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Interferometric measurement of refractive index inhomogeneity on polished sapphire substrates: application to LIGO-II

Oreb, Bob and Leistner, Achim and Billingsley, Garilynn and Kells, Bill and Camp, Jordan (2001) Interferometric measurement of refractive index inhomogeneity on polished sapphire substrates: application to LIGO-II. In: Optical Manufacturing and Testing IV. Proceedings of the SPIE. No.4451. Society of Photo-optical Instrumentation Engineers (SPIE) , Bellingham, WA, pp. 414-423. ISBN 9780819441652.

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In order to improve the detection sensitivity of the Laser Interferometer Gravitational-wave Observatory (LIGO) the use of 40-kg sapphire test masses is being considered for the next instrument upgrade. Currently, sapphire material of adequate size is only available with the optical axis aligned with the m axis of the crystal. To determine the material's suitability it is necessary to characterize the refractive index inhomogeneity of the sapphire substrates for two orthogonal directions of polarisation, to a fraction of a part per million (ppm). We report on a method used to measure the refractive index inhomogeneity which requires three separate measurements of the polished sapphire blank in a Fizeau interferometer. These measurements are of the surface shapes or figures of the two polished sides of the blank and that of the wavefront entering side one propagating through the blank, reflected off side two and exiting through side one. The phase maps corresponding to these three measurements are combined to obtain the refractive index inhomogeneity map distribution. Measurements were carried out on two sapphire substrates (m axis) produced by the heat exchange method. The inhomogeneity maps show features which depend on polarisation direction. The physical origin of the inhomogeneities is discussed as well as the probable impact on the detection of a gravitational wave signal.

Item Type:Book Section
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Billingsley, Garilynn0000-0002-4141-2744
Additional Information:© 2001 Society of Photo-Optical Instrumentation Engineers (SPIE). We acknowledge the assistance of C.J. Walsh, J.A. Seckold, Z.S. Hegedus and N. Savvides. Discussions with Chandra Khattak of Crystal Systems and Roger Route of Stanford University are gratefully acknowledged. This material is based upon work supported by the National Science Foundation under the Co-operative Agreement with the California Institute of Technology, No. PHY-9210038.
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Subject Keywords:Interferometry, gravitational waves, LIGO, metrology, optical inhomogeneity, sapphire
Series Name:Proceedings of the SPIE
Issue or Number:4451
Record Number:CaltechAUTHORS:20181213-143631858
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Official Citation:Bozenko F. Oreb, Achim J. Leistner, GariLynn Billingsley, William P. Kells, Jordan Camp, "Interferometric measurement of refractive index inhomogeneity on polished sapphire substrates: application to LIGO-II," Proc. SPIE 4451, Optical Manufacturing and Testing IV, (27 December 2001);doi: 10.1117/12.453639
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91803
Deposited By: George Porter
Deposited On:19 Dec 2018 22:56
Last Modified:16 Nov 2021 03:44

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