Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 μm

The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than ± (30.2 11.1) ppm, which is equivalent to ± (3.6 1.3) ppm for a coating stack that is highly reflective at 1530 nm. While this is a very promising low absorption result for alternative low-loss coating materials, further work will be necessary to reach the requirements of <1 ppm for future GW detectors. Classical and Quantum Gravity Class. Quantum Grav. 32 (2015) 105008 (12pp) doi:10.1088/0264-9381/32/10/105008 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 0264-9381/15/105008+12$33.00 © 2015 IOP Publishing Ltd Printed in the UK 1