Three hundred microsecond optical cavity storage time and <b>10⁻⁷</b> active RAM cancellation for <b>10⁻¹⁹</b> laser frequency stabilization

Frequency stabilization of lasers to optical reference cavities is an established method to achieve state-of-the-art stability. The strengths of this method are the high discriminator coefficient of optical cavities and the low-noise extraction of the stabilization signal using modulation techniques. In this Letter, we report beyond state-of-the-art performance on both of these fundamentals, unlocking 10 −19 fractional frequency laser stabilization. We employ a 68 cm long cavity to realize an optical storage time of 300 µs, achieving ultrahigh frequency discrimination. We develop a simple and robust scheme to actively cancel residual amplitude modulation (RAM) at the 10 −7 level in an annealed-proton-exchanged (APE) lithium-niobate waveguide electro-optic modulator (EOM).