基于飞秒光梳激发铷、氢原子双光子跃迁的光频率标准

The optical frequency standard, as the most stable and precise atomic frequency standard, is widely used in the time and frequency fields. In typical optical frequency standards, ultrastable optical resonantcavity is applied to narrow the laser linewidth, and atomic cooling and trapping systems are used to suppress the Doppler-broadening of atom transition lines. In this project, instead of using the complex resonant cavity and cooling systems,we propose a simple, long-term operating optical frequency standard based on the atomic two-photon transitions excited by a femtosecond optical combs.We first study on the evolution mechanism of ultrashort pulse in time domain and frequency domain in the process of pulses generation, amplification and frequency doubling, and the generation mechanism of Doppler broadening in the two-photon transitions excited by femtosecond combs, then propose schemes to optimize the femtosecond pulses and eliminate the Doppler broadening via quantum coherence control. Furthermore, a simple, long-term operating optical frequency standard for time-keeping based on two-photon transitions of rubidium atoms is to be built in our project. Based on the former work of rubidium, we study the theory of two-photon transition of hydrogen, the transition rate of the condition of pulsed excitation, and calculate the center frequency and relative intensity of two-photon transition in hydrogen. We intend to research the technology of ultraviolet optical comb to excited 1S-3S and 1S-2S two-photon transitions of hydrogen atom, which has much more scientific significance, in order to realize an optical frequency standard based on two-photon transitions of hydrogen atoms.

光频标是现今稳定度最高的原子频率标准，基于光频标实现的频率标准在时频领域获得了广泛应用。传统光频标的实现要利用高稳光学谐振腔压窄激光器线宽，同时要利用原子冷却和囚禁技术去除多普勒展宽等效应以获得超窄原子谱线。本课题提出一种基于飞秒光梳激发原子双光子跃迁的光学频率标准，省去传统光频标中复杂的冷原子系统和超级谐振腔。我们将以铷双光子跃迁为基础，研究飞秒激光产生、放大和倍频过程中脉冲时域频域形态演化机理及操控优化方法；多普勒基底和展宽等效应的产生机理及基于量子操控方法的频移消除策略研究；建立结构简单、可长期运行的基于铷原子双光子跃迁的守时型光学频率标准。同时本课题在铷的工作基础上，研究更具科学价值的氢原子双光子跃迁。研究氢原子双光子跃迁几率及其谱线位置和强度，研究紫外光梳的产生原理，利用紫外光梳激发氢原子1S-3S、1S-2S双光子跃迁，最终实现基于氢原子双光子跃迁的光学频率标准。

光频标是现今稳定度最高的原子频率标准，基于光频标实现的频率标准在时频领域获得了广泛应用。然而，传统光频标的实现要利用高稳光学谐振腔压窄激光器线宽，同时要利用原子冷却和囚禁技术去除多普勒展宽等效应以获得超窄原子谱线，系统结构复杂，实现难度大。本项目研究了基于飞秒光梳激发原子双光子跃迁的光学频率标准，省去传统光频标中复杂的超级谐振腔。课题组以铷双光子跃迁为研究基础，以氢双光子跃迁为重点，研究了飞秒光梳光源产生、放大和倍频过程相关理论及操控优化方法；研究了多普勒基底和展宽等效应的产生机理及基于量子操控方法的频移消除策略；建立了高稳定度铷原子双光子频标；设计并实现了紫外光梳，实现了氢原子双光子跃迁谱线探测。.具体内容为：.1、以光纤光梳作为光源，通过两级掺铒光纤放大器实现光放大，使用棱镜对色散补偿以压缩展宽的脉冲，通过PPLN进行倍频，得到778nm的锁模光，用于铷5S-5D跃迁。.2、基于光纤光梳，分别实现基于热铷原子与冷铷原子5S-5D跃迁的双光子频率标准。具体地，建立了秒稳8.0×10-12，千秒稳4.3×10-13的热铷双光子频标，能够持续运行三小时以上；使用磁光阱冷却囚禁铷原子，建立了更高稳定度的冷铷双光子频标，秒稳1.4×10-12，千秒稳达到4.8×10-14。.3、利用量子相干操控方式优化双光子光谱。通过压电陶瓷调节与泵浦光强调节相结合的方式调节光梳的重复频率，以及裁剪光谱的方式，实现对同向双光子跃迁噪底的抑制，提高跃迁谱线的信噪比。.4、自主搭建了钛宝石锁模激光器，基于820nm锁模光四倍频的方式，实现205nm的紫外光梳；基于729nm锁模光三倍频的方式，实现243nm的紫外光梳。对钛蓝宝石光梳与倍频光路进行了优化，提高了紫外光梳的功率。.5、设计并制造了一套氢原子束制备与冷却系统，将氢原子冷却至7.8K。利用氢原子装置与紫外光梳，完成氢原子双光子跃迁谱线探测。