Coherent light from a field of microlasers.

A single pinpoint of light streams from a microscopic laser built into a specially prepared semiconductor water. It follows, then, that a water dotted with an array of 400 such lasers would sprout 400 individual light beams. But by carefully controlling the size and spacing of these tiny semiconducter lasers, researchers can now lock them into operating in step. Although each laser emits its own light, the individual beams merge just above the wafer's surface into a single, coherent beam.

"What we get out of this work is a very clear, clean beam coming out of a semiconductor wafer - cleaner that has ever been seen before for a two-dimensional, surface-emitting laser array," says Paul L. Gourley of the Sandia National Laboratories in Albuquerque, N.M. He and his co-workers describe their techniques for fabricating, operating and testing such arrays in the March 4 Applied Physics Letters.

To elucidate the details of how such laser arrays function, the Sandia team used light pulses to activate microscopic lasing elements fabricated in semicongallium consisting of alternating layers of gallium aresenide and aluminum gallium arsenide. By chaning the area illuminated by the pulse, the researchers could activate different numbers of lasers in the array, then observe the beam emerging from the wafer surface.

The researchers found that individual laser beams merge above the wafer surface into a single beam with a distinctive four-lobed pattern resembling a four-leaf clover. Such a pattern in consistent with that expected from an array in which each laser produces light that is 180 [degrees] out of phase with its nearest neighbors. Expanding the array from four to 400 lasers generates the same basic pattern but produces a sharper beam with significantly less spreading.

"These observations provide further impetus and guidance for the development of [two-dimensional] laser diode arrays," the researchers conclude.

Because semiconductor laser arrays can span wide areas, they may prove useful as light sources for applications such as medical imaging or optical computing, in which photons rather than electrons carry information. Moreover, by utilizing much of the area on a wafer's surface, researchers foresee the possibility of fabricating high-power light sources for semiconductor applications.