Development of a high sensitivity absorption measurement system for low-loss novel glasses

Abstract

This thesis describes the development of a wavefront measurement system and analysis to quantify absorption in low-loss glasses at 2 μm. Motivated by the proposed gravitational wave detectors at 2 μm and potential glasses which could revolutionise future telecommunication fibres, it will be essential to precisely measure the incredibly small losses due to absorption in the short-wave infra-red. Key limitations for 2 μm technologies are power loss and the associated reduced transmission distance and absorption induced thermal lens. A solution to these issues is the development and implementation of ZBLAN, a glass which has theoretically predicted losses which are orders of magnitude below the low-loss glasses currently employed. ZBLAN is prevented from reaching the theoretical limits due to extrinsic scatter, resulting from crystallisation in the sample and absorption. Both losses must be understood to improve the glass but the mechanisms for the former are largely unknown and absorption has not been precisely measured. However, if absorption losses can be measured the losses due to scatter can then be inferred. I describe in this thesis a photo-thermal system to measure the absorption coefficient of a sample independently. This was completed by imaging the thermally induced wavefront distortion of an angled incident beam caused by heating in the glass due to the absorption of a 2 μm laser beam. A value for the bulk and surface absorption coefficients was extracted through comparison to a model which considers the thermo-elastic and thermo-refractive effects. The multi-parameter model was verified for this system on a N-BK7 glass sample of known parameters as this system does not require calibration and novel glasses may have variations in glass parameters from published values. Furthermore, it was demonstrated that modelling using the incorrect thermo-expansion and thermo-optic coefficients reduces the quality of fit, such that I propose in cases of very high signal to noise the quality of fit could be used determine the correct sample coefficients and limit uncertainty in the absorption coefficients. Bulk absorption in the 10s of ppmcm−1 was measured in ZBLAN and upper bound of surface absorption was determined. Angled incidence of a probe beam was shown to allow errors in bulk absorption to be reduced when surface is not considered as well as enable surface and bulk absorption to be separately measured in a single measurement. These results show that this system can be used as a tool to characterise absorption losses at 2 μm and by increasing the pump beam power to quantify absorption in lowest loss ZBLAN.