Characteristics of fiber-optic magnetic-field sensors employing metallic glasses

A variety of extremely high-sensitivity, room-temperature magnetic-field sensors have been fabricated and tested using single-mode fibers in conjunction with highly magnetostrictive metallic glasses. Minimum detectable fields of 5 X 10-9 Oe/m fiber are reported. The typical characteristics of the fiber magnetometers are summarized. It has been shown by Dandridge et al.1 that nickel magnetostrictive jackets and coatings on single-mode fibers deployed in a Mach-Zehnder interferometer can be utilized in the laboratory to measure magnetic fields in the range of 10-6 Oe for sensor lengths of the order of 1 m. Subsequently, both phase modulation2 and magnetic sensing3 using metallic glasses in conjunction with optical fibers have been reported. This Letter describes in detail the use of commercial metallic-glass magnetostrictive materials on fibers for very-low-field magnetic-sensor applications. This approach has re-sulted in a dramatic improvement of magnetic-field sensitivity with potentially reduced magnetic noise and low bias fields while both simple fiber-sensor configu-rations and the excellent linearity of response versus applied magnetic field have been maintained. The data reported here represent by far the highest sensitivities achieved to date to our knowledge for fiber magne-tometers and would appear to make these devices seri-ous candidates for a variety of magnetic-sensor appli-cations. The experimental details of the all-fiber interfer-ometer employed to measure small phase shifts have been described. 4 The basic approach for magnetic-field detection is to bond a fiber in one arm of the interfer-ometer to a magnetostrictive material that changes linear dimension in an applied magnetic field. Figures 1(a) and 1(b) show schematically two of the specific configurations utilized in the laboratory to evaluate metallic-glass fiber magnetometers. In Fig. 1(a), two pairs of the Helmholtz coils are used to generate ac and dc magnetic fields, respectively, for a fiber sensor con-sisting of a bare single-mode (o.d.- 80 ptm) fiber bonded to a metallic-glass strip. Commercially available me-tallic glasses, such as the Allied Chemical # 2605 SC