In situ strain sensing at high-temperature environment is crucial in the aerospace field. Silica fibers will soften at 700 ℃, which can hardly be utilized for strain sensing at higher temperatures. Notably, single crystal sapphire fiber is a promising material for high-temperature sensing due to the high melting point (~2045 ℃). Here, we report the strain sensing at 800 ℃ of sapphire fiber Bragg gratings (SFBG) inscribed by a femtosecond laser lineby- line scanning technique. At first, a line-by-line sapphire fiber grating was inscribed using femtosecond laser direct writing technique. The sapphire fiber ends were polished into bevels to reduce Fresnel reflections, and the signal-to-noise ratio of the SFBG was improved from 9 dB to 17.2 dB. And then, strain characteristics of the SFBG were investigated at room temperature. It was found that the maximum strain of SFBG was decreased to 3600 με (64% reduction) comparing with 9714 με of the pristine sapphire fiber, which is due to the micro-damage introduced by femtosecond laser pulses. In addition, the strain sensitivity of the SFBG is 1.42 pm/με. Subsequently, a strain sensing experiment of the SFBG was carried out at 1100 ℃ using a high-temperature tensile testing system. After annealing at 1100 ℃ for 4 h to improve the high-temperature stability, the SFBG exhibited a strain sensitivity of 1.6 pm/με (R2=0.998) at 1100 ℃. As a result, strain sensing at 1100 ℃ environment was realized based on the SFBG, which indicates a promising application in the aerospace field, especially in strain sensing for structural safety monitoring of hypersonic aircraft at high-temperature.
We propose and demonstrate a fiber Bragg grating (FBG) array inscribed with femtosecond laser point-by-point (PbP) method for quasi-distributed high-temperature sensing. Via optimization of grating length, femtosecond laser energy and grating pitch, a wavelength-division-multiplexed (WDM) FBG array including nine FBGs with various Bragg wavelengths ranging from 1510 nm to 1590 nm and an identical ultra-weak FBG array including 60 FBGs with a peak reflectivity of ~0.1‰ were fabricated. After annealing at 700°C for nearly 200 hours, the FBG exhibited a an extremely low Bragg wavelength shift (i.e., -2 pm/h). In addition, the modulation of WDM FBG array was studied. A generalized calibration curve was applied to the FBG array and the maximum fitting error of 27 pm and the temperature measurement accuracy of ± 1.8°C were achieved. And then, the fabricated WDM FBG array was used to realize the quasi-distributed high-temperature sensing up to 700°C in the static and dynamic environment. Such WDM FBG array could be developed for quasi-distributed high-temperature sensing in metallurgical, chemical, and aviation industries.
This paper reports a new silica fiber-tip Fabry-Perot interferometer with thin film and large surface area characteristic for high pressure and vacuum degree detection simultaneously, which is fabricated by etching a flat fiber tip into concave surface firstly, with subsequent arc jointing the concave fiber into a inline Fabry-Perot cavity, then drawing one surface of the F-P cavity into several micrometers scale by arc discharge and finally etching the surface into sub-micrometer scale integrally. As the silica fiber-tip Fabry-Perot interferometer film thickness could be tailored very thinly by HF acid solution, plus the surface area of thin film could be expanded during the chemical etching process, the variation of the bubble cavity length is very sensitive to the inner/outer pressure difference of the fiber-tip Fabry-Perot interferometer. Experimental result shows an high sensitivity of 780nm/MPa is feasible. Such configuration has the advantages of lowcost, ease of fabrication and compact size, which make it a promising candidate for pressure and vacuum measurement.
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