Measurement of stray light in the LISA instrument

TL;DR

This work tackles stray light in LISA-like interferometry by developing the SL-OGSE method, which uses a scanned-frequency laser in an FMCW framework to generate fringes whose frequencies encode the optical path-length differences $OPD$. The core approach combines a $2\, ext{nm}$ frequency sweep around $1064.5\,$nm, Fourier analysis of multi-channel signals, and the relation $f_{SL} = \frac{OPD}{c}\frac{\Delta\nu}{\Delta t}$ to quantify each stray-light path. Optical simulations with ray-tracing (FRED) map observed $OPD$ peaks to concrete physical paths and help predict stray-light contributions. Demonstrations on the ZIFO bench show the method can identify major stray-light paths and guide mitigation, though accuracy hinges on precise knowledge of the DUT and parameters, with a detectable floor around $1\times10^{-6}$ in fractional amplitude and reliable identification above roughly $2\times10^{-5}$.

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.

Figures (5)

• Figure 1: Schema of the SL-OGSE prototype
• Figure 2: Schema of the ZIFO (left). Dotted purple and green areas indicate the three interferometers. The two laser paths are indicated in blue and green. On the right is a photo of the instrument.
• Figure 3: Spectrum obtained from a SL-OGSE measurement on the ZIFO bench. There are contributions due to perturbations and defects (electrical grid, SL inside the SL-OGSE) and optical contribution due to the ZIFO design. Measurement is repeated at two different scan rates, 5.1 GHz/s (blue dots) and 5.3 GHz/s (red dots). As a result, a peak for which blue and red dots gather at the same OPD indicates a genuine SL contribution, while contributions that appear as two, blue and red, peaks with a $\sim 4\%$ difference in OPD are readily identified as foreign perturbations (coupling from the line voltage or from the power supply).
• Figure 4: ZIFO optomechanical setup used for FRED simulations. The two nominal beams are shown in red and green.
• Figure 5: Example of comparison between measure and simulation. Right: SL peaks measured with the SL-OGSE are shown as orange dots. Simulated SL paths are represented by blue triangles. For the biggest SL contribution, the corresponding path is shown on the left.