Ultra-high laser frequency stabilization using spectral holes in a cryogenically cooled crystal as a frequency reference

具有超高频率稳定性的激光源在精密原子分子光谱学、时频计量学以及洛伦兹不变性检验等基础物理学中具有重要意义。目前最好的方法是伺服稳定激光频率到一个超低损耗的光腔模式。这在1616年产生了一些相对不稳定的部分。然而，即使是最好的光学腔在原则上也受到热噪声和振动灵敏度的限制，这些都是实现更高性能的严重困难。在这项工作中，我们将寻求一种基于掺杂稀土离子的晶体材料实现超稳定光频率的替代方法。固态频率基准的吸引人的特点是紧凑（尺寸~1 cm3），易于操作，相对不受环境变化（磁场，温度，加速度）的影响，温度低于5 K时非常窄的均匀线宽（0.1 - 100 kHz）。一种特殊类型的光谱学，光谱孔燃烧（SHB），被用来克服固体（0.1 - 10 GHz）中光学跃迁的非均匀加宽所带来的限制。虽然这种方法已经研究了很长时间，但直到最近，申请人的小组和NIST（美国）的一个小组才独立地表明，在580 nm的掺铕钇正硅酸钇晶体（Eu3+:Y2SiO5）中的光学跃迁实际上在短时间和长时间尺度上都具有超高频稳定的潜力。这种方法的基本稳定性极限不同于参考腔体，因此在短期和长期尺度上都有可能超过性能最高的腔体。因此，我们建议充分发展这种方法，目标是在短时间和长时间尺度上展示比最先进的光学腔更好的性能（目标1×10-16在1到10秒的时间尺度上的不稳定性，1×10-15在100 -104秒的时间尺度上）。如果实现这一目标，该方法（与飞秒频率梳相结合）可能成为广泛使用的氢脉泽的替代方案。使用低温本身并不矛盾的简单性和可靠性，因为现代低温技术提供紧凑，易于使用的桌面低温恒温器。

Laser sources with ultra-high frequency stability are of fundamental importance for precision atomic and molecular spectroscopy, time and frequency metrology, as well as fundamental physics, such as tests of Lorentz Invariance. The currently best method is servo-stabilization of the laser frequency to a mode of a ultra-low-loss optical cavity. This yielded relative instabilities of a few parts in 1016. However, even the best optical cavities are limited in principle by thermal noise and vibration sensitivity, and these are serious difficulties on the way to even higher performance. In this work, we will pursue an alternative approach for realizing ultra stable optical frequencies, based on a crystalline material doped with rare earth ions. Appealing features of a solid-state frequency reference are compactness (size ~1 cm3), ease of handling, relative immunity to environmental variations (magnetic field, temperature, acceleration), very narrow homogeneous linewidths (0.1 – 100 kHz) at temperatures below 5 K. A particular type of spectroscopy, spectral hole burning (SHB), is used to overcome the limitations imposed by inhomogeneous broadening of the optical transitions in solids (0.1 - 10 GHz). While this approach has been investigated for a long time, only very recently the applicants’ group and a group at NIST (USA) have independently shown that the optical transitions in the Europium-doped Yttrium orthosilicate crystal (Eu3+:Y2SiO5) at 580 nm do, in fact, have the potential for ultra-high frequency stabilization both on short and on long time scales. The fundamental stability limits of this approach are different from those of a reference cavity and therefore have the potential to surpass even the highest performance cavities on both short and long time scales. Therefore, we propose to fully develop this approach with the goal of demonstrating a performance better than state-of-the art optical cavities both on short times and on long time scales (goal 1×10-16 instability on time scales between 1 and 10 s, 1×10-15 for 100 -104 s). If this is achieved, the approach could become (in combination with a femtosecond frequency comb) an alternative to the widely used Hydrogen maser. The use of cryogenics is not in itself a contradiction to simplicity and reliability since modern cryogenic techniques offer compact, easy-to-use tabletop cryostats.