Abstract
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.
Highlights
In this paper a non-destructive technique for determining the refractive index and thickness of hydroxide-catalysis bonds from reflectivity measurements is reported. This technique strikes two birds with one stone: 1) it will be invaluable for enabling us to tailor the optical properties of hydroxidecatalysis bonds for optical applications such as fabrication of high power laser crystals assembled using hydroxide-catalysis bonding [1] and optical fibre injection components [2], in which low reflectivity is required; 2) it could give in situ measurements of bond thickness in the mirror suspensions of ground-based gravitional wave detectors to more accurately predict the thermal noise from these bonds in these suspensions [3]
Hydroxide-catalysis bonding is such a technique which has been instrumental in the assembly of opto-mechanical parts of the recently extremely successful aLIGO gravitational wave detectors which recently made the first two direct detections of gravitational waves both caused by two black holes merging [6,7]; as well as the ultrastable interferometer readout system [8] of the successful LISA Pathfinder mission of which results were reported recently [9]
The analysis of flatness maps shows minimum bond thickness ranging from 80 nm in positions ‘C’ and ‘L’ to 110 nm in position ‘R’, whereas the analysis suggests that the thickness after curing ranges between 140 in position ‘R’ and 300 nm in location ‘L’ and ‘C’ suggesting that the bond thickness is in the same order of magnitude as the derived minimum bond thickness, but that it is not necessary determined by the conformity of the substrates bonded together
Summary
In this paper a non-destructive technique for determining the refractive index and thickness of hydroxide-catalysis bonds from reflectivity measurements is reported. First measurements of the reflectivity of a hydroxidecatalysis bonds between silica substrates (using sodium silicate solution) were presented giving a value less than 7.08·10−4 at normal incidence: these bonds were resistant to 70 J/cm of laser power for 25 ns pulses at a wavelength of 1064 nm These first results make this technique highly attractive for further development of optical applications, such as fabrication of high power laser crystals assembled using hydroxide-catalysis bonding [1] and optical fibre injection components [2]. In this paper the method of taking reflectivity measurements and extracting the information on the bond thickness and refractive index from hydroxide-catalysis bonds is explained as well as a demonstration of the technique through measurements of reflectivity of a hydroxide-catalysis bond between two silica windows as a function of curing time
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