Diffractively coupled Fabry-Perot resonator with power-recycling

TL;DR

This paper demonstrates an all-reflective coupling method for Fabry-Perot resonators using a 3-port diffraction grating, avoiding substrate absorption and thermal issues, with potential applications in gravitational wave detection.

Contribution

The authors experimentally realize and validate an all-reflective cavity coupling scheme with a 3-port diffraction grating, confirming theoretical predictions and enabling broader bandwidths for gravitational wave detectors.

Findings

Experimental validation of all-reflective cavity coupling.

Agreement with theoretical models of 3-port diffraction gratings.

Potential for improved gravitational wave detector sensitivity.

Abstract

We demonstrate the optical coupling of two cavities without light transmission through a substrate. Compared to a conventional coupling component, that is a partially transmissive mirror, an all-reflective coupler avoids light absorption in the substrate and therefore associated thermal problems, and even allows the use of opaque materials with possibly favourable mechanical and thermal properties. Recently, the all-reflective coupling of two cavities with a low-efficiency 3-port diffraction grating was theoretically investigated. Such a grating has an additional (a third) port. However, it was shown that the additional port does not necessarily decrease the bandwidth of the coupled cavities. Such an all-reflective scheme for cavity coupling is of interest in the field of gravitational wave detection. In such detectors light that is resonantly enhanced inside the so-called power-recycling cavity is coupled to (kilometre-scale) Fabry-Perot resonators representing the arms of a Michelson interferometer. In order to achieve a high sensitivity over a broad spectrum, the Fabry-Perot resonators need to have a high bandwidth for a given (high) power build-up. We realized such an all-reflective coupling in a table-top experiment. Our findings are in full agreement with the theoretical model incorporating the characteristics of the 3-port grating used, and therefore encourage the application of all-reflective cavity couplers in future gravitational wave detectors.