| The idea that the Q switch can act as a mode-selector had been proposed in the 1960s,and some related theoretical analyses and experimental studies had been presented thereafter.The passively Q-switched laser has a simple structure and a wide range of applications.The mode selection effect of passive Q-switching also has aroused the interests of many researchers.In this dissertation,through the combination of theoretical analysis and experimental verification,the influence of laser parameters such as saturable absorber initial transmittance,output mirror reflectivity and laser cavity length on the formation of single-longitudinal mode oscillation in passively Q-switched lasers is studied.The specific laser parameters for realizing the single-longitudinal mode oscillation are clarified and a Cr:YAG passively Q-switched Nd:YAG laser system is built for experimental verification.The research results have important theoretical significance and application value.Theoretically,the stimulated emission cross-section ratio of adjacent longitudinal modes in the cavity is estimated by replacing the Lorentzian profile of the Nd:YAG gain medium with a simplified "triangular model".Taking into account the gain difference between adjacent longitudinal modes,the relationship between the intensity ratio of the adjacent longitudinal modes and the number of round trips of the fluorescence before the establishment of the longitudinal mode pulse is given.Then,the establishment process of the adjacent longitudinal mode pulse is numerically solved based on the laser rate equations.Finally,the effects of the initial transmittance of the saturable absorber,the reflectance of the output mirror,and the cavity length of the laser on the single longitudinal mode oscillation is analyzed qualitatively and quantitatively.The results show that in order to obtain single-longitudinal mode laser output under non-microcavity condition,there is an optimal combination of the initial transmittance of the saturable absorber,the reflectivity of the output mirror,and the length of the laser cavity.Secondly,lower reflectance of the output mirror is more conducive to achieving a single longitudinal mode.Thirdly,for a given output mirror reflectivity and laser cavity length,there is an optimal initial transmittance value of the saturable absorber for the best output characteristics of single longitudinal mode,and the optimal value increases with the decrease of the reflectivity of the output mirror.Fourthly,a shorter laser cavity length is more favorable for achieving single longitudinal mode.According to the theoretical simulation results,a Cr:YAG passively Q-switched Nd:YAG flat-concave-cavity laser is built with an optical cavity length of 65mm.When the initial transmittance of Cr:YAG is 0.7 and the reflectivity of the output mirror is 0.6,the threshold pumping current of the laser is 16 A.When the pumping current is 17.5 A,a single-longitudinal mode pulse laser output is obtained with an output power of 490 mW.With the increase of the pumping current,the number of oscillating longitudinal modes gradually increases,but overall it is in the state of 2 to 5 longitudinal modes and the intensity difference between the primary and secondary modes is large.The frequency beat of the spectrum analyzer showed that the laser output was operated in unstable single-longitudinal mode state,and the unstable single-longitudinal mode output power at the pump current of 20A is 972 mW.The effects of laser cavity length and output mirror reflectance on the output mode are observed experimentally,and the experimental results are consistent with the theoretical simulation before.The experimental results generally verify the correctness of the theoretical model.The results of the study are theoretically valuable for the use of saturable absorbers as passive Q-switches to achieve single-longitudinal mode laser output,and also meaningful for the implementation of single-longitudinal mode oscillations with other Q-switch realization. |
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