A temperature-compensated sensor architecture for a fiber optic hydrogen sensor consisting of a partly palladium-coated pi-shifted fiber Bragg grating was modeled and compared with measurements. The transfer matrix formalism was used to calculate the spectral line shape of the pi-shifted FBG with a hydrogen-induced, non-homogeneous strain distribution along the grating axis. The temperature response of the grating itself can be compensated by referencing the notch to the flank wavelength. In addition, the hydrogen solubility in Pd shows a non-linear temperature dependence that was also included in the sensor performance calculations. For the investigated H2 concentration range of 200 ppm to 20000 ppm and between 15 °C and 40 °C, measurement data fit well to the simulation above 3000 ppm but become diffuse below, indicating deviations from the expected dependence according to Sieverts’ square root law.
An aluminum embedded fiber Bragg grating (FBG) sensor configuration for a decoupled temperature and strain measurement was proposed and demonstrated. This configuration consists of a three-point regenerated FBG (RFBG) sensor array with a distance of 8 mm in one fiber. One RFBG was in direct contact with the aluminum, whereas the other RFBGs were protected from the aluminum by a steel capillary. The RFBG in contact with the aluminum was influenced by temperature and strain, and the other RFBGs only by temperature. The casting mold used here had a second symmetric arm used for an additional temperature reference measurement with three RFBGs in one capillary. Two casting experiments with this configuration were made, and decoupled temperature and strain measurements based on one sensing fiber were demonstrated. The temperature values in the strain sensing fiber showed good agreement with the reference temperatures in the second casting arm. Thereby, it could be demonstrated that one fiber was sufficient for both temperature and strain measurement and that the temperature compensation for the strain sensor was successful. This is a significant step towards the application of smart casts.
The temperature distribution during a metal casting process is important because it influences structural properties and residual stresses of the casted part. In this paper, we report on multipoint temperature monitoring based on a regenerated fiber Bragg grating (RFBG) array during a copper casting process, with a maximum temperature exceeding 1100°C. A temperature calibration function up to 800°C was obtained experimentally and was linearly extended to 1200°C. According to the experimental results, the temperature information from an RFBG temperature sensor agreed well with that from a thermocouple that was located close to the respective RFBG sensor element, even at temperatures above 1100°C. With the RFBG temperature sensor array in a single fiber, the temperature distribution and its temporal development were obtained, which can be used to better understand the copper casting process and improve the casting mold design.
Optical fiber sensors based on multimode interference (MMI) have been widely used and developed into various applications. The sensing principle is mostly based on the induced wavelength shift of selected dips (or peaks) in the transmission spectrum. A simple structure to obtain MMI devices is a so-called single- mode—multimode—single-mode (SMS) fiber structure, which is composed of a short section of multimode fiber (MMF) fusion spliced between two single-mode fibers (SMFs). However, most of these MMI-based fiber sensors are related to circular core MMFs. Although the sensitivity is enhanced and the fabrication process is improved, some common problems still exist and need to be solved. To solve current challenges, such as linearity, crosstalk, and compactness, it is essential to find solutions like using a new structure or a new type of fiber. Recently, different new fibers, like hollow annular core fiber (HACF) and square no-core fiber, have been demonstrated to have advantages to overcome some of these limits. The applications of these fibers provide possibilities to study specifically shaped core multimode fibers. In this contribution, we propose a compact MMI-based fiber sensor for temperature measurement. Instead of the standard MMF used in the SMS structure, a square-core fiber (SCF) with a circular cladding is implemented as the sensing element. To the best knowledge of the authors, the SCF has not been investigated yet for sensing. Therefore, the sensing characteristics are studied experimentally. The proposed fiber sensor reaches a sensitivity of 45 pm/◦C. It proves the sensing capability of SCF is promising, provides great potential for further works.
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