Locking the frequency of lasers to an optical cavity at the $1.6 \times 10^{-17}$ relative instability level

TL;DR

This paper demonstrates ultra-stable laser frequency locking to an optical cavity, achieving a relative instability of 1.6×10⁻¹⁷, crucial for advanced optical clocks and fundamental physics experiments.

Contribution

The authors achieved record low frequency instability in laser locking by stabilizing two Nd:YAG lasers to a high-finesse cavity, reaching a relative instability of 1.6×10⁻¹⁷.

Findings

Beat frequency instability of 6.3 mHz at 40 s

Relative instability of 1.6×10⁻¹⁷ compared to laser frequency

Beat signal FWHM of 7.8 mHz

Abstract

We stabilized the frequencies of two independent Nd:YAG lasers to two adjacent longitudinal modes of a high-finesse Fabry-P\'erot resonator and obtained a beat frequency instability of 6.3 mHz at an integration time of 40 s. Referred to a single laser, this is $1.6\times10^{-17}$ relative to the laser frequency, and $1.3\times10^{-6}$ relative to the full width at half maximum of the cavity resonance. The amplitude spectrum of the beat signal had a FWHM of 7.8 mHz. This stable frequency locking is of importance for next-generation optical clock interrogation lasers and fundamental physics tests.