L. Pinard, C. Michel, B. Sassolas, J. Degallaix, D. Forest, M. Granata, L. Mereni, J. Teillon, Laboratoire des Matériaux Avancés – CNRS, Villeurbanne France
For the first time in September 2015, a direct detection of gravitational wave occurred in the LIGO inter-ferometers. These advanced detectors need large fused silica mirrors (34-35 cm diameter, 20 cm thick) having optical and mechanical properties never reached up to now. LMA has developed and optimized these IBS (Ion Beam Sputtering) coatings on the mirrors of the Fabry-Perot arm cavities of the LIGO or Virgo gravitational wave detectors to get:
• the lowest optical losses (0.3 ppm absorption at 1064 nm, around 5 ppm of scattering)
• the lowest mechanical losses (thermal noise reduced by the use of Ti:Ta₂O₅ as high index layer)
• the best coating uniformity (<0.1 % on 150 mm diameter).
To improve the detector sensitivity, the laser power has increased during the following scientific runs and a problem appeared in the high reflective coating: some very high absorbing points (several hundreds of ppm) can be present. Some investigations were done to understand and find their origin, and a solution was found to suppress them. This optimization was helpful for other projects using high finesse cavi-ties. :
The other noise limiting the detector performances in the 100 Hz region is the coating thermal noise, com-ing from the high index layer. Some R&Ds started at LMA (Working Group between LIGO and Virgo) to find a new material able to reduce by a factor 2 this noise. The best candidate is the Ti:GeO₂. Some final results will be presented.