Study On Characteristics Of Yb:YAG Crystal Waveguide Laser With Large Core Size

The rectangular waveguide laser with all crystal materials prepared by diffusion thermal bonding is one of the main development directions of laser in the future,and is a effective way to realize the small volume,high brightness and compact laser.This paper mainly focuses on the research of the rectangular crystal waveguide laser with diffusion thermal bonding,including the design of crystal waveguide structure,the preparation process and the experiment of laser output characteristics.The specific work includes:(1)In the design of crystal waveguide structure,the core layer is expanded to332?m by using the method of refractive index matching and mode competition,and good beam quality output is guaranteed.Among them,on the basis of the research on the measurement method of micro refractive index difference,by adopting refractive index matching,i.e.selecting 1at.%Yb:YAG as the core material and 0.5at.%Er:YAG as the core material,the refractive index difference can be reduced to 4×10^-6,and the existing single-mode core layer of 40-60?m crystal rectangular waveguide can be expanded to 185?m,which is 5 times that of conventional YAG as the inner cladding design.Considering that the crystal waveguide is an active device,a mode competition method is proposed and introduced to design the core layer size of crystal waveguide.The core layer size of basic mode output is expanded to 332?m,which is1.79 times of the refractive index matching design and 8 times of the conventional design(40-60?m).The above design is calculated and analyzed by analytical method and simulation method,and the results are consistent.The upper limit of inner cladding thickness is determined to be less than 190?m according to the principle of absorption efficiency>95%.According to the condition of single mode,the lower limit of inner cladding is larger than 73?m.There is no specific requirement for the design of the thickness of the cladding,but it is necessary to ensure sufficient mechanical strength and small thermal stress.(2)In the preparation of crystal waveguides,based on the theory of gap closure of bonding surface,we give the bonding condition of laser crystal,i.e.?-R curve,and deduce the general conditions that large surface can be bonded:the requirements of large surface bonding are higher,and the basic requirements of roughness less than0.5nm,surface errorless than?/20,and peak radius 0.16-0.18mm are determined.Through continuous exploration,a set of complete preparation technology from pretreatment,grinding and polishing,cleaning,photoresist,heat treatment and post-treatment has been developed.The dynamic detection method is used to detect all kinds of problems in the process in time to ensure the accuracy and quality of each process.Finally,Yb:YAG/Er:YAG single clad crystal waveguide with large core size was fabricated by four times of bonding,and Yb:YAG/Er:YAG/spinel double clad crystal waveguide was fabricated by eight times of bonding,realizing the same and different large surface crystal bonding.(3)The laser output characteristics of crystal waveguide are mainly verified by experiments.The core material is 1at.%Yb:YAG,the section size is 320?m×400?m,the inner cladding material is 0.5at.%Er:YAG,the section size is 7mm×30mm,and the crystal waveguide length is 77mm.The output power is 26W and the optical to optical conversion is 48.5%when the pump power is 102.8W and the absorption pump power is 53.6W.The measured beam quality,M²=1.22×1.05,is neardiffraction-limit.The experimental results are consistent with the theoretical design.When the pump power is 58W,the absorption pump power is 29W,the output energy is 0.36m J@20k Hz,and the pulse width is 23ns.When the power is 7.2W,the measured beam quality is M²=1.12×1.06.The pulse laser output with high peak power and brightness is realized by electro-optic active Q-switch.When the pump power is 122W,the absorption pump power is 60W,the output pulse energy is1.3m J@10k Hz,o-o(optical to optical efficiency)is 22%,the pulse width is 10ns.In the process of increasing the pump power,the phenomenon of power saturation is found.Through analysis and comparison,the reason of parasitic oscillation consumption in the cladding is deduced.A layer of infrared absorbing material Ge is sputtered on the surface of crystal waveguide to reduce the parasitic oscillation in the cladding.When the pump power is 68W,the pulse energy is 1.75m J@10k Hz,and the corresponding optical efficiency is 26%.The measured pulse width and beam quality are basically the same.The effect of parasitic oscillation is suppressed by sputtering Germanium on the the crystal waveguide.The peak power is 175k W,the brightness is1304MW/cm²·sr,and the power density is 54.8MW/cm²,which is higher than that of common fiber and solid-state lasers.