Research On High-Repetition-Rate Passively Q-Switched Solid-State Laser

With its high-repetition-frequency,narrow-pulse-width,and high-peak-power output pulse,the passively Q-switched laser is regarded as one of the ideal light sources in the application fields of laser radar（LIDAR）,laser micro-machining,environmental monitoring,and laser communications.The passively Q-switched laser doesn’t need an external driving source.And it can achieve the modulation of the resonant cavity loss only through the Q-switched crystal in the resonant cavity.It has the advantages of simple structure and low cost,and can achieve light and miniaturization.However,the output pulse stability of the passively Q-switched laser is poor,and there are fluctuations in the pulse amplitude and the pulse repetition frequency.It greatly limits the wider application of the passively Q-switched laser.To address the problem of the poor pulse stability and to meet the application requirements of LIDAR and the other fields,this thesis conducts a study of the high-repetition-frequency passively Q-switched laser in the 1.06μm band and 1.5μm band based on the pulse control technology.Based on the rate equations of the passively Q-switched laser,we discussed and analyzed the effect of the Q-switch on the output pulse stability.And we completed the simulation of the pulse building process under the pre-pumping technology.The effects of four factors on the pulse stability of the passively Q-switched laser are summarized,such as the uncertainty of the Q-switch,the polarization orientation characteristics of the saturable absorber,the thermal effect of the laser crystal,and the mode matching between the pumping beam and the fundamental mode beam.The 1.06μm high-repetition-frequency passively Q-switched laser with Nd:YAG+Gr⁴⁺:YAG crystal configuration,combined with appropriate resonant cavity parameters,achieves the output pulse characteristics of high-repetition-frequency and narrow pulse width.According to the influence factors of the pulse stability summarized in Chapter 2,precision temperature control of the pump source and the resonant cavity is performed by TEC to reduce the effect of thermal effects on the pulse stability,and the effect of the polarization orientation characteristics of the saturable absorber on the pulse stability is reduced by using a suitably oriented Cr⁴⁺:YAG crystal setup.Through theoretical analysis and specific experiments,the influence of the mode matching between the pumping beam and the fundamental mode beam on the pulse stability of the passively Q-switched laser was investigated,and the optimal position of the coupling focusing system was determined.Under continuous pumping,the output of the 1064 nm passively Q-switched pulse with a repetition-frequency of 280 k Hz and a pulse time jitter of 200 ns was realized.To further reduce the uncertainty of the Q-switch,the pre-pumping technology was used.In the process of using the pre-pumping technology,the"method of determining the pre-pumping current parameters"is invented.The time jitter of the 280 k Hz repetition frequency pulse is reduced to 100 ns by using the pre-pumping power module.This is the lowest time jitter value reported in the results of 1064 nm passively Q-switched laser with 280 k Hz and above repetition frequency.The 1.5μm high-repetition-frequency passively Q-switched laser uses a crystal configuration of Er,Yb:YAB+Co²⁺:Mg Al₂O₄.Compared with the Nd:YAG crystal,the thermal effects of Er,Yb:YAB crystals are more pronounced.It becomes a major factor affecting the output pulse characteristics of the passively Q-switched laser.Therefore,we simulated the temperature field distribution of the Er,Yb:YAB crystal under LD end-pumped by the COMSOL software,and further simulated the heat dissipation effect of the"sapphire/Er,Yb:YAB/sapphire"and the"YAB/Er,Yb:YAB/YAB"composite crystal under the same pumping conditions.In the experimental process based on the single Er,Yb:YAB crystal,the output pulse was only achieved under the pulse pumping due to the thermal effect of the crystal,and then the effect of pump parameters on the laser pulse establishment and output characteristics was investigated.In an experiment based on the"sapphire/Er,Yb:YAB/sapphire"composite crystal,because the composite structure enhanced the crystal heat dissipation,the output pulse with a repetition frequency of 50 k Hz is realized under the continuous pumping.By controlling the coating parameters of the output lens under the pulse pumping,a single wavelength pulse output with 119 k Hz repetition frequency is obtained at 1522 nm.In the case of the multi-wavelength pulse output,the output pulse of 208 kHz repetition frequency was achieved.