Optical coherence tomography (OCT) is a nonintrusive optical imaging method offering micrometer-scale spatial resolution, enabling the generation of internal structural cross-sectional images. Since the first OCT was invented in the early 1990s, its utilization has become prevalent in the field of ophthalmology and has become one of the golden standards in clinical diagnosis. Beyond the extensive research in biomedical fields, because of the accomplished exceptional spatial resolution and rapid imaging acquisition rate, the OCT has also been explored as an emerging solution for characterizing structure of new materials, detecting defects of industrial products in situ. In this paper, we reviewed and summarized recent progresses of OCT in several industrial applications, including structural analysis of composite materials, quality inspection of printed electronic devices, and analysis of industrial fluids. Meanwhile, the potential challenges and future development of OCT application in these fields are also outlined.

Optical excitation is widely acknowledged as one of the most effective means of balancing sensor responses and response/recovery properties at room temperature (RT, 25 °C). Moreover, noble metals have been proven to be suitable as photosensitizers for optical excitation. Localized surface plasmon resonance (LSPR) determines the liberalization of quasi-free electrons in noble metals under light irradiation, and numerous injected electrons in semiconductors will greatly promote the generation of chemisorbed oxygen, thus elevating the sensor response. In this study, pure SnO₂ and Au/SnO₂ nanowires (NWs) were successfully synthesized through the electrospinning method and validated using XRD, EDS, HRTEM, and XPS. Although a Schottky barrier led to a much higher initial resistance of the Au/SnO₂ composite compared with pure SnO₂ at RT in the dark, the photoinduced resistance of the Au/SnO₂ composite became lower than that of pure SnO₂ under UV irradiation with the same intensity, which confirmed the effect of LSPR. Furthermore, when used as sensing materials, a detailed comparison between the sensing properties of pure SnO₂ and Au/SnO₂ composite toward NO₂ in the dark and under UV irradiation highlighted the crucial role of the LSPR effects. In particular, the response of Au/SnO₂ NWs toward 5 ppm NO₂ could reach 65 at RT under UV irradiation, and the response/recovery time was only 82/42 s, which far exceeded those under Au modification-only or optical excitation-only. Finally, the gas-sensing mechanism corresponding to the change in sensor performance in each case was systematically proposed.

Efficient fiber-to-chip coupling is a key issue in the field of integrated optics and photonics due to the lack of on-chip silicon light source at present. Here, we propose a silicon-based fiber-to-chip edge coupler by use of subwavelength grating (SWG)-assisted structure. The key conversion region is composed of a trident-shaped SWG in the center and two matched strip waveguides on both sides. To achieve high mode match between fiber mode and silicon waveguide mode and to realize low-loss transmission on-chip, we have divided the conversion region into three parts and determined their optimum dimensions. From results, the total device length is only 60 μm from input fiber to output silicon waveguide, and the insertion loss (IL) is as low as 0.23 dB at the wavelength of 1.55 μm. For the working bandwidth, its value can be enlarged to 240 nm (or 390 nm) by keeping IL < 1 dB (or 1.5 dB), which is quite promising for on-chip broadband devices. Based upon these advantages, we hope such a device could be applied in light coupling between optical fiber and on-chip silicon waveguide.

The complicated manufacturing process and poor industrial environment inevitably produce unacceptable scratches on glass screen surface. In this study, a machine vision system is developed to automatically detect fine scratch, which has a width smaller than 30 $\mu \text{m}$ , on glass screen surface for the purpose of appearance quality assessment. The imaging hardware of the proposed machine vision system is made up of a high-resolution camera and a wide field lens so that a high-quality image is achieved in a relatively large image area. The captured glass screen images were first classified with YOLOv3 to exactly distinguish fine scratches in two other kinds of defects including dusts and stains. Then, the U-Net network was applied on the selected scratches to estimate their dimensions approximately. The proposed algorithms combination has a fast image processing speed and it is able to effectively reduce the false segmentation of other defects. The results of the developed machine vision system show a relatively good agreement with the well-established microscopic method. It demonstrates our machine vision method is able to image and quantitatively analyze scratches with dimensions between 25 and 100 $\mu \text{m}$ .

A compact asymmetrical directional coupler (ADC), which is composed of a strip and a tilted subwavelength gratings (TSWG) waveguide, is proposed to realize a broadband and high-extinction-ratio polarization beam splitter (PBS). In addition to conventional SWG structural parameters, such as its period and duty cycle, a TSWG waveguide provides an extra degree of freedom to solely adjust the TE modal effective index for more accurate phase matching. Hence the TE polarization state is coupled more efficiently in the ADC system, and then the extinction ratio (ER) is consequently improved. The simulation of our proposed device demonstrated a higher coupling efficiency and an ultrahigh ER of 34.2 dB for the TM polarization state at the center wavelength around 1550 nm. The operating bandwidth of the TM polarization state defined as the spectral range, with ER higher than 20 dB, is determined to be as broad as 75 nm. And, with an assisted hybrid plasmonic waveguide at the end of the cross port, the achieved bandwidth of the TE polarization state is as broad as 133 nm. They are much broader than those of other DC/ADC implemented PBS devices. Moreover, further analysis shows that the tilting angle of the TSWG waveguide has much higher fabrication tolerance than changing SWG duty cycle.

An electro-optic modulator (EOM) is an indispensable component to connect the electric and optical fields. Here, we propose a high-performance, thin-film lithium niobate-based EOM, where the modulation waveguide is formed by an etching slot on the lithium niobate film and the deposit of an ultrathin silicon film in the slot region. Therefore, a small mode size and high mode energy can be simultaneously achieved in the LN region with a high EO coefficient, which will be beneficial to increase the EO overlap and gradually decrease in the mode size. Further, we employed a waveguide structure to construct a typical Mach–Zehnder interference-type EOM. According to the requirements of high-speed traveling wave modulation, we conduct the index matching, impedance matching, and low-loss operation. From the results, the key half-wave voltage length product and 3 dB modulation bandwidth are, respectively, 1.45 V cm and 119 GHz in a modulation length of 4 mm. Moreover, a larger 3 dB bandwidth also can be achieved by shortening the modulation length. Therefore, we believe the proposed waveguide structure and EOM will provide new ways to enhance the performance of LNOI-based EOMs.