Output Characteristics Of Nanosecond Mid-infrared Optical Parametric Oscillation Based On Acousto-optic Modulation

As one of the four major inventions of the 20th century,the laser has been highly regarded since its inception,and researchers have been continuously innovating laser technology.Many scholars are devoted to the direction of wavelength-tunable and ultra-short and ultra-fast lasers.Through existing research proposals,optical parametric oscillators（OPOs）have been proven to be an extremely important tunable laser generator.Its unique advantages include high output power,strong conversion efficiency,excellent beam quality,wide tuning range,and the ability to support multiple operating modes（including continuous wave,nanosecond pulse,picosecond pulse,and femtosecond pulse,etc.）.These characteristics have made OPOs highly regarded and become one of the indispensable laser devices in many fields.The pulse peak power and pulse width of a laser are two parameters that are interrelated.When the total output energy of the laser is unchanged,the shorter the pulse width,the higher the pulse peak power that can be obtained.With the continuous advancement of Q-switching and mode-locking technologies,lasers are moving towards ultra-short pulse direction and are continuously increasing the laser peak power.These advancements provide necessary support for the in-depth exploration of ultrafast and ultra-strong technologies.Currently,research on high-peak-power nanosecond mid-infrared optical parametric oscillators has received extensive attention at home and abroad.It is worth noting that 3-5μm lasers have important application values in medical diagnosis and treatment,environmental monitoring,free-space optical communication,laser radar,and infrared interference and other fields.This paper studies the application of laser Q-switching technology in transforming continuous laser output into high peak power pulse lasers.Based on this,we introduce the use of acousto-optic Q-switches as functional elements in laser cavities.The acousto-optic Q-switch has the advantages of small size,low optical loss,fast response speed,and stable operation,and can actively Q-switch and control the repetition frequency of the output pulse laser.In the past,the acousto-optic Q-switch used in research was purchased externally,and there were uncontrollable factors such as size,beam quality,and switch time.Therefore,we independently designed,selected crystals,and optimized processes to produce an acousto-optic Q-switch suitable for applications.In the experiment,it was found that a single acousto-optic Q-switch was not enough to achieve excellent nonlinear conversion,so we introduced two-dimensional nanomaterial Platinum Disulfide（Pt S₂）.This material has unique photoelectric properties,photo-induced luminescence effects,and good nonlinear absorption effects,which can act as a saturable absorber（SA）for passive Q-switching and are used in this study.By combining the acousto-optic Q-switch and Pt S₂saturable absorber,we achieved dual loss modulation and obtained 3.4μm idler light.In addition,the dual loss modulation technique can also be used in a resonant cavity to achieve high peak power,narrow pulse width,and high stability intracavity optical parametric oscillator（IOPO）output.It was demonstrated that both the acousto-optic Q-switch and Pt S₂saturable absorber for the laser cavity were beneficial to the conversion process of OPOs.The main research contents of this paper are summarized as follows:（1）This paper introduces the use of acousto-optic Q-switches as the Q-switching device for continuous solid-state lasers,and details its entire process from principle,design to production and application.The acousto-optic Q-switch works under conditions of a wavelength of 1064nm,a frequency of 40.68MHz,and an optical aperture of 1.5mm.The device uses quartz crystal as the acousto-optic medium,36°Y-cut lithium niobate crystal as the transducer material,and Hf O₂-Si O₂coated anti-reflective film,eventually achieving 96%diffraction efficiency,and switch time less than 29ns.The paper mentioned measures such as selecting crystal materials,improving the polishing quality,and designing rational heat dissipation structure,to better maintain the diffraction light’s beam mode.（2）Through theoretical research,a complete set of technical parameters and process steps is set up and organized.The quartz crystal acousto-optic medium with a size of13×5×32mm³and the piezoelectric transducer lithium niobate crystal with a size of34×4×1mm³are cut by X-ray directional cutting.Then,the quartz crystal is manually and mechanically polished and coated with anti-reflective film in a vacuum coating machine,and the lithium niobate chip is thinned,polished and coated with electrodes,then pressure-welded to the quartz crystal,and finally the circuit module is debugged,the casing is assembled and bonded,and the finished device is obtained and tested for performance,validating the practicality of the device.（3）Through experiments,we studied the laser output characteristics of the 1064nm fundamental cavity acousto-optic Q-switch at different modulation frequencies.The experimental results showed that the maximum single-pulse energy and peak power were36.6μJ and 1.26k W,respectively,and the minimum pulse width was 29ns.At the same conditions,we also compared the output parameters of the single acousto-optic Q-switch actively Q-switched laser and the AO+Pt S₂passively Q-switched laser.The AO+Pt S₂passively dual loss-modulated laser obtained narrower pulse widths,higher peak powers,and greater instantaneous energy,which is advantageous for the nonlinear frequency conversion process of OPOs.（4）The dual Q-switching OPO is realized by using the acousto-optic Q-switch and the Pt S₂SA prepared in this work,and nanosecond mid-infrared pulses are obtained.Through the analysis of the output results,it is found that Pt S₂saturable absorber has a significant optimizing effect on improving wavelength conversion efficiency,including a 240%improvement in pulse train stability,a 59.7%reduction in idler light pulse width,a 198%improvement in peak power,and a 16.9%increase in nonlinear conversion efficiency.The experimental results show that applying the acousto-optic Q-switch and Pt S₂saturable absorber to the laser cavity can effectively improve the conversion efficiency of optical parametric oscillators.