Cancellation of lateral displacement noise of 3-port gratings for coupling light to cavities

TL;DR

This paper demonstrates experimentally that lateral displacement noise in 3-port gratings used for coupling light to optical cavities can be canceled by combining both diffracted output ports, maintaining high signal-to-noise ratio.

Contribution

The study shows that combining both output ports of 3-port gratings cancels lateral displacement noise, enabling low-noise cavity coupling in gravitational wave detectors.

Findings

Lateral displacement noise cancels when both ports are combined.

Achieved signal-to-shot-noise ratio comparable to conventional couplers.

Validates 3-port gratings as a low-absorption alternative for cavity coupling.

Abstract

Reflection gratings enable light coupling to optical cavities without transmission through substrates. Gratings that have three ports and are mounted in second-order Littrow configuration even allow the coupling to high-finesse cavities using low diffraction efficiencies. In contrast to conventional transmissive cavity couplers, however, the phase of the diffracted light depends on the lateral position of the grating, which introduces an additional noise coupling. Here we experimentally demonstrate that this kind of noise cancels out once both diffracted output ports of the grating are combined. We achieve the same signal-to-shot-noise ratio as for a conventional coupler. From this perspective, 3-port grating couplers in second-order Littrow configuration remain a valuable approach to reducing optical absorption of cavity coupler substrates in future gravitational wave detectors.