Research On Key Devices Of Linear Cavity High-order Transverse Mode Fiber Laser

At present,most of the research on fiber lasers that can realize high-order vector mode output is based on single-mode fiber lasers,which indirectly realize high-order mode output through fundamental mode conversion.However,fiber lasers based on all-few-mode devices still lack detailed research reports.In this paper,theoretical and experimental studies on key devices in linear cavity high-order transverse mode fiber lasers are carried out.The main structure of the fiber laser is composed of photosensitive few-mode erbium-doped fiber,vortex fiber and a pair of fiber gratings,wherein the pair of fiber gratings serve as vector mode selectors to undertake the screening of specific vector modes.The main research contents are as follows:（1）The modes in photosensitive few-mode erbium-doped fiber and double-ring vortex fiber are studied.The simulation results show that the self-made photosensitive few-mode erbium-doped fiber supports the transmission of LP₀₁ and LP₁₁ modes.The effective refractive index difference between the second-order vector modes in the self-made double-ring vortex fiber can reach the order of 1×10^-4,so that the degenerate separation of the second-order vector modes can be realized.The degree of separation of the vector modes in the double-ring vortex fiber is affected by the thickness of the low-index ring in the middle of the fiber,the thickness and refractive index of the outer high-index ring,and the outer diameter of the fiber.（2）The spectral properties of photosensitive few-mode erbium-doped fiber gratings and vortex gratings are studied.The resonance wavelength of each coupled mode and the spectrum of the gratings under different excitation modes are simulated and calculated respectively,and the spectrum of the self-made gratings is measured through experiments.By comparing the simulation results with the experimental measurement results,the coupling mode types at each resonance wavelength of the self-made photosensitive few-mode erbium-doped fiber gratings and the self-made vortex fiber gratings are determined.（3）An experimental device was built,and the experimental research of the linear cavity high-order transverse mode fiber laser was carried out using key devices such as photosensitive few-mode erbium-doped fiber,vortex fiber,photosensitive few-mode erbium-doped fiber grating,and vortex fiber grating.According to the matching conditions of the resonance wavelength,the grating pair composed of photosensitive few-mode erbium-doped fiber grating and vortex fiber grating can be used for vector mode screening as a vector mode selector.By analyzing the experimental results,the conditions that the grating must satisfy to realize the second-order vector mode selection function are analyzed,and the optimization of the vector mode selector is completed.