A novel demodulation method for interferometric fiber sensor is proposed in this paper. The physical parameters to be measured by the sensor is obtained by calculating the phase variation of the interference components. The phase variation is computed with the assist of the fast Fourier analysis. For fiber interferometers, most of the energy is contained in the few spatial frequencies corresponding to the components that produce the interference. Therefore, the information of the interference fringe can be presented by the Fourier results at those intrinsic frequencies. Based on this assumption, we proposed a novel method to interrogate the fiber interferometer by calculating the Fourier phase at the spatial frequency. Theoretical derivation proves that the Fourier phase variation is equal to the phase change of the interferometer. Simulation results demonstrate the ability of noise resistance of the proposed method since the information of all wavelength sampling points are adopted for the demodulation process. A Sagnac interferometer based on a section of polarization-maintaining photonic crystal fiber is utilized to verify the feasibility of the phase demodulation technique by lateral pressure sensing. Experimental results of -0.069rad/kPa is acquired.
In this article, we propose a fiber displacement sensor based on a few mode fiber loop sandwiched between two single mode fibers (SMF). The proposed sensor is flexible due to the tunable resolution and dynamic range. The FMF is coiled to a fiber loop by making a knot. The in-line MZI sensing structure is fixed on a two dimensional (2D) translation stages. By moving one stage while another stage is fixed, the displacement is applied on the sensing structure. The resolution of the translation stage is 10μm. The few mode fiber loop acts as the transducer for the displacement sensing. The displacement will change the radius of the few mode fiber loop, which leads to a wavelength shift of the interference pattern. When the fiber loop has different initial radius, the same displacement will cause a different curvature variation. So the sensitivity of the wavelength shift to the displacement is dependent on the initial radius. A smaller initial radius of the loop will lead to a larger sensitivity, higher resolution but smaller dynamic range, so it is proper for micro displacement sensing. On the contrary is the lager initial radius that is proper for sensing in a large dynamic range. By simply adjusting the initial radius of the transducer loop, different sensitivity and resolution can be reached. Experimental results show the sensitivities of 0.267nm/mm, 0.384nm/mm, 0.749nm/mm and 1.06nm/mm for initial loop radius of 1.9cm, 1.5cm, 1cm and 0.75cm, respectively.