Harsh environments and confined spaces are no match for an optical fiber-based current sensor that researchers at the Hoya Corp. R&D Center are developing. Electric companies use bulky devices such as gas-insulated circuit breakers to measure current at power plants. The researchers are investigating ways to replace those sensing devices with small flint glass fiber-based instruments. The photonics technology relies on the Faraday effect: When light goes through a fiber that is in a magnetic field, the field rotates its polarization. Measuring the change in polarization gives the strength of the magnetic field. The technique is more accurate with flint glass fibers than with fused quartz because the glass has a larger Verdet constant and thus maintains linear polarity. Prototype devices use laser diodes emitting at 1550 and 850 nm for large and smaller currents, respectively. The lab is testing use of light-emitting diodes as a way to reduce device costs. Yoshihiko Matsuoka, manager of Hoya's Fiber Photonics R&D Group in Tokyo, said the company expects to have a commercial product in three to five years. Developmental prototypes have achieved results that exceed Japanese electrical standards.