Development of an optical bench detecting absorbing micro-scale defects for large and highly reflective mirrors.

To further improve the sensitivity of the laser interferometers detecting gravitationnal waves one strategy is to increase the optical power circulating inside the kilometers long arm Fabry-Perot cavities. Next generation of gravitational wave detectors, aims to achieve optical power in the order of the Megawatt incident on the arm cavity mirrors. However, previous increases in optical power have highlighted a major limitation : the presence of one to a few microscopic absorbing defects in the coating of the mirrors. Theses defects mainly of metalic nature causes local absorption inducing optical aberations and excess optical losses. This not only limits the maximum power achievable in the high-finesse cavities but also degrade the control of the instrument. The presentation will describe the development of a new optical measurement bench able to detect these microscopic defects on dielectric mirrors on surfaces up to 620 mm in diameter using wavefront measurements to observe the aberations induced from an absorbing point excited by a 200W laser at 1064nm. This bench will ensure that the most critical mirrors of the gravitationnal wave detectors has no locally absorbing defects, but will also allow to further improve the coating deposition process conditions to prevent the occurrence of this type of defect. The latest results obtained on 350 mm mirrors will be showcased.