In September 2015, a direct detection of gravitational wave occurred in the LIGO interferometers for the first time. These advanced detectors need large fused silica mirrors (34 cm diameter, 20 cm thick) having optical and mechanical properties never reached up to now. Over five years, 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 Ta2O5 doped Ti as high index layer)
• the best coating uniformity (<0.1 % on 150 mm diameter).
The mirror substrates are made in ultra-pure fused silica (Suprasil from Heraeus) on which a Ti-Ta2O5/SiO2 IBS (Ion Beam Sputtering) multilayers is deposited. Moreover, in the advanced detectors, the two Fabry-Perot cavities (arms of the Michelson interferometer) must be as identical as possible (same mirror transmission, same finesse). Even if our large IBS coater using RF ion sources is very reproducible, it is almost impossible to reach the symmetry specifications for the High Reflective (HR) coating transmission, especially for the Input Mirrors (Transmission of 1.4+/- 0.1% at 1064 nm with ΔT<0.01%). The only solution is to coat the two substrates in the same run using a planetary motion. In addition, the transmission value must be uniform on each mirror on a large area implying a strong improvement of the coating uniformity on a large scale (better than 0.1%). This last requirement is particularly difficult for the HR coating of the EM whose total thickness is close to 6μm. We describe how we managed to improve the mirror absorption and the scattering at a level much lower than the first generation of VIRGO mirrors. Secondly, the way to improve the multilayers (Ti-Ta2O5/SiO2) thickness uniformity on a large diameter is detailed. This last point was particularly difficult for the HR coating of the EM whose total thickness is close to 6μm.
We describe how we managed to improve the mirror absorption and the scattering at a level much lower than the first generation of VIRGO mirrors. Secondly, the way to improve the multilayers (Ti-Ta2O5/SiO2) thickness uniformity on a large diameter is detailed.
This last point was particularly useful to develop a large Fabry-Perot Etalon, in collaboration with ZYGO (US) and the Kiepenheuer-Institut fur Sonnenphysik (Freiburg, Germany). This etalon will be used as a visible tunable filter for the DKIST solar telescope (Hawaii). The coating is a reflective coating (95%) over the band 520-870 nm. The main tricky requirement concerns the RMS air gap variation between the two plates which must be lower than 1 nm RMS over 250 mm diameter. The development made to achieve these performances will be detailed and particularly the stress compensation procedure.