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

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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.