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  • Researchers Develop Electro-Optic Lightning Detector

Photonics Spectra
Dec 1999
Dr. James P. Smith

HUNTSVILLE, Ala. -- Lightning strikes during space launch preparation can be very dangerous. Information provided by detectors can predict a lightning hazard, but time is of the essence.

An array of 31 electrometers are in place at Cape Canaveral in Florida that can predict the danger of electric charge buildup during a thunderstorm. But these devices measure current flow from a metal flat-plate antenna to the ground rather than directly measuring electric field intensities. The electric field or voltage changes during lightning discharges must be calculated from the electrometer readings. The selection of a time constant for electrometer units requires a compromise between the time resolution of lightning events and the sensitivity of the measurement.

Researchers William J. Koshak of NASA's Marshall Space Flight Center and Richard J. Solakiewicz of Chicago State University have developed a Pockels-cell-based electro-optic lightning detector that can provide a direct measurement of electric potential and can resolve fast transient events with high sensitivity. Their work was described in the July 20 issue of Applied Optics.

Koshak explained that the heart of this cell is an electro-optic crystal. The refractive index of a crystal changes when voltage crosses it and when light that is being transmitted through the crystal is modified. Pockels cells are commonly used for high-speed light choppers and for fiber optic communications. They can be modulated at high rates -- up to the gigahertz range.

In the electro-optic lightning detector, laser light passing through a potassium dihydrogen phosphate crystal is monitored while the electric field between a flat-plate antenna and ground is applied to the crystal. As lightning discharges produce changes in this electric field, the laser light modulates. The crystal needs a high impedance to hold the voltage during the measurement of a lightning event.

Koshak acknowledged that at this time the electrometer units are more practical. The electro-optic system is an engineering model, built to assess the attributes of the crystals. The major cost of these devices is the crystal -- as much as $2000 -- but this can be reduced, he said. Koshak described the unit as "bulky and expensive," but as the power and optical characteristics of diode lasers continually improve and the development of the electro-optic crystals continues, the electro-optic lightning sensors will become competitive.

He added that the model had only one laser light pass through the crystal, and that performance will increase with multiple passes or multiple crystals. The possibility of miniaturization offers another advantage because fiber optics could be used to send the laser light to a remote sensor, consisting only of the plate antenna and crystal. The modulated light could then be brought back for detection and measurement.

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