Optimal Alignment Sensing of a Readout Mode Cleaner Cavity

TL;DR

This paper introduces a modified dither alignment sensing method that enhances the signal-to-noise ratio in optical mode cleaner cavities by making the error signals sensitive to the signal sideband, demonstrated with a threefold SNR improvement in a gravitational-wave detector.

Contribution

The paper presents a novel alignment sensing technique that improves SNR by modulating the signal field, enabling better alignment control in optical cavities.

Findings

Achieved a 3x SNR improvement in a gravitational-wave detector.

Demonstrated the method's applicability to kilometer-scale optical systems.

Enhanced alignment sensitivity to signal sidebands.

Abstract

Critically coupled resonant optical cavities are often used as mode cleaners in optical systems to improve the signal to noise ratio (SNR) of a signal that is encoded as an amplitude modulation of a laser beam. Achieving the best SNR requires maintaining the alignment of the mode cleaner relative to the laser beam on which the signal is encoded. An automatic alignment system which is primarily sensitive to the carrier field component of the beam will not, in general, provide optimal SNR. We present an approach that modifies traditional dither alignment sensing by applying a large amplitude modulation on the signal field, thereby producing error signals that are sensitive to the signal sideband field alignment. When used in conjunction with alignment actuators, this approach can improve the detected SNR; we demonstrate a factor of 3 improvement in the SNR of a kilometer-scale detector of the Laser Interferometer Gravitational-wave Observatory. This approach can be generalized to other types of alignment sensors.