Wavefront Sensing with a Coupled Cavity for Torsion-Bar Antenna

TL;DR

This paper proposes a novel wavefront sensing scheme using a coupled cavity to measure tiny pendulum rotations in a gravitational wave detector, demonstrating feasibility through experimental locking techniques.

Contribution

A new wavefront sensing method with a coupled cavity is introduced, enhancing angular signal detection for torsion-bar antennas in gravitational wave observation.

Findings

Feasibility of angular signal amplification confirmed.

Coupled cavity successfully locked using Pound-Drever-Hall technique.

Prototype demonstrates potential for improved torsion pendulum measurements.

Abstract

Torsion-Bar Antenna (TOBA) is a ground-based gravitational wave detector using torsion pendulums. TOBA can detect intermediate-mass black hole binary mergers, gravitational wave stochastic background, and Newtonian noise, and is useful for earthquake early warning. A prototype detector Phase-III TOBA with 35 cm-scale pendulums is under development to demonstrate noise reduction. The target strain sensitivity is set to $1\times10^{-15}\,{/\sqrt{\rm Hz}}$ between 0.1 Hz--10 Hz. A new scheme of wavefront sensing with a coupled cavity was proposed to measure the pendulum rotation as low as $5\times10^{-16}\,{{\rm rad}/\sqrt{\rm Hz}}$ for Phase-III TOBA. In our method, an auxiliary cavity is used to enhance the first-order Hermite--Gaussian mode in a main cavity. Experimental demonstration is ongoing to confirm the feasibility of angular signal amplification and establish a method for locking a coupled cavity. We evaluated the performance of the coupled cavity and concluded that angular signal amplification would be feasible with this sensor. The coupled cavity was successfully locked to the resonance by the Pound--Drever--Hall technique with two modulation frequencies.