Mode matching error signals using radio-frequency beam shape modulation

TL;DR

This paper introduces a novel RF beam shape modulation technique for precise mode matching in interferometers, demonstrating high accuracy and insensitivity to misalignment, with potential application in Advanced LIGO.

Contribution

A new mode matching sensing scheme using RF sidebands and single photodiodes, insensitive to misalignment, suitable for closed-loop control in gravitational wave detectors.

Findings

Achieved 99.9% mode matching in tabletop experiments.

Demonstrated first-order insensitivity to misalignment.

Proposed implementation for Advanced LIGO.

Abstract

Precise mode matching is needed to maximize performance in coupled cavity interferometers such as Advanced LIGO. In this paper we present a new mode matching sensing scheme that uses a single radio frequency higher order mode sideband and single element photodiodes. It is first order insensitive to misalignment and can serve as an error signal in a closed loop control system for a set of mode matching actuators. We also discuss how it may be implemented in Advanced LIGO. The proposed mode matching error signal has been successfully demonstrated on a tabletop experiment, where the error signal increased the mode matching of a beam to a cavity to 99.9%.