An optical fiber-mounted field sensor for measuring an electric or magnetic field includes an optical fiber configured to receive light from a laser source, a polarizer, a polarization manipulator, electro-optical material or magneto-optical material adjacent to the polarization manipulator, and a high reflection coating. The polarizer is adjacent to an output of the fiber, while the polarization manipulator is adjacent to the polarizer and opposite of the optical fiber. The electro-optical material or magneto-optical material is adjacent to the polarization manipulator, and the high reflection coating is adjacent to the electro-optical material or magneto-optical material. An optical mainframe for sending and receiving optical beams to and from the optical fiber-mounted field sensor is also described.