Optical fiber sensor to measure the bending of a flexible sheet

Abstract

We propose an in-line all-fiber Mach-Zehnder interferometer based on two adiabatic fiber tapers separated by a distance L for measuring bending of a flexible sheet. While the fiber tapers are kept straight, the transmission spectra are unaltered. When the fiber is bent the RI profile becomes asymmetric, allowing the coupling between the fundamental core and cladding modes that propagate as whispering gallery (WG) modes. The core and WG modes propagate through fiber section L, acquiring a differential phase delay that determines the degree of interference or coupling between them when they reach the second taper. This differential phase delay depends on the curvature, which induces geometrical and elasto-optic changes in the index profile. In the second taper, the cladding modes are coupled back to the core and eventually will interfere with the core mode, producing a wavelength modulated signal. The amplitude of the modulation, expressed as fringe visibility, is used to measure the changes in the curvature radius.