Residual amplitude modulation at the $10^{-7}$ level for ultra-stable lasers

TL;DR

This paper demonstrates an active system to reduce residual amplitude modulation in laser stabilization, achieving a RAM level of 1.4×10⁻⁷, significantly below the thermal noise limit of ultra-stable cavities.

Contribution

It introduces a digital servo-based active RAM reduction method using a free space EOM for ultra-stable laser stabilization.

Findings

Achieved RAM stability of 1.4×10⁻⁷

RAM contribution to frequency instability is below 5×10⁻¹⁹

Enables laser stabilization at the thermal noise limit

Abstract

The stabilization of lasers on ultra-stable optical cavities by the Pound-Drever-Hall (PDH) technique is a widely used method. The PDH method relies on the phase-modulation of the laser, which is usually performed by an electro-optic modulator (EOM). When approaching the $10^{-16}$ level, this technology requires an active control of the residual amplitude modulation (RAM) generated by the EOM in order to bring the frequency stability of the laser down to the thermal noise limit of the ultra-stable cavity. In this article, we report on the development of an active system of RAM reduction based on a free space EOM, which is used to perform PDH-stabilization of a laser on a cryogenic silicon cavity. A RAM stability of $1.4 \times 10^{-7}$ is obtained by employing a digital servo that stabilizes the EOM DC electric field, the crystal temperature and the laser power. Considering an ultra-stable cavity with a finesse of $2.5\times 10^5$, this RAM level would contribute to the fractional frequency instability at the level of about $5\times 10^{-19}$, well below the state of the art thermal noise limit of a few $ 10^{-17}$.