Demonstration of the optical AC coupling technique at the advanced LIGO gravitational wave detector

Steffen Kaufer,Valera Frolov,Benno Willke,Marie Kasprzack

doi:10.1088/1361-6382/aa7119

Abstract

Lasers for gravitational wave detectors need to fulfill tight requirements in amplitude stability, which can only be met by means of feedback control loops. Ultimately, power stabilization control loops are limited by the shot noise of their sensor. The power noise increases linearly with the amount of detected power, while the shot noise grows with the square root. Increasing the detected power is therefore a suitable means to reach a lower sensing noise but it is limited by the power handling capabilities of the photodiodes.An alternative way of improving the sensitivity is the optical AC coupling technique, which exploits the high pass behavior of an optical resonator to reduce the optical power on the detector without compromising its sensitivity above the corner frequency.In this paper we investigate the optical AC coupling technique at the aLIGO Livingston gravitational wave detector. We measured an optical AC coupling gain of 10 dB in the gravitational wave detection band, which offers the potential to improve the laser power stability by the same factor.

Highlights

The first detection of gravitational waves in 2015 started the age of gravitational wave astronomy [1, 2]
We report on the first measurement of an optical AC coupling transfer function in the gravitational wave detection band, performed at the aLIGO Livingston gravitational wave detector
We will start this article with a summary about the optical AC coupling theory before we describe how we measured the OAC transfer function at the aLIGO Livingston detector

Summary

Introduction

The first detection of gravitational waves in 2015 started the age of gravitational wave astronomy [1, 2]. Current gravitational wave interferometers are large-scale Michelson interferometers enhanced with advanced interferometry techniques like signal and power recycling or arm cavities [3]. In the second generation gravitational wave detectors several effects induce a coupling from technical laser power noise into the readout channel of the interferometers [4,5,6]. The relative power stability reached by the currently used feedback control loops is limited by their detector shot noise. To reach the required relative power noise of 2 · 10−9 Hz−1/2 at 10 Hz a shot noise limited photodiode array, detecting 200 mW, had to be developed [9]. While it will become increasingly difficult to reach better stabilities by detecting more optical power, alternative techniques become more attractive

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Conclusion