Measurement of stray light in the LISA instrument

Abstract

Measurement of stray light in a complex optical system can be a complex task. We developed a method to measure coherent stray light inside an assembled device, determining all stray light sources and their relative contribution. The method is based on the insertion of a laser with a scanned frequency and the detection of all the electrical and optical signals obtained from the instrument. The spectra calculated from these signals show fringes due to interference between each stray light contribution and the nominal beam. The frequency of these interference peaks indicates the difference in path length between the stray light path and the nominal path. To have a description of the measured data we realized optical simulations, which link the measured path length to a possible path throughout the system. In the following, we will show some measurements made on a test bench realized to simulate the performance of LISA interferometers and describe how accurate simulations are in predicting and explaining the measured results.