Curvature sensor based on two cascading abrupt-tapers modal interferometer in single mode fiber

A curvature sensor based on two cascading abrupt-tapers modal interferometer in a single mode fiber (SMF) is proposed and experimentally demonstrated in this paper. The device consists of two cascading abrupt-tapers with different sizes, manufactured by CO2 laser in a standard SMF. The cladding modes will be excited in the first abrupt-taper section, partly coupled with the core mode. After propagation of these coupled modes, they will recombine at the second abrupt-taper section. The experimental results show that the optimum visibility of interference fringe can be got only when the two abrupt-tapers are at different sizes. If we bend the proposed modal interference, part of the cladding modes will be leaked out, making the interference pattern having a shift. Thus it can be used to measure curvature. The experimental results show that the shift of the dip wavelength is almost linearly proportional to the change of curvature, and the curvature sensitivities are −13.176 nm/m−1 in the measurement ranges of 4.8 m−1 to 6.38 m−1. As the curvature continues to increase, ranging from 6.38 m−1 to 7.98 m−1, the sensitivity is doubled to −25.946 nm/m−1. Moreover, this curvature sensor can be seen low sensitive to either the external refractive index or the temperature when employing in a relatively stable environment. The proposed curvature sensor is simple-fabricated and inexpensive, which is very suitable for curvature measurement in practical applications.