# Design and experimental demonstration of a laser modulation system for future gravitational-wave detectors

## TL;DR

This paper presents a laser modulation system using Mach-Zehnder interferometers designed to reduce quantum noise in future gravitational-wave detectors, with experimental validation of its noise reduction capabilities.

## Contribution

It introduces a novel laser modulation system with two Mach-Zehnder interferometers that reduces excess noise and enhances control robustness in gravitational-wave detectors.

## Key findings

- Successfully demonstrated noise reduction in a proof-of-principle experiment.
- Derived displacement noise requirements for the Mach-Zehnder Modulator.
- Showed potential for improved quantum noise performance in future detectors.

## Abstract

Detuning the signal-recycling cavity length from a cavity resonance significantly improves the quantum noise beyond the standard quantum limit, while there is no km-scale gravitational-wave detector successfully implemented the technique. The detuning technique is known to introduce great excess noise, and such noise can be reduced by a laser modulation system with two Mach-Zehnder interferometers in series. This modulation system, termed Mach-Zehnder Modulator (MZM), also makes the control of the gravitational-wave detector more robust by introducing the third modulation field which is non-resonant in any part of the main interferometer. On the other hand, mirror displacements of the Mach-Zehnder interferometers arise a new kind of noise source coupled to the gravitational-wave signal port. In this paper, the displacement noise requirement of the MZM is derived, and also results of our proof-of-principle experiment is reported.