based on randomly distributed fiber grating arrays,and at the same time provides a new path for sensing applications such as distributed acoustic waves or pressure.