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Schematic illustration of active FBG which is heated by in-fiber high-power laser light.
Sketch of a grating level sensor array, light from a high-power laser diode was coupled into a multimode (MM) fiber that is fusion spliced to the SM fiber. High-power 910 nm laser light in the multimode fiber leaks into the cladding of the single-mode fiber through the spliced junction and heats the silver coating of FBGs. The actual fusion spliced SM to MM junction is shown in the figure.
Bragg grating response in and out of water when it was heated by 115 mW 910 nm diode laser power.
Resonance wavelength shifts of heated gratings in air, water, and liquid as a function of input laser power. The grating wavelength shifts in water and liquid re-plotted using a reduced vertical scale is shown in the inset of the same figure.
Spectra responses of a four-grating sensor array when the fiber was pulled out in serial from water. Sensor 1 was the topmost grating and Sensor 4 was the lowest sensor. The dot traces are the spectral responses for the unheated sensor.
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