Stray light in the LISA instrument: characterization and modelling of the coherent perturbation of a homodyne interference

In the LISA mission, the presence of stray light is a concern since a perturbation of the heterodyne phase measurement at the level of several µradian can give rise to significant noise in the LISA gravitational wave measurement. The perturbation scales as the square root of the stray light to nominal beam optical power ratio. Reaching very low deviation of the phase measurement may require a very strong rejection of coherent stray light. Apart from coherent interference in the heterodyne phase measurements, stray light can also shift measurements through an incoherent contribution. Various detectors, such as the CAS (constellation acquisition sensors) could receive light due to backscattering in the telescope optics, or from stars or planets, with a non-negligible power. We present work undertaken to set-up experiments for characterizing stray light at component and system level, including calibrated measurements of coherent back-scatter (BS) from a target with surface roughness, and interpretation of the data by a model description of the BS amplitude. The latter displays speckle-type variations when the sample is translated in front of the incident beam and fast variations when the sample orientation changes with respect the laser beam. These observations are interpreted using an electromagnetic model of the fields scattered by the surface, taking into account the measured surface topography and the design (thicknesses & optical indexes) of the coating constitutive of the mirror.