DKDP晶体光学特性及频率转换技术研究
文献类型：学位论文
| 作者 | 崔子健 |
| 文献子类 | 博士 |
| 导师 | 刘德安 |
| 关键词 | 非线性光学，频率转换，电光效应，温度不敏感，电压调谐相位匹配 Nonlinear optics, Frequency conversion, Electro-optic effect, Temperature insensitive, Voltage-tuning phase matching |
| 其他题名 | Research on Optical Characteristics and Frequency Conversion Technology of DKDP Crystal |
| 英文摘要 | 激光的问世和二次谐波现象的发现开启了非线性光学研究的大门，使其得到了巨大地发展并形成了一门崭新的光学分支学科。作为非线性光学中最重要的技术之一，非线性光学频率转换已成为惯性约束聚变(Inertial Confinement Fusion，ICF)激光装置和超短超强激光系统中不可或缺的关键技术。高效稳定的频率转换主要取决于光波在非线性相互作用过程中严格相位匹配条件的保持。光学性能良好的非线性晶体，如：β-BaB2O4 (BBO)，LiB3O5 (LBO)、KH2PO4 (KDP)、KD2PO4 (DKDP)等，为频率转换技术提供了核心材料，在激光驱动惯性约束聚变和超短激光脉冲产生等诸多领域有着重要的应用与推广。 由于晶体自身存在光谱色散这一固有秉性问题以及晶体温度变化、角度偏差、光束倾斜、激光的热效应与波长漂移等客观实际问题，使得在实际应用中，特别是在高重复频率、高平均功率激光以及高峰值功率激光装置的频率转换系统中，相位匹配条件难以始终精确地保持，由此导致相位失配产生、转换效率下降、光束质量恶化、脉冲波形畸变等问题，从而对激光装置的整体输出性能造成严重的影响。针对以上问题，本文从多方面对KDP/DKDP晶体在非线性光学中应用的关键技术问题展开研究，重点围绕温度、角度等因素不敏感的频率转换新技术、自然补偿热致相位失配的新方案、光波非线性相互作用过程中实现严格相位匹配的新方法进行研究。主要工作和创新性研究成果包括以下内容： 1. 针对二阶非线性光学频率转换过程，在详细研究激光脉冲时空域演变特性的基础上，编写了频率转换仿真程序，并依据实际激光装置的实验数据进行了校核。该程序具有完整的物理模型，综合考虑了二阶非线性效应三波混频所涉及的多个物理过程，包括：空域的衍射与走离、时域的群速失配与群速色散、晶体的光学吸收与表面反射损耗等，实现了纳秒量级至飞秒量级全时间尺度的三维光学频率转换仿真计算，涵盖了单轴晶体、双轴晶体、不同相位匹配类型、多晶体级联、多种光学效应共同作用等多种情况，可以为频率转换系统的设计和晶体参数的选取提供可靠的依据。 2. 提出了把电光效应和非线性光学频率转换相结合，利用电光效应补偿相位失配的新技术。由于电压是非机械式的调节，并且具有灵活的调节特性，因此利用电光效应可以对温度、角度变化导致的相位失配进行有效地补偿，从而实现温度、角度等因素不敏感的频率转换。在建立该技术方案理论模型的同时，积极展开了实验研究，对该方案进行了实验验证。在实验上演示了同时实现温度、角度不敏感的频率转换技术，晶体的温度、角度接受带宽分别增加了2.1和2.4倍，证明了该设计方案的有效性和多用性。同时，结合高功率激光装置的实际情况，以功率密度为2 GW/cm2的纳秒量级基频脉冲为例，对高功率密度情况下基于该技术方案的频率转换系统进行了深入分析和优化设计。 3. 从热致相位失配产生的根本物理机理出发，提出了普遍适用的能够自然补偿热致相位失配的新型频率转换设计方案，构建了这一方案的理论模型，给出了基于该方案设计频率转换系统的一般规律和准则。该方案基于三块相同类型的晶体级联结构，通过配置光波在晶体中传输的偏振态，使相邻晶体的相位失配对温度的一阶导数符号相反，从而产生相反的相位失配，因此可以实现频率转换系统中激光热效应导致的相位失配的自然补偿。为了验证这一方案，对KDP晶体实现温度不敏感的1053 nm激光二次和三次谐波产生(Second and Third Harmonic Generation，SHG和THG)进行了理论分析和实验验证。实验结果表明：与传统方案相比，基于该设计的频率转换系统能有效地实现热致相位失配的自然补偿，SHG和THG的温度接受带宽分别实现了2.0和1.7倍的增大。由于使用相同类型的晶体，因此这一设计方案完全不受晶体类型、相位匹配类型和激光波段的限制，具有极好的通用性，可以广泛地并且几乎没有任何限制地应用于不同波段的不同频率转换中； 4. 创新性地提出并实验验证了一种全新的相位匹配方法——电压调谐相位匹配，建立了利用电光效应实现相位匹配的完整理论框架，并设计了巧妙的实验验证方案。电压调谐相位匹配通过引入外电场，把电光效应和非线性频率转换同时应用在单块非线性晶体中，利用电光效应对晶体的折射率和激光脉冲的传输进行精确地调控，使非线性频率转换过程始终能满足严格的相位匹配。在原理性验证实验中，通过把电光效应和非临界相位匹配四次谐波产生(Fourth Harmonic Generation，FHG)同时应用在单块部分氘化的KDP晶体中，实现了在初始相位匹配温度±2°C的变化范围内，FHG过程始终能保持准完美的相位匹配。该实验成功地演示了利用电光效应对光波之间非线性相互作用过程的操控，证明了这一新相位匹配方法的正确性与可行性，从理论和实验的角度系统性地为非线性光学中相位匹配的实现提供了全新的原理方法和技术方案。该方法可以从根本上克服传统技术中转换效率对角度、温度、波长变化敏感的问题，这不仅为设计新颖的非线性光学器件提供了新的途径，也为进一步深入研究非线性光学相互作用提供了新的研究方向。更重要的是，这一方法可以人为地改变材料的折射率特性，实现传统技术不能实现的功能，从而提高了传统非线性材料、甚至低双折射和各向同性材料在非线性光学中的应用范围。; The invention of the laser and the discovery of the second harmonic phenomenon have opened the research of nonlinear optics, which has greatly developed and formed a new branch of optics. As one of the most important techniques in nonlinear optics, nonlinear optical frequency conversion has become an indispensable key technology in Inertial Confinement Fusion (ICF) laser facilities and ultra-short ultra-intense laser systems. The efficient and stable frequency conversion depends mainly on the maintenance of the strict phase matching condition during the process of nonlinear interaction between the optical waves. Nonlinear crystals with excellent optical properties such as β-BaB2O4 (BBO), LiB3O5 (LBO), KH2PO4 (KDP), KD2PO4 (DKDP) provide the core material for the frequency conversion technology, which have important applications in many fields, such as laser-driven ICF and the ultra-short laser pulse generation. Because of the inherent nature of spectral dispersion in crystal, and the objective reality, including crystal temperature variation, angular deviation, beam tilt, laser thermal effect and wavelength shift, it is difficult to always maintain accurate phase matching in practical applications, especially in the frequency conversion systems of high repetition rate, high average power laser and high peak power laser facility. Therefore, the problem of phase mismatch will occur, resulting in unfavorable effects on the frequency conversion such as conversion efficiency decrease, beam quality deterioration, and pulse waveform distortion, which have serious impact on the holistic output performance of laser facility. Aiming at these problems hereinbefore, the key technical problems of KDP/DKDP crystal used in nonlinear optics are researched from many aspects. The research focuses on the new temperature- and angle-insensitive frequency conversion technology, novel scheme that is capable of self-compensating thermally induced phase mismatch and new method which can achieve strict phase matching during the process of nonlinear interaction between the optical waves. The main work and innovative research results include the following: 1. Aiming at the second-order nonlinear optical frequency conversion process, the simulation program used for frequency conversion is accomplished based on the detailed study of the evolution characteristics of laser pulse in the time-space domain, and is checked according to the experimental data of the actual laser facilities. The program has a complete physical model. Multiple physical processes involved in the three-wave mixing of the second-order nonlinear optical effect are taken into account, which include the diffraction and walk off effects in the spatial domain, the group velocity mismatch and dispersion in the time domain, and the optical absorption and the surface reflection loss of the crystal. The program achieves three dimensional space numerical simulation of optical frequency conversion in the time scale from nanosecond to femtosecond, and covers many cases such as the uniaxial crystal, the biaxial crystal, various phase matching types, multi crystal cascade, multi optical effects. It could provide reliable basis for the design of frequency conversion system and the selection of crystal parameters. 2. Based on the combination of electro-optic effect and nonlinear optical frequency conversion, a new technology that can compensate the phase mismatch using electro-optic effect is proposed. Because the voltage is nonmechanically tunable with a flexible adjustment characteristic and a high degree of precision, the phase mismatch induced by temperature and angle variation can be effectively compensated via the electro-optic effect. Therefore, temperature- and angle- insensitive frequency conversion can be achieved. In this dissertation, the theoretical model of the proposed scheme is established, and the scheme is experimentally demonstrated by temperature- and angle-insensitive frequency conversion. The experimental results show that the temperature and angle acceptance bandwidths are 2.1 and 2.4 times larger, respectively, than that of the traditional method employing a single crystal, which verify the availability and versatility of the scheme. Meanwhile, according to the actual situation of high power laser facility, we considered a fundamental wave power density of 2 GW/cm2 as an example to analyze and optimize the frequency conversion system with high power density based on the proposed scheme. 3. Based on the fundamental physical mechanism of thermal-induced phase mismatch, a novel frequency conversion scheme which can naturally compensate the thermal-induced phase mismatch is proposed. The theoretical model of the scheme is constructed, and the general rules for designing frequency conversion systems based on the proposed scheme are given. The basic design of this scheme is that three crystals with the same types are cascaded. By configuring the polarization states of the interacting waves in the crystal, the signs of the first temperature derivative of phase mismatch between adjacent crystals are the opposite, hence resulting in opposite phase mismatch. Thus, the natural compensation of phase mismatch caused by laser thermal effect in the frequency conversion process can be achieved. In order to verify this scheme, temperature-insensitive second and third harmonic generation (SHG and THG) at wavelength of 1053 nm using KDP crystal are theoretically analyzed and experimentally demonstrated. The results show that the thermal-induced phase mismatch can be effectively compensated for in frequency conversion system based on the proposed scheme. The temperature acceptance bandwidths of SHG and THG are 2.0 and 1.7 times larger, respectively, than that of the traditional method. Since the used crystals are the same type, this scheme is almost completely free from the limitations of the laser wavelength, crystal type and phase matching type. This new scheme with excellent universality can be widely apply to different frequency conversion processes at various laser wavelengths with scarcely any limitations. 4. A new phase-matching method based on the linear electro-optic effect, termed “voltage-tuning phase matching” is innovatively proposed and experimentally demonstrated. A basic theoretical framework for achieving phase matching using electro-optic effect is established, and an ingenious experiment is designed to verify the proposed method. By introducing an external electric field, voltage-tuning phase matching method applies the electro-optic effect and nonlinear frequency conversion simultaneously in a single nonlinear crystal, so that the refractive index of crystal and the propagation of laser pulse can be precisely controlled using the electro-optic effect. Therefore, the phase-matching condition can be always satisfied during the frequency-conversion process. In the proof-of-principle experiment, by applying the electro-optic effect and fourth harmonic generation (FHG) simultaneously in a partially deuterated KDP crystal, quasi-perfect phase matching is achieved systematically over a temperature range of the initial phase-matching temperature ±2°C during the FHG process. The experiment successfully demonstrates that the nonlinear interaction between optical waves can be effectively controlled using the electro-optic effect, and proves the validity and feasibility of the proposed phase matching method. Voltage-tuning phase matching provides a completely new method and technique for achieving phase matching in nonlinear optics, both in theory and experiment. This method allows quasi-perfect phase matching to be achieved systematically and the associated sensitivities to the angle, temperature, and wavelength in the traditional techniques to be fundamentally overcome. It not only paves the way for the design of novel nonlinear optical devices but also provides a new perspective for the further study of nonlinear optical interactions. Moreover, this idea can artificially change the refractive index characteristics of materials to achieve the functions that normally impossible using traditional techniques. The applications of conventional nonlinear materials and even low-birefringence and isotropic materials in the field of nonlinear optics can thus be extended. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31007]  |
| 专题 | 中国科学院上海光学精密机械研究所 |
| 作者单位 | 中国科学院上海光学精密机械研究所 |
| 推荐引用方式 GB/T 7714 | 崔子健. DKDP晶体光学特性及频率转换技术研究[D]. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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