Sol–Gel Waveguide-Based Sensor for Structural Health Monitoring on Large Surfaces in Aerospace Domain

Abstract

The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two different organic–inorganic sol–gels were selected to fabricate the photonic device: TiO2–SiO2 and ZrO2–SiO2, acting as the waveguide core and the cladding, respectively. A systematic study was performed to determine the manufacturing parameters controlling their properties. The results show that large surfaces can be functionalized via sol–gel methods using the direct laser-writing approach. The structures are characterized in terms of refractive index, and the guiding properties were investigated through simulations and experiments, indicating an excellent behavior regarding the light guidance in a straight waveguide or in the 2D matrix structure grid. Additionally, preliminary tests show that the presence of impact can be easily detected after damage through the induced optical losses on large surfaces. This proof of concept sensor is a promising tool for structural health monitoring. To achieve the ultimate goal, the integration of this photonic sensor will be later performed on aircraft wings.

Multi-Mode Interferometry: Application to TiO2–SiO2 Sol-Gel Waveguide-Based Sensing in the Aerospace Domain

“…We present results regarding the creation of MMI couplers, confronting simulation and an experimental point of view, revealing the feasibility of multiplying the number of TiO 2 -SiO 2 guides probed with a unique optical source, leading to the creation of more compact components, while the coupling efficiency from optical fiber to sol-gel waveguides is simulated in different configurations. The mechanical resistance of these waveguides was also evaluated in our previous article [10], from 80 • C down to −40 • C, showing that the integrity of those waveguides was not affected by temperature changes between the takeoff and landing for a single thermal cycle. To complete these preliminary results, further experiments have been performed during 24 complete thermal cycles by also monitoring the output light of the waveguides.…”

“…Alternative ways, related to guided optics, can be used regarding SHM applications. For example, we have recently evidenced the potential of a low-weight, low-cost, and non-intrusive solgel sensor, operating in the aerospace domain, to detect and localize damage on large surfaces [10]. This latter is investigated in the framework of the Clean Sky 2 (Horizon 2020) ADD-ON project (Advanced Damage Detection through Optical Sensor Network).…”

“…), the zones exposed to photons are resistant to alcohol while the unexposed zones are dissolved, allowing us to reveal sol-gel components for which applications can be achieved in many fields [12][13][14]. The SHM device, already largely introduced in [10], was manufactured using the direct laser writing technique and is schematically illustrated in Figure 1a. It consists of TiO2-SiO2 sol-gel waveguides on large surfaces (30 cm × 35 cm) arranged in a 2D matrix and based on light transport while a ZrO2-SiO2 cladding acts as a buffer layer.…”

Sol-Gel Coating Membranes for Optical Fiber Sensors for Concrete Structures Monitoring

“…Monitoring of structures is mainly carried out by piezoelectric [74][75][76], strain [77,78], and OFS sensors [79][80][81][82]. The OFS are based on interferometric principles (Fabry-Perot and MZI) and optical (Bragg Fibers) or stimulated (Brillouin) dispersion principles [83,84].…”

High Refractive Index Silica-Titania Films Fabricated via the Sol–Gel Method and Dip-Coating Technique—Physical and Chemical Characterization

“…The greatest possibilities in terms of shaping the refractive index are provided by the use of titanium dioxide ( n TiO 2 ~2.5) with silicon dioxide ( n SiO 2 ~1.45). Sample applications of thin films fabricated using the sol–gel method are presented in Refs [ 34 , 35 , 36 , 37 ].…”