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题名	用于汞原子光晶格钟的大功率连续深紫外激光技术研究
作者	赵儒臣
文献子类	博士
导师	王育竹
关键词	汞原子光晶格钟 neutral mercury optical lattice clock 深紫外激光 deep-ultraviolet laser 外腔谐振倍频技术 enhanced cavity frequency doubling 激光冷却 laser cooling
其他题名	Research on high-power continuous-wave deep-ultraviolet laser techniques for mercury optical lattice clock
英文摘要	高精度的时间频率标准是精密测量物理和验证基本物理模型的关键和基础。近年来，光频标的研究已经成为原子频标领域的热点，光钟频率的不确定度和稳定度均已大幅超过了最好的微波原子钟，其不确定度已经进入10-18水平。中性原子光晶格钟由于其探测的原子数更多，在量子投影噪声极限上比单离子光钟小一个数量级。其中，汞原子光晶格钟与其他中性原子光晶格钟相比，其黑体辐射频移要小一个量级以上，因此汞原子成为下一代光频标最热门的候选元素之一。实现汞原子光晶格钟的挑战在于，用于汞原子的冷却激光，魔术波长激光以及窄线宽钟激光的波长均处于深紫外波段，难以直接获得，这些波长通常需要由红外或者可见波段的连续激光进行倍频或者四倍频产生。除了汞原子，很多原子（离子）和分子的激光光谱及应用也需要大功率的连续紫外激光。合适波长的高功率基频激光，倍频效率及紫外损伤等都是限制大功率深紫外激光产生的重要因素。 本文主要针对汞原子光晶格钟所需的大功率连续深紫外激光系统开展了一系列研究工作，解决了大功率连续深紫外激光系统中的一些关键问题。其中，建立了一套基于室温下光纤激光放大器和两个级联高效率倍频的大功率连续可调谐的253.7 nm冷却激光系统，实现了最高1.4 W的深紫外激光输出，稳定输出功率为760 mW；完成了265.6 nm钟频激光系统中的两级倍频，获得了10.6 mW的深紫外激光输出。 实现大功率连续深紫外激光输出的关键是高效率的外腔倍频。根据Boyd-Kleinman的聚焦高斯光束倍频理论和环形谐振腔增益倍频理论，设计并优化了环形倍频腔的多个参数，获得了高效率的倍频。其中，在冷却激光系统中，第一级LBO倍频器的倍频效率可以达到60%以上，最大可以获得3.9 W的507.4 nm绿光输出，达到该倍频器的理论极限；第二级BBO倍频器的倍频效率约35%，最大可以获得1.4 W的253.7nm深紫外激光输出及760 mW的稳定输出。在钟频激光系统中，第一级采用PPLN晶体的单次通过倍频，获得了275 mW的531.2nm激光输出，第二级采用BBO晶体腔倍频，获得了10.6 mW的265.6 nm激光输出。 另外，大功率连续深紫外激光系统还需要采取一系列的技术手段来保障激光的稳定输出。为进一步压窄外腔半导体激光器（ECDL）的线宽，采用了自制的低噪声电流源，并通过PDH稳频技术将ECDL的频率锁定到自制的Fabry-Perot（FP）参考腔，激光线宽被压窄到23 kHz，倍频后的功率噪声也大大降低，同时保证四倍频后紫外激光的线宽小于100 kHz，远小于汞原子冷却所用的61S0 - 63P1跃迁的自然线宽。为了提高倍频腔的连续运行能力，设计了基于模拟电路的自动锁腔装置，实现了谐振腔失锁后的自动重新锁定。253.7nm深紫外激光易对晶体表面和镀膜表面形成损伤，为了提高第二级倍频器的长期运行能力，选择了双色片耦合输出的方式来避免镀膜损伤，并采用全封闭充高纯氧气的倍频腔结构来防止紫外损伤和紫外诱导的有机污染，实现了在100 mW输出功率运行1小时以上输出功率无明显衰减。为了提高PPLN晶体的输出稳定性，设计并加工了PPLN晶体的热沉和温控装置，利用自制的温控电路使PPLN晶体的温度稳定性优于6mK。 汞原子的激光冷却和参数测量需要对激光、磁场、CCD相机等进行时序控制。本实验中，利用NI公司的DAQ板卡等建立了冷原子产生和测试的时序控制系统，并用LabWindows/CVI软件编写了具有四个模拟通道输出的时序控制程序。该系统已经应用于汞原子磁光阱（MOT）的参数测量，获得了超冷汞原子的原子数和温度等参数， 稳定输出的大功率深紫外激光系统为汞原子光晶格钟的研究提供了可靠的光源，冷原子时序控制系统及其程序可以扩展并应用到汞原子光晶格钟的系统中。这些工作是汞原子光晶格钟研究的关键步骤，也为进一步的研究奠定了基础。相关的倍频技术还可以应用到其他的原子物理研究中。; The high accuracy time and frequency standard is the key factor and the basic foundation for precision measurement physics and the test of the fundamental physical models. Recently, optical frequency standard became a hot topic in the researches of atomic frequency standard. The frequency uncertainty and stability of optical clocks, whose frequency uncertainty has reached the 10-18 level, have greatly surpassed the best microwave atomic clocks. Due to the numerous atoms can be detected in optical lattice clocks, its quantum projection noise is much smaller than that in the single ion optical clocks. Compared with the other optical lattice clocks, the blackbody radiation shift is more than one order of magnitude lower for neutral mercury (Hg) optical lattice clock. Therefore, Hg atom is a good candidate for the next generation of optical frequency standard. To achieve the Hg optical lattice clock, it requires to construct the cooling laser, the magic-wavelength lattice laser and the narrow-linewidth clock laser. However, the wavelengths of these lasers are all in the deep-ultraviolet (DUV) region, so they cannot generate directly, therefore have to frequency double or quadruple from continuous-wave (CW) infrared laser or visible laser. In addition to Hg atom, the high-power CW DUV lasers are widely used in the atomic (ionic) and molecular laser spectroscopy and their applications. The important factors to generate high-power CW DUV laser are the high-power fundamental laser, high efficient frequency-doubling and preventing of DUV induced damage. In this paper, to realize the high-power CW DUV laser sources in the Hg optical lattice clock, some key technical problems have been studied and solved. A high-power tunable CW DUV laser at 253.7 nm is developed based on a high-power 1014.8 nm room-temperature fiber laser amplifier and two cascaded efficient frequency-doubling stages. The maximal output power reaches 1.4 W initially, and it is stabled at 760 mW. Two cascaded efficient frequency-doubling stages have been demonstrated for the clock laser system, and 10.6 mW output power is obtained at 265.6 nm. The high-efficient cavity-enhanced frequency doubling is important to generate high-power CW DUV lasers. According to the Boyd-Kleinman theory and the theory of cavity enhanced frequency doubling, the parameters of the enhanced bow-tie ring cavities are designed and optimized, and high efficient frequency doublings are realized. In the first stage of cooling laser system with LBO crystal, the conversion efficiency reaches 60% in and the maximal output is 3.9 W at 507.4 nm, which agreed well with the design. In the second stage with BBO crystal, the conversion efficiency reaches 35%, and 760 mW stable output at 253.7 nm is obtained. In the clock laser system, single pass frequency doubling with PPLN crystal is utilized in the first stage, then an enhanced cavity with BBO crystal is applied in the second frequency doubling stage. The maximum output power of 10.6 mW clock laser at 265.6 nm is demonstrated. In addition, a series of techniques have been applied to maintain the long-term operation of these high-power CW DUV laser systems. By using a home-made low-noise current controller and PDH locking the ECDL at 1014.8 nm to a home-made temperature-controlled Invar Fabry-Perot (FP) cavity, the linewidth of the ECDL is narrowed to 23 kHz and the relative intensity noise is suppressed. The linewidth of the DUV laser is reduced to less than 100 kHz, which is narrower than the natural linewidth of the 61S0-63P1 transition. In order to improve the long-term operation, an analog auto-relock servo is designed to relock the ring cavity automatically. Due to the proneness of DUV damage on the optical coatings at 253.7 nm, a dichromatic mirror is placed directly after the BBO crystal as an output coupler. The shield box for the BBO crystal and the enhanced cavity is filled with filtered clean dry oxygen (O2) gas with 99.9% purity to reduce the possibility of organic contamination on the optical surfaces. The laser system is demonstrated a 100 mW continuous operation without any power-degradation in one hour. A home-made temperature controller for PPLN crystal is developed to keep it working at the optimum temperature, and the stability of the temperature is below 6 mK. For laser cooling and measurement of mercury atom, it is necessary to use a time-sequence to control the elements in the MOT, such as the laser, the magnetic field and EMCCD camera et al. In our experiment, a time sequence control system is built to generate and measure the cold atoms with NI DAQ cards et al., and a time-sequence controlling program is written with the NI LabWindows/CVI software to generate four analog signals. This system has already used to measure the atom number and temperature of cold mercury atoms in the MOT. The development of stable high-power CW DUV laser sources is important for research of Hg optical lattice clock. The time-sequence controlling system and program can be extended and applied for the next step of Hg optical lattice clock. These works are the key steps for the research on Hg optical lattice clock, and laid a good foundation for future works. The techniques on frequency doubling can also be applied to the other researches of atomic physic.
学科主题	光学工程
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/31023]
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	赵儒臣. 用于汞原子光晶格钟的大功率连续深紫外激光技术研究[D].

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