Radar distorts light-based electronics.

For years, engineers puzzled over a problem: How to build machines with sensitive electronic components that will not suffer distortion when operated in an electromagnetic field. Such fields show up around radar and radio transmitters, causing unwanted currents to flow in metal wires and cables.

Then, an answer emerged: Use photonic devices, which send signals through fiberoptic cables or free space, media unaffected by electromagnetism.

Not so fast, say John K. Daher and his colleagues at the Georgia Tech Research Institute in Atlanta. Their new study shows that electrical connections in photonic devices suffer the same troubles as the electronic components themselves. "It's myth to say that installing a photonic system means you don't have to worry about electromagnetic interference," says Daher, who described the study at a July symposium at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The researchers tested four types of photonic devices -- electro-optic, acousto-optic, magneto-optic, and charge-coupled devices -- all of which use light as the primary means of information transmission. These devices sense energy in one form (sound, magnetic pulses, etc.) and convert it into another (light, electrical pulses, etc.). Yet, when exposed to radio frequency or microwave fields, they all experienced interference, in some cases disrupting equipment. The magneto-optic device, used in aircraft instrument panels, showed inadvertent switching. In charge-coupled devices, commonly used in videocameras, even 1 microwatt of energy distorted the video signal. With the electro-optic device, used to modulate a laser, the beam stopped shining entirely.

Both military and civilian systems share these weaknesses, the researchers explain, so engineers must shield photonic devices from hazardous fields. "You can't let down your guard," says Daher, "just because you're using an optical system."