We have designed, fabricated, and characterized a fiber-optic three-dimensional vector accelerometer for vibration measurement. The accelerometer is based on fiber Bragg gratings (FBGs) inscribed with three cores of seven-core fiber by femtosecond laser with a phase mask. Through monitoring the wavelength shifts of two of the seven cores, we obtain the vibration azimuthal angle and the acceleration in the XY plane by angle reconstruction and algorithmic calculation. Similarly, by monitoring the wavelength shift of the core fixed along the Z-axis, we obtain the vibration acceleration in the Z-axis direction. The full 3D vector information about the vibration signal is obtained by vector synthesis. The accelerometer has achieved a working frequency bandwidth from 10 to 220 Hz with the maximum sensitivity of 355 pm/g, and the best azimuthal angle accuracy of 0.269 ^° in the linear range of 0 ^° -100 ^° and 170 ^° -280 ^° . Moreover, it is possible to increase the overall bandwidth, sensitivity and accuracy of the azimuth of sensors in the XY plane by adjusting the length of the optical fiber and adjusting mechanical parameters to increase the bandwidth and sensitivity of the sensor in Z-axis. Such improvements will further enhance application to small-scale seismic surveys.

Fiber Bragg grating (FBG) accelerometers measure vibrations by demodulating wavelength shifts induced by dynamic strain. Their high-temperature resistance and multiplexing capabilities provide them with opportunities for quasi-distributed applications in harsh environments. The basic principle of an FBG accelerometer is based on vibration-strain coupling. In the design of a sensor's mechanical structure, improving the coupling efficiency can effectively enhance the sensitivity of the accelerometer. Moreover, an FBG accelerometer can be developed as a multidimensional sensor, which is a unique feature compared to distributed-fiber vibration-sensing technology. In this paper, we review the recent advances in FBG accelerometer developments and by focusing on two main trends: sensitivity enhancement and orientation identification. We emphatically introduce new techniques for FBG accelerometers including hybrid strain loading, thin-cladding fiber FBGs, multicore FBGs and cladding FBGs.

We demonstrate a fiber-optic strain sensor based on a seven-core fiber (7-CF) for accurate extrusion bending and tensile strain sensing. The sensor structure we designed is the 7-CF spliced to a piece of multimode fiber on each side with a lateral core-offset and then conventionally fused to single mode optical fiber as the input and output ports. The experimental results indicate that the optimal curvature sensitivity of the sensor is achieved 25.96 nm/m-1, and the sensitivity of the tensile strain is 0.094 nm/με, which allows us to monitor bending and stretching strain respectively with high precision by the simple, low-cost, and effective sensor.

In this letter, we proposed a gas refractometer based on optical fiber Fabry-Perot interferometer with an open cavity. The Fabry-Perot cavity is a short section capillary tube spliced between a single-mode fiber and a side-hole fiber (SHF). The unique cladding of the SHF enables gas entering or leaving the Fabry-Perot cavity freely. The measurement of pressure-induced refractive index change of nitrogen gas has been demonstrated by monitoring the wavelength shift of the interference pattern. The robust and microsize architecture make the interferometer a good candidate for gas sensing applications.

An optical fiber humidity sensor based on an optical Fabry–Perot interferometer is proposed and experimentally demonstrated. The sensor is constructed by a short section of hollow-core fiber coated with a polyimide (PI) film. Taking advantage of the direct response of the PI film, a sensitivity of up to 1.309 nm/%RH can be achieved in the humidity change range from 40% RH to 80% RH. The temperature sensitivity is measured to be 43.57 pm/°C when the temperature changes from 25°C to 55°C. Because of its simple structure, fast response time, convenient production, and good reproducibility, the proposed sensor will be competitive in the field of cultural relic humidity monitoring and pharmaceutical storage.

We present an integrated in-fiber coupler for excitation of whispering-gallery modes of a microsphere resonator. The coupler is simply fabricated by chemical etching away the holey area of a photonic crystal fiber, leaving a freestanding solid core enclosed in a silica housing. Light is coupled into a microsphere through the suspended core with a diameter of 2.1 μm. Since the coupler itself performs as a Fabry–Perot interferometer, asymmetric Fano resonances can be observed in the mixed reflection spectrum. The silica housing of the coupler provides a robust mechanical support to the microsphere resonator. The new Fano resonance coupler shows great potential in biochemical sensing and optical switching applications.