【作者】 吴波；

【导师】 沈永行；

【作者基本信息】 浙江大学 ， 光学， 2007， 博士

【摘要】 光参量振荡器具有高转换效率、宽调谐范围的特点,可在传统的基于受激光放大原理工作的激光器所不能达到的波段有效地工作,在国防、生化、环保及医疗等领域都有重要的用途。利用双折射相位匹配的方法制作的光参量振荡器,由于对晶体切割方向、泵浦光的入射方向及工作环境,例如工作温度等有较为严格的要求,存在着转换效率不够高、调谐范围小和调谐困难等缺点,限制了光参量振荡器的应用。利用准相位匹配技术(QPM)制作的光参量振荡器可以有效地消除以上的限制,是目前光参量振荡器研究的热点。本论文所涉及到的研究工作是在成功地自行制备的大尺寸、均匀周期性畴反转掺镁铌酸锂晶体(PPMgLN)的基础之上,制作具有高输出功率、高转换效率和宽调谐范围的光参量振荡器。我们设计、制作了一个基于单谐振荡的光参量振荡器的实验系统,其泵源为声光调Q的Nd3+:YAG激光器,其调Q频率在10kHz到50kHz之间可调,输出波长为1.064μm,输出脉冲宽度为20-30ns。通过仔细调节光参量振荡器的一系列参数,我们已经获得了最高输出功率为4.8W、转换效率为44%,调谐范围为1.42-1.73μm和2.76-4.27μm的输出激光。该光参量振荡器可以通过简单地调整晶体的位置,使之工作在不同的反转周期来达到调谐的目的,同时可利用辅助的温度调谐,使光参量振荡器具有大范围连续调谐的功能,已经具备了实际应用的价值。实验中,我们还对光参量振荡器的一些输出特性,例如光束质量、光谱宽度等参数进行了测量,测量结果显示,基于准相位匹配技术的光参量振荡器具有较好的输出特性。本论文同时还利用经典电磁理论,在振幅慢变化条件下,采用数值模拟的方法,就泵浦光为单色平面波的近似情况对由行波腔组成的基于单谐振荡和双谐振荡的光参量振荡器进行了较为系统的分析。分析的结果表明了在高强度的泵浦光源下,为提高单谐振荡器的输出功率和转换效率,应该尽可能地采用低反射率的输出镜和采用束腰半径较大的泵浦光。模拟还对双谐振荡器作了较为仔细的分析,通过对腔内光强的仿真模拟,指出在双谐振荡器中,由于腔内同时存储了大量的信号光和闲散光,阻碍了泵浦光向信号光和闲散光的转换,是造成双谐振荡器在高强度泵浦光的情况下转换能力不高的主要原因。与之相对应的是,在单谐振荡器中由于只有信号光能发生谐振,因此,在单谐振荡器中,三光波总是按照尽可能快的速率实现三波间的相互转换。通过大量的实验和测量表明,较之双折射相位匹配方法制作的光参量振荡器,基于准相位匹配的光参量振荡器在转换效率和调谐等方面均胜一筹,具有很高的实际应用价值。更多还原

【Abstract】 The optical parametric oscillators(OPO) are widly used in the field of national defence, environment protection, physic and biochemistry etc. because of its high conversion efficiency and wide tunability. Besides, the optical parametric oscillators are often used in the applications where the required wavelength of laser sources using normal amplifying media can not be reached. Before 1990’s, the optical parametric osicallators were normally based on the technique of angle-phase matching, which required a rigorous working condition such as the inputing angle of the beam and the temperature of the environment. Because of the restriction of the accepting angle of the pump beam, the conversion efficiency of the optical parametric oscillators based on the technique of angle-phase matching is poor, and the the tunability scale small. These disadvantages limited the use of OPO. With the development of quasi-phase-matching(QPM) technique, the OPO based on the QPM eliminates the limitation very well, and become one of the attractive focus in the domains of nonlinear optics in recent years.The objective of our research work is to fabricate an OPO system based on the periodically poled MgO doped lithium niobate crystal (PPMgLN) with a good conversion efficiency and a high output power. The PPMgLN wafers were frabricated by means of high voltage pulse trigged domain reversal technique. By using a Q-switched Nd3+:YAG laser with a modulation frequency between 10 kHz to 50 kHz, and a pulse width between 20-30ns, we established a singly resonant optical parametric oscillator(SRO) system. Up to today, we have obtained a power output of 4.8W from the OPO (including the signal and the idler) at an input pump power of 10.8W. The efficiency of the OPO was 44% and the slope efficiency was 47%. The tunability of the OPO was from 1.42μm to 1.73μm and 2.76μm to 4.27μm. To perform the tuning, it is very convenient just to shift the nonlinear crystal so that it could work at different poled area with different reversing periods. When the function of continous tunabilty was required, the temperature-tunability could be used together with the position tuning approach. On account of the benefits of the high conversion efficiency, high output power, wide tunability and convenient operating method, the optical parametric oscillator we fabricated had reached a practically usable extent. Some important parameters of the OPO were measured, such as the beam quality and the spectrum of the beam output. The results obtained showed that the OPO was of good performance.In the last chapter of this thesis, we simulated the characteristic of an OPO based on the technique of QPM by means of numerical analysis with the classical theory of electromagnetism under the condition of slowly-varying-envelope waves. All the simulation was processed with the assumption that all the waves which coupled with each other were all the monochromatic plane waves. The result obtained from simulation showed that if high output power and hight conversion efficiency was intended, the reflection coefficient of the output mirror should be reduced while the waist size of the pump beam be increased. In this chapter, we also simulated the working state of the doubly resonant optical parametric oscillator(DRO). The result showed that due to the storage of the signal and the idler light in the resonant cavity, the pump power was suffocated to transform to the signal and the idler to some extent. These were different from the SRO where the pump, the signal and the idler were transformed to each other as fast as it could be.After a lot of experiments and theoretical analysis, a clear conclusion was obtained that the performance of the QPM tecnique based OPO was much better than the normal angle-phase matching based OPO, especially in its conversion efficiency and its tunability.更多还原