Property of Bent Few-Mode Fiber and Its Application in Displacement Sensor

The intermodal interference between LP
₀₁
and LP
₀₂
modes of few mode fiber (FMF) under bending is studied theoretically and experimentally. The shift of critical wavelength caused by fiber curvature is measured in the experiment and analyzed theoretically for the first time. The sensitivity of the critical wavelength shift to curvature variation has been obtained from the measurement and calculation results. Based on the wavelength shift effect induced by the curvature of FMF, a novel displacement sensor employing a helix coil, as an example of the application, is proposed in this letter. With present fiber and structure parameters, 120-mm displacement with 0.172-nm/mm maximum sensitivity has been achieved by changing the fiber curvature through adjusting the pitch of the helix coil. The effect of curvature-induced wavelength shift provides various potential applications of FMF such as displacement and curvature sensors.