A simplified cryogenic optical resonator apparatus providing ultra-low frequency drift

A system providing an optical frequency with instability comparable to that of a hydrogen maser is presented. It consists of a 5 cm long, vertically oriented silicon optical resonator operated at temperatures between 1.5 K and 3.6 K in a closed-cycle cryostat with a low-temperature Joule-Thomson stage. We show that with a standard cryostat, a simple cryogenic optomechanical setup, and no active or passive vibration isolation, a minimum frequency instability of 2.5 × 10^-15 at τ = 1500 s integration time can be reached. The influence of pulse-tube vibrations was minimized by using a resonator designed for low acceleration sensitivity. With reduced optical laser power and interrogation duty cycle, an ultra-low fractional frequency drift of -2.6 × 10^-19/s is reached. At 3.5 K, the resonator frequency exhibits a vanishing thermal sensitivity and an ultra-small temperature derivative 8.5 × 10^-12/K². These are favorable properties that should lead to high performance also in simpler cryostats not equipped with a Joule-Thomson stage.