Research On Mode-locked Thulium-Doped Fiber Laser Based On Nonlinear Fiber Loop Mirror

Fiber lasers have the advantages of relatively simple structure,high conversion efficiency,good heat dissipation characteristics,and high stability.The laser in the 2 μm band is in the band of human eye safety,atmospheric window,strong absorption of water molecules and polymer materials,etc,and has important applications in air pollution detection,wind speed measurement,military confrontation,laser medical treatment,laser processing and other fields.In addition,the laser in the 2 μm band can be used as the pump source of the mid-infrared band laser,and the wavelength can be extended to the mid-infrared laser in the 3-5 μm band through technologies such as optical parametric oscillation.Through mode locking,fiber lasers can have higher peak power and higher efficiency,making the laser application in the 2 μm band more extensive.In the thulium-doped fiber laser,the laser beam can be modulated by passive mode-locking technology to achieve ultrashort pulse output from the fiber laser.However,fiber lasers that use nonlinear fiber loop mirrors for mode-locking are more compact,relatively inexpensive,more stable in operation,and can generate high-quality ultrashort pulses.Therefore,this paper has carried out theoretical and experimental research on it.The main research contents are summarized as follows:(1)Based on the nonlinear Schr?dinger equation,a mathematical model of the fiber laser based on the nonlinear fiber loop mirror mode-locking is established,and the simulation is carried out.Finally,a stable pulsed laser output is obtained.The effects of different lengths of gain fibers,different output coupling ratios,different lengths of NOLM loops and other parameters on the laser output results were explored.Based on the simulation results,it can guide the construction of lasers with higher power and better performance.(2)An all polarization-maintaining fiber laser based on nonlinear fiber ring mirror mode-locking is designed and constructed.The total cavity length of the laser is 33.1 m,and the laser works in the negative dispersion region.The output of a noise-like pulsed laser with a maximum pulse width of 1.82 ns,a maximum average power of 29.6 m W and a repetition rate of 6.21 MHz was obtained by using a 16.5 cm long gain fiber.At this time,the signal-to-noise ratio of the laser is as high as 64.5 d B,and it has very high stability.In addition,the length of the gain fiber and the length of the NOLM loop are optimized,so that the laser can obtain higher power output without basically changing other structures of the laser.When the total cavity length of the laser is 34.7 m,the laser obtains a maximum pulse width of 3.2 ns and a maximum average power of 74.6 m W noise-like pulse output under the length of a 9.5 cm short gain fiber.At this time,the power stability of the laser is extremely high.(3)The parameters of different dispersion compensating fibers are compared,and PM2000 D fiber is selected to perform dispersion compensation on the fiber laser,so that the laser works in the positive dispersion region.At this time,the cavity length of the laser becomes 22.52 m,and the laser obtains a noise-like pulsed laser output with a maximum average power of 20.5 m W when the length of the gain fiber is still 16.5 cm,and the pulse width is reduced to 566 ps.The power stability of the laser is also high.