Ultra-high-temperature monitoring is of great significance in many extreme environments, such as aerospace and power industries. Since the melting temperature of single-crystal sapphire fibers is up to 2045 ℃, sapphire fiber Bragg gratings (SFBGs) can serve as an ideal ultra-high-temperature sensors. However, SFBGs suffer from lossy spots due to high temperature oxidation, resulting in a significant deterioration of spectral signal-to-noise ratio (SNR) and a decrease in demodulation accuracy. Here, an ultra-high-temperature sensor based on an SFBG, which was inscribed by a femtosecond laser line-by-line scanning technique and sealed in a sapphire tube infiltrated with argon gas, is proposed and demonstrated. Moreover, since the spectrum of SFBG exhibits multiple peaks and large bandwidth, the conventional peak detection algorithm of single-mode FBG is not desirable for realizing the high-precision demodulation of SFBG. We proposed an high-precision temperature demodulation for SFBG sensors, including spectral data de-noising, a Gaussian-like profile fitting and peak search. Furthermore, the ultra-high-temperature response of the sensor in the range of 800 ℃ to 1900 ℃ was studied by utilizing a high-temperature blackbody radiation source. Additionally, the results showed that the temperature sensitivity of SFBG sensor was 44.3 pm/℃ at 1900 ℃ and the temperature uncertainty with one σerror bar was less than ±1.5 ℃. As such, the proposed SFBG sensor is desirable for harsh environments in aviation, nuclear power and smelting industries.
High-temperature sensing is in great demand in the aviation, nuclear power and petroleum industries. Single-crystal sapphire fiber is a promising candidate for the fabrication of ultra-high temperature sensor due to its high melting temperature of 2045 °C. However, sapphire fiber usually exhibits multimode operation owing to it having no cladding. We demonstrate a new method for fabricating single-mode helical Bragg grating waveguides (HBGWs) in a multimode sapphire fiber based on femtosecond laser direct writing technique. Such a helical Bragg waveguide can be obtained by using merely one fabrication step. The negative refractive index changes region works as a depressed cladding waveguide, and the periodical structure yields Bragg resonance. And hence, a single-mode HBGW created in sapphire fiber was successfully fabricated by using the proper parameters, such as a diameter of 10 μm and a single-pulse energy of 29.9 nJ, and the bandwidth of its reflection spectrum was merely 0.68 nm. Subsequently, the temperature response of the fabricated HBGW created in sapphire fiber was tested and it could withstand the high temperature of 1800 °C and its temperature sensitivity was 41.2 pm/°C.
The measurement of ultra-high temperatures is crucial for making meaningful advancements in the aerospace and power industries. Single-crystal sapphire fibers are desirable for the fabrication of ultra-high temperature sensors due to its high melting temperature of 2045 °C. Sapphire fiber Bragg gratings (SFBGs) suffer from a significant deterioration in their spectra following an ultra-high temperature exposure due to high-temperature oxidation. Here, an ultra-high-temperature sensor based on SFBG created by femtosecond laser inscription and inert gas-sealed packaging is proposed and demonstrated. The SFBG high-temperature sensor consists of a sapphire tube infiltrated with argon gas and an SFBG inscribed with a femtosecond laser line-by-line technique. Moreover, a standard FC/APC connector was set up at the end of the sapphire tube. The ultra-high temperature sensor was isothermally annealed for 55 hours at 1600 °C. Furthermore, the long-term thermal stability and temperature response of the sensors was evaluated, and then Savitzky-Golay smoothing and interpolation of the reflection spectrum were utilized to increase the accuracy in detecting the peak wavelength. It has been found that the stabilized ultra-high-temperature sensor can withstand temperatures up to 1600 °C for up to 20 hours. Furthermore, a third-order polynomial fitted to the response was used for calibrating the sensor from room temperature to 1800 °C. Additionally, its temperature sensitivity at 1800°C was 41.9 pm/°C. These results make it eminently suitable for utilization for ultra-high temperature measurements in power, smelting, and aviation industries.
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.
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