Regenerated Bragg Grating Sensor Array for Temperature Measurements During an Aluminum Casting Process

Fiber-Optic Multipoint Sensor System with Low Drift for the Long-Term Monitoring of High-Temperature Distributions in Chemical Reactors

“…As can be seen, the Bragg wavelengths exhibited a blue shift of about 1.1 nm due to the regeneration procedure and the regeneration efficiencies, expressed by the ratios of peak reflected power to pristine peak reflected power, ranged from 1% to 6%. This is lower than the typical values reported for regeneration experiments with seed grating arrays, where the gratings regenerated to about 10% of their pristine peak power [26,27]. Despite the low peak reflectivity values of the RFBGs, the signal-to-noise ratio (SNR) was still larger than 25 dB, enabling precise detection of the Bragg peaks ( Figure 3b).…”

“…For the large RFBG sensor arrays used here, no calibration device with a sufficiently large heat zone was available to directly calibrate the produced RFBG sensor arrays. Lindner et al [26] showed that different RFBGs, which were fabricated with the same type of optical fiber with the same grating In order to protect the fiber from mechanical degradation and to avoid that external strain is transferred to the fiber, the fiber sensor array was loosely installed in a stainless-steel capillary, with an outside diameter of 0.8 mm and about three meters in length. Glue sealed the proximal end of the capillary and fixed the fiber to the capillary at this point.…”

“…For the large RFBG sensor arrays used here, no calibration device with a sufficiently large heat zone was available to directly calibrate the produced RFBG sensor arrays. Lindner et al [26] showed that different RFBGs, which were fabricated with the same type of optical fiber with the same grating period (the same Bragg wavelength of~1548 nm at reference temperature), which experienced the same annealing procedure, exhibited equal dependencies of their Bragg wavelength shifts on temperature. They reported a generalized non-linear calibration function for temperatures ranging from about 20 • C to 800 • C, with an uncertainty of ±1.5 K [26], which is expressed as follows: where λ B (T) is the Bragg wavelength of a given RFBG at temperature T in • C, λ B (0) is an individual parameter, which represents the Bragg wavelength of the respective RFBG at zero centigrade, and A to E are general parameters whose values are listed in Table 1 [26].…”

“…For the RFBG sensor arrays used in the research described here, the same type of optical fiber and fabrication process was used as in [26]. Only the Bragg wavelengths of the RFBG arrays were in the range of 1533 nm to 1558 nm (see Table 2).…”

“…As mentioned, the generalized calibration function is valid for RFBGs comprising a Bragg wavelength of 1548 nm. Compared to [26], the RFBG wavelengths used here differed by up to −15 nm at the reference temperature. In this case, a dependency of the RFBG temperature sensitivity on the wavelength due to thermal expansion of the fiber and the grating period has to be considered.…”