In this paper, fiber Bragg grating (FBG) sensors for strain monitoring at cryogenic temperatures were proposed and demonstrated. The strain sensitivity, strain repeatability and temperature compensation effect of FBG sensors were studied, within the temperature range of -196°C to 20°C. The strain sensitivity of FBG sensors at cryogenic temperatures was tested by quartz specimens with low thermal expansion coefficient. Experimental results show that the strain sensitivity of the relative change of the central wavelength for the FBG is 0.784, which almost does not change with the temperature. The repeatability of the adhesive used for FBG strain sensors at cryogenic temperatures was tested. Experimental results show that FBG sensors have good adaptability to cryogenic temperatures, and the strain repeatability is within 1.0% FS. The zero drift of the FBG strain sensor was compensated by using the FBG temperature sensor, and the strain error is less than 5% of the measured values with the compensation at cryogenic temperatures. Compared with the traditional electrical strain gauge used at cryogenic temperatures, FBG strain sensors have advantages in non-sensitivity drift and good zero drift compensation effect, and they can play an important role in the structural health monitoring at cryogenic temperatures such as spacecraft tanks, fuel pipelines, and so on.
A micro all-glass fiber-optic accelerometer based on the extrinsic Fabry–Perot interferometer (EFPI) is developed. A microcantilever beam is fabricated using a femtosecond laser and assembled with a silica tube and a silica inertial mass. Two mirrors, the end face of the leading single-mode fiber and the inner glass/air interface of the cantilever beam, form an EFPI. When the accelerometer is subjected to the vibration along the fiber axis, the vibration is detected by interrogating the variation in the cavity length of the EFPI. The proposed accelerometer has a compact structure and high signal-to-noise ratio. Experimental results show that the sensitivity of 2.9 nm/g@500 Hz within the acceleration range of 0 to 3 g is achieved. The accelerometer can work within the frequency range of 100 to 1500 Hz.
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