Suppression Of Stimulated Raman Scattering In Fiber Laser Based On Long Period Fiber Gratings

In recent years,laser technology has developed rapidly with the joint efforts of researchers at home and abroad.Among them,the fiber laser has become the representative of the new generation of lasers because of its excellent beam quality,high quantum efficiency,high light conversion efficiency,high power,wide band,compact structure,reliable operation,high cost performance and full cure.However,with the improving of the fiber laser power output,effect of stimulated Raman scattering(SRS)has become a damage fiber laser output characteristic and hinder the main factor to further improve the laser power.So how to effectively suppress continuous SRS effect in high power fiber laser has become the major problem to be solved in the field of high power laser facility.At present,researchers mostly suppress the SRS effect of fiber laser by using large mode field fiber or special structure fiber.However,the above methods are difficult to be widely used due to the complexity of manufacturing process and high cost.Therefore,a method based on long period fiber grating(LPFG)to suppress the stimulated Raman scattering(SRS)effect of fiber lasers is proposed and studied in this paper.The specific research contents are as follows:In view of the problem that the SRS effect is easily excited due to the high output power of the fiber laser,this paper puts forward a method to make LPFG by ultraviolet lithography to suppress the SRS effect based on the basic principle of the fiber laser and the mechanism of the SRS effect.In order to design and write LPFG with SRS suppression effect,the coupling theory and mathematical model of LPFG are studied in this paper.Based on the coupling model,the effects of grating period,fiber index modulation depth,grating period number and other important parameters on LPFG spectrum are simulated.Finally,combining the LPFG mathematical model and simulation results,the LPFG with SRS suppression function was designed.In order to write LPFG with SRS suppression function,a point-by-point UV laser marking system with real-time online monitoring function is designed and built in this paper.Based on this system,LPFG that meets the index requirements is written.In the process of writing LPFG,the influence of grating period,UV exposure,number of grating period and other parameters on a loss peak spectrum of LPFG was analyzed experimentally.At the same time,in order to make LPFG capable of carrying high-power laser,LPFG was annealed by the heat treatment method combining low-temperature slow change and high-temperature gradual change.The experimental results show that the annealing method optimizes the temperature coefficient of LPFG from 0.452℃/W to 0.017℃/W,which greatly improves the bearing capacity of LPFG with high power laser.Based on the above studies,the LPFG with high power laser bearing characteristics and all parameters meeting the expected requirements was successfully produced in this paper.In order to verify the inhibition effect of LPFG on SRS effect of continuous fiber laser,LPFG was applied to single oscillation stage fiber laser system and MOPA type fiber laser system respectively for experimental research.The experimental results show that the inhibition rate of LPFG on SRS effect of single-oscillation stage fiber laser exceeds 15 d B(96.8%).The inhibition rate of LPFG(placed in seed source)on the SRS effect of MOPA fiber laser exceeded8.3d B(85.2%).