Experimental Research Of High Energy Short Pulse Fiber Laser Based On Novel Saturable Absorber

Recently,fiber lasers based on doping rare earth ions have played an increasingly important role in a number of applications,such as communications,medical,industrial processing,spectral sensing and so on.Fiber lasers have the advantages of good beam quality,high light-to-light conversion efficiency,compact structure,long-term stability,and small external interference.The passively Q-switched or mode-locked technology using the saturable absorbers is one of the important ways to obtain pulsed laser in the field of fiber lasers.This thesis involes the experimental study of the generation of high-energy short-pulsed laser beams in Yb-doped fiber lasers based on the novel saturable absorbers.We have built the nonlinear polarization evolution?NPE?passively mode-locked ytterbium doped fiber laser with space structure,which is used as the seed source for the all-fiber double-arm detection system to measure the saturation absorption capacity of the saturable absorbers.We inserted the novel saturable absorber?tungsten disulfide,bismuth telluride,black phosphorus?with saturated absorption ability into the all-fiber Yb-doped oscillator to obtain the high-energy stable passively Q-switched operation,respectively.The main research contents are divided into the following three parts:1.We have built the NPE passively mode-locked ytterbium-doped fiber laser?YDFL?based on space structure.With the increase of pump power and careful adjustment of three the wave plates and the birefringent filter in the cavity,the stable mode-locked pulse is obtained with the repetition rate of 49.60 MHz,the shortest pulse width of 4.1 ps,and the maximum output power of 151 mW.At the pump power of 580 mW,the second harmonic mode-locked pulse is obtained with the repetition rate of 99.13 MHz,which is about twice the fundamental repetition rate.After a week of observation,the laser has been working in the state of mode-locking,indicating its good stability.We used this laser as the seed source for the double-arm detection system to measure the nonlinear saturation absorption of the novel saturable sbsorbers.2.The tungsten disulfide?WS₂?and the bismuth telluride?Bi₂Te₃?wallpapers prepared by the vertical evaporation method have obtained the stable high-energy Q-switched operation in the all-fiber ytterbium-doped oscillating cavity.The nonlinear saturation absorption characteristics of WS₂ and Bi₂Te₃ wallpapers are as follows:Modulation depth?MD?of the WS₂ is evaluated to be 10.17%,the saturating intensity is about 8.89 MW/cm²,and the non-saturable loss is about 3.41%.The Bi₂Te₃ has the MD of 23.04%,the saturable intensity of 22.55 MW/cm² and the unsaturated loss of 48.39%.?1?The experimental results of the high energy Yb-doped Q-switched fiber lasers based on WS₂ thin film are as follows:When pump power increases from 50 mW to 94 mW,the repetition rate varies from 36.78 kHz to 81.75 kHz,the single pulse width turns from 3.61?s to 1.0?s and the maximum single pulse energy is 56.50 nJ.?2?The experimental results of the high energy Yb-doped Q-switched fiber lasers based on Bi₂Te₃ thin films are as follows:In the pump power range from 60 mW to 116 mW,the minimum single pulse width is 1.07?s,the maximum single pulse energy is 61.80 nJ,and the repetition rate tuning range is 53.31 kHz to 106 kHz.At the pump power of 92 mW,the Bi₂Te₃ thin film achieves dual-wavelength passively Q-switched at 1026.52 nm and 1029.46 nm by fine tuning the polarization controller.Therefore,Bi₂Te₃ thin films are superior to WS₂ thin films in Yb-doped passively Q-switched fiber lasers.3.The black phosphorus saturable absorber?BP-SA?prepared by mechanical exfoliation method and liquid phase stripping method achieved the stable passively Q-switched operation in all-fiber Yb-doped fiber lasers.?1?We first built passively Q-switched Yb-doped fiber laser based on the mechanically exfoliated BP-SA.The shortest pulse width of the Q-switched pulse is 1.9?s,the maximum single-pulse energy is 24.36 nJ,and the repetition rate varies from 52.54 kHz to 58.79 kHz.?2?We designed the passively Q-switched Yb-doped fiber laser based on BP solution with micro-fiber.The pump power range of the Q-switched pulse is 45 mW to 165 mW,corresponding to the range of the repetition rate from 48.22 kHz to 96.79 kHz,the shortest single pulse duration of 950 ns,the maximum output power of 9.9 mW and the maximum single pulse energy of 106.69 nJ.Therefore,compared with the results of the two lasers,it can be seen that the interaction between the liquid-phase stripping BP solution and the evanescent field generated by the micro-fiber makes it easier to achieve high-energy Q-switched operation.