Researchers at the University of Illinois at Urbana-Champaign have found a way to significantly improve the performance of vertical-cavity surface-emitting lasers (VCSEL) by drilling holes in their surfaces. The development could lead to cheaper long haul optical communication systems and photonic integrated circuits.
Low-cost VCSELs are currently used in data communication applications where beam quality is said to be 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.’
The two-dimensional photonic crystal, created by drilling holes in the semiconductor surface, introduces a periodic change in the index of refraction, 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 of the laser beam 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 silicon dioxide mask layer on the semiconductor surface using focused-ion beam 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.’
The next step, he said, is to push VCSEL performance toward higher power by considering designs that are much larger in diameter.
‘Looking beyond that, we also have fundamental problems with high-speed data communication on our circuit boards and in our chips,’ Choquette said. ‘This is a technology that could serve as the foundation for a new way of looking at optical interconnects and photonic integrated circuits.’