Light scattered from optical components is a source of critical noise for interferometric gravitational wave detectors such as LIGO, Virgo, KAGRA, or the future space antenna LISA. Therefore it is important to accurately quantify the amount of light coherently back-reflected or back-scattered by optical components involved in the design of such instruments. As an example, for a good quality plane silica window (1 nm RMS roughness) illuminated by a Gaussian beam with a waist of 1 mm, the amount of light back-scattered by each interface is between -95 dB and -125 dB following the angle of incidence. This defines the stringent sensitivity requirements we have to satisfy for such applications. In our communication, we present the measurements performed on a Silver coated mirror using a low coherence interferometer implemented with balanced detection.
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