  Inelastic back-scattering of stray light is a long-standing and fundamental
problem in high-sensitivity interferometric measurements and a potential
limitation for advanced gravitational-wave detectors. The emerging parasitic
interferences cannot be distinguished from a scientific signal via conventional
single readout. In this work, we propose the subtraction of inelastic
back-scatter signals by employing dual homodyne detection on the output light,
and demonstrate it for 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
and other back-scatter limited devices.
