In this study, the response of fiber Bragg gratings (FBGs) embedded in cast aluminum parts under thermal and mechanical load were investigated. Several types of FBGs in different types of fibers were used in order to verify general applicability. To monitor a temperature-induced strain, an embedded regenerated FBG (RFBG) in a cast part was placed in a climatic chamber and heated up to 120 ${}^{\circ }\mathrm{C}$ within several cycles. The results show good agreement with a theoretical model, which consists of a shrink-fit model and temperature-dependent material parameters. Several cast parts with different types of FBGs were machined into tensile test specimens and tensile tests were executed. For the tensile tests, a cyclic procedure was chosen, which allowed us to distinguish between the elastic and plastic deformation of the specimen. An analytical model, which described the elastic part of the tensile test, was introduced and showed good agreement with the measurements. Embedded FBGs - integrated during the casting process - showed under all mechanical and thermal load conditions no hysteresis, a reproducible sensor response, and a high reliable operation, which is very important to create metallic smart structures and packaged fiber optic sensors for harsh environments.

Regenerated fiber Bragg gratings (RFBGs) have been created in large mode area (LMA) fibers, and their key properties for their application as high-temperature sensors were experimentally characterized. UV written Type-I seed gratings in standard single-mode fibers (SMF-28) and in low numerical aperture LMA fibers with 125 μm and 250 μm cladding diameters were regenerated, and their wavelength drift characteristics at high temperatures, their temperature sensitivities, and their mechanical strengths were tested and compared with each other. We found that low dopant concentrations in the fiber cores led to lower wavelength drift rates during and after regeneration. A temperature calibration from room temperature to 800 $^\circ$ C showed that LMA fibers have generally similar but slightly lower temperature sensitivities than SMF-28 fibers. The tensile strength tests showed that LMA fibers with 250 μm cladding diameter have similar tensile fracture stresses as SMF-28 fibers but four times larger tensile force tolerances because of their twofold fiber diameter. Our research shows that using LMA fibers for RFBGs offers significant advantages for high-temperature sensing than their counterparts in standard fibers, which are also applicable to other silica fiber-based high-temperature sensors.

A new calibration methodology for regenerated fiber Bragg grating (RFBG) temperature sensors up to 700 °C is proposed and demonstrated. A generalized, wavelength-dependent temperature calibration function is experimentally determined that describes the temperature-induced wavelength shifts for all RFBG sensor elements that are manufactured with the same fabrication parameters in the wavelength range from 1465 nm to 1605 nm. Using this generalized calibration function for absolute temperature measurements, each RFBG sensor element only needs to be calibrated at one reference temperature, representing a considerable simplification of the conventional calibration procedure. The new calibration methodology was validated with 7 RFBGs, and uncertainties were found to be compliant with those of Class 1 thermocouples (< ±1.5 K or < ±0.4% of the measured temperature). The proposed calibration technique overcomes difficulties with the calibration of spatially extended multipoint RFBG sensor arrays, where setting up an adequate calibration facility for large sensor fibers is challenging and costly. We assume that this calibration method can also be adapted to other types of FBG temperature sensors besides RFBGs. An accurate and practical calibration approach is essential for the acceptance and dissemination of the fiber-optic multipoint temperature sensing technology.

The high-temperature performance of a sapphire fiber Bragg grating has been characterized by a commercial single-mode interrogator. By using the mode field matching fusion splicing method, we proposed before, high-order modes in the reflection spectrum of the sapphire fiber Bragg grating could be suppressed and a quasi-single mode reflection spectrum could be obtained from a single-mode fiber-based demodulation system. A 48-hour isothermal annealing at 900°C and two temperature calibration processes, one up to 800°C and one up to 1200°C, were carried out successively. There was no obvious reflectivity decay or wavelength drift during the 900°C annealing process, and no hysteresis during the two temperature calibration processes. The precisions of the sapphire fiber Bragg grating were ±0.25°C in the 800°C calibration process, while increased to ±5°C in the 1200°C calibration process.

Intra-spectral referencing comprises a single-sensor solution, where simultaneous measurements of hydrogen concentration and temperature are achieved within a single, partially palladium-coated π-phase-shifted Fiber Bragg Grating (FBG). Highly localized, hydrogen-induced strain sections provoke the notch and the envelope of the π-FBG spectrum to yield different hydrogen responses, while temperature affects both features almost identically. The precise temperature compensation in sub-Kelvin ranges was used to characterize the functional layer in the range of 20-50°C and at 250-20,000 ppm H₂ in N₂. Upon initial hydrogen exposure after fabrication, a gradually decreasing sensitivity drift was observed, which relaxed and stabilized after sufficiently long initialization with hydrogen. The desorption consisted of at least two processes, a fast desorption when large amounts of hydrogen are present in the nanofilm, followed by a notably slow desorption (~5 hrs) until the baseline was recovered. A zero-point referenced calibration scheme of the PTFE-capped Pd₉₁:Ni₀₉ fiber optic hydrogen sensor revealed a strong non-linear response especially at low concentrations <1,000 ppm. This required linearization, which led to effective decoupling of the temperature cross-sensitivity through iterative data processing. Decoupled concentrations and temperatures show a lower detection limit of 250 ppm, a significant enhancement in baseline stability, and time constants of t₉₀ = 19s for ab- and t₁₀ = 18s for desorption. This work paves the way for both practical applications of fiber optic H₂ sensors and fundamental research in Pd nanofilm engineering.