Investigation on the Dependence of Directional Torsion Measurement on Multimode Fiber Geometry

The torsion measurement and torsion direction discrimination characteristics of a multimode fiber (MMF) geometry in a singlemode-multimode-singlemode (SMS) fiber structure are investigated theoretically and experimentally. Three different geometries of MMFs, including circular symmetric MMF, axisymmetric MMF, and asymmetric MMF were fabricated based on a fiber polishing technique. The torsion response characteristics of a MMF depends on the birefringence within the MMF, as birefringence is related to the twist rate of the fiber. The experimental results demonstrate that the unpolished MMF with a circular asymmetric refractive index (RI) distribution has no wavelength response to torsion, and the single-side polished MMF with an axisymmetric RI distribution can measure torsion, but it does not have the ability to discriminate torsion direction. It is found that an MMF section with two polished surfaces at an angle to each other results in a departure from the symmetrical RI distribution, which introduces elliptical birefringence into the MMF. As a result, directional torsion measurement is achieved with sensitivities of 0.162 nm/(rad/m) clockwise and -0.139 nm/(rad/m) counterclockwise. The MMF geometry presented provides a new approach to torsion direction discrimination and shows good application potential in practical torsion vector measurement.