Femtosecond Laser‐Written Invisible Sensors in Architectural Glass and Their Impact on Strength

Abstract Monitoring architectural glass is becoming increasingly important due to its transition from small infill panels to large, load‐bearing applications, but traditional sensors are visually unappealing for use with transparent materials. This study explores the integration of waveguide Bragg gratings (WBG) into 4 mm thick soda lime silicate architectural glass using femtosecond laser technology, creating invisible optical sensors within the bulk. This method furthermore protects the sensors from surface damage. We report on the optimization and characterization of laser‐written waveguides in this type of glass, achieving low‐loss single‐mode waveguides up to 25 cm long with a propagation loss of 0.52 dB cm ⁻¹ at 1550 nm wavelength. Furthermore, WBGs are realized showing a 12.8% peak reflectivity and 115 pm ⁻³ dB bandwidth. A demonstrator consisting of a 25 cm‐long architectural glass plate with a WBG in the center connected to the edge via a single‐mode waveguide, shows a strain sensitivity of 1.20 pm μɛ ⁻¹ and a temperature sensitivity of 13.5 pm °C ⁻¹ . Importantly, mechanical testing confirms that the waveguides do not compromise the strength of the glass which is crucial when subjected to loads. These findings underscore the potential of this technology for structural health monitoring of glass structures.