An active method for coupling laser with a high-finesse Fabry–Pérot cavity in ultra-stable lasers

Abstract

An active method for coupling a laser with a high-finesse Fabry–Pérot cavity in ultra-stable lasers is proposed. This method uses the centroid of the reflected laser spot and the power ratio of the resonant modes to detect the misalignment between the laser and the Fabry–Pérot cavity. Subsequently, four actively rotating wedge prisms are used to compensate for the misalignment. Based on ray tracing and table lookup, the detection and compensation systems can form a closed loop for actively recoupling the laser into the Fabry–Pérot cavity when a misalignment occurs. Through experiments, the laser can be re-coupled with the Fabry–Pérot cavity by four 0.1° wedge prisms within a 0.9 mm decentration or a 0.1° tilt. The re-alignment of the laser and Fabry–Pérot cavity by active optical coupling may provide an alternative solution for the issue of support and vibration sensitivity of the Fabry–Pérot cavity. This solution is essential to improve the frequency stability of transportable and space-borne ultra-stable lasers.