Research On SRS And TMI Effects And Suppression Of High Power Ytterbium-Doped Double-Clad Fiber Lasers

High-power Yb-doped fiber lasers have the advantages of excellent beam quality,high conversion efficiency and easy maintenance,and are widely used in industrial and military fields.With the continuous increase of output power,stimulated Raman scattering（SRS）effect and transverse mode instability（TMI）effect have become the main obstacles to the power improvement of single-mode fiber lasers.The use of large-mode field single-mode optical fibers is the fundamental way to solve the SRS and TMI effects,and is also the key to achieving higher power near-single-mode laser output.Therefore,this paper focuses on the study of large mode field ytterbium-doped fiber,aiming to suppress the SRS and TMI effects to the greatest extent through the design of the fiber’s axial structure and radial structure.This paper firstly introduces the preparation of Yb-doped fiber by modified chemical vapor deposition（MCVD）method and drawing technique,and the basic parameters and testing methods of Yb-doped fiber are described,which provides the basis for the preliminary design and preparation of optical fiber.Theoretical studies on SRS and TMI effects in high-power fiber lasers are carried out.Firstly,the formation mechanism of SRS is expounded,the theoretical model of SRS in fiber amplifier is deduced,and the influence of fiber parameters and laser parameters on SRS threshold is studied based on the theoretical model.Then,the formation mechanism of TMI is expounded,the theoretical model of TMI in fiber amplifier is deduced,and the influence of fiber parameters and laser parameters on TMI threshold is simulated and analyzed.The results show that the suppression methods of SRS and TMI effects are largely difficult to consider simultaneously in conventional fiber.The research on the axial structure design of optical fiber is carried out.Firstly,based on the structure of traditional graded cladding tapered fiber,a constant cladding tapered fiber is proposed,and the SRS and TMI effects in graded cladding tapered fiber,constant cladding tapered fiber and uniform fiber are studied by theoretical models.The simulation results show that the two types of tapered fibers have inhibitory effects on SRS compared with uniform fiber under similar comparison conditions,and the inhibition effect of the constant cladding tapered fiber is more significant.Regarding the suppression of the TMI effect,the TMI thresholds of the two tapered fibers are better than that of the uniform fiber in most cases.Next,these two types of tapered fibers are prepared and characterized based on a high-power fiber laser system.For the graded cladding tapered fiber,combining the tapered fiber with low numerical aperture（NA）and partial doping design,a25/400-37.5/600-25/400 spindle-shaped fiber with core NA～0.05 and Yb³⁺ion doping diameter ratio of 77%was successfully prepared.Based on this fiber,a high beam quality laser output with an output power of 4.1 k W was realized.For the constant cladding tapered fiber,the 31.2/400-52.5/400 tapered fiber was prepared first.Based on this fiber,the experimental comparison of different signal transmission directions was carried out.The results show that when the signal light is transmitted from the big end to the small end,the quality optimization effect of the output beam is very obvious.Then,a 24/400-31/400tapered fiber was prepared,and a laser output of 2.7 k W was achieved based on this fiber.Finally,the laser performances of 20/400-30/600 graded cladding tapered fiber and20/500-30/500 constant cladding tapered fiber with the same equivalent core diameter under large-bend coiling condition were experimentally compared.The results show that the graded cladding tapered fiber has a higher TMI threshold,but further optimization of the constant cladding tapered fiber is still expected to achieve greater power output.In order to further suppress the SRS and TMI effects,the radial structure design of the optical fiber was studied.The design of the M-type refractive index profile is the simplest and most effective method to suppress the SRS and TMI effects simultaneously.The structural characteristics of the M-type fiber were explored by the finite element method,and then a 25/400 M-type Yb-doped fiber was prepared in line with the theoretical design.A high-power fiber amplifier was built to test the M-type fiber,and compared it with the common commercial 30/400 fiber.The results show that the M-type fiber has a higher TMI threshold and achieves a laser output of 2285 W,which is the highest output power achieved based on the M-type fiber so far.