Gravitational wave detectors such as Virgo and LIGO use long-baseline Michelson interferometers with high
finesse Fabry-Perrot cavity in the arms. The symmetry of these cavities is essential to prevent the interferometer
from sensitivity to laser fluctuations. For this purpose the difference between the transmissions of the two input
mirrors has to be minimized. Advanced LIGO, the upgrade of LIGO, plans a transmission matching between the
two input mirrors as high as 99%. A small deviation in the process fabrication from run to run might induce
transmission mismatch larger than 1%. Consequently, the two input mirrors have to be coated during the same
coating run. That requires ability to deposit the reflective coating, based on a stack of titanium doped tantala
(Ti:Ta2O5) layers and silica layers, uniformly over a 800 mm diameter aperture. This paper presents the study to
improve the thickness uniformity of a reflective coating and the preliminary results achieved on two Ø350mm
substrates coated in the run.
The device presented in this paper is designed for coupling a free space optical wave under quasi-normal incidence in and out of a highly multimode waveguide with high efficiency. It uses two resonant diffraction gratings at the substrate-waveguide interface that are made of a shallow metal grating, covered with a high refractive index layer. It is shown that the resonant structure can theoretically diffract up to 90% of the incident energy in and out of the waveguiding layer. The geometrical parameters of the structure and the tolerances can easily be achieved by conventional technology means.