Spectrally Narrow, Long-Term Stable Optical Frequency Reference Based on a Eu3+:Y2SiO5 Crystal at Cryogenic Temperature

Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu3+:Y2SiO5 at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5 +/- 3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.