One of the current challenges in Structural Health Monitoring (SHM) is the validation of new processing algorithms and the development of new methods. To achieve this, it is relevant to have complete control over the entire chain, from instrumentation to processing algorithms.

To address this challenge, CEA List has developed an opto-electronic acquisition system using Fiber Bragg Gratings (FBGs) to measure ultrasonic waves. This system called “Optogero” has seven optical channels to enable simultaneous interrogation of multiple gratings. (cf. Figure 1).

In several SHM applications, FBGs offer numerous advantages over the more commonly used piezoelectric transducers (PZT). For instance, they can operate at much higher temperatures (above 1,000°C) than conventional PZT (around 200°C). Optical fibers can also be integrated directly into a structure with very limited intrusiveness. However, the interrogation of FBGs presents several technical challenges, and their sensitivity currently remains lower than that of PZTs.

The Optogero optoelectronic interrogation system is based on the edge-filtering measurement method. It can scan up to 7 Bragg gratings in parallel to identify their optimal operating point in order to maximize their sensitivity to ultrasonic waves. Real-time control of the laser source wavelength eliminates the consequences of varying environmental conditions, such as quasi-static strain and temperature changes in the structure. These effects would otherwise alter the response of the FBG and drive it away from the optimal operating point of the system. Various analog and digital processing techniques are integrated into the system to improve the signal to noise ratio. The Optogero system also includes a middleware architecture that allows users to easily embed data processing algorithms (imaging, artificial intelligence) for specific applications.

The system has been tested over a two-year period in the laboratory to validate its operation. Measurements were done on various materials such as aluminum, polymer-based composites or concrete, using several different testing methods (acoustic emission measurements, guided wave imaging, at room temperature and high temperatures). We will present the operating principles of Optogero and the system's performance in a few representative use cases.

Optical fiber sensors can be considered as cutting-edge sensors in many SHM applications since they can be seamlessly embedded within complex structures to provide tens (quasi-distributed) to thousands (distributed) measurements over a single optical fiber, while being able to withstand harsh environments (from a few K up to 1800 K). In particular, Fiber Bragg Gratings (FBG) are promising transducers for ultrasound measurements, but the efficient simulation of their behaviour for such purpose requires to address specific challenges since optical fiber diameters and FBG pitches are small compared to ultrasonic wavelengths, leading in general to small size mesh elements, therefore to heavy computational burdens without any dedicated optimized model.

In this work, we propose a one-way coupling scheme leading to a negligible computational burden overhead linked to the simulation of the FBG spectral response. It assumes a perfect transduction between the host structure and the optical fiber, and a negligible impact of the fiber on the ultrasonic wave propagation within the monitored structure. Ultrasound propagation within the host structure is simulated using transient high-order spectral finite elements, available in the SHM module of CIVA [1], enabling low memory and efficient computations. From this computation, strain at the sensor location is extracted. This strain field is then used to compute the FBG spectral response using a transfer matrix approach [2]. Furthermore, to mimic edge filtering, this response is computed only for one optical wavelength, corresponding to the interrogation laser wavelength. By doing so, no explicit meshing of the FBG is introduced, and the computations corresponding to the transfer matrix approach have only a limited impact on the total computational cost.

Computational performances of the scheme are investigated, and validations regarding specificities of the FBG response, such as its varying sensitivity with respect to the angle between incident wave polarization and fiber orientation, as well as the FBG length compared to the ultrasonic wavelength, will be presented. This model will be added in future versions of the SHM module of CIVA with automatic parametrization to enable reliable parametric studies including FBGs.

[1] MESNIL, Olivier, RECOQUILLAY, Arnaud, DRUET, Tom, et al. Experimental validation of transient spectral finite element simulation tools dedicated to guided wave-based structural health monitoring. Journal of Nondestructive Evaluation, Diagnostics and Prognostics of Engineering Systems, 2021, vol. 4, no 4, p. 041003.

[2] YAMADA, Makoto et SAKUDA, Kyohei. Analysis of almost-periodic distributed feedback slab waveguides via a fundamental matrix approach. Applied optics, 1987, vol. 26, no 16, p. 3474-3478.

Structural health monitoring (SHM) has a potential 13.1 times return on investment as it allows reduction in maintenance costs as well as increased service life. Several different SHM techniques have been developed trying to detect damage in the structure making use of different damage sensitive features. Based on the application demand, different sensor systems have been employed for the collection of the data. Each sensor system brings their own set of advantages and disadvantages.

In the last 20 years, immense work has been carried out using optical fiber sensors (OFS) for the SHM applications. OFS are light in weight, easy to multiplex, electro-magnetically neutral and may be embedded in structures. Hence are seen as ideal sensor for structural health monitoring (SHM). Fiber Bragg grating sensors have been utilized for guided wave sensing in the edge filtering configuration, they offer several interesting opportunities such as acoustic coupling, mode filtering, and self-referencing. But they suffer from drawback such as directional sensitivity, and limited bandwidth (upto few hundred kHz). To overcome these shortcomings, a new class of OF sensors based on the micro ring resonator (MRR) is proposed. The MRR works on the principle of constructive interference where integer multiples of particular wavelength of light constructively interfere to give large reflection peaks (similar to that of the FBG sensors). The key difference, is that the FBG sensors are typically of the order of few mm (typically 10mm) while the MRR is typically about 200 times smaller (45-50 µm). This makes the MRR a really point sensor thus allowing to overcome the directional sensitivity, as well as increase the bandwidth by about 100 or so times. The MRR response under different excitations frequencies for guided wave sensing is investigated in this study.