LEDs Offer Fast Data Transmission

Aaron J. Hand

With properties somewhere between a standard LED and a laser diode, resonant-cavity LEDs produce intense light from quantum wells. Compared with conventional LEDs, the resonant-cavity variety produces a brighter, more directional beam with higher spectral purity and modulation speed, making it suitable for use in plastic optical fiber short-haul communications.

To optimize the performance of the devices, however, the researchers must make trade-off decisions regarding several parameters, including output power, modulation speed and temperature coefficient, said Markus Pessa, a professor at the Tampere University of Technology and director of its Optoelectronics Research Centre.

Tampere University's 650-nm resonant-cavity LED could be useful for high-speed, short-haul data transmission through plastic optical fibers.

Pessa's group has developed a 650-nm resonant-cavity LED that achieves a transmission rate of 1 Gb/s while keeping the power level at 200 to 400 µW. According to Pessa, the European car industry has expressed considerable interest in the device. Although the bandwidth is acceptable, the temperature coefficient remains a problem, causing considerable fluctuations in power output. Typical temperature coefficient is 1%/K, he said. "The most demanding automotive industry demands [temperature coefficient] of about 0.3%/K between 40 and +85 °C without using any thermoelectric cooler."

Like vertical-cavity surface-emitting lasers (VCSELs), resonant-cavity diodes emit light perpendicular to the surface. The device is based on a quantum well within a microcavity, with two parallel mirrors placed a wavelength apart. However, VCSELs tend to exhibit even higher temperature coefficients and are not available at 650 nm, Pessa said.

And though their complex layer structures make resonant-cavity LEDs more expensive than their conventional counterparts, their similar processing and packaging make them a relatively low-cost solution.

In cooperation with Nordic Epitaxy Inc., which is also based in Tampere and which makes semiconductor wafers for the telecommunications market, Pessa and his fellow researchers are evaluating the commercial potential of these devices. Prospects look good, he said, for cost-effective data transmission up to 620 Mb/s.