| Single longitudinal mode laser in visible band has great application potential in spectroscopy and medicine because of its narrow output spectral line width,low phase noise and long coherent length.The common realization method of single longitudinal mode laser in visible band is based on the nonlinear frequency transformation of infrared band,combined with methods such as volume Bragg grating,torsional cavity mode,F-P etalons,prelase Q-switch,etc,to realize the operation of a single longitudinal mode.However when using volume Bragg grating cavity loss due to the existence of the diffraction effect is higher,and the resonator structure of twisted-mode cavity method is complex,the requirement of F-P etalon is very strict,and the excessive superposition of F-P etalon will cause greater loss in the cavity,and the mode selection capability of prelase Q switching technology is limited and the mode is very unstable under high pump injection.In order to obtain stable and efficient single longitudinal mode output in visible light band,this paper proposes a composite mode selection technology which uses Pr:YLF as gain medium and combines the prelase Q-switching mode selection technology with the line width compression technology of F-P etalon.The mode selection scheme is dominated by prelase Q-switching and supplemented by a single F-P etalon,which not only reduces the manufacturing requirements of F-P etalon components,but also improves the mode selection capability under high pump injection.It solves the problems of complex structure,low output efficiency and poor output line width narrowing effect of single longitudinal mode Q-switched pulse laser in visible band fundamentally.The main research contents and achievements of this paper are as follows:In terms of theoretical research:(1)The physical model of Pr:YLF single longitudinal mode laser based on prelase Q-switch and F-P etalon composite mode selection technology is established.In order to understand the operation mechanism of single longitudinal mode by the composite mode selection technology more clearly,the fourth-order Runge-Kutta method is used to solve the differential equations,and the variation law of the number of upper level inversion particles in the cavity and the number of photons in each mode is obtained.(2)The longitudinal model evolution model of mode competition process and mode competition intensity among longitudinal modes under the operation mechanism of composite mode selection was established.The computation of the theoretical model is carried out to obtain the computation of the number of longitudinal modes and the power ratio between adjacent modes over time,and to describe the intensity of mode competition at different values of modes,and the distribution of the number of modes at different times.The intensity of mode competition at different values of modes and the distribution of the number of modes at different times are described from a side.(3)A single longitudinal mode Q-switched pulse laser output performance optimization model was established with the maximum single pulse energy as the objective function.According to the evaluation and optimization theory,the boundary conditions and the logical relations among the key parameters in the operation process of the composite die selection were determined,and the key parameters of the selection components were optimized when the prelase Q-switched and F-P etalons were combined.By solving the model,the effects of different evaluation efficiency,different energy injection conditions and resonator parameters on the output characteristics of Pr:YLF laser,such as peak power,pulse width and single pulse energy,are obtained.In terms of experimental research:(1)Experiments of Pr:YLF continuous laser output and Q-switched pulse laser output at 639 nm band were carried out to analyze the output characteristics of continuous and high repetition pulse light.In the experimental study of 639 nm output of Pr:YLF laser under continuous condition,when the maximum absorption power of the crystal is 2.8 W and the length of the resonator is 85 mm,the measured continuous output power of 639 nm red is 823 m W.In the experimental study of Pr:YLF laser under pulse condition,the peak power and single pulse energy and pulse width were measured at the repetition frequency from 100 Hz to 20 k Hz.The single pulse energy was measured at the high repetition frequency of 20 k Hz and the maximum absorption power of the crystal was 2.8 W,the pulse widths were 7.12 μJ and 78.4 ns,respectively.(2)Prelase Q-switched and Q-switched pulse laser experiments based on F-P etalons were carried out respectively,and the compression limits of output linewidth were measured to be 228 GHz and 151 GHz,respectively,for the purpose of minimizing the output linewidth.It is found that both of them can compress the output linewidth of the laser,but can not realize the operation of a single longitudinal mode.(3)On the basis of(1)and(2),the composite mode selection Q-switching single longitudinal mode pulse laser experiment was carried out,and the output line width and output characteristics were studied respectively.The single longitudinal mode Q-switched pulse laser output based on composite mode selection technology was realized by using the linkage mode of pre-lase Q-switched and F-P etalon.The single pulse energy,pulse width and line width of single longitudinal mode were measured at different repetition rates.When the high repetition frequency is 20 k Hz,the measured maximum single pulse energy of single longitudinal mode is 3.85 μJ,the pulse width is 105 ns,the output line width near the threshold value is 26 MHz,and the output line width is 36 MHz under the maximum output condition,which verifies the effectiveness of the prelase Q-switching technology and the composite mode selection technology of F-P etalon. |
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