For second-generation gravitational-wave interferometric detectors such as Advanced LIGO, the thermal noise of the test mass mirror coatings is a significant limit in a critical midband around 100 Hz.
The optical cavities at the heart of the length-sensing mechanism of gravitational wave interferometers use mirrors made with multilayer dielectric coatings to produce the high reflectivities that are required. The conventional design of high-reflectivity multilayer dielectric coating consists of quarter-wavelength alternating layers made of silica (low-index) and tantala (high-index) materials. Such design, though optimal from the reflectivity viewpoint, does not minimize the thermal noise since there is a substantial difference in the mechanical loss of the two material constituents.
In [1], we proposed a systematic procedure for designing minimal-noise coatings featuring a prescribed reflectivity. Such design is based on a periodic stack of identical high/low index ‘‘doublets,’’ with total thickness of half-wavelength and optimized material fractions, with the exception of the terminal (top/bottom) layers.
Mirrors based on such a design were fabricated at the Laboratoire des Matériaux Avancés (Lyon, France) and experimentally characterized by measuring the broadband noise floor of the Thermal Noise Interferometer (TNI) at the California Institute of Technology [2].
The figure shows the optimized design (top), and the measured thermal noise (bottom) compared with the standard quarter-wavelength design. The measurements indicated a reduction in thermal noise in line with modeling predictions.
Our optimized mirror coatings have been implemented in Advanced LIGO [3], and have contributed to attain the required strain sensitivity for the direct detection of gravitational waves.
@inproceedings{ISI-000241947200007, author = {Agresti, Juri and Castaldi, Giuseppe and DeSalvo, Riccardo and Galdi, Vincenzo and Pierro, Vincenzo and Pinto, Innocenzo M.}, editor = {Ellison, MJ}, title = {Optimized multilayer dielectric mirror coatings for gravitational wave interferometers}, booktitle = {Proceedings of SPIE}, year = {2006}, month = aug, volume = {6286}, organization = {Advances in Thin-Film Coatings for Optical Applications III, SPIE Photonics West, San Diego, CA, USA, Aug. 13-14, 2006}, doi = {10.1117/12.678977}, pages = {628608} }
A standard quarter-wavelength multilayer optical coating will produce the highest reflectivity for a given number of coating layers, but in general it will not yield the lowest thermal noise for a prescribed reflectivity. Coatings with the layer thicknesses optimized to minimize thermal noise could be useful in future generation interferometric gravitational wave detectors where coating thermal noise is expected to limit the sensitivity of the instrument. We present the results of direct measurements of the thermal noise of a standard quarter-wavelength coating and a low noise optimized coating. The measurements indicate a reduction in thermal noise in line with modeling predictions.
@article{IJ86_PRD_81_122001_2010, title = {Measurement of thermal noise in multilayer coatings with optimized layer thickness}, author = {Villar, Akira E. and Black, Eric D. and DeSalvo, Riccardo and Libbrecht, Kenneth G. and Michel, Christophe and Morgado, Nazario and Pinard, Laurent and Pinto, Innocenzo M. and Pierro, Vincenzo and Galdi, Vincenzo and Principe, Maria and Taurasi, Ilaria}, journal = {Physical Review D}, volume = {81}, issue = {12}, pages = {122001}, numpages = {8}, year = {2010}, month = jun, publisher = {American Physical Society}, doi = {10.1103/PhysRevD.81.122001}, url = {http://link.aps.org/doi/10.1103/PhysRevD.81.122001} }
The Laser Interferometer Gravitational Wave Observatory (LIGO) consists of two widely separated 4 km laser interferometers designed to detect gravitational waves from distant astrophysical sources in the frequency range from 10 Hz to 10 kHz. The first observation run of the Advanced LIGO detectors started in September 2015 and ended in January 2016. A strain sensitivity of better than <img style=’vertical-align:middle;’ src="/assets/images/equations/imgtemp_x9wfkh-1.svg" border="0"/> was achieved around 100 Hz. Understanding both the fundamental and the technical noise sources was critical for increasing the astrophysical strain sensitivity. The average distance at which coalescing binary black hole systems with individual masses of 30 \(M_⊙\)could be detected above a signal-to-noise ratio (SNR) of 8 was 1.3 Gpc, and the range for binary neutron star inspirals was about 75 Mpc. With respect to the initial detectors, the observable volume of the Universe increased by a factor 69 and 43, respectively. These improvements helped Advanced LIGO to detect the gravitational wave signal from the binary black hole coalescence, known as GW150914.
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