Fiber-Optic Curvature Sensor Based on Cladding-Mode Bragg Grating Excited by Fiber Multimode Interferometer

A sensing structure consisting of a short multimode fiber (MMF) spliced in just upstream of a uniform fiber Bragg grating (FBG) is proposed and experimentally demonstrated for temperature-independent curvature measurement. The short MMF section generates cladding modes in the fiber that contains the FBG. Several of these modes get reflected back by the FBG at shorter wavelengths and re-enter the launch fiber after passing through the MMF section. The net recoupling efficiency between the incident forward core mode and reflected cladding modes is -20 dB when the fiber is straight, but decays when a curvature applied on the sensing structure. A maximum sensitivity of 0.74 dB/m^-1 is obtained in a large measurement range up to ~15.5 m^-1, with no orientation dependence of the sensitivity. The reflected coupled power variation is measured to be ±0.15 dB over the temperature range from 0 to 70 °C, allowing for temperature-independent curvature measurements.