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.
In this paper, we introduce an optimal bias voltage searching strategy and maintaining method in BOTDA system based on dual sideband modulation. The system utilizes both up shifted and down shifted continuous wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain a clean Brillouin shifted curve along the fiber under test, the probe light must be in good signal to noise ratio. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel control method of frequency shift in sideband modulation of a BOTDA system, which keeps the first order frequency-shifted light at its maximum signal to noise ratio and minimum light intensity fluctuation. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out to evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 50 km fiber cable with spatial resolution of 0.5m.