Electricity Over Glass
A major fiber-optics firm buys into transmitting electric power with light
Running a live wire into a passenger jet's fuel tank seems like a bad idea on the face of it. Still, sensors that monitor the fuel tank have to run on electricity, so aircraft makers previously had little choice. But what if power could be delivered over optical fiber instead of copper wire, without fear of short circuits and sparks? In late May, the big laser and optics company JDS Uniphase Corp., in San Jose, Calif., bought a small Silicon Valley firm with the technology to do just that.
Photonic Power Systems Inc., in nearby Cupertino, has developed a system that uses a laser to inject power in the form of light into a fiber-optic cable and a photovoltaic (PV) array to convert the light back into electricity for powering devices. This method of transferring power can be highly advantageous in situations where sparks or shorts can be a fatal problem [see photo, " Fatal Short"], where electromagnetic interference is more than just an inconvenience--in cellphone base stations, for example, or in pacemakers--and where conventional methods are bulky and cumbersome.
Already, a Photonic Power device is replacing instrument transformers used in the power grid to measure high currents. But lacking the backing of a big company, aircraft makers and other potential customers have been hesitant to design power over optical fiber into their systems, says Jan-Gustav Werthen, the founder of the company and now the engineering director for the photonic power unit at JDS Uniphase. After all, power over fiber is still a relatively unfamiliar technology.
At the heart of the system is an array of PV cells on a 2-by-2- or 1-by-1-millimeter chip of gallium arsenide, indium phosphide, or indium-gallium arsenide, depending on the wavelength of the laser to be used. With an efficiency of 40 to 50 percent, the array is twice as good at turning light into electricity as the typical silicon PV cells found on the sides of buildings and significantly better than the advanced-material cells used in solar concentrators. The high efficiency is achieved partly because the lasers providing power are tuned to produce light at the frequencies best converted to electricity by the chip, while PV arrays have to convert light at whatever frequencies the sun provides.
A device similar to that commercialized by Photonic Power was first reported in 1979 by Peter Borden at a Varian Associates laboratory in Palo Alto, Calif. But it took more than two decades for its promise to be recognized by a large company, points out John P. Benner, a research manager at the U.S. National Renewable Energy Laboratory, in Golden, Colo. Werthen, the former project leader at Varian, founded Photonic Power in 1992. The firm has shipped 10 000 units since then and at present is the only commercial provider of power over optical fiber, according to Werthen. Why is there no competition yet? The PV chips are not easy to fashion, Photonic Power owns key patents, and the photonics industry has generally not paid much attention to power, Werthen observes.
Though Werthen expects to find customers for power over fiber in a variety of industries, including aerospace, communications, defense, and medical equipment, as well as in industrial sensors, the company's fastest growing sector is currently electric power transmission. One important application is eliminating the transformers used to step down high currents and voltages to measurable levels.
Such transformers are large and necessarily heat up, which can lead to hot spots. To prevent equipment temperatures from rising to dangerous levels and to reduce power leaks, oil and gas are used as insulators. But oil is flammable and can make the transformers explode at high temperatures. The transformers are also expensive to install and maintain.
Photonic Power offers the option of measuring high currents by placing a transducer directly on the line, obviating the use of transformers to overcome voltage differences, as the power-over-fiber system converts electricity directly to light. The photonic sensor system is two-way: it sends an optically powered signal to the sensor at the transducer, where a laser returns a data signal of a different wavelength back over the same fiber.
Vincent Lui, a senior research analyst at IDC, in Framingham, Mass., estimates that today's market for power over fiber is nearlyUS $15 million. But with future improvements in high power delivery and the development of more scalable androbust processing and packaging, he thinks the potential market could expand into the hundreds of millions.