A High-precision Tunable Thulium-doped Fiber Laser In The 2μm Band Based On Mathematical Micromirror Devices

Space laser communicamion has the advantages of large transmission capacity,anti-electromagnetic interference and strong mobility.It is an important means of spatial bandwidth information transmission in the future.It not only has great application potential in spaceflight test control,space test,navigation and positioning,aviation transmission and other aspects,but also can be used in broadband access,emergency communications,optical fiber backup,military classified communications,regional environment,cost constraints and other communication fields.The 2μm-band is the safe band of human eyes and the low loss atmospheric light transmission window.At present,semiconductor lasers in this band are not mature,while thulium-doped fiber lasers operate in 2μm-band.Generally,the line width of lasers determines the modulation rate of signals,and the fluctuation of the output power and spectrum,leading to the accumulation of amplitude noise and the deterioration of bit error rate at the receiving end.Therefore,to persuit the high stability,narrow line-width and multi-wavelength tunable ability has become the research hotspots to broaden the current application field of optical fiber lasers.Therefore,this thesis presents a high-precision tunable Tm3+-doped fiber laser in 2μm-band based on digital microirror device(DMD).The main research work is as follows:The physical model of two-dimension DMD grating is established.The optical diffraction characteristics of different DMD types available are analyzed in 2μm-band,and the necessary conditions for the near-blazed diffraction and the return of the diffraction beam along the original path are determined.The results show the suitable type for the tunable fiber laser in 2μm-band is the 0.7-inch DMD with13.68 μm-pixel and the inclination angle of the micro mirror±12°.Based on the energy level of thulium ion,the laser generation under 3H6→3H4 pumping is analyzed.According to the rate equation and transmission equation theory of Tm3+-doped fiber laser,the Tm3+-doped double-clad fiber laser is modeled and simulated by Matlab software.The influence of different parameters on the laser output is analyzed,and the device parameters is determined in this experiment.The optimized length of Tm3+-doped fiber is 4.5 m.On this basis,we fabricate the Tm3+-doped fiber amplifier and analyze the effect of discharge intensity,duration and times on the fusion quality of double-clad Tm3+-doped fiber.The spontaneous emission spectrum of the amplifier cover 230 nm bandwidth around the 2μm-band.Based on DMD chip,a tunable Tm3+-doped fiber laser with 2μm-band is designed and optimized by Zemax software.The experimental results show that the laser can be tuned from 1930 nm-2000 nm.The tuning range is 70 nm and the tuning accuracy is less than 0.1 nm,The 3dB-linewidth of the output laser is less than 0.05 nm(limited by the spectrometer resolution of 0.05 nm),and the side-mode suppression ratio is greater than 40 dB.At room temperature,the maximum power fluctuation of the laser within 1 hour is 0.1 dB,and the maximum wavelength drift is less than 0.05 nm.This research work can be used in future large-capacity space laser optical communication systems.