We report the effect of coating thickness on the sensitivity of a relative humidity (RH) sensor based on an Agarose coated photonic crystal fiber interferometer for the first time. An experimental method is demonstrated to select an optimum coating thickness to achieve the highest sensitivity for a given RH sensing range. It is shown that the Refractive Index (RI) of the coating experienced by the mode interacting with the coating depends on the thickness of the coating. It is observed that the spectral shift of the interferometer depends on both the bulk RI change and the thickness change of the Agarose coating with respect to an RH change. The RH sensitivity of the sensor has a significant dependence on the thickness of the coating and the sensor with highest sensitivity shows a linear response for RH change in the range of 40-90% RH with a humidity resolution of 0.07%RH and a fast response time of 75 ms for an RH change from 50% to 90%.
Single-mode optical fibers with thin nickel coatings (outer diameter ∼350 μm) are successfully embedded into stainless steel (SS) 316 components using bespoke laser-based additive manufacturing technology. In our approach, we manufacture SS 316 components using selective laser melting, incorporating U-shaped grooves with dimensions suitable to hold nickel-coated optical fibers. Coated optical fibers containing fiber Bragg gratings for strain monitoring and temperature sensing are placed in the groove. The embedding is completed by melting subsequent powder layers on top of the fibers. Cross-sectional microscopy analysis of the fabricated components, together with analysis of the Bragg gratings behavior during fabrication indicates a strong substanceto-substance bond between coated fiber and added SS 316 material. Temperature and strain cycling of the embedded sensors demonstrates the ability of gratings to survive the embedding process, and act as sensing elements in harsh environments. In situ strain and temperature measurements from within the component are demonstrated for high dynamic stress levels and elevated temperatures (<400°C).Index Terms-Bragg gratings, embedded fiber sensors, laser melting, laser sintering, optical fiber sensors, stainless steel, strain sensors, temperature sensors.
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