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

玻璃和玻璃类材料由于其优异的光学、机械和美学特性而广泛用于各种应用中。对于许多这些产品，如工业尺寸的太阳能电池或大窗玻璃，玻璃的高透明度和高机械强度的组合是至关重要的，不能妥协。虽然玻璃材料已经使用了几个世纪，但对其结构完整性的测定和监测仍然是一项科学挑战。测量玻璃强度的直接方法由于所获得的结果的非常宽的分布而固有地受到限制，所述结果通常使用统计分析（例如威布尔分布）来解释。虽然概率方法在描述大数据集的行为方面可以相当准确，但它不能用于评估给定样本的强度。因此，对于安全性具有高度优先级的关键应用，采用了将最坏情况视为典型情况的方法，以确保每个部件都符合规定的标准。这通常导致使用额外的材料，以确保安全裕度，代价是增加重量和成本。一种有吸引力的替代方法是开发一种非破坏性测试方法，该方法可以提供关于给定玻璃部件的强度的信息，从而允许基于测试结果和应用要求来接受或丢弃该玻璃部件。可以通过将传感器结合到玻璃材料中来尝试这种测试，使得传感器响应与材料强度相关。为此，我们建议使用的结构健康传感器的基础上，回音壁模式谐振器直接写入玻璃材料，使用超快激光微加工。通过在环形波导结构中的全内反射来限制光的回音壁模式谐振器被期望允许通过确定透射/反射光谱中的偏振变化和/或变化的程度来对局部诱导的双折射进行灵敏的非破坏性测量，并且因此对应力进行灵敏的非破坏性测量。由于激光直写方法的灵活性，回音壁模式谐振器可以嵌入不同的深度和位置，这可能允许对关键应用的玻璃强度进行非破坏性分析和表征。

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.