An arm length stabilization system for KAGRA and future gravitational-wave detectors

TL;DR

This paper introduces a simplified arm length stabilization system for gravitational-wave detectors like KAGRA, demonstrating its effectiveness in lock acquisition and low residual noise through simulations and experimental tests.

Contribution

A new ALS design compatible with long-arm interferometers that simplifies control configuration and eliminates the need for optical fibers along the arms.

Findings

Lock acquisition achieved with the new ALS in KAGRA

Residual noise measured at 8.2 Hz, below the arm cavity linewidth

Successful experimental demonstration of the ALS in a single arm cavity

Abstract

Modern ground-based gravitational wave (GW) detectors require a complex interferometer configuration with multiple coupled optical cavities. Since achieving the resonances of the arm cavities is the most challenging among the lock acquisition processes, the scheme called arm length stabilization (ALS) had been employed for lock acquisition of the arm cavities. We designed a new type of the ALS, which is compatible with the interferometers having long arms like the next generation GW detectors. The features of the new ALS are that the control configuration is simpler than those of previous ones and that it is not necessary to lay optical fibers for the ALS along the kilometer-long arms of the detector. Along with simulations of its noise performance, an experimental test of the new ALS was performed utilizing a single arm cavity of KAGRA. This paper presents the first results of the test where we demonstrated that lock acquisition of the arm cavity was achieved using the new ALS and residual noise was measured to be $8.2\,\mathrm{Hz}$ in units of frequency, which is smaller than the linewidth of the arm cavity and thus low enough to lock the full interferometer of KAGRA in a repeatable and reliable manner.