CHARACTERIZATION OF LIGHT SCATTERING FROM A LIGO-INPUT TEST MASS
Adrian Avila-Alvarez, Daniel Vander-Hyde, Joshua Hacker, Erik Muniz, Joshua Smith.
California State University, Fullerton, Fullerton, CA.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a major project aimed at detecting astrophysical gravitational waves using laser interferometry. During quality control, one of LIGO’s input test masses, named ITM06, was found to have an abnormal cloudy anti-reflection coating on its secondary surface. It was not known how important this defect would be for scattering at the near infrared wavelength of 1,064 nm used by the LIGO lasers. A concern was that this flaw may alter the test mass’s scattered-light character and thereby decrease LIGO’s sensitivity to gravitational waves by adding nonlinear noise in the instrument readouts or by diminishing the overall laser power. To determine if the defect was visible in the infrared and to quantify its scatter, we measured ITM06 using the Fullerton imaging scatterometer. This apparatus illuminates the secondary surface with a 1,064 nm laser and takes pictures of the scattering for one-degree increments of the viewing angle. These pictures were then processed through a custom MATLAB script in which the bidirectional reflectance distribution function (BRDF) was computed for each angle to quantify its scatter. In these data, it was obvious that the infrared light lit up the cloudy defect. Despite this, the BRDF values were an acceptably low 4 x 10-7 str-1 (above 10 degrees). Our measurements confirmed that the abnormal coating does increase light scattering at 1,064-nm wavelengths, but the total scattering of ITM06 is still low enough for use in LIGO.