Optical layout for a 10m Fabry-P\'erot Michelson interferometer with tunable stability

TL;DR

This paper presents an optical design for a 10-meter Fabry-Pérot Michelson interferometer with adjustable stability, enabling exploration of stability limits and Brownian noise measurement in high-precision interferometry.

Contribution

It introduces a tunable stability optical layout allowing stepwise increase of arm cavity length with minimal hardware changes.

Findings

Design enables initial high stability operation.

Allows measurement of coating Brownian noise at low frequencies.

Facilitates exploration of stability limits with realistic mirrors.

Abstract

The sensitivity of high-precision interferometric measurements can be limited by Brownian noise within dielectric mirror coatings. This occurs, for instance, in the optical resonators of gravitational wave detectors where the noise can be reduced by increasing the laser beam size. However, the stability of the resonator and its optical performance often impose a limit on the maximally feasible beam size. In this article we describe the optical design of a 10\,m Fabry-P\'erot Michelson interferometer with tunable stability. Our design will allow us to carry out initial commissioning with arm cavities of high stability, while afterwards the arm cavity length can be increased stepwise towards the final, marginally stable configuration. Requiring only minimal hardware changes, with respect to a comparable "static" layout, the proposed technique will not only enable us to explore the stability limits of an optical resonator with realistic mirrors exhibiting inevitable surface imperfections, but also the opportunity to measure coating Brownian noise at frequencies as low as a few hundred Hertz. A detailed optical design of the tunable interferometer is presented and requirements for the optical elements are derived from robustness evaluations.