A fiber optic whispering gallery mode (WGM) resonator was proposed and realized by integrating an inline polymer waveguide with a microsphere mounted on it. The polymer waveguide with a diameter of 1 μm was printed with femtosecond laser-assisted multiphoton polymerization in a section of a grooved hollow-core fiber, which was sandwiched between two single-mode fibers. Two WGW resonators assembled with microspheres of different sizes were prepared. The transmission spectra of those stimulated WGMs were investigated both in simulation and experimentally. The temperature response of the resonators was particularly studied, and a linear sensitivity of −593 pm/°C was achieved from 20 °C to 100 °C.
In this paper, we propose and experimentally demonstrate a simple technique to enhance the curvature sensitivity of a bending fiber optic sensor based on anti-resonant reflecting optical waveguide (ARROW) guidance. The sensing structure is assembled by splicing a segment of capillary hollow-core fiber (CHCF) between two single-mode fibers (SMF), and the device is set on a steel sheet for measuring different curvatures. Without any surface treatment, the ARROW sensor exhibits a curvature sensitivity of 1.6 dB/m−1 in a curvature range from 0 to 2.14 m−1. By carefully coating half of the CHCF length with polydimethylsiloxane (PDMS), the curvature sensitivity of the ARROW sensor is enhanced to −5.62 dB/m−1, as well as an increment in the curvature range (from 0 to 2.68 m−1). Moreover, the covered device exhibits a low-temperature sensitivity (0.038 dB/°C), meaning that temperature fluctuations do not compromise the bending fiber optic sensor operation. The ARROW sensor fabricated with this technique has high sensitivity and a wide range for curvature measurements, with the advantage that the technique is cost-effective and easy to implement. All these features make this technique appealing for real sensing applications, such as structural health monitoring.
Over decades, fiber-optic temperature sensors based on conventional single-mode fibers (SMF) have been demonstrated with either high linearity and stability in a limited temperature region or poor linearity and thermal hysteresis in a high-temperature measurement range. For high-temperature measurements, isothermal annealing is typically necessary for the fiber-optic sensors, aiming at releasing the residual stress, eliminating the thermal hysteresis and, thus, improving the high-temperature measurement linearity and stability. In this article, an annealing-free fiber-optic high-temperature (1100 °C) sensor based on a diaphragm-free hollow-core fiber (HCF) Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The proposed sensor exhibits an excellent thermal stability and linearity (R2 > 0.99 in a 100–1100 °C range) without the need for high-temperature annealing. The proposed sensor is extremely simple in preparation, and the annealing-free property can reduce the cost of sensor production significantly, which is promising in mass production and industry applications.
Abstract
In this work, a fiber-optic fluoride-ion-detection Michelson interferometer based on the thin-core fiber (TCF) and no-core fiber (NCF) coated with α-Fe2O3/ZrO2 sensing film is proposed and presented. The single-mode fiber (SMF) is spliced with the TCF and NCF in turn, and a waist-enlarged taper is spliced between them. Then, a silver film is plated on the end face of NCF to enhance the reflection. After the absorption of fluoride ion by the sensing film, the effective refractive index (RI) of the coated cladding will change, which leads to the regular red shift of the interference dip with the increasing fluoride-ion concentration. Thus, the fluoride-ion concentrations can be determined according to the corresponding dip wavelength shifts. The results show that the sensor has an excellent linear response (R
2 = 0.995) with good sensitivity (8.970 nm/ppm) when the fluoride-ion concentration is in the range of 0–1.5 ppm. The response time is about 15 s. The sensor has the advantage of good selectivity, good temperature and pH stabilities, and can be applied to detect fluoride ion effectively.
Abstract
An intensity modulated fiber-optic carbon monoxide (CO) sensor by integrating in-situ solvothermal-growth Ag/Co-MOF sensing film is fabricated and evaluated. The Michelson interference sensing structure is composed of single-mode fiber (SMF), enlarged taper, thin-core fiber (TCF), and Ag film as the reflector. Ag/Co-MOF was coated on the cladding of the TCF as the sensing material, and the enlarged taper is located between TCF and SMF as the coupler. The structure, morphology, compositions and thermal stability of the Ag/Co-MOF sensing film were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), etc. The sensitivity of the sensor is 0.04515 dB/ppm, and the fitting parameter of the CO concentration is 0.99876. In addition, the sensor has the advantages of good selectivity, good signal and temperature stability, and it has potential application in trace CO detection.
An experiment is proposed to show the feasibility of using hollow-core photonic bandgap fibers (HC-PBF) in the fiber-optic interferometric stethoscopes to generally improve the sensitivity and overcome the problems associated with the electronic stethoscopes. In the experiment, the HC-1550 is used as a measuring arm of an unbalanced Mach-Zehnder interferometer (MZI) and the conventional single-mode optical fiber (SMF) is used as an isolated reference arm. Detection and demodulation of the relative phase shift is performed passively using phase-generated carrier homodyne technique (PGC). The proposed results indicate the significance of using HC-PBFs in the future stethoscopes.
In-Fiber BaTiO3 Microsphere Resonator for High-Sensitivity Temperature Measurement