Scattered Light Measurements for Advanced LIGO's Output Mode-Cleaner Mirrors
Authors:Adrian Avila-Alvarez, Fabian Magana-Sandoval, Erik Muniz, Joshua Smith, Daniel Vander-Hyde
Mentor:Joshua Smith, Assistant Professor of Physics, California State University, Fullerton
The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), with sites in Livingston LA and Hanford WA, and its international partners, Virgo, GEO600, and KAGRA, are being built to detect gravitational waves, a phenomenon theorized by Einstein in his Theory of General Relativity (GR). A direct prediction of GR, gravitational waves are ripples in space-time that propagate at the speed of light and are created by violent astrophysical processes. The gravitational-wave detectors are all based on the Michelson interferometer, which has an input laser, a beam splitter, and two perpendicular arms with mirrors at each end. However, their configurations have significantly more complexity to augment the sensitivity. Higher order spatial modes can create 'junk light' that decreases the shot-noise limited sensitivity of the detectors. To combat this, each LIGO detector has an Output Mode-Cleaner (OMC) at its detection 'dark port'. Scattered light from the OMC mirrors can reduce the shot-noise limited sensitivity of the instruments, and add noise via stray and counter-propagating light. Thus it is important that the light scattering from the OMC mirrors in Advanced LIGO be minimal. The experiment involved using an imaging scatterometer to measure the light scattered from the surface of an output mode-cleaner optic. A 1064 nm laser beam is incident on the optic at an angle of four degrees. A lens and iris are used to form an image of the optical surface on a CCD camera. A rotation stage moves in one degree increments and the camera records a photo for each angle. These images are processed in Matlab to extract the scattered power, a factor needed to calculate scattered light. Using the Bidirectional Reflectance Distribution Function (BRDF), we were able to calculate scattered light on the OMC. The results are then passed to the LIGO lab.