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Abstract

Inelastic back-scattering of stray light is a long-standing problem in
high-sensitivity interferometric measurements and a potential limitation for
advanced gravitational-wave detectors, in particular at sub-audio-band
frequencies. The emerging parasitic interferences cannot be distinguished from
a scientific signal via conventional single readout. In this work, we propose
and demonstrate the subtraction of inelastic back-scatter signals by employing
dual homodyne detection on the output light -- here -- of a table-top Michelson
interferometer. The additional readout contains solely parasitic signals and is
used to model the scatter source. Subtraction of the scatter signal reduces the
noise spectral density and thus improves the measurement sensitivity. Our
scheme is qualitatively different from the previously demonstrated vetoing of
scatter signals and opens a new path for improving the sensitivity of future
gravitational-wave detectors.