An optical fiber cable and magnetic field detector magnetostrictively reave to the presence of an external magnetic field is presented, and methods thereof. The optical fiber cable comprises a elongated optically transparent core sheathed by a magnetostrictively responsive jacket disposed about the periphery of the core. The jacket responds to magnetic fields present and strains the core effecting the light transmission of the core. The effect upon the light transmission by the jacket is detected by interferometry for determining the presence of the magnetic field.