| The mid-infrared laser has a strong transmission in the atmosphere and becomes theminimum attenuation window in the atmospheric transmission, which also covers theabsorption peaks of many atomics and has a strong penetration in the fog and smoke.Meanwhile, the absorption of the gas molecules and the scattering of the suspendedsolids have little influence on the mid-infrared laser. Therefore, the mid-infrared laserhas import application value and prospect in spectrum measurement, remote sensing,environment protection, military and many other fields. Nowadays, the generationtechniques of the mid-infrared laser become a research focus in the world. In addition,the OPA technique has attracted great interests because of its high gain, low thermaleffect, wide tuning range, high signal-to-noise ratio, high quantum efficiency and highbeam quality.The main contents of this dissertation are as follows:1. The research background and generation techniques of the mid-infrared laser aresummarized. The advantages and disadvantages of the generation techniques arealso given. The advantages and classification of the optical parametric oscillator,the commonly used mid-infrared nonlinear crystal and the research status of themid-infrared laser at home and abroad are highlighted. Finally, the research statusof OPCPA in recent years is reviewed.2. The coupled wave equations of the three-wave mixing are obtained from theMaxwell equations and the general solutions of the normalized coupled waveequations are also obtained; The phase matching curves of the commonly usedmid-infrared nonlinear crystals are theoretically analyzed and studied; Thethreshold characteristics of the singly resonant and doubly resonant OPOs aretheoretically analyzed and simulated; The accept angle, accept wavelength andwalk-off angle of the biaxial crystal are theoretically analyzed and studied.3. The theoretical model of the time modulation caused by the mode beating is built.Then the influence of the time modulation caused by mode beating on optical parametric process is theoretically analyzed and studied. It is found that thepower of the signal wave and idler wave fluctuates significantly with the timewhen the pump laser oscillates on several random longitudinal modes. However,the power of the signal wave and idler wave becomes relatively stable when thepump laser oscillates on hundreds of random longitudinal modes.4. The theoretical model of the beam quality of OPA is built. Then the influence ofthe wave-front mismatch, spatial chirp and multi-mode pump on the beam qualityof OPA is theoretically analyzed and studied. It is found that the beam quality andthe wave-front distribution of the amplified signal become worse as the mismatchangle of the wave-fronts increases or the spatial chirp becomes more complex. Itis also found that the beam quality of amplified signal becomes worse and theconversion efficiency descends a lot when the transverse distribution of the pumplaser becomes worse.5. The KTA-OPO is designed and experimentally studied, which is pumped by aseed-injected single-longitudinal mode Q-switched Nd:YAG laser and amulti-longitudinal mode Q-switched Nd:YAG laser, respectively. The pulseenergy of10mJ with a conversion efficiency of14%from the pump laser to themid-infrared laser is obtained.6. The signal-to-noise ratio improved by a fiber Sagnac loop is theoreticallyanalyzed and simulated. It is found that the signal-to-noise ratio could beenhanced by104magnitude when a fiber Sagnac loop is used.7. The experimental system of high beam quality and high stability OPA isoptimally designed. Then the OPA system which is pumped by a seed-injectedsingle longitudinal mode Q-switched Nd:YAG green laser and seeded by apassive mode-locked fiber laser as the seed signal is experimentally studied. Thepulse energy greater than20mJ, M2<1.4and signal-to-noise ratio greater than108are obtained. |
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