Frequency stabilization to 6 x 10-16 via spectral-hole burning

We demonstrate two-stage laser stabilization based on a combination of Fabry-Perot and spectral-hole burning techniques. The laser is first pre-stabilized by the Fabry-Perot cavity to a fractional-frequency stability of sigma(y)(tau) < 1 x 10(-13). A pattern of multiple spectral holes written in the absorption spectrum of Eu3+:Y2SiO5 serves to further stabilize the laser to sigma(y)(tau) <= 6 x 10(-16) for 2 s <= tau <= 8 s. We also measure the frequency sensitivity of Eu3+:Y2SiO5 spectral holes to environmental perturbations including temperature (16 kHz K-2), pressure (211.4 Hz Pa-1) and acceleration (7 x 10(-12) g(-1)). Each spectral hole sensitivity parameter is lower than the corresponding parameter for Fabry-Perot cavities, suggesting that spectral holes can be more frequency-stable.