Optical frequency reference based on a cryogenic silicon resonator

We present the development and in-depth characterization of an optical reference based on a 1.5 μm laser stabilized to a cryogenic silicon optical resonator operated at 1.7 K. The closed-cycle cryostat is equipped with a cryogenic passive vibration isolation. At τ = 1 s integration time the frequency instability is 2 × 10^-14, predominantly due to residual vibrations. At τ = 100 s the frequency instability is 6.2 × 10^-15. The lowest instability of 3.5 × 10^-16 occurs at τ = 6000 s, and is limited by the stability of the hydrogen maser used in the comparison. The mean fractional frequency drift rate over 190 days was -3.7 × 10^-20/s. In conjunction with a frequency comb and a GNSS receiver this optical reference would be suitable to provide optical frequencies with accuracies at the low 10^-14 level. We show that residual vibrations affect the resonator and the optical fiber delivering the laser light to it, and that laboratory temperature variations contribute to frequency instability at short and medium integration times. Mitigation of these issues might in the future allow for demonstration of the thermal-noise-limited performance of the resonator.