Method for the interrogation of FBG thermo-hygrometer through full analog circuit

Abstract

The proposed work has the aim to investigate a full analog electrical circuitry to convert the wavelength-encoded signal coming from a pair of Fiber Bragg Grating (FBG) sensors into a single monotonic electrical signal. The latter can be used either to be read from a PLC system (or directly by a switch) if a 4-20 mA signal is needed (e.g. for safety application) or to have an instantly conversion without employing the classical interrogation system with a post-processing by means of a digital unit. Since its peculiarities (robust, reliable and completely free from any digital processing section) the proposed system has the aim to overcome the classical interrogator, with the aim to pave the way to a wider employment of FGB sensor in that environment where the reliability given by the interrogator based on multiple digital processing unit, handled by an operative system, may be subjected to failure. In the proposed manuscript, the system was studied analytically and numerically, taking advantage of its characteristic to behaves linearly in a range of 200pm Bragg wavelength shifting, due to the Arrayed Waveguide Grating (AWG) device, used as optical filter. As results, the capability to perform compensated measurement, by means of 2 FBG subjected to different physical quantities, was investigated. The obtained formula comprises FBGs linear coefficient in function of the physical phenomenon to measure and the system output.