Detecting low-intensity magnetic fields with a magnetostrictive fiber optic sensor

A magnetostrictive fiber optic Mach-Zehnder interferometric sensor that detects low-intensity magnetic fields was realized. The sensor utilizes 850 nm single-mode fiber optic cable and couplers. Four classes of sensing arms were fabricated by coupling the magnetostrictive materials to a length of fiber optic cable as: ribbons, cylinders, sandwiches, and sputter coated sheaths. The sensing arms were evaluated using Metglas, nickel, and a combination of Metglas and nickel. A Helmholtz coil was used to generate AC magnetic test and evaluation field. Optimization techniques were applied to the sensor to maximize its sensitivity, including: annealing the magnetostrictive material; enclosing the Helmholtz coil, sensing arm, and reference arm in a magnetic field shielded chamber; and applying a DC bias to the Helmholtz coil's AC current component. The measurements indicated that the lowest magnetic flux density that could be detected was 0.3 mG. This was achieved using a Metglas ribbon-fiber configuration where ribbon's magnetization direction was aligned perpendicularly with respect to the applied 52 kHz DC magnetic field and a 5 volt DC bias. When this configuration was correspondingly operated with a 10 Hz AC magnetic field and 5 volt DC bias, an 8.9 mG magnetic flux density was detected.<>