Coatings or films are applied to a substrate for several applications, such as waterproofing, corrosion resistance, adhesion performance, cosmetic effects, and optical coatings. When applying a coating to a substrate, it is vital to monitor the coating thickness during the coating process to achieve a product to the desired specification via real time production control. There are several different coating thickness measurement methods that can be used, either in-line or off-line, which can determine the coating thickness relative to the material of the coating and the substrate. In-line coating thickness measurement methods are often very difficult to design and implement due to the nature of the harsh environmental conditions of typical production processes and the speed at which the process is run. This paper addresses the current and novel coating thickness methodologies for application to chromium coatings on a ferro-magnetic steel substrate with their advantages and limitations regarding in-line measurement. The most common in-line coating thickness measurement method utilized within the steel packaging industry is the X-ray Fluorescence (XRF) method, but these systems can become costly when implemented for a wide packaging product and pose health and safety concerns due to its ionizing radiation. As technology advances, nanometer-scale coatings are becoming more common, and here three methods are highlighted, which have been used extensively in other industries (with several variants in their design) which can potentially measure coatings of nanometer thickness in a production line, precisely, safely, and do so in a non-contact and non-destructive manner. These methods are optical reflectometry, ellipsometry and interferometry.

A novel cyclic olefin copolymer (COC)-based polymer optical fiber (POF) with a rectangular porous core is designed for terahertz (THz) sensing by the finite element method. The numerical simulations showed an ultrahigh relative sensitivity of 89.73% of the x-polarization mode at a frequency of 1.2 THz and under optimum design conditions. In addition to this, they showed an ultralow confinement loss of 2.18 × 10⁻¹² cm⁻¹, a high birefringence of 1.91 × 10⁻³, a numerical aperture of 0.33, and an effective mode area of 1.65 × 10⁵ μm² was obtained for optimum design conditions. Moreover, the range dispersion variation was within 0.7 ± 0.41 ps/THz/cm, with the frequency range of 1.0–1.4 THz. Compared with the traditional sensor, the late-model sensor will have application value in THz sensing and communication.

A parallel Fabry–Perot interferometer (FPI) optical fiber sensor, enhanced with UV glue, was proposed for environmental temperature detection. The UV glue is applied to the fiber’s sensing region using a coating method, forming an FP cavity through misalignment welding, allowing the FP to function as a temperature sensor. In parallel, a reference FPI with a similar free spectral range (FSR) is connected, generating a Vernier effect that amplifies small changes in the refractive index (RI) of the sensing region. The study demonstrates that UV glue enhances the temperature-sensing capabilities of the FP, and when combined with the Vernier effect, it significantly improves the sensitivity of a single interferometric sensor. The temperature sensitivity of the parallel-connected FPI is −2.80219 nm/°C, which is 7.768 times greater than that of a single FPI (−0.36075 nm/°C). The sensor shows high sensitivity, stability, and reversibility, making it promising for temperature-monitoring applications in various fields, including everyday use, industrial production, and the advancement of optical fiber temperature-sensing technologies.

This paper presents a sensor that leverages a cloverleaf hollow fiber (CHF) for optofluidic surface-enhanced Raman spectroscopy (SERS) detection within the fiber. Unlike other microstructured fibers, this design features four air holes, providing a larger contact area between the test liquid and the enhanced substrate at the fiber core, which simplifies liquid injection. By taking advantage of this unique structure, silver nanoparticles were modified inside the fiber to serve as the Raman enhancement substrate. The Raman signals are excited via the evanescent field, enabling highly sensitive SERS detection. Rhodamine 6G (R6G) was used as the target analyte, achieving a detection limit of 10 ^-11 M, an enhancement factor (EF) of 1.59×10 ⁸ , and a relative standard deviation (RSD) of 4.8%, demonstrating both high sensitivity and reproducibility in fiber-based detection. Additionally, SERS detection of enrofloxacin solution was carried out, further validating the fiber’s capability in detecting antibiotic substances with excellent stability. The successful detection achieved with this fiber underscores its significant potential for applications in biomedical, agricultural, food safety, and environmental monitoring.

We built a reflective parallel tapered optical fiber structure and used it as a biosensor for highly sensitive in situ detection of label-free Mucin 1 (MUC1). A sensing tapered optical fiber probe was prepared, and a reference probe of similar Free Spectral Range (FSR) was connected in parallel to establish the optical Vernier-effect. This arrangement amplified the refractive index (RI) sensitivity to 8.4 times that of a single sensing probe with 19845.22 nm/RIU. Meanwhile, the aptamer was chemically bonded to the surface of sensing probe to specifically detect MUC1. This biosensor has a linear wide detection range of 10^-10 - 10^-1 mg/mL, with a high sensitivity of 12.56 nm/log(C/C0) and a low limit of detection (LOD) of 0.012 pg/mL. Significantly, the reflective structures, unlike transmissive ones, could be inserted into samples rather than requiring samples to be moved to the sensing area, which not only reduces contamination of samples during movement, but also enables the measurement of trace samples that cannot be transferred. Further, this novel sensing structure with high sensitivity holds promise for the preliminary diagnosis and prognostic monitoring of cancer and has broad development prospects in clinical and therapeutic applications.