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CSIRO is manufacturing the `core' optical substrates for LIGO, a Michelson interferometer with arms up to 4 km in length each containing a Fabry Perot cavity. The beam splitter and input test mass mirrors (the entrance mirror to each cavity) have specifications not only for the optical surfaces but also for the radius of curvature of the wave front transmitted through the optical substrate. Our approach to manufacturing the substrates is to calculate the quadratic component of refractive index gradient (Delta) n from measurement of the transmitted wave front and the surface relief of the two substrate surfaces. After one of the surfaces (S1) is polished to specification, the radius on the second side required to achieve the specification on the transmitted wave front is calculated (using the measured value of (Delta) n, the actual value of S1 and the target value of the transmitted wave front). Results of this work and complications of the measurement procedure due to the thermal inertia and poor thermal conductivity of the silica substrates will be presented.
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Christopher J. Walsh, Achim J. Leistner, Bozenko F. Oreb, Jeffrey A. Seckold, David I. Farrant, Edita Pavlovic, "Metrology of transmission optics for LIGO," Proc. SPIE 3745, Interferometry '99: Applications, (17 August 1999); https://doi.org/10.1117/12.357795