Measurement of megampere currents with optical fibers

Fiber optic sensors are routinely used to measure multi-megampere currents. The sensors are low noise, absolutely calibrated, and electrically decoupled from the pulsed power source. Polarized light from a HeNe laser is guided past the current carrier by a single-mode, low-birefringence fiber. The magnetic field from the current causes a Faraday rotation of the light polarization which is detected by a polarization analyzer and photodiode at the end of the fiber. A rotation of about 2500/MA + or - 5%, slightly less than the Verdet constant for non-birefringent silica glass was observed. It was found that highly birefringent (polarization preserving) optical fibers do not work in this application. The sensor is being ruggedized for field use with high-explosive-driven current sources by using a diode laser and single mode fiber couplers to replace the laboratory system of lenses and spatial filters.