Minho Song

Conventional acoustic emission (AE) sensors, such as piezoelectric and capacitive sensors, are susceptible to electromagnetic interference (EMI), while previously reported fiber–optic AE sensors encounter issues with reproducibility and high cost. To overcome these limitations, we have developed a micro-electromechanical systems (MEMS) fiber-optic Fabry-Perot (FP) acoustic sensor using bulk-silicon-micromachined 710-nm diaphragms. The thin diaphragm was fabricated in a multilayer (Au/Ti/Si3N4) configuration to achieve high sensitivity and mechanical strength for acoustic sensing applications. More than 250 diaphragms with high reflectivity and easy-to-align features, such as sloped walls and alignment grooves, could be produced from a single batch process. Using a tunable laser, we confirmed the consistent spectral properties of the integrated FP sensors, including a uniform free spectral range (FSR) and an interference contrast greater than 21 dB. From acoustic sensing experiments, the sensitivity and minimum detectable pressure (MDP) were measured to be 179.3 mV/Pa and 247.6 μPa/√Hz at 140 kHz, respectively. The frequency response was almost flat in the 20–150 kHz range. We constructed a 4-point sensor array to test its performance in detecting and localizing acoustic events. Using the generalized cross-correlation with phase transform (GCC-PHAT) algorithm, average errors of 0.95 cm (1.34 %) and 1.4 cm (1.82 %) were obtained in two-dimensional (2D) and three-dimensional (3D) location tests, respectively.

Owing to unstable multi‐wavelength lasing, robust measurement is difficult with a fibre Bragg grating (FBG) laser sensor system that uses FBGs as laser cavity mirrors as well as sensors. A stabilising technique is essential for proper data acquisition. Here, it is demonstrated that polarisation scrambling (PS) stabilises the output of laser sensor system in combination with a spectrometer demodulation. The state of polarisation (SOP) of a fibre‐optic laser cavity is changed by a 1 MHz PS. A 5 kHz spectrometer traces the distribution of output laser wavelengths. The rapid changes in SOP make all the FBG wavelengths lase as long as necessary to determine the Bragg wavelength shifts in the sampling windows of a photo‐diode array. A laser sensor system with eight FBG sensors is used to demonstrate robust measurement and data collection under environmental perturbations.

This study proposes a fiber-optic temperature sensor with a single-mode fiber tip covered with a thermo-sensitive polymer resin. The temperature is sensed by measuring the Fresnel reflection from the optical fiber/polymer interface. Because the thermo-optic coefficients differ between the optical fiber and the polymer, the in situ temperature can be measured even in curing composite materials. In initial experiments, the proposed sensor successfully measured and recovered the temperature information. The measured sensor data were linearly correlated, with an R² exceeding 0.99. The standard deviation in the long-term measurements of constant temperature was 2.6%. The durability and stability of the sensor head material in long-term operation was validated by Fourier transform infrared spectroscopy and X-ray diffraction analysis. In further experiments, the suggested miniature temperature sensor obtained the internal temperatures of curing composite material over a wide range (30–110 °C).