Research On LD End Pumped Double-pass Gain Tm:YAG Q-switched Laser

Diode Laser(LD)end-pumped Tm-doped laser provide excellent atmospheric permeability and safety to the human eye near output wavelength of 2μm,so the output2μm laser can be used in many fields such as medical treatment.In view of the defects of Tm:YAG crystal,such as small emission cross section and low gain,a double-pass gain mode is proposed to make the oscillating beam pass through the laser gain medium four times in a round-trip cycle in the resonant cavity,so as to improve the output performance of 2μm laser.The main contents are:Theoretically,based on the Tm:YAG continuous and Q-switched rate equation model of LD end pumped,the double-pass gain Tm:YAG continuous and Q-switched rate equation of LD end pumped are established.Compared with the conventional Tm:YAG CW laser,the output power of the double-pass gain Tm:YAG CW laser is higher;compared with the conventional Tm:YAG Q-switched laser,the output energy of the double-pass gain Tm:YAG Q-switched laser is larger and the output pulse width is narrower.It is proved theoretically that the double-pass gain technique is feasible to improve the output performance of Tm:YAG Q-switched laser.Based on the distribution model of optical and thermal fields in Tm:YAG crystal,the distribution of optical and thermal fields in the crystal is simulated.Using MATLAB software to simulate thermal focus of Tm:YAG crystal.The thermal lens focal length of Tm:YAG crystal is 67mm when the pump power is30W and the pump spot radius is 0.4mm.On this basis,a laser resonator with good optical and thermal stability is designed.The theoretical results lay a foundation for the subsequent experiments and the analysis of experimental phenomena.In the experiment,the LD end pumped Tm:YAG CW laser was studied.When the cavity length is 40mm,the laser oscillation threshold power is 4.5W,and the maximum output power is 4.2W.When the injection power is 13.23W,the slope efficiency is 48.1%.In order to realize the output of Q-switched pulsed laser and compare with the results of subsequent double-pass gain experiments,the cavity length is lengthened to 220mm.When the laser operates freely,the oscillation threshold power is 5.45W,the output power is0.643W at 10.02W pump power,and the slope efficiency is reduced to 14.07%.When the Q-switched repetition frequency is 50Hz and the pump power is 9.80W,the single pulse energy is 5.82m J and the pulse width is 130ns;When the repetition frequency is 5k Hz and the pump power is 11.11W,the single pulse energy is 113.2μJ and the pulse width is789.5ns.Thirdly,LD end pumped double-pass gain Tm:YAG CW laser is studied.The oscillation threshold power of the laser is 5.45W,the highest output power of 0.656W is obtained when the injection power is 8.94W,and the slope efficiency is 18.80%.Finally,the LD end pumped double-pass gain Tm:YAG acousto-optic Q-switched laser is experimentally studied.When the Q-switched repetition frequency is 50Hz and the pump power is 10.56W,the laser output with single pulse energy of 8.32m J and pulse width of119.3ns is obtained.The slope efficiency is 14.54%,the central wavelength is 2014.10nm,and the beam quality M~2 is 1.2.When the Q-switched repetition frequency is 5k Hz and the pump power is 11.11W,the laser output with single pulse energy of 124.44μJ and pulse width of 685.2ns is obtained.The slope efficiency is 18.27%,the central wavelength is2014.10nm,and the beam quality M~2 is 1.22.The experimental results show that the output power and slope efficiency of the laser can be effectively improved by using the double-pass gain technique,which provides a new idea for realizing high-power laser output in small gain laser medium.