Simple Signal Processing Method to Enlarge the Dynamic Range of the Fresnel Reflection-Based Fiber Fabry–Perot Refractive Index Sensors

In this work, we propose a signal processing algorithm to enlarge the refractive index dynamic range (RIDR) of the Fresnel reflection-based fiber Fabry-Perot (FR-FFP) sensors. In this approach, the reflected spectrum of the FR-FFP interferometer is modulated by an impulse function. The processed optical spectrum includes the superposition of the original wavelength-modulated interference pattern and an ultra-narrow peak, produced by the impulse function. The wavelength of that peak coincides with a maximum of the interference fringe. In Fourier domain, the spectrum exhibits the characteristic interference peaks superimposed to a broad step. In Fourier domain the amplitude of one of the interference peaks is modulated by refractive index (RI) of the external medium, but the amplitude of the step is not affected. It was possible to stablish a simple correspondence between the amplitude of the interference peak and the RI of the external medium in a wide dynamic range [1.000, 2.000]. The inclusion of the impulse function, which produces the step in the Fourier domain, eliminates the RI ambiguity of the FR-FFP allowing the enlargement of the RIDR.