we demonstrate a fiber-optic Fabry-Perot pressure sensor with embedded MEMS micro-cavity for ultra-high pressure detection. The embedded type structure (ETS) is first proposed and analyzed to meet the requirements of ultra-high pressure detection. The geometry parameters of sensor have been optimized by analyzing mechanical and optical characteristics. From the analysis results based on the ETS, an ultra-high pressure sensor is fabricated and experimentally demonstrated for ultra-high pressure detection. The ultra-high pressure experiment illustrated response relationship between the absolute phase and pressure in the range of 2~120MPa, and full-scale errors less than 0.079%F.S. at room temperature. The other characteristics index of sensor, such as pressure sensitivity, hysteresis error, temperature's influence factor are superior to 1.071rad/MPa, 1.397%, 2.665×10 $^-3$ rad/°C. The transient response time of system is 0.27s. The sensor can be used for ultra-high pressure detections, which is necessary in deep-sea and oil exploration.

A novel design method for a fiber-optic Fabry–Perot (FP) acoustic sensor based on a flywheel-like diaphragm has been proposed and experimentally demonstrated. Through theory analysis and simulation, we have established a sensing model based on the flywheel-like diaphragm and illustrated the acoustic sensor’s resonant frequency regulation method. A flywheel-like diaphragm-based acoustic sensor has been fabricated and a dual-wavelength system is used to demodulate the phase fluctuation caused by the acoustic wave. Experimental results show that the proposed acoustic sensor has an acoustic pressure sensitivity of 1.3967 rad Pa ⁻¹ and noise-limited minimum detectable pressure of ∼ 16.43 μ P a / H z @ 9 k H z . The signal to noise ratio (SNR) of 1 kHz, 9 kHz, and 20 kHz are achieved to be 10.39 dB, 60.51 dB, and 37.14 dB, respectively. Over a fairly broad response range (1 kHz to 20 kHz), the average SNR is up to 45.57 dB. Moreover, our acoustic sensor’s directivity map approximates to a heart shape. Its excellent frequency response characteristics, sensitivity, and directivity pattern are believed to have bright application prospects.

A flywheel-like sensitive diaphragm, which breaks the sensitivity limitations imposed by the increased thickness and the decreased radius in edge-clamped circular structure, has been proposed and analyzed in fiber Fabry-Perot (FP) acoustic sensor. A new model is built around the effect of diaphragm’s structure parameters on sensitivity of a fiber FP acoustic sensor with common ductile material. The proposed sensor based on flywheel-like stainless diaphragm in 5mm diameter and 0.035mm thickness has been fabricated and its acoustic performances were tested. The test results show that our acoustic sensor can realize acoustic measurement from 0.1 kHz to 19 kHz. The sensor exhibits an acoustic pressure sensitivity of 1.525nm/Pa at the frequency of 4.5 kHz. The noise-limited minimum detectable pressure (MDP) level of ~13.06μPa / √Hz@4.5kHz and the acoustic pressure signal-to-noise ratio (SNR) of 70.42dB@4.5kHz are achieved, respectively. And the mean SNR of cavity length variation of 62.43dB can be acquired over the entire frequency range. Comparing with the edge-clamped circular structure, the flywheel-like structure endows the common ductile sheet material the ability to response to acoustic pressure. Cost-effective and compact size give the proposed acoustic sensor a competitive edge, something of crucial importance to commercial applications.

In this work, we have developed a novel fiber sensor based on optical path multiplexing (OPM) for measuring ultra-low chemical oxygen demand (COD) concentration. The cumulative effect brought by the novel OPM structure could amplify sensor’s sensitivity, which significantly improves the detectability in UV-Vis spectrometry method. Meanwhile, structural optimization method based on maximum coupling efficiency have been demonstrated. In the ultraviolet-visible (UV–Vis) absorption spectroscopy experiment, the absorbance of 215nm wavelength and the area under absorption spectrum were used to estimate the COD concentration, respectively. The results shown that they all increased linearly with the change of COD concentration, and kept the strictly linear relationship in the ranges of 0~6mg/L. Their linear correlation coefficients are above 0.9977. Surprisingly, the COD measurement curve represented by the area under absorption spectrum gave a better performance in this study, whose RMSE, LOD and LOQ were superior to 0.0165mg/L, 0.0362mg/L and 0.1207mg/L, respectively. The good performance indicators revealed accurate measuring ability of the OPM-based fiber sensor for ultra-low COD levels. More importantly, the open architecture in OPM-based fiber sensor has shown a good application prospect for in-situ water quality assessment.

An integrated optical waveguide-type laser velocimeter has been proposed and demonstrated at the central wavelength of 1550 nm. The micro-fabricated and photo-lithographically defined the integrated sensor head has dimensions in the range of 0.25 mm $\times3.50$ mm. The velocities in the range of $5.0\times 10 ^{-2} \mu \text{m}\cdot \text{s}$ ⁻¹ to $2.2\times 10\,\,^{6} \mu \text{m}\cdot \text{s}$ ⁻¹ , and a resolution of $5.0\times 10\,\,^{-2} \mu \text{m}\cdot \text{s}$ ⁻¹ have been achieved in the experiment. Through three experimental measurements and data calculation in the measured velocity range, the average relative error is less than 0.52%. This small size semiconductor sensor velocimeter uses semiconductor materials (silicon dioxide, SiO ₂ ) to make key sensor elements. The refractive indexes of waveguide core, upper cladding and lower cladding material are 1.448, 1.442 and 1.444. The optical interference fringes were used for velocity measurement, and the collected scattered light signal was used for velocity analysis. The apparent advantages of small size, high precision with large dynamic range, low cost, and high batch-fabrication uniformity make the velocimeter suitable for non-invasive measurement of ultra-low speeds of gases, liquids or solids at the near-surface.