A compact fiber Bragg grating （FBG） diaphragm accelerometer based on L-shaped rigid cantilever beam is proposed and experimentally demonstrated. The sensing system is based on the integration of a flat diaphragm and an L-shaped rigid cantilever beam. The FBG is pre-tensioned and the two side points are fixed, efficiently avoiding the unwanted chirp effect of grating. Dynamic vibration measurement shows that the proposed FBG diaphragm accelerometer provides a wide frequency response range （0-110 Hz） and an extremely high sensitivity （106.5 pm/g）, indentifying it as a good candidate for embedding structural health monitoring and seismic wave measurement.
A simple fiber tip sensor for refractive index （RI） measurement based on Fabry-Perot （FP） interference modulated by Fresnel reflection is proposed and demonstrated. The sensor head consists of an etching- induced micro air gap near the tip of a single-mode fiber. The microgap and the fiber tip function as two reflectors to form a FP cavity. The external RI can be unambiguously measured by monitoring the fringe contrast of the interference pattern from the reflection spectrum. The experimental results show that the proposed sensor achieves temperature-independent RI measurement with good linear response. The proposed sensor achieves a high RI resolution of up to 3.4× 10^-5 and has advantages of low cost and easy fabrication.
A metal bellows-based fiber Bragg grating （FBG） accelerometer is proposed and experimentally demonstrated. The optical fiber （containing the FBG） is pre-tensioned, and the two ends of the optical fiber are fixed directly from the shell to the inertial mass. In this design, the FBG is uniformly tensioned to obtain a constant strain distribution over it. By employing this configuration, the FBG always has a sharp reflection characteristic with no broadening in its reflection spectrum during wavelength shifting. Dynamic vibration measurements show that the proposed FBG accelerometer has a wide frequency response range （5-110 Hz） and an extremely high sensitivity （548.7 pro/g）. The two important indicators of FBG accelerometer can be tuned by the addition of mass to tailor the sensor performance to specific applications, identifying it as a good candidate for structural health monitoring.
The intelligent structural health monitoring method, which uses a fiber Bragg grating （FBG） sensor, is a new approach in the field of civil engineering. However, it lacks a reliable FBG-based accelerometer for taking structural low frequency vibration measurements. In this letter, a fiextensional FBG-based accelerometer is proposed and demonstrated. The experimental results indicate that the natural frequency of the developed accelerometer is 16.7 Hz, with a high sensitivity of 410.7 pm/g. In addition, it has a broad and flat response over low frequencies ranging from 1 to 10 Hz. The natural frequency and sensitivity of the accelerometer can be tuned by adding mass to tailor the sensor performance to specific applications. Experimental results are presented to demonstrate the good performance of the proposed FBG-based accelerometer. These results show that the proposed accelerometer is satisfactory for low frequency vibration measurements.
The reflective spectrum power and the bandwidth of the fiber Bragg grating （FBG） under gradient strain are researched and experimentally demonstrated. The gradient strain is applied on the FBG, which can induce FBG bandwidth broadening, resulting in the variation of reflective power. Based on the coupled-mode theory and transfer matrix method, the segmental linear relationship between the gradient strain, the reflective power, and the bandwidth is simulated and analyzed, and the influence of the FBG length on the reflective spectrum is analyzed. In the experiment, the strict gradient stain device is designed; the experimental results indicate that the reflective optic power and the bandwidth of the FBG under gradient stain are concerned with the length of the FBG. Experimental results are well consistent with the theoretical analysis, which have important guiding significance in the FBG dynamic sensing.
A micro Fabry-Perot interferometer （M-FPI） is constructed by splicing a short section of polarization- maintaining photonic crystal fiber （PM-PCF） to an end-cleaved single-mode fiber with controllable offset. Due to the high effective optical path difference induced by the solid core of the PCF, the M-FPI has an ultrasmall cavity of approximately 110 μm. The temperature sensitivity within a range from 33 ℃ to approximately 600 ℃ is measured to be 13.8 pm/℃, which shows good agreement with the theoretical result. This proposed sensor has the advantages of ultracompact size and high stability. Therefore, it is suitable for various space-limited sensing applications in harsh environments.
