A low-drift fiber-optic sensor system, consisting of 24 regenerated fiber Bragg gratings (RFBG), equally distributed over a length of 2.3 m, is presented here. The sensor system can monitor spatially extended temperature profiles with a time resolution of 1 Hz at temperatures of up to 500 • C. The system is intended to be used in chemical reactors for both the control of the production ramp-up, where a fast time response is needed, as well as for production surveillance, where low sensor drifts over several years are required. The fiber-optic sensor system was installed in a pilot test reactor and was exposed to a constant temperature profile, with temperatures in the range of 150-500 • C for more than two years. During this period, the temperature profile was measured every three to five months and the fiber-optic temperature data were compared with data from a three-point thermocouple array and a calibrated single-point thermocouple. A very good agreement between all temperature measurements was found. The drift rates of the 24 RFBG sensor elements were determined by comparing the Bragg wavelengths at a precisely defined reference temperature near room temperature before and after the two-year deployment. They were found to be in the range of 0.0 K/a to 2.3 K/a, with an average value of 1.0 K/a. These low drift rates were achieved by a dedicated temperature treatment of the RFBGs during fabrication. Here, the demonstrated robustness, accuracy, and low drift characteristics show the potential of fiber-optic sensors for future industrial applications. gratings strongly decay at high temperatures [1,2]. Amongst others, type II FBGs inscribed with femtosecond (fs) lasers [3][4][5] and regenerated FBGs (RFBGs) [6][7][8] have been reported to be suitable for temperatures up to 1200 • C. Most often, type II FBGs are inscribed with high-intensity femtosecond laser beams by phase masks (type II-PM) [3,9] or by a point-by-point (type II-PbP) [10] technique, and their main features are a high grating reflectivity, inherent temperature stability, and high initial tensile strength. The disadvantages of type II FBGs arise from their polarization sensitivity [10,11] and strong cladding mode coupling [5,10], which can limit their multiplexing capabilities. Type II-PbP gratings show significant wavelength drift when exposed to high temperatures [12,13]. For type II-PM FBGs, a stabilization of the wavelength drift after an annealing procedure of 100 h at 1000 • C has been reported [14]. Due to temperature treatments, a thermal-induced glass corrosion and thus a reduction of the tensile strength has to be taken into account.Regenerated fiber Bragg gratings (RFBGs) emerge from type I gratings in H 2 -loaded fibers after an annealing process at temperatures in the range of 800 • C to 1100 • C [7,15]. Their uniform spectral line shapes and the lack of cladding mode coupling make RFBGs perfectly suited for multiplexing applications. When compared to type II FBGs, RFBGs typically exhibit lower values of grating reflectivity. However, ...

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A new iterative matrix algorithm has been applied to improve the precision of temperature and force decoupling in multi-parameter FBG sensing. For the first time, this evaluation technique allows the integration of nonlinearities in the sensor's temperature characteristic and the temperature dependence of the sensor's force sensitivity. Applied to a sensor cable consisting of two FBGs in fibers with 80 µm and 125 µm cladding diameter installed in a 7 m-long coiled PEEK capillary, this technique significantly reduced the uncertainties in friction-compensated temperature measurements. In the presence of high friction-induced forces of up to 1.6 N the uncertainties in temperature evaluation were reduced from several degrees Celsius if using a standard linear matrix approach to less than 0.5°C if using the iterative matrix approach in an extended temperature range between -35°C and 125°C.

In this paper, the deployment of a newly developed, multipoint, fiber-optic temperature-sensor system for temperature distribution measurements in a 6 MW gas turbine is demonstrated. The optical sensor fiber was integrated in a stainless steel protection cable with a 1.6 mm outside diameter. It included six measurement points, distributed over a length of 110 mm. The sensor cable was mounted in a temperature probe and was positioned radially in the exhaust-gas diffusor of the turbine. With this temperature probe, the radial temperature profiles in the exhaust-gas diffusor were measured with high spatial and temporal resolution. During a test run of the turbine, characteristic temperature gradients were observed when the machine operated at different loads.