The continuous development of high-temperature fiber Bragg grating technology (FBG technology) promotes a significant increase in novel applications. For instance, nowadays FBG applications include such fields as “the temperature profiling of high-temperature manufacturing equipment, monitoring of fuel combustion machinery, temperature regulation of large diesel engines in trains, as well as assessment the structural integrity of a building post-fire”.
Additionally, high-temperature FBG technology is used in oil and gas industries where the resistance to the temperatures higher than 500 °C is totally recommended. To be more precise, the sensors based on fiber Bragg gratings are able to stand temperature conditions below and above 800 °C. Herewith, the thermal stability of FBG sensors depends closely on the intrinsic thermal stability of the core-cladding materials.
This is the reason why the development of fiber optic technology with higher thermal resistance, for example, the molten core technique, is still required. Thus, it was decided to apply a circular core/cladding glass optical fiber containing a yttrium-doped aluminosilicate core and a silica cladding in FBG sensors that may withstand about 900 °C.
The following types of FBG sensors are based on the nature of refractive index modifications induced by laser irradiation. The following types of FBGs are distinguished:
- The type I in fiber Bragg gratings produces a laser irradiation regime that emits an isotropic increase of the refractive index.
- The type II in FBGs, in its turn, has a connection with the creation of an anisotropic index change upon irradiation, generally emitted by the presence of nanogratings, and leads to the observation of form birefringence.
- Ultra-high temperature regenerated fiber Bragg gratings are able to operate above 800 °C in silica optical fibers. Therefore, these FBGs find their application in such areas as the profiling of high-temperature manufacturing equipment, dual pressure/temperature sensing for gas turbines, sodium-cooled nuclear reactors, high-temperature air flow meters for internal combustion engines and train engine temperature regulation.
- Femtosecond fiber Bragg gratings are made by ultrafast laser systems usually in the NIR spectral range, resulting in their use as temperature sensors for monitoring fluidized bed combustors, as well as for radiation-resistant temperature sensors.
- Sapphire fiber Bragg gratings allow achieving even higher temperature operation by using materials with higher melting points.
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