IMAGE CALIBRATION BASED ON RAYLEIGH SCATTER IN SIO2
Erik Muñiz, Joshua Smith, Fabian Magana-Sandoval.
California State University, Fullerton, Fullerton, CA.
The Laser Interferometer Gravitational-wave Observatory (LIGO) is a major project designed to directly measure gravitational waves from the cosmos. Increasing the astrophysical range of the LIGO detectors requires the use of state-of-the-art optics with as little scattering as possible. Using a custom-built imaging scatterometer, we measured the scattered light from a high quality fused silica viewport of the type used in LIGO. We found that the total scattering of the front and back surfaces was less than the scattering in the substrate material. We hypothesized that the origin of the substrate scattering was Rayleigh scattering, like the atmospheric scattering that makes the sky blue. Here we report the characteristics of scatter in our fused silica sample and compare it to theory and the measured scattering values in the literature. We measured the maximum scattered light intensity ratio to be about 0.5 ppm cm-1 at 1,064 nm, which agrees with the theoretical value for Rayleigh scattering in fused silica. We also confirmed that the substrate scattering exhibits a cos2 polarization angle dependence in accordance with Rayleigh scattering law. We conclude that the bulk scattering we observed is Rayleigh scattering and that this sets a fundamental limit to the total scattering in fused silica viewports.