Dual parameter detection without cross-sensitivity is a commonly important need but challenging, as most fiber detection components have a single sensitive mechanism. In this article, an optical fiber probe based on a three-beam Fabry-Perot interferometer (FPI) for temperature and refractive index (RI) measurement was proposed and experimentally realized. Theoretically, such structure has three reflective surfaces, forming three FPIs, and can obtain a hybrid sensing mechanism: wavelength-sensitive to temperature and intensity-sensitive to RI. Experimentally, the chemical etching method was used to form a concave in the fiber tip. Fused together with another section of un-etched fiber, an air bubble can be formed inside the fiber. Then, a cleavage near the bubble can shape the three-beam FPI. Using the fast Fourier Transform, the first-order spatial frequency is corresponding to the air-FPI, and the ratio of the second-order spatial frequency to first-order spatial frequency can describe how many smaller interference periods have been created within one original period due to the cleavage. The test temperature range is 30-40°C with a step of 2°C. The band pass filtering method was used to analyze different frequency components. Experimental results validate that the silica-FP was dominate for temperature sensing, and the average temperature sensitivity was 8.97 pm/°C with repeated linearity over 0.95. The test RI range was 1.3333-1.3908, and average RI sensitivity was 10.05 dB/RIU with repeated linearity over 0.92. Therefore, the proposed structure is a compact and efficient sensing component with hybrid sensitive mechanism that can achieve a temperature-RI dual-parameter measurement.

Capacitive micromachined ultrasonic transducers (CMUTs) are one of the appealing MEMS devices. Most studies treat CMUTs as rigid plates vibrating in open air, ignoring the mechanical boundary conditions for simplification and resulting in cumulative errors in coupled fields. This paper presents a new analytical model for the pull-in characteristics of the flat circular CMUT cell featuring sealed cavity. Utilizing the plate theory coupled with Boyle’s law, the paper establishes a strong relation between the pressures inside the sealed cavity and the pull-in characteristics for the first time. Not only did we point out that the existence of the pressure inside the sealed cavity cannot be omitted, but we also quantified the direct effect of the pressure ratios on the pull-in phenomenon. The pull-in voltages increase while the pull-in ratios decrease with the pressure ratios of the pressure inside the sealed cavity to the ambient pressure. The proposed calculation process delivers a good approximation of the pull-in voltages and displacements, which are consistent with COMSOL simulation results. Particularly, the percentage error of our calculation process is 6.986% for the worst case. Therefore, our proposed analytical model accurately and efficiently predicts the pull-in characteristics and this paper offers new perspectives and reference value in designing and modeling the CMUTs.

A temperature fiber sensor based on tapered fiber Bragg grating (tapered FBG) fabricated by femtosecond laser has been proposed and realized with good reproducibility. Firstly, the fiber taper with 25 μm diameter and 1000 μm length is fabricated by arc-discharge elongation using two standard single-mode fibers. Secondly, two first-order FBGs are fabricated in tapered and non-tapered fiber regions for comparison. Both FBGs are point-by-point direct-written by femtosecond laser, and the grating lengths are 1000 μm. Thirdly, a temperature experiment is performed using a heating chamber, and experimental results show that in the range of 30~350 °C, the temperature sensitivity of the tapered FBG has increased from 11.0 pm/°C to 12.3 pm/°C. The tapered FBG proposed here can be further configured for sensing other parameters in physical, chemical, and biomedical applications.