An optical-fiber sensor based on Faraday Effect was developed for measuring total lightning current. Designed for aircraft installation, it is lightweight, non-conducting, structure conforming, and is immune to electromagnetic interference, hysteresis, and saturation. It can also be used on windmills, lightning towers, and can help validate lightning detection network measurements.

Faraday Effect causes light polarization to rotate when the fiber is exposed to a magnetic field in the direction of light propagation. Thus, the magnetic field strength can be determined from the light polarization change. By forming closed fiber loops and applying Ampere’s law, measuring the total light rotation yields the total current enclosed. The broadband, dual-detector, reflective polarimetric scheme allows measurement of both DC component and AC waveforms with a 60 dB dynamic range.

Three sensor systems were built with different sensitivities from different laser wavelengths. The first system operates at 1,310 nm, uses spun polarization maintaining fiber, and can measure approximately 300 A to 300 kA. High-current measurements up to 200 kA were demonstrated at a commercial lightning test facility. The system was recently installed on an aircraft and flown near icing weather conditions. Operating at 850 nm, the second system uses twisted single-mode fiber and has a 150 A to 150 kA range. The third system operates at 1,550 nm, uses spun polarization maintaining fiber, and can measure 400 A to 400 kA. Both systems were validated with rocket-triggered lightning measurements and achieved excellent comparison results when compared to a resistive shunt.

The fiber-optic sensor provides many unique lightning measurement capabilities not possible with traditional sensors. It represents an important new tool where low-weight, large structure dimension, large total current, and low frequency capabilities are important considerations.

This work was done by Truong Nguyen of Langley Research Center. LAR-18043-1