Optical sensitivities of current gravitational wave observatories at higher kHz, MHz and GHz frequencies

TL;DR

Current gravitational wave observatories possess a high-frequency optical sensitivity that extends into the MHz and GHz range, which is relevant for detecting potential high-frequency cosmological signals.

Contribution

This paper calculates and explains the high-frequency noise spectral densities of existing GW observatories, revealing their sensitivity beyond the audio band.

Findings

High sensitivity at integer multiples of the free spectral range

Sensitivity extends into MHz and GHz frequencies

Photon shot noise dominates high-frequency noise spectral density

Abstract

GEO 600, Kagra, LIGO, and Virgo were built to observe gravitational waves at frequencies in the audio band, where the highest event rates combined with the largest signal to noise ratios had been predicted. Currently, hypothetical sources of cosmological origin that could have produced signals at higher frequencies are under discussion. What is not widely known is that current interferometric GW observatories have a frequency comb of high optical sensitivity that encompasses these high frequencies. Here we calculate the high-frequency noise spectral densities of operating GW observatories under the justified assumption that photon shot noise is the dominant noise source. We explain the underlying physics of why high sensitivity is achieved for all integer multiples of the free spectral ranges of the observatory's resonators when an interferometer arm is not orientated perpendicular to the propagation direction of the GW. Proposals for new concepts of high-frequency GW detectors must be compared with the high-frequency sensitivities presented here.