Reduction of coating thermal noise by using an etalon

TL;DR

This paper investigates reducing coating thermal noise in optical interferometers, like gravitational-wave detectors, by using an etalon with a specific coating configuration to minimize phase fluctuations.

Contribution

It provides a thermal-noise analysis of an etalon with coatings on both surfaces, proposing a practical method to lower thermal noise without excessive complexity.

Findings

Etalon configuration reduces thermal noise compared to traditional coatings.

Analysis using Fluctuation-dissipation theorem confirms noise reduction potential.

Design considerations for implementing etalons in high-precision measurements.

Abstract

Reduction of coating thermal noise is a key issue in precise measurements with an optical interferometer. A good example of such a measurement device is a gravitational-wave detector, where each mirror is coated by a few tens of quarter-wavelength dielectric layers to achieve high reflectivity while the thermal-noise level increases with the number of layers. One way to realize the reduction of coating thermal noise, recently proposed by Khalili, is the mechanical separation of the first few layers from the rest so that a major part of the fluctuations contributes only little to the phase shift of the reflected light. Using an etalon, a Fabry-Perot optical resonator of a monolithic cavity, with a few coating layers on the front and significantly more on the back surface is a way to realize such a system without too much complexity, and in this paper we perform a thermal-noise analysis of an etalon using the Fluctuation-dissipation theorem with probes on both sides of a finite-size cylindrical mirror.