Wave Front Sensing demodulated at the difference frequency between two phase-modulation sidebands in a compound interferometer configuration for a gravitational-wave detector

TL;DR

This paper introduces a novel wave front sensing technique that demodulates at the difference frequency between two phase-modulated sidebands, enabling better decoupling of alignment signals in gravitational-wave detectors.

Contribution

The paper proposes and demonstrates a new PM sideband difference frequency wave front sensing method, improving alignment control in interferometric gravitational-wave detectors.

Findings

Effective decoupling of multiple alignment signals achieved

Stable interferometer locking for over one hour demonstrated

Experimental validation using KAGRA's PRXARM configuration

Abstract

Precise alignment sensing and control are essential for maintaining the stability of laser interferometric gravitational-wave detectors. Conventional Wave Front Sensing technique (WFS), which relies on the beat between the carrier and phase-modulated (PM) sidebands, is dominated by arm-axis signals when the carrier resonates in the full interferometer. This dominance limits the detection of other optical axes, such as the Power Recycling Cavity (PRC) and incident beam axes. To address this problem, we propose a novel sensing technique, "Phase-Modulated-sideband $\times$ Phase-Modulated-sideband Wave Front Sensing" (PMPMWFS), which demodulates the beat signal at the difference frequency between two anti-resonant PM sidebands. We derived the theoretical response of PMPMWFS and experimentally demonstrated it using the Power-Recycled X-arm (PRXARM) configuration of KAGRA. The results show that PMPMWFS effectively decouples angular fluctuation signals of the PRC and incident beam from those of the arm cavity and provides orthogonal signal components for the end mirror of the arm cavity. Furthermore, feedback control using PMPMWFS achieved stable interferometer locking for over one hour. These results demonstrate that PMPMWFS offers an effective sensing method for decoupling multiple alignment degrees of freedom in future gravitational-wave detectors.