Ultrafast laser machining of embedded glass stress/fatigue sensors based on whispering gallery mode resonators

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Glass and glass type materials are widely used in various applications due to their excellent optical, mechanical, and esthetic properties. For many of those products, such as industrial size solar cells or large windowpanes, combination of high transparency and high mechanical strength of glass is of fundamental importance and cannot be compromised. Although glass materials have been used for centuries determination and monitoring of their structural integrity is still a scientific challenge. A straightforward approach of measuring the glass strength is inherently limited due to a very broad distribution of the obtained results that are typically interpreted using statistical analysis such as the Weibull distribution. Although the probabilistic approach can be quite accurate in terms of describing behavior of a large set of data, it cannot be applied to evaluate the strength of a given sample. As a result, for critical applications where safety is of high priority an approach of treating the worst case scenario as a typical one is adopted to ensure that each part meets the imposed standards. That often leads to use of extra material in order to ensure the safety margin at the expense of additional weight and cost. An appealing alternative approach would be to develop a non-destructive testing method that could provide information about the strength of a given glass part allowing either to accept or discard it based on the test results and application requirements. Such testing can be attempted via incorporation of a sensor into glass material such that the sensors response correlates with the material strength. For that purpose we propose to use a structural health sensor based on a whispering gallery mode resonator directly written into glass material using ultrafast laser micromachining. Whispering gallery mode resonators that confine light via total internal reflection in a looped waveguide structure are expected to allow for sensitive non-destructive measurement of the locally induced birefringence, and thus the stress, by determining the degree of polarization change and/or variation in the transmission/reflection spectra. Owing to flexibility of the laser direct-write approach, whispering gallery mode resonators can be embedded at different depths and locations that potentially permit non-destructive analysis and characterization of the glass strength for critical applications.