Virtual Waveguide Offers a Real Optical Switch

Daniel C. McCarthy

Waveguides used as optical switches do not lend themselves easily to reconfiguration. But researchers at the University of Arkansas may have found a way around these rigid structures. By applying a direct current electric field to a semiconductor crystal, they were able to channel the path of a laser beam much the same way an optical fiber would, except that by manipulating the electric field they could also control the beam's direction.

"[The crystal] starts out with no waveguide present," explained Scot Hawkins, a physics student and researcher on the team. Predictably, the light from a 1320-nm diode laser expands as it travels through the indium phosphide (InP). "Then we turn on the electrical field, and it forms an optical wire." The beam does not diffuse in the crystal but rather simulates the function of an optical fiber.

In a written explanation of the team's research, leader Gregory Salamo explains that since these self-induced waveguides are erased as a new optical beam enters the crystal and passes over the same region, the optical interconnects are reconfigurable. Hawkins said the beam could be steered through the 5-mm cubic crystal at angles between 5^° and 10^°. Switching speed is on the order of a millisecond.

The technique relies on the photorefractive properties of InP, which is most sensitive to wavelengths around 1300 nm -- a range useful to data communications applications. But another advantage InP offers over other photorefractive materials is speed. "The light with which we formed the waveguide has an intensity of 30 mW/cm²," said Hawkins. "At that intensity, it takes several minutes to form the waveguide in other materials. InP takes about 10 ms."

More than one "optical wire" can carry signals through the same crystal as long as the distance between them is twice the 20-µm diameter of their beams. Otherwise, crosstalk between signals is possible. "But it's more than just crosstalk," Hawkins added. "If they're in phase they can actually merge."

If the beams are in phase and within two beam diameters of each other, they will collide and form a single beam. If they are out of phase, they will deflect each other; the closer together the beams are, the larger the angle of deflection, Hawkins said.