| Currently,nanosecond pulsed 3-μm lasers are interest for many scientific research and practical applications.At present,Q-switched technologies are the main approaches to obtain laser with high peak power and short pulse duration at 3-μm wavelength range,including active Q-switched such as electro-optical(EO)Q-switched,acousto-optical(AO)Q-switched.Due to the limitation of absorption characteristics and low damage threshold of photoelectric materials,the development of photoelectric switches with excellent performance has been a difficult problem to be solved in the laser technology at 3-μm wavelength range.Ten years ago,we innovated LGS EO Q-switch to achieve laser output with short pulse and high peak power at 2-3-μm wavelength range.Because the EO Q-switched technology only regulates the linearly polarized light,the thermal birefringence of the laser crystal results in thermal depolarization of the linearly polarized light,and the thermal depolarization effect is enhanced with the increase of the pumped power,so high repetition rate operation cannot be realized.The unique features of AO Q-switched technology are low insertion loss,compact structure and suiting for high repetition rate operation.Acousto-optic(AO)modulators as Q-switches are widely used in lasers to obtain high repetition rate and narrow pulse duration laser output.Although several novel AO Q-switches of 3-μm waveband have achieved initial results,in these crystals functioning as the acousto-optic medium and the transducer are usually different materials.This leads to a complex manufacturing process and high auxiliary cost to realize the electric and acoustic impedances matching,and has become a key obstacle for improving the device manufacturing process and performance.Therefore,to develop Er,Cr:YSGG acousto-optic Q-switched laser with excellent performance,the acousto-optic switch was made by the most commonly used bulk lithium niobate crystal with excellent ultrasonic performance for the first time,and the output characteristics of LiNbO3 acousto-optic Q-switched Er,Cr:YSGG at high repetition rate was study.The main work of this paper can be divided into the following aspects:1.Analysis and verification of LiNbO3 acousto-optic Q switch.The acousto-optic and piezoelectric properties of lithium niobate crystals were analyzed theoretically.And acousto-optic modulator and transducer can be made by LiNbO3 crystal.The optimal acoustic wave mode and propagation direction in the acousto-optical medium were determined,so that the LiNbO3 acousto-optical Q-switch was designed to obtain the high diffraction efficiency close to the theoretical prediction.The results showed that when the longitudinal wave excited by the 36°-Y-cut LiNbO3 transducer propagated along the crystal axis z---[001],the energy loss during ultrasonic propagation can be reduced and the diffraction efficiency of the device can be improved.This study demonstrates for the first time that the LiNbO3 acousto-optic Q-switch has excellent acousto-optic characteristics,and the diffraction efficiency of 2.79 μm wavelength was up to 57%at the RF frequency of 40.68MHz.LiNbO3 crystal is the most common transducer material,and the bonding can simplify the fabrication process and reduce the auxiliary cost.This study is also a step forward to explore the possibility of direct bonding of homogeneous materials into integrated acousto-optic devices.2.The compensation of thermal focal lensing effect and the optimal design of laser resonator.The thermal focal lensing effect is always an important factor affecting the output performance of all solid-state laser at high repetition rate.To compensate or reduce the strong thermal focal lensing effect in the laser crystal to achieve higher output energy and better beam quality,the thermal focal length of Er,Cr:YSGG laser crystal was calculated theoretically and measured experimentally,the thermal focal lensing effect compensation resonator structure was designed,and the plane-convex resonator was used to compensate the thermal focal lensing effect of Er,Cr:YSGG laser.The designed thermal compensated plane-convex resonator obviously improves the beam quality and the output capacity of the laser at high repetition rate.When the repetition rate was 50 Hz,the maximum output energy of Er,Cr:YSGG laser were Eout=107.1 mJ in the plane-parallel resonator,and Eout=236 mJ in the plane-convex resonator,and the corresponding output energy was increased by 2.2 times.When the repetition rate was 100 Hz,the maximum output energy of Er,Cr:YSGG laser were Eout=26.1 mJ in the plane-parallel resonator,and Eout=52.6 mJ in the plane-convex resonator,and the corresponding output energy was increased by 2 times.3.Study on the output characteristics of Er,Cr:YSGG LiNbO3 acousto-optic Qswitched laser.The influence of different reflectance,different RF driving power and thermal focal lensing effect on the output performance of LiNbO3 acousto-optic Qswitched Er,Cr:YSGG laser were studied.The results showed that when the repetition rate was 100 Hz and the RF drive power was 30 W,the optimal transmission rate of the laser output mirror is 70%.The LiNbO3 acousto-optic Q-switch achieved the maximum diffraction efficiency and stable Q-switching characteristics.The thermal focal lensing effect compensation was realized in the plane-convex resonator,and the maximum pulse energy and the shortest pulse duration were obtained.Compared with planeparallel resonator,plane-convex resonator can increased the output energy of laser by 1.5 times.The developed LiNbO3 acousto-optical Q-switched Er,Cr:YSGG laser can achieve stable output of high pulse energy and narrow pulse width at high repetition rate,and the pulse energy stability of 30 min is 2.55%.4.The narrowest pulse and peak power of LiNbO3 acousto-optic Q-switch performance.The output performance of LiNbO3 and TeO2 acousto-optic Q-switches were compared under same experimental conditions at 60 Hz.Compared with TeO2 acousto-optical Q-switch,LiNbO3 acousto-optical Q-switch has a higher damage resistance threshold and can obtain laser output with shorter pulse duration.At 60 Hz,the maximum output energy of the laser was 8.5 mJ,the minimum pulse duration was 60.8 ns,and the corresponding peak power was about 140 kW,which sets the narrowest pulse duration and the maximum peak power of acousto-optic Q-switch in this waveband.This shows that LiNbO3 acousto-optic Q-switch has good acousto-optic Qswitched performance,and shows that LiNbO3 can become a new type of high power acousto-optic Q-switch at 3-5 μm waveband. |
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