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Published February 2015 | public
Journal Article Open

Broadband measurement of coating thermal noise in rigid Fabry–Pérot cavities


We report on the relative length fluctuation of two fixed-spacer Fabry–Pérot cavities with mirrors fabricated from silica/tantala dielectric coatings on fused silica substrates. By locking a laser to each cavity and reading out the beat note v = v_1− v_2 of the transmitted beams, we find that, for frequencies from 10 Hz to 1 kHz, the power spectral density of beat note fluctuation is S_v(f)=(0.5Hz)^2/f. By careful budgeting of noise sources contributing to the beat note, we find that our measurement is consistent with the fluctuation in this band being dominated by the Brownian noise of the mirror coatings. Fitting for the coating loss angle ⌽_c, we find it equal to 4 × 10^(−4). We then use a Bayesian analysis to combine our measurement with previous observations, and thereby extract estimates for the individual loss angles of the silica and tantala constituents of these coatings. With minor upgrades, the testbed described in this article can be used in the future to measure the length noise of cavities formed with novel mirror coating materials and geometries.

Additional Information

© 2015 BIPM & IOP Publishing Ltd. Received 8 August 2014, revised 30 October 2014. Accepted for publication 4 November 2014. Published 9 December 2014. The authors wish to thank R Abbott, D Sigg, D Yeaton-Massey, L Price, P King, M Abernathy, M Daily, R Cervantes, S Terry, and N Smith-Lefebvre for useful discussions and technical help. We thank Borja Sorazu for careful reading of the manuscript. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation and operates under cooperative agreement PHY-0757058. This paper has LIGO Document Number LIGO-P1400072.

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August 20, 2023
August 20, 2023