A sub-40 mHz laser based on a silicon single-crystal optical cavity

State-of-the-art optical oscillators are limited by thermal noise that causes fluctuations of the cavity length. This laser performance is currently setting the limit for the stability of optical clocks. We demonstrate that single crystal silicon as resonator material can potentially reduce the thermal noise limit for optical cavities by an order of magnitude. We present an experimental realization of such a cavity where the spacer and the mirror substrates are both machined from single crystal silicon and operated at 124 K where the thermal expansion coefficient is zero. We present the setup and performance of this cavity. The stability was analyzed in a three-cornered-hat analysis of heterodyne beat signals among three independent stable lasers. The silicon cavity demonstrates a fractional frequency stability of 1 × 10^-16 at short time scales and supports a world-record laser linewidth of <;40 mHz at 1.5 μm.