Single and coupled cavity mode sensing schemes using a diagnostic field

TL;DR

This paper introduces a new mode sensing scheme for simple and coupled optical cavities that enhances mode matching without moving parts, aiding gravitational-wave detectors with high squeezing levels.

Contribution

A novel diagnostic field-based mode sensing method for simple and coupled cavities that does not require tuning or moving parts, improving mode matching in quantum experiments.

Findings

Effective error signals derived for mode mismatch detection.

Demonstrated feasibility with Einstein Telescope optical design.

Supports high squeezing levels in gravitational-wave detection.

Abstract

Precise optical mode matching is of critical importance in experiments using squeezed-vacuum states. Automatic spatial-mode matching schemes have the potential to reduce losses and improve loss stability. However, in quantum-enhanced coupled-cavity experiments, such as gravitational-wave detectors, one must also ensure that the sub-cavities are also mode matched. We propose a new mode sensing scheme, which works for simple and coupled cavities. The scheme requires no moving parts, nor tuning of Gouy phases. Instead a diagnostic field tuned to the HG20/LG10 mode frequency is used. The error signals are derived to be proportional to the difference in waist position, and difference in Rayleigh ranges, between the sub-cavity eigenmodes. The two error signals are separable by 90 degrees of demodulation phase. We demonstrate reasonable error signals for a simplified Einstein Telescope optical design. This work will facilitate routine use of extremely high levels of squeezing in current and future gravitational-wave detectors.