Design of Backscatter Tailored Optical Fibers for distributed magnetic field sensing using Fiber Optic Pulsed Polarimetry

Abstract: Fiber optic pulsed polarimetry is a LIDAR-like fiber sensing technique that uses a backscatter enhanced single mode backscatter-tailored optical fiber(BTOF) to measure the distributed B fields on all Magnetic Fusion Energy devices. The BTOF has a series of wavelength resonant reflection fiber Bragg gratings written uniformly along its length. The fiber's Verdet constant determines the strength of the Faraday effect which effectuates the measurement of local B along the fiber placed intimately next to or within a magnetized plasma volume. A robust measurement of the field distribution along the fiber is obtained at high rep rates, 5 MHz, high spatial resolution(1-10cm), high B field accuracy(<1%) and temporal response (ns). Multipathing in the BTOF produces 3rd order reflections that contaminate the LIDAR signal. Algorithms are given for calculating the level of contamination for uniform and flat reflection designs, in particular and any reflection series in general. The contamination is bracketed giving confidence in designing and implementing a BTOF. Applications include magnetic fusion devices, rail guns, high temperature superconducting magnets and magnetized target fusion research.