Prospects for High-Frequency Gravitational-Wave Detection with GEO600

Current ground-based interferometers are optimized for sensitivity from a few tens of Hz to about 1 kHz. While they are not currently utilized for GW detection, interferometric detectors also feature narrow bands of strong sensitivity at higher frequencies where the sideband fields created by a GW are resonantly amplified in the optical system. Small changes to system parameters allow the narrow band of high sensitivity to be scanned over a much larger range of frequencies. In this paper, we investigate whether simply modifying the detuning angle of the signal-recycling mirror of the GEO600 interferometer can make this experiment sensitive to GWs in the kilohertz frequency range. We compute the strain sensitivity for GEO600 across a frequency range from several kHz to tens of kHz for various detuning angles. We also show that LIGO cannot attain the same effect assuming that the optical components are not changed due to the narrow band response of the Fabry-Perot cavities. We then calculate the sensitivity of GEO600 to various proposed high-frequency GW sources and compare it to the sensitivity of other ground-based detectors.

💡 Research Summary

This paper investigates the potential of modifying the operational parameters of the existing GEO600 gravitational-wave (GW) interferometer to detect high-frequency GWs in the kilohertz to tens of kilohertz range, a regime largely inaccessible to current ground-based detectors like LIGO and Virgo.

The core proposal is remarkably simple: by finely detuning the position of the signal-recycling mirror (MSR), one can shift a narrow band of resonant sensitivity to much higher frequencies. This effect arises because a GW generates sideband fields in the interferometer’s laser light, which can be resonantly amplified within the signal-recycling cavity (SRC). The resonant frequency is determined by the SRC length and the MSR detuning angle (φ). For GEO600’s folded-arm Michelson design (with an effective arm length of 1200 m), adjusting φ allows this high-sensitivity peak to be scanned across a range from a few kHz to over 60 kHz. The study uses the Finesse 3.0 software to model GEO600’s optical configuration and compute its strain sensitivity, incorporating dominant noise sources like quantum shot noise, laser noise, seismic noise, and thermal noise.

A critical finding is that this technique is uniquely suited to GEO600-like detectors and is not effectively transferable to detectors like LIGO. LIGO’s high-finesse Fabry-Perot arm cavities drastically increase the effective length of the SRC, confining the tunable resonance to very low frequencies (~200 Hz). Furthermore, optical spring effects in LIGO broaden the sensitivity feature at larger detunings, preventing the formation of a sharp, high-frequency peak achievable in GEO600.

The results demonstrate that with an MSR detuning of approximately 86 degrees, GEO600 can achieve a sensitivity peak around 10 kHz that is orders of magnitude better than its standard tuned configuration. The “scanned configuration” sensitivity, representing the envelope of optimal sensitivity across all detuning angles, shows GEO600’s potential broadband sensitivity from ~2 kHz to beyond 60 kHz.

The paper then evaluates the detection prospects for two candidate high-frequency GW sources: superradiant boson clouds around black holes and sub-solar-mass compact binary coalescences. Signal-to-noise ratio (SNR) calculations indicate that even with the enhanced sensitivity, detection would be limited to very nearby sources (within roughly 1 kiloparsec) due to the expected weakness of these signals. Therefore, while not immediately promising for a high yield of detections, this study establishes a novel, low-cost method for probing the high-frequency GW spectrum using existing infrastructure. It positions GEO600 and similar interferometer designs as valuable testbeds for high-frequency GW science and potentially as specialized detectors for extreme nearby events, paving a complementary path alongside future dedicated high-frequency detectors.