| Fiber laser has the advantages of good beam quality,simple structure,strong heat dissipation,energy concentration and easy implementation,and is widely used in laser medicine,material processing,laser radar,optical communication and other fields.Among them,the thulium-doped fiber laser working in the 2μm band is relatively safe for the human eye,which has received widespread attention and has been important applications in the fields of medical and space remote sensing.Nowadays,the power of fiber laser has been greatly improved.However,with the continuous increase of power,nonlinear effects will appear in fiber,which will affect the further improvement of fiber output power.Studies have shown that large-mode-field fiber can reduce the optical power density in the fiber core and reduce the nonlinear effect,Therefore,a new type of thulium-doped large mode area fiber with heterogeneous helical cladding（HHC）is proposed in this paper.The parameters of the fiber structure were optimized by COMSOL Multiphysics software;The laser characteristics of the fiber were analyzed by Matlab;The same type of optical fiber was prepared,and the optical properties of the optical fiber glass material and the optical fiber were tested experimentally.Firstly,the helical coordinate transformation theory was used to convert the three-dimensional simulation model of optical fiber into two-dimensional model in this paper.Based on the finite element method,the simulation research of optical fiber were carried out by using COMSOL Multiphysics software.The transmission characteristics of the optical fiber are analyzed,and the structural parameters are optimized to realize single-mode transmission.The core diameter of single mode transmission can reach 66μm,and the mode field area can reach 2360μm2.The structural parameters of optical fiber bending are optimized,the minimum bending radius of optical fiber single-mode transmission is 33cm.LP01mode loss is 0.1 d B/m,LP11mode loss is 10.08d B/m,which meets the industry recognition level.The slit width is reduced from 14μm to 9μm than non-bending optical fiber,and the pitch of thread is increased from 22mm to 25mm than non-bending fiber.The large-mode field fiber proposed in this paper has a simple structure and is all-solid-state,and the parameters can be coordinated with each other,which reduces the preparation difficulty compared with most of the large-mode field fibers with complex structures,and the mode resolution ability is strong enough,which is expected to be well used in high-power fiber lasers.Secondly,in order to better apply the fiber to the laser field,the pump threshold power characteristics and laser output characteristics of the fiber laser are analyzed based on the rate equation and transmission equation of Tm3+.The calculation results show that the pumping threshold power Pthof the fiber laser decreases with the increase of the reflectivity R2sof the output mirror.There are optimal Tm3+doping concentration N and fiber length L to minimize Pth,and the Pthis 1.997 W.There is an optimal fiber length to maximize the output power,and the laser slope efficiency is up to close to 36.5%.The calculation results provide theoretical support for the practical application of optical fiber in related fields.Finally,the same type of thulium-doped heterohelical clad large-mode field optical fiber was prepared.The experimental results show that the hydroxyl content of the thulium-doped silicate glass used in the preparation of optical fiber in this paper is 3.65*1019 cm-3,the fluorescence lifetime is 302μs,the four absorption peaks of the absorption spectrum are 680 nm,790 nm,1210nm and 1620 nm,and the peak wavelength of the emission spectrum is 1845 nm.The helical fiber was successfully drawn at a temperature of 830℃.The fiber transmission loss coefficient tested use the sniping method is 0.0832 d Bcm-1.The transmission characteristics of the optical fiber in the2μm band were tested,and the results were basically in line with expectations after removing various unfavorable factors. |
