Improving VCSEL Performance

Researchers at the University of Illinois (Urbana-Champaign, Ill.) have found a way to significantly improve the performance of vertical-cavity surface-emitting lasers by drilling holes in their surfaces. Faster and cheaper long-haul optical communication systems, as well as photonic ICs, could be the result.

Low-cost VCSELs are currently used in data communication applications where beam quality is of little importance. To operate at higher speeds and over longer distances, the devices must function in a single transverse mode with a carefully controlled beam. "These characteristics are normally found only in very expensive lasers, not in mass-produced VCSELs," said Kent D. Choquette, an Illinois professor of electrical and computer engineering and a researcher at the university's Micro and Nanotechnology Laboratory. "By embedding a two-dimensional photonic crystal into the top face of a VCSEL, however, we can accurately design and control the device's mode characteristics."

Choquette and his colleagues at Illinois and at the Furukawa Electric Co. (Yokohama, Japan) reported their findings in the Feb. 16 issue of Applied Physics Letters.

The 2-D photonic crystal, created by drilling holes in the semiconductor surface, introduces a periodic change in the refractive index, Choquette said. The holes represent regions of low refractive index, surrounded by semiconductor material where the index is higher. A particular combination of refractive indices will produce a single-mode waveguide that permits only one transverse wave to propagate.

"Our photonic crystal consists of a triangular array of circular holes that have been etched into the top of a VCSEL," Choquette said. "Because the index variation has to be on the length scale of light, the periodicity of the holes must be on the order of several hundred nanometers."

To create such a precise array of holes, the researchers first lithographically define the desired pattern into a SiO₂ mask layer on the semiconductor surface using focused ion-beam (FIB) etching. The holes are then bored into the semiconductor material using inductively coupled plasma etching.

"By selectively varying parameters such as depth, diameter and spacing of the holes, we can control the modal characteristics of the laser," Choquette said. "This means we can accurately design and fabricate single-mode VCSELs for high-performance optical communication systems."

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