All-reflective coupling of two optical cavities with 3-port diffraction gratings

TL;DR

This paper proposes a novel all-reflective coupling scheme for two optical cavities using a 3-port diffraction grating, eliminating substrate transmission and potentially reducing thermal noise in laser interferometers.

Contribution

It introduces a theoretical design for coupling cavities with a 3-port diffraction grating, avoiding substrate transmission and thermal noise issues.

Findings

Power-recycling of high-finesse cavities is feasible without substrate transmission.

The power splitting ratio of the grating's output ports is noncritical.

The scheme potentially reduces thermal noise in interferometric measurements.

Abstract

The shot-noise limited sensitivity of Michelson-type laser interferometers with Fabry-Perot arm cavities can be increased by the so-called power-recycling technique. In such a scheme the power-recycling cavity is optically coupled with the interferometer's arm cavities. A problem arises because the central coupling mirror transmits a rather high laser power and may show thermal lensing, thermo-refractive noise and photo-thermo-refractive noise. Cryogenic cooling of this mirror is also challenging, and thus thermal noise becomes a general problem. Here, we theoretically investigate an all-reflective coupling scheme of two optical cavities based on a 3-port diffraction grating. We show that power-recycling of a high-finesse arm cavity is possible without transmitting any laser power through a substrate material. The power splitting ratio of the three output ports of the grating is, surprisingly, noncritical.