Absolute and Relative Stability of an Optical Frequency Reference Based on Spectral Hole Burning in Eu3+:Y2SiO5.

We present and analyze four frequency measurements designed to characterize the performance of an optical frequency reference based on spectral hole burning in Eu3+:Y2SiO5. The first frequency comparison, between a single unperturbed spectral hole and a hydrogen maser, demonstrates a fractional frequency drift rate of 5 ⨰ 10-18 s-1. Optical frequency comparisons between a pattern of spectral holes, a Fabry-Pérot cavity, and an Al+ optical atomic clock show a short-term fractional frequency stability of 1 × 10-15τ-1/2that averages down to 2.51 1.1 -0.5 × 10-6 at τ = 540 s (with linear frequency drift removed). Finally, spectral-hole patterns in two different Eu3+:Y2SiO5 crystals located in the same cryogenic vessel are compared, yielding a short-term stability of 7 × 10-16τ-1/2 that averages down to 5.5+18-0.9 × 10-17 at τ = 204 s (with quadratic frequency drift removed). [ABSTRACT FROM AUTHOR]