FSR-Speedmeter Readout: Sub-Seismic Periodic GW Sensing with Sideband Injection

Melissinos (2014) pointed out that injecting an FSR-offset sideband can render a large interferometer sensitive to very low-frequency periodic signals, because the sideband’s DC-region readout is less contaminated by typical low-frequency technical noise. Meanwhile, speedmeter interferometers (Spencer et al. 2021) suppress quantum backaction at low frequencies, and cryogenic silicon designs (Adhikari et al. 2020) reduce thermal noise. We propose the first integrated “FSR-speedmeter” readout: an FSR sideband is injected into a polarization Sagnac speedmeter operated at cryogenic temperature and stabilized by adaptive thermal correction (Tao et al. 2025), with simulations and tabletop prototyping to quantify the achievable sub-seismic sensitivity. This goes beyond the original FSR proposal by marrying it to a topology that intrinsically rejects radiation-pressure noise and to cryogenic thermal-noise suppression. Targets include Earth normal modes, ultralight dark-matter-driven periodic strains, and low-frequency continuous-wave sources otherwise inaccessible to current ground-based detectors. If validated, this approach offers a practical pathway to recover the 0.1–5 Hz “dead zone” from the ground without relying solely on environmental subtraction.

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