High-power Short-pulse Solid-state Laser And Its Pumped Mid-infrared Periodically Polarized Lithium Niobate Optical Parametric Oscillator

Study on high power short pulse solid lasers and mid-infrared periodically poled Lithium Niobate optical parametric oscillators (PPLN-OPO) has important practical guidance value for the optimization of pulse solid laser and mid-infrared laser light sources. In this dissertation, we have studied the gain-switching, active and passive Q-switching pulse solid laser output characteristics, when considering the influence of thermalization and lower multiplet relaxation of the laser medium, Q-switch turn-on time and laser transverse mode; Then, the features of mid-infrared PPLN-OPO pumped by pulse solid lasers were studied theoretically and experimentally. The main research content is summarized below:(1) Characteristics of laser relaxation oscillation process and gain-switching pulsesInfluence of thermalization and lower multiplet relaxation (collectively called as energy level relaxation) of the laser medium on turn-on relaxation oscillation process and steady state output characteristics of1064nm Nd:YAG CW laser was studied; And further influence of laser energy level relaxation on gain-switching laser pulse building-up process and output characteristics was also analyzed. Results show that the energy level dynamic relaxation process has obvious effect on turn-on relaxation oscillation process of CW laser and gain-switching pulse laser output characteristics, especially in the case of high power short pulses; Compared with the situation regardless of the energy level relaxation, it is found that under the same pump duration gain-switching pulse width is wider, and pulse energy and peak intensity are lower when considering energy level relaxation. However, the energy level relaxation hasn't any impact on the final steady state output of CW laser. Therefore, we can conclude that energy level relaxation mainly has influence on laser dynamic process especially short pulse laser building-up process, and it is needed to consider the role of energy level relaxation in the optimal design of short pulse lasers.(2) Characteristics of active Q-switching laser pulsesInfluence of energy level relaxation process on fast active electro-optical Q-switching Nd:YAG pulse laser output characteristics and the adverse influence of slow-switch effect on acousto-optical Q-switching pulse lasers were studied. For the electro-optical Q-switching pulse lasers, the approximate treatment can be considered as fast-switch; However, as electro-optical Q-switching laser pulses have a short pulse width (ns magnitude), it is needed to consider the influence of energy level relaxation on its output characteristics. For the acousto-optical Q-switching pulse lasers, the pulse width is wider, so it isn't needed to consider the effect of energy level relaxation; But the acousto-optical Q-switch is opened with a long time, it is easy to generate multiple pulses under high power situation, greatly reducing the laser energy conversion efficiency, deteriorating laser pulse uniformity and beam quality at the same time, which should be avoided.(3) Characteristics of passive Q-switching laser pulsesInfluence of energy level relaxation process and laser transverse mode on Cr4+:YAG passive Q-switching laser output characteristics was studied, which has some guiding significance in obtaining stable and reliable Q-switching laser pulses. The output characteristics of LD end-pumped Nd:YVO4Cr4+:YAG passive Q-switching laser were measured experimentally, and then when introducing energy level relaxation into passive Q-switching rate equations, it was found that the theoretical simulation results agreed well with the experimental results when thermalization time tt=5ns. Then, formation of axisymmetric TEM01*modes caused by thermal birefringence effect was studied in a LD side-pumped Nd:YAG CW laser. When inserting Cr4+:YAG saturable absorber, due to transverse mode selection effect of the saturable absorber, four symmetric distributed phase-locked Gaussian beams were produced in the critical work area of the laser resonantor, and high-resolution sine and cosine-modulated Gaussian interference fringes were observed in the far field; In addition, it was found that when the laser transverse mode was phase locked stably, the passive Q-switching laser pulses were also stable.(4) Theoretical study of mid-infrared PPLN-OPOThree waves interaction coupled wave equations of nonlinear optics, quasi-phase matching principle and its advantages, wavelength tuning of PPLN-OPO, oscillation threshold and conversion efficiency of external-cavity, singly resonant OPO were analyzed theoretically.(5) Experimental study of external-cavity, singly resonant mid-infrared PPLN-OPOUsing acousto-optical Q-switching LD side-pumped1.064?m Nd:YAG pulse lasers as the pump sources, the power output and spectral characteristics of external-cavity, singly resonant mid-infrared MgO:PPLN-OPO were studied, with the idler wavelength temperature-tunable around3.8?m. First of all, related experimental parameter designs were analyzed for the high power mid-infrared PPLN-OPO, mainly including pump light source, PPLN crystal, OPO cavity mirror coating and mechanical design. Then, through inserting an aperture of different sizes into the LD side-pumped1.064?m Nd:YAG pulse laser cavity, the influence of the pump beam quality on OPO output characteristics was analyzed. The experimental results show that the better is the pump beam quality, the higher is the parametric conversion efficiency. In the experiments, the highest3.8?m idler beam power of5.5W and1.064?m pump beam to3.8?m idler beam conversion efficiency of27%were obtained. At the same time, we analyzed the influence of the PPLN crystal thermal effect on the pump beam transverse mode. When the pump power was higher than a certain value, the thermal lens effect was converted to thermal waveguide effect, the pump beam was restricted within the crystal, and thus the parametric beams overlap better with the pump beam, which is beneficial for the efficient and stable operation of OPO. Finally, we briefly analyzed the parametric light spectrum and parasitic oscillation visible light spectrum of the mid-infrared PPLN-OPO. In addition to the near-infrared signal light and mid-infrared idle light spectrum, parasitic oscillation of green and red visible light was also observed by human eyes.