【作者】 钟凯；

【导师】 姚建铨；

【作者基本信息】 天津大学 ， 光电子技术， 2010， 博士

【摘要】 1.5-1.6μm人眼安全波段、3-5μm和8-12μm中红外波段是光学频段中三个非常重要的大气窗口,此外,近年来随着人们对太赫兹（THz）波段的逐渐了解,THz辐射也已经被广泛应用到各个基础研究与应用研究领域。实用可靠的光源是这几个波段的应用技术发展的基础。与其他方法相比,利用非线性光学频率变换方法如光学参量振荡器（OPO）、差频（DFG）等将近红外激光转换到以上几个波段,具有可实现全固态、结构紧凑、室温运转、低阈值、高效率、高功率（能量）、可调谐的相干光输出的优点。本文对人眼安全波段的KTP/KTA OPO、3-5μm的中红外KTA OPO以及利用GaSe、ZGP晶体差频实现8-12μm的中红外激光和THz辐射的技术进行了理论和实验研究,并对实验中发现的新现象进行了探索。本文的主要创新点可以归纳如下:1.对低重复频率、高脉冲能量的人眼安全波段KTP/KTA OPO进行了研究。通过对1064nm激光泵浦的KTP/KTA OPO的相位匹配关系、有效非线性系数、走离角等相关参数进行理论计算和比较,获得了OPO的最佳运转条件。在实验中对电光调Q的脉冲Nd:YAG激光器泵浦的单谐振非临界相位匹配的KTA/KTP OPO进行了研究,采用近内腔泵浦的复合腔结构,实现了高效率、高单脉冲能量的人眼安全波段激光输出。其中KTP OPO在1572nm处的单脉冲输出能量最高为66.5mJ,光光转换效率12.1%,电光转换效率4.47%;KTA OPO在1536nm的单脉冲输出能量的最高能量达74.9mJ,光光转换效率12.14%,电光转换效率4.74%。两实验结果中转换效率均为已报道类似方案的最高值。2.利用电光调Q运转的Nd:YAG激光器内腔泵浦KTA OPO实现高能量3.47μm的中红外激光输出。为了使OPO产生的中红外激光有效输出腔外,我们采用双端输出的折叠腔结构,中红外激光的输出总能量达到31mJ,光光转换效率4.76%,电光转换效率约为1.82%,从基频光到闲频光的光子转换效率达87%,这也是已报道的中红外内腔泵浦KTA OPO的最高输出能量和转换效率。3.对LD端面泵浦、声光调Q的激光器内腔泵浦的高重复频率人眼安全波段KTA OPO进行了理论和实验研究。在理论上对内腔泵浦的OPO（IOPO）谐振腔进行了分析,并在实验中利用Nd:YVO₄激光器泵浦KTA OPO实现了30kHz、1.03W的1536nm激光输出,光光转换效率12.26%,泵浦阈值只有0.75W。对Nd:YLF和Nd:YVO₄作为泵浦源的KTA IOPO的动力学过程进行了理论分析,并在实验中证明了上能级寿命更长的Nd:YLF晶体作为泵浦激光增益介质更有利于IOPO实现低阈值、高峰值功率的输出。4.利用LD端面泵浦、声光调Q的Nd:YVO₄激光器泵浦KTA IOPO实现了高重复频率、高效率的3.47μm中红外激光输出。采用信号光在腔内振荡、闲频光单端输出的直腔结构,LD泵浦功率为9.9W时闲频光的输出功率为435mW,光光转换效率4.4%,从基频光到中红外闲频光的光子转换效率可达64%。5.在LD端面泵浦的Nd:YAG激光器泵浦KTA IOPO的实验中发现了KTA的受激拉曼散射效应,并对此现象进行了进一步研究。利用Nd:YAG激光器泵浦KTA晶体,我们首次观察到了1178nm的斯托克斯光输出,并基于KTA晶体的级联受激拉曼散射和倍频效应,实现了从可见光到近红外的多波长激光同时输出,包含573nm、1064nm、1091nm、1120nm、1146nm、1178nm六条谱线,总功率约600mW,每个波长的功率为几十到几百毫瓦。优化实验方案实现了470mW的573nm黄光输出。6.对利用GaSe和ZGP晶体差频产生8-12μm中红外激光进行了理论和实验研究。以1064nm激光泵浦的简并点附近双波长KTP OPO在GaSe及ZGP中差频产生中红外激光的理论计算为基础,通过实验在GaSe晶体中差频获得了调谐范围8.28-18.365μm的中红外激光输出,在8.76μm处最高单脉冲能量为31.7μJ,中红外激光的峰值功率为7kW,能量转换效率约为0.9%;在ZGP晶体中差频,获得了7.2-12.2μm的可调谐中红外激光输出,波长为9.22μm时最大输出能量为10μJ,中红外激光的峰值功率约为2.2kW,能量转换效率为0.45%。7.对双波长KTP OPO在GaSe晶体中差频产生THz辐射进行了理论和实验研究。通过求解差频的耦合波方程,在理论上分析了晶体吸收系数、晶体长度、泵浦强度等条件对GaSe晶体差频产生THz波过程的影响。在实验中,首次将走离补偿结构的KTP OPO作为差频泵浦源,在GaSe晶体中差频实现了THz波输出,THz波的调谐范围0.186-3.7THz,在1.68THz处峰值功率最高达到11W,OPO的阈值、稳定性以及差频产生THz波的功率等与采用普通的KTP OPO作为泵浦源时有了明显改善。8.对利用ZnTe晶体级联差频产生THz辐射的原理和过程进行了理论研究。通过对级联差频耦合波方程的求解,得出了ZnTe晶体中级联差频的最佳泵浦条件和ZnTe晶体的最佳长度,并且分析了晶体吸收、波矢失配及泵浦强度对级联差频的影响。根据我们的计算结果,当泵浦强度为50MW/mm2,泵浦光频率分别为369THz和368THz时,级联差频产生的THz波的强度可达0.815MW/mm2,与普通差频相比提高了7.7倍,能量转换效率为0.815%,光子转换效率超过600%,远远超过Manley-Rowe关系的限制。更多还原

【Abstract】 The wavelength regions of 1.5-1.6μm eye-safe band, 3-5μm and 8-12μm mid-infrared bands are three important atmospheric windows in the optical spectrum. Moreover, the terahertz （THz） radiation has been widely exploited in basic and application research areas in recent years with the increase of knowledge for people in this area. Practical radiation sources in these bands are the basic elements for applications. Compared with other methods, nonlinear optical frequency conversion technologies such as optical parametric oscillator （OPO） and difference frequency conversion （DFG） can provide us with all-solid-state, compact, room-temperature operating, low-threshold, high-efficiency, high-power （high-energy）, tunable and coherent sources, by converting near-infrared lasers to the required wavelength regions mentioned above. In this dissertation, the eye-safe and 3-5μm mid-infrared KTP/KTA OPOs, as well as the 8-12μm and THz radiation sources based on DFG in GaSe and ZGP crystals, are theoretically and experimentally studied. Some new phenomena observed in the experiments are also further explored.The main contents and key innovative points are as follows:1. The low-repetition-rate, high-pulse-energy eye-safe KTP/KTA OPOs are researched. The optimum operating conditions are derived based on the calculations and comparisons on the phase-matching relations, effective nonlinear coefficients and walk-off angles of KTP OPO and KTA OPO pumped by 1064-nm lasers. In the experiment, high-efficiency, high-pulse-energy eye-safe lasers are achieved from noncritically phase-matched KTP and KTA OPOs pumped by electro-optically Q-switched pulsed Nd:YAG lasers in a compound cavity configuration. The maximum pulse energy of KTP OPO at 1572nm is 66.5mJ, corresponding to the optical-to-optical conversion efficiency of 12.1% and electrical-to-optical conversion efficiency of 4.47%; the maximum pulse energy of KTA OPO at 1536nm is up to 74.9mJ, corresponding to the optical-to-optical conversion efficiency of 12.14% and electrical-to-optical conversion efficiency of 4.74%. The conversion efficiencies are the higher than any other reported results of similar configuration.2. The high-pulse-energy mid-infrared KTA OPOs at 3.47μm intracavity pumped by an electro-optical Q-switched Nd:YAG laser are achieved. A folded cavity is adopted in order to fully extract the generated idler wave of the OPO in two directions. The total output energy reaches 31mJ, corresponding to the optical-to-optical conversion efficiency of 4.76% and electrical-to-optical conversion efficiency of 1.82%. The photon conversion efficiency from the fundamental to idler wave is 87%. This is the highest output energy and conversion efficiency of mid-infrared KTA OPOs that have been reported.3. Eye-safe KTA OPOs intracavity pumped by end-pumped acousto-optically （A-O） Q-switched high-repetition-rate lasers are theoretically and experimentally researched. Based on the theoretical analysis on the cavity parameters of intracavity pumped OPOs （IOPOs）, we achieve a 30-kHz, 1.03-W KTA OPO at 1536nm in the experiment, pumped by an A-O Q-switched Nd:YVO₄ laser. The optical-to-optical conversion efficiency is 12.26% and the threshold is as low as 0.75W. The dynamic processes of KTA IOPOs using Nd:YLF and Nd:YVO₄ as the pump lasers are compared, which conclude that laser materials with higher energy storage ability are more suitable in low-threshold, high-peak-power IOPOs.4. A high-repetition-rate 3.47-μm mid-infrared KTA IOPO is achieved, pumped by an LD-end-pumped A-O Q-switched Nd:YVO₄ laser. With a linear cavity in which the signal wave is resonant and the idler is coupled out in one direction, the maximum mid-infrared output power is 435mW when the LD pump power is 9.9W. The optical-to-optical conversion efficiency is 4.4% and the photon conversion efficiency from fundamental to idler wave reaches 64%.5. Stimulated Raman scattering （SRS） in KTA is observed in the experiment of eye-safe IOPO pumped by a Nd:YAG laser and further experiments are performed on this phenomenon. We first report the Stokes wave at 1178nm in KTA pumped by a Nd:YAG laser. Based on cascaded SRS and self-frequency-doubling in KTA, we achieve a multi-wavelength laser with wavelengths from visible to the near-infrared, including spectra of 573nm, 1064nm, 1091nm, 1120nm, 1146nm and 1178nm. The output power for each line is from tens to hundreds of milliwatts. The yellow laser at 573nm reaches 470mW with an optimized scheme.6. The difference frequency generation （DFG） of 8-12μm mid-infrared laser in GaSe and ZGP crystals is theoretically and experimentally researched. Firstly we calculated the phase-matching relations, effective nonlinear coefficients and walk-off angles of the DFG process based on a dual-wavelength KTP OPO working near the degenerate point pumped by a 1064nm laser. In the experiment, the tuning range of 8.28-18.365μm is achieved using a GaSe crystal. The maximum generated mid-infrared pulse energy is 31.7μJ at 8.76μm, corresponding to the peak power of 7kW and the energy conversion efficiency of about 0.9%. With a ZGP crystal for DFG, the maximum mid-infrared pulse energy is 10μJ at 8.76μm, the peak power of which is 2.2kW and the energy conversion efficiency is about 0.45%.7. THz difference frequency generation （DFG） in GaSe based on a dual-wavelength KTP OPO is researched. The DFG process is theoretically analyzed considering the effects of absorption, crystal length and pump intensity, etc. A walk-off compensated dual-wavelength KTP OPO is firstly employed as the pump source for DFG. The generated THz wave is tunable from 0.186 to 3.7THz, with the maximum peak power of 11W at 1.68THz. The stability and threshold of the KTP OPO, as well as the generated THz power is obviously improved compared with that using a common KTP OPO.8. Theoretical analysis is performed on the principles and process of cascaded DFG for THz wave in a ZnTe crystal. We obtain the optimum pump condition and crystal length through solving the coupling wave equations of cascaded DFG, and analyze the influence of absorption, wave-number mismatch and pump intensity on the cascade DFG process. Based on our calculations in which the pump intensity of 50MW/mm2 and the pump frequencies of 369THz and 368THz are taken for example, the generated THz intensity at 1THz reaches 0.815MW/mm2. The output intensity is increased by 7.7 times, corresponding to the energy conversion efficiency of 0.815%. The photon conversion efficiency reaches 600%, which goes far beyond the Manley-Rowe limit.更多还原