Study Of Acousto-optic Q-switched Pr:YLF Double-pulse Narrow-linewidth Laser At 639 Nm

All-solid-state double-pulse narrow-linewidth lasers in visible land have the characteristics of outputting double pulses,narrow spectral line width,high peak power,and long coherence length,so they can be widely employed in many fields,such as atmospheric environment monitoring,precision parts machining and fine measurement.In this paper,Pr:YLF crystal is used as the gain medium,which can realize the spectral characteristics of direct lasing visible light without frequency doubling,avoiding the spatial deviation and thermal lens effect of frequency doubling crystal.And the cavity structure is compact,which effectively improves the optical-optical conversion efficiency of the system.On this basis,the stable double-pulse narrow-linewidth laser output at 639 nm can be realized by combing the two-step acousto-optic Q-switching technology and double F-P etalon group linewidth compression technology.In the aspect of theoretical research,the theoretical model of Pr:YLF CW laser is established at first,and the optimal transmittance of output mirror and output power are simulated.In this foundation,by introducing the boundary conditions for step loss of acousto-optic Q-switch and the micro-loss term between adjacent modes caused by the F-P etalon,the physical model of the double-pulse narrow-linewidth laser based on the two-step acousto-optic Q-switching and the double F-P etalon group is established to effectively analyze the change law of the number of reversed particles and photons during the operation of the laser.The logical relationship among the parameters in the resonant cavity is optimized,and the optimal values of the key parameters are determined.The above process provides the theoretical basis for subsequent experiments.Finally,the thicknesses of the double F-P etalon group used for the experiments are determined to be 1.0 mm and 0.5 mm,with the corresponding optimum angles of 0.67°and 1.35°.In the aspect of experimental research,the 639 nm continuous laser output experiment of Pr:YLF laser is firstly carried out.When the absorbed power is 7.44 W,the maximum output power is 2.694 W,and the spectral line width is 0.51 nm.Then,the acousto-optic Q-switched crystal is placed in the resonant cavity to conduct the 639 nm pulsed laser output experiment of Pr:YLF laser.Under the condition of repetition frequency of 10 k Hz,the single-pulse laser output with pulse energy of 14.61μJ and pulse width of 53.4 ns was obtained when the absorbed power was 5.51 W.Keeping the repetition frequency constant and setting the Q-switched duration ₁t=40μs and t₂=40μs,the equal-amplitude double-pulse laser output with master-slave pulse interval of 40μs,pulse energy of 13.01μJ and pulse width of 76.2 ns and 95.2 ns in a single Q-switched period is obtained.Meanwhile,it is proved that the pulse interval of double-pulse laser output is adjustable,and the equal-amplitude double-pulse output with pulse interval of 30μs and 20μs is obtained,respectively.Finally,on this basis,the double F-P etalon group is inserted into the resonator,and the double-pulse narrow-linewidth laser output with master-slave pulse interval of 40μs,pulse energy of 7.8μJ and pulse width of 110.3 ns and 140.2 ns is obtained.It is verified by F-P interferometer that when the pulse energy is 2.3μJ,the laser operates in a single longitudinal mode with the spectral line width of 37.5 MHz.