Researches On Femtosecond Thulium-doped Fiber Laser Based On Large-mode-area Fiber

2 ?m ultrafast pulse with high peak power and high pulse energy has a wide range of applications in the fields of atmospheric detection,biomedical,different frequency generation(DFG),military radar,and so on.Therefore,it is of great significance to study the generation of 2 ?m high-energy ultrashort pulses.The utilization of large-mode-area fiber in fiber lasers can greatly increase the fiber capacity of the pulse energy and peak power,which is beneficial for generating high-energy ultrafast pulses.This article has done the following researches based on these reasons:1.The research progress of 2 ?m high-energy thulium-doped fiber laser is introduced.The research is mainly investigated from two aspects: obtaining high-energy ultrashort pulse from amplifier and directly outputting high-energy ultrashort pulse from oscillator.Then the development of large-mode-area fibers utilized in 1 ?m,1.5 ?m and 2 ?m regime are investigated.And the current technical bottlenecks and directions for further researches are proposed.2.The principle of generating pulse when the laser's different oscillation modes phase locked is reviewed,followed by the main methods of passively mode locking,the working mechanisms of physical saturable absorber and virtual saturable absorber.3.The structure and fusion of large-mode-area fibers is discussed;then the output modes of large-mode-area fibers according to the theory of optical waveguides are calculated,and the 2D and 3D light field distributions of each mode through MATLAB are obtained;next the bending loss of each mode and the single-mode transmission method of large-mode-area fiber are studied,and the conclusions are that the maximum bending radius of single-mode transmission is 35 mm for LMA-TDF-25/250 andthe maximum bending radius of single-mode transmission is 24 mm for LMA-EYDF-25/300.4.The formation of high-order solitons and the principle of obtaining ultrafast pulses using the evolution of high-order solitons are demonstrated.Based on the principle of high-order soliton self-compression,a thulium-doped fiber laser with high peak power and narrow pulse width has been built.Firstly,a single-mode fiber oscillator has been set up based on carbon nanotube mode-locking.The output pulse is in stable mode-locked state with a central wavelength of 1.96 ?m.After nonlinear amplification to generate high-order solitons,the pulse duration is shortened 4 times and the pulse energy is increased 40 times.The final output pulse width is 196 fs,and the pulse energy is 5.6 n J.5.Based on the NPR technique,a large mode-area fiber soliton mode-locked laser has been constructed.When the pump power is increased to 7 W,the output pulse is stable with fundamental repetition frequency of 53.7 MHz,pulse energy of 2.9 n J,and the pulse duration of 715 fs.Continually to increase the pump power,the laser can still operate at stable modelocked state.The pulse energy is up to 4.5 n J,which greatly improves the energy of the soliton pulse output from oscillator.Subsequently,a single-mode fiber laser has been built on the basis of the large mode-area fiber laser system,and the output performance of the laser has been characterized to compared with the large-mode-area fiber laser.It proves the excellent performance of the large-mode-area fiber in generating high-energy ultrashort pulses.