Study On Weak Stimulated Raman Scattering High Power Laser Oscillators Based On Distributed Side-coupled Cladding-pumped Fiber

High-power fiber lasers have become more and more important in civil and military fields and become a research hot spot in the world in recent years,owing to their excellent performance.At present,stimulated Raman scattering(SRS),which is known as the major obstacle that limit the development of high-energy fiber laser systems,have attracted more and more attention.SRS ruins the efficiency of the laser system,the quality of the output beam and the stability of the system.How to improve the SRS threshold is one of the most important issues for the further power scaling of high power fiber laser systems.Distributed Side-Coupled Cladding-Pumped(DSCCP)fiber,because of its novel distributed pump injection pattern based on evanescent wave coupling,is seen as a breakthrough in the realization of power scalability.By far,there are only a few relevant literatures published on the novel fibers and mostly are reported in the field of amplifiers.In addition,the study of the SRS threshold in the subsequent amplifier system resulted from Raman noise of the oscillator is also rarely reported.This dissertation focuses on the study of SRS suppression in fiber lasers,and analyzes the influence of the output characteristics of the seed oscillator in the MOPA structure on the threshold Raman characteristics of the subsequent amplifier.Combined with the analysis results,a multi-stage cascaded oscillator system based on DSCCP fiber is constructed.And the output characteristics of the oscillator is studied by means of numerical simulation and experimental measurement,which provides guidance for the subsequent research of high power fiber laser.The main contents of the dissertation are as follows:1.Study the effect of seed light on the threshold of stimulated Raman scatteringThe stimulated Raman threshold characteristics in high power fiber amplifiers are studied by theoretical analysis and numerical simulation.This work mainly focuses the effects of the injection of seed signal optical power,injection of seed Raman optical power and forward and backward pumping method on the stimulated Raman threshold.2.Numerical simulation and experimental study of high power fiber optic oscillator based on DSCCP fiberThe oscillator system is experimentally studied in view of the theoretical and simulation results that the output characteristics of the seed oscillator could influence the stimulated Raman characteristics of the subsequent amplification system.It is proposed to use distributed pump injection method to suppress the possible thermal effects under high power state.The ultimate goal is to optimize the output characteristics of the fiber oscillator and provide an excellent seed source for the overall high power fiber laser system.Then,a three-cascaded fiber oscillator is built based on the domestic 25/250-250μm DSCCP fiber.Firstly,the output characteristics of the forward pumped,backward pumped and bidirectional pumped structure oscillators are analyzed separately by means of numerical simulation.And then the three-stage backward-pumped cascaded oscillator are used to achieve one kilowatt and fifteen hundred watts of laser output power,proving the power capability of backward-pumped cascaded oscillator systems based on DSCCP fibers in power expansion and the inhibition of SRS and the residual pump light.