Determination of the refractive index and thickness of a hydroxide-catalysis bond between fused silica from reflectivity measurements

Hydroxide-catalysis bonding is a high precision jointing technique producing
strong, transparent and thin bonds, the use of which in the delicate fused silica mirror
suspensions of aLIGO have been instrumental in the first detections of gravitational radiation.
More sensitive future gravitational wave detectors will require more accurate (ideally in situ)
measurements of properties such as bond thickness. Here a non-destructive technique is
presented in which the thickness and refractive index of a bond are determined from
measurements of optical reflectivity. The reflectivity of a bond made between two fused silica
discs using sodium silicate solution is less than 1⋅10−3 after 3 months. The thickness decreases
to a constant value of around 140 nm at its minimum and the refractive index increases from
1.36 to 1.45. This proves that as well as determination of bond thickness in situ this bonding
technique is highly interesting for optical applications.