| 题名 | 前端预放中光谱控制和应用研究 |
| 作者 | 张鹏 |
| 学位类别 | 博士 |
| 答辩日期 | 2012 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 李学春 |
| 关键词 | 光谱控制 相位调制 偏振控制 幅频调制效应 布里渊激光器 布里渊放大 |
| 其他题名 | Spectrum control technology of the front end and its applications |
| 中文摘要 | 高功率激光驱动器是进行惯性约束聚变研究的基本工具。为实现激光脉冲对聚变材料的有效压缩，要求主激光脉冲在时间形状、同步精度、空间均匀性、频域、系统稳定性、能量等方面满足特定的要求。光谱控制技术 作为一种重要的技术手段，对实现激光的全域控制起到了关键性的作用。一方面，出于后向SBS的抑制和束匀滑的需要，前端预放系统中采用单纵模种子光与相位调制相结合的技术手段来展宽激光的光谱。另一方面，宽谱光在系统传输放大过程中发生的幅频调制效应，会引起光脉冲时间波形的畸变，影响激光脉冲的整形能力，因此，需要对脉冲光谱进行预补偿来减小幅频调制的影响。此外，SBS放大技术，作为提高脉冲能量一种手段，要求泵浦光和种子光的频差等于介质的布里渊频移，这同样依赖于光谱控制技术。 本论文围绕高功率激光驱动器前端预放系统中的光谱控制技术，分别从束匀滑中的相位调制、幅频调制补偿、瞄准冲击点火的主动式SBS短脉冲放大以及布里渊光纤激光器等方面开展了一系列理论和实验的研究工作： 1. 针对SSD中具有角色散的宽谱光通过空间滤波器时发生的“堵孔效应”，采用脉冲拼接的方法，在国内首次开展了时变光谱调制的研究。实验中发现，由于光开光有限消光比以及两部分脉冲相位不锁定，得到的时变光谱调制脉冲前部会出现明显的幅度调制，并且在拼接处会出现抖动的尖峰。针对此问题，提出了解决方案，为时变光谱调制的最终应用奠定了基础。 2. 提出了一种基于保偏光纤偏振模色散的幅频调制效应补偿方案。根据理论分析，通过调节入射光的偏振态、合理选择保偏光纤的长度，调节光纤挤压器的驱动电压，可实现对振幅失配型FM-AM的补偿。 3. 针对前端预放系统中光脉冲低重频、窄脉宽的特点以及系统中振幅调制器等偏振敏感器件对脉冲偏振态的要求，提出主动偏振控制技术。该项技术的关键在于对光信号能量的获取。我们采用电流积分放大的方法，将纳秒以及百皮秒脉冲的脉宽展宽到10μs量级，再用高速采集卡采集脉冲波形，成功获取了光脉冲的能量。该技术可以实现对重复频率为1Hz、脉冲宽度大于150ps的光脉冲主动偏振控制。采用该技术后，振幅调制器4小时输出光脉冲能量抖动的峰峰值为9.25%，均方根值为1.66% RMS。 4. 针对冲击点火对低时间抖动、高峰值功率的超强短脉冲的需求，提出了一种基于SBS原理的短脉冲主动式放大技术并进行了理论建模。经过数值计算得到以下结论：(1)，对于窄带泵浦光，SBS的能量转移效率与泵浦光和Stokes光的初始相位差无关；(2)，为充分利用泵浦光的能量，泵浦光的长度应等于SBS放大池长度的二倍；(3)，在相同的泵浦功率密度下，泵浦脉宽越宽，能量转移效率越高；(4)，在相同的泵浦能量下，使用脉宽较宽的泵浦脉冲有利于提高能量转移效率。这些结论为以后的实验工作提供了理论依据。 5. 作为光谱控制技术应用探索，开展了对布里渊激光器的研究。介绍了传统布里渊激光器的物理机制。分析了布里渊激光器的光谱特性。自行研制了一台单纵模混合掺镱布里渊光纤激光器，并分析了其相对于传统布里渊激光器所具备的优点。当注入Brillouin泵浦光功率为9.68mW时，激光器最大输出功率可达70.1mW。输出光的光谱信噪比较入射Brillouin泵浦光提高了约10dB。根据理论上的估算，其线宽约为1.27KHz。最后，针对激光器受环境中温度和机械噪声以及Brillouin泵浦光频率稳定性的影响，而发生的跳模现象，提出了解决方案。 |
| 英文摘要 | The high power laser system is the basic tool used for inertial confinement fusion (ICF). In order to compress thermonuclear fuel efficiently, the laser pulse should meet the requirements of temporal pulse shape, synchronous precision, spatial shape, spectrum, energy and so on. Spectrum control technology is critical for the realization of full control of the main laser pulse. On the one hand, considering the need of suppressing the SBS effect in the main laser optics and of beam smoothing, the technique, which combines the single-mode seed laser and phase modulation, is used to expand the laser spectrum. On the other hand, the transmission of the laser with broad spectrum in the high power laser system will gives rise to frequency modulation to amplitude modulation (FM-AM) conversion, which will lead to aberration of temporal pulse shape and finally influence the pulse shaping ability of the system. Therefore, FM-AM effect needs to compensate in frequency domain. Moreover, SBS amplification, as a technique for improve pulse energy, requires that the frequency difference between pump and Stokes wave should equal the Brillouin shift, which also depends on the spectrum control technology. This thesis is dedicated to studying the spectrum control technology for the purpose of realizing full control of the main laser pulse. It focuses on the theoretical and experimental research of these key techniques: phase modulation technique in beam smoothing, the compensation of FM-AM effect, short pulse amplification based on active SBS and the Brillouin fiber laser. 1. To reduce the occurrence of pinhole closure when the laser with broad spectrum passed the pinhole, the technology of dynamically bandwidth reduction after sufficient beam smoothing was investigated for the first time domestically. Here, “optical pulse splicing” method was adopted. We have found that due to the limited extinction ratio of the optical switch and the fact that phase difference between the parts of the pulse was not locked, amplitude modulation appeared in the anterior part of the laser pulse whose bandwidth was dynamically reduced. Finally, posssible solutions were proposed, which laid the foundation for the application of this technology. 2. According to the demand of polarization control in the front end, an automatic polarization compensation method for low-repetition frequency short optical pulse was proposed. The key of this method is acquisition of the laser pulse energy. By combined use of an integral and amplification circuit (IAC) and a high-speed acquisition card, the laser pulse energy can be obtained. The IAC is able to broaden the pulsewidth of a nanosecond and even sub-nanosecond pulse to tens of microsecond. This technique can stabilize the polarization of the optical pulse with the repetition frequency of 1 Hz, whose pulsewidth is larger than 150 ps. By using this method, the amplitude jitter of the amplitude modulator output in 4 hours is 1.66% RMS and the peak-to-peak value of it is 9.52%. 3. A new FM-AM compensation method based on polarization mode dispersion (PMD) in a polarization-maintaining (PM) fiber was proposed. According to the theoretical analysis, by adjusting polarization direction of the input pulse, choosing the suitable length of the PM fiber and changing the drive voltage of fiber squeezer, specified FM-AM effect that results from spectrum amplitude transmission variation can be compensated. 4. Aiming at the demand of a spike pulse with low time jitter and high peak power in shock ignition, a technique of short pulse amplification based on active SBS was proposed and its theoretical model was established. According to results of numerical calculations, conclusions listed below were obtained. (1) For the bandwidth limited pump pulse, initial phase difference between pump and Stokes light dose not influence the energy transfer efficiency of the SBS; (2) In order to make full use of pump energy, the length of SBS cell should be at least half of the length of pump pulse; (3) If pump intensity remains constant, energy transfer efficiency will be higher when the pulse width of pump light is wider. (4) If pump energy remains constant, pump light with wider pulsewidth can improve energy transfer efficiency. 5. As an application of the spectrum control technology, we studied the Brillouin fiber laser theoretically and experimentally. The physical mechanism of conventional Brillouin fiber lasers was investigated. The spectrum characteristics of Brillouin fiber lasers were analyzed. Then a hybrid Brillouin/Ytterbium fiber laser (BYFL) was demonstrated and its advantages were elucidated compared with conventional Brillouin fibers. When the Brillouin pump (BP) was 9.68 mw, the BYFL maximum output power achieved 70.1 mw. The optical signal-to-noise ratio of the BYFL increased by 10 dB compared to that of the BP. According to the numerical calculation, the linewidth of this BYFL was 1.27 KHz. Finally, possible solutions were proposed to solve the mode hopping problem resulting from mechanical and thermal noise in environment. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15707]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 张鹏. 前端预放中光谱控制和应用研究[D]. 中国科学院上海光学精密机械研究所. 2012. |
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