Laser stabilization at the millihertz level

TLDR

Laser sources can theoretically achieve millihertz linewidths when optimally locked to a high‑finesse Fabry–Perot cavity, as shown by their basic stability characteristics. The paper aims to identify the physical problems that must be addressed to stabilize laser linewidths well below 1 Hz. The authors analyze optical feedback, modulation errors, mechanical support and isolation, and thermal stabilization of the cavity, and discuss interim solutions. Experimentally, they achieved locking accuracy better than 2 × 10⁻⁵ of the linewidth (≈1.5 Hz) and maintained phase coherence between two lasers for 8 s, corresponding to a 50 mHz linewidth.

Abstract

The main task of this paper is to identify a number of physical problems that must be successfully addressed to achieve stabilized laser linewidths well below 1 Hz. After presentation of the basic stability characteristics of available laser sources, we show that if any of these lasers were optimally locked to a high-finesse Fabry–Perot cavity it would be theoretically possible to obtain a laser linewidth in the millihertz domain. Problems of optical feedback, modulation waveform errors, mechanical support and isolation of the reference cavity, thermal stabilization of the environment, etc. are considered, and interim solutions are discussed. Experimentally, locking accuracy to successive cavity orders of <2 × 10−5 linewidths (±1.5 Hz) was achieved; mirror birefringence pulled the lock point approximately 10-fold more. Relative phase coherence between two independent lasers locked onto adjacent cavity orders was preserved for 8 sec, corresponding to a linewidth of each optical source of ∼50 mHz.