LIGO Document T1400252-v3

Imaging Scatterometer Characterization of Advanced LIGO Optic ITM06

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T - Technical notes
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In this document we report on the scatter characteristics of the Advanced LIGO test mass ITM06~\cite{C1000474,E1300886}. After it was coated by the vendor LMA, the vendor characterization found that the Anti-Reflective (AR) coating showed a cloudy or leopard-print scattering pattern when seen under visible light, but that its scattering BRDF values were reasonably low~\cite{C1103260}. We performed an imaging scatterometer characterization of the AR-coated surface in order to confirm the LMA measurements and provide more information about the spatial character of the scatter, particular the impact of the leopard pattern when illuminated by 1064\,nm light, the primary wavelength of light used in the Advanced LIGO interferometers. We find that ITM06 does have an abnormal anti-reflection coating and we present images of the leopard pattern illuminated by low levels of 1064\,nm light. However, we find that the scatter level, while higher than that from other AR-coated optics~\cite{padilla2014}, appears to be low enough (similar to LMA's measurements) to be suitable for use in Advanced LIGO.
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