Various full-field optical techniques such as digital shearography, digital holography, electronic speckle pattern interferometry (ESPI), thermography, and digital image correlation have been applied in recent years for full-field nondestructive testing (NDT).¹ 2. 3. 4. 5. ^{– 6} Of these techniques, digital shearography has demonstrated many advantages for real-world applications.⁷ 8. 9. 10. ^{– 11} Unlike other techniques, digital shearography uses a special shearing device to directly measure a displacement gradient rather than the displacement itself. This significant feature is particularly important for field NDT applications. Displacement gradients contain strain information and defects in materials generate strain concentration. As a result, digital shearography displays defects more directly. Rigid body motion also does not produce a displacement derivative. The relative insensitivity of digital shearography to environmental noises makes it better suited for field applications. Currently, digital shearography has become an industrial tool for NDT, especially, for NDT of delaminations in composite materials and honeycomb structures.¹² 13. 14. 15. 16. ^{– 17}

<!-- *** Custom HTML *** --> A planar lightwave circuit-based multi-channel arrayed waveguide grating (AWG) is used as a critical part of the fiber Bragg grating (FBG) interrogation unit and designed to be integrated into the fiber grating sensing system. We designed and fabricated a high-performance, 40 channels AWG in the device, enabling whole C-band wavelength demodulation while maintaining high-measurement resolution. The AWG was designed to have 100-GHz channel spacing; the channel crosstalk was measured to be −45 dB. An AWG-based FBG interrogation system was built to analyze the characteristics of our unit. The testing result shows that our system has the capability of demodulating wavelength from 1526.438 to 1563.863 nm with wavelength accuracy within ±10 pm; the wavelength resolution is measured to be 1 pm. Our testing result confirms that an AWG-based FBG interrogator is an excellent choice for FBG sensing systems.

The convolution neural network (CNN) is a classical neural network with advantages in image processing. The use of multiport optical interferometric linear structures in neural networks has recently attracted a great deal of attention. Here, we use three 3 × 3 reconfigurable optical processors, based on Mach-Zehnder interferometers (MZIs), to implement a two-layer CNN. To circumvent the random phase errors originating from the fabrication process, MZIs are calibrated before the classification experiment. The MNIST datasets and Fashion-MNIST datasets are used to verify the classification accuracy. The optical processor achieves 86.9% accuracy on the MNIST datasets and 79.3% accuracy on the Fashion-MNIST datasets. Experiments show that we can improve the classification accuracy by reducing phase errors of MZIs and photodetector (PD) noises. In the future, our work provides a way to embed the optical processor in CNN to compute matrix multiplication.

<!-- *** Custom HTML *** --> A Mach–Zehnder interferometer based on processed hollow-core fiber (HCF) is proposed and experimentally demonstrated for gas pressure sensing. The fiber sensor is fabricated by splicing a segment of HCF between the two coreless fibers. One end of the HCF is processed into an open cavity, which allows gas pressure of the air-core to be equivalent to that of the external environment. Gas pressure change induces refractive index change of the air-core, thus the measurement of gas pressure can be achieved by monitoring wavelength shift of the interference pattern. The proposed fiber sensor exhibits a high gas pressure sensitivity of 9.35 nm / MPa in the range from 0.1 to 0.04 MPa and a temperature sensitivity of 30 pm / ° C in the range from 30°C to 90°C. Furthermore, the fiber sensor processes the advantages of compactness, low cost, and all-fiber configuration, which makes it suitable for gas pressure measurement.

The flagship monthly journal of SPIE, Optical Engineering (OE) publishes peer-reviewed papers reporting on research and development in all areas of optics, photonics, and imaging science and engineering.

For the improvement of monitoring accuracy, a vibration monitoring for aircraft wing model using a fiber Bragg grating (FBG) array packaged by vacuum-assisted resin transfer molding (VARTM) is proposed. The working principle of the vibration monitoring using FBG array has been explained, which can theoretically support the idea of this paper. VARTM has been explained in detail, which is suitable for not only the single FBG sensor but also the FBG array within a relatively large area. The calibration experiment has been performed using the FBG sensor packaged by VARTM. The strain sensitivity of the VARTM package is 1.35 pm/μϵ and the linearity is 0.9999. The vibration monitoring experiment has been carried out using FBG array packaged by VARTM. The measured rate of strain changes across the aluminum test board used to simulate the aircraft wing is 0.69 μϵ/mm and the linearity is 0.9931. The damping ratio is 0.16, which could be further used for system performance evaluation. Experimental results demonstrate that the vibration monitoring using FBG sensors packaged by VARTM can be efficiently used for the structural health monitoring. Given the validation and great performance, this method is quite promising for in-flight monitoring and holds great reference value in other similar engineering structures.