Short-wavelength lasers save costs
stephen m. hardy
Fiber-optic system developers accustomed to choosing between two light sources--edge-emitting lasers and light-emitting diodes (leds)--may soon find themselves forced to consider a third alternative for high-speed premises applications. Vertical-cavity surface-emitting lasers (vcsels), say proponents, could prove the low-cost component of choice to provide short-wavelength light for Fibre Channel and Gigabit Ethernet networks (see photo on page 21).
vcsel construction differs from that of conventional lasers in that while edge emitters produce light through one of the cleaved sides of a wafer, vcsels emit from a point at the top of the wafer. The active element in a vcsel is mounted horizontally within a cavity designed to match the element`s performance. The element rests between a pair of Bragg reflectors with differing reflectivities. The bottom reflector has a reflectivity that approaches 99.9%; the top mirror`s reflectivity is slightly less. As the light loops from top to bottom between the two mirrors and through the active element, photons escape through the upper reflector for eventual broadcast through the optical fiber.
vcsel proponents, which include suppliers such as Hewlett-Packard, Honeywell, Micro Optical Devices, Mitel Semiconductor, Motorola, and Vixel, say this architecture offers several benefits. The vcsel`s symmetrical, well-collimated beam enables easy coupling to optical fiber via an imaging lens. vcsel packaging offers the promise of being less complex than edge-emitting lasers. In large part because they can be tested on the wafer, vcsels also can be produced less expensively than edge emitters, in much the same way as leds.
"The bottom line," says Bob Steele, director of optoelectronics at market-research firm Strategies Unlimited in Mountain View, CA, "is you get something that approaches the cost of an led with the performance of a laser." That performance includes speeds as high as 20 GHz, according to some sources.
To illustrate Steele`s point, Dan Rausch, strategic marketing program manager at Hewlett-Packard`s Components Group based in San Jose, CA, reveals that his company`s vcsel transceiver sells in high-volume quantities for less than $100, compared to the $240 that the company plans to charge for the same quantities for its new high-speed transceiver based on a Fabry-Perot edge-emitting laser.
vcsels are not expected to replace edge-emitting lasers--or even comparatively slower leds--in all applications, however. vcsels currently are limited to 850-nm wavelengths, which cannot meet the power requirements of the long-haul networks presently served by 1300-nm conventional lasers and leds. They also are presently limited to use in multimode fiber networks. But at shorter distances, such as the premises applications targeted by Fibre Channel and Gigabit Ethernet equipment, vcsels promise an ideal solution, particularly when stacked against the 622-Mbit/sec speed limitations suffered by leds and the comparatively high cost of 1300-nm edge-emitting lasers.
Potential of vcsels
Technology forums and standards bodies have already begun to notice the potential of vcsels. Both the Fibre Channel community and the atm Forum have issued specifications for 850-nm performance; for example, with help from the Fibre Channel community, the atm Forum has established specifications for 300 meters at 622 Mbits/sec and 1 kilometer at 155 Mbits/sec in multimode fiber. (The 155-Mbit/sec specification would have covered a longer distance, says Rausch, if the atm Forum had not encountered eye-safety problems when it attempted to accommodate 780-nm CD lasers within the same specification.) As these bodies and the Gigabit Ethernet community consider even higher speeds, vcsels should achieve greater prominence.
"I think you`ll find that many of the initial Gigabit Ethernet products that are beginning to show up in the marketplace will have this technology built into them," says Rausch. "And there are some Fibre Channel people who are also using the technology. We would expect the same to occur in atm [Asynchronous Transfer Mode] as they progress the 1244- and 2488-Mbit/sec standards forward."
"The coupling to fiber is easy, and the packaging is simpler," agrees Steele. "I think most transceiver products at a gigabit [speed] and above will go to vcsels."
Even without such a speed advantage, say some sources, vcsels would outperform leds in these applications. For example, because vcsels have a narrow 0.5-nm linewidth (much narrower than the 100-nm linewidth typical of leds operating at the same wavelengths), they offer greater resistance to dispersion loss. vcsels also consume only 20 mW of power, compared to the 200 mW associated with leds.
In the future, says Rausch, advances in vcsel technology should even further simplify its packaging, eliminating the optical feedback loop requirement it currently shares with edge-emitting lasers, for example. The resulting simplified drive circuits will make it possible to construct vcsel-based devices in even smaller, less-expensive packages. vcsels can also be used in multichannel devices for optical backplane replacements or very-high-density optical interconnects, he says.
Thus, vcsels should be expected to carve a significant niche in high-speed optical data networks and perhaps be used side-by-side with 1300-nm lasers when necessary. "It is the most cost-effective technology for multimode fiber links at gigabit rates," concludes Rausch. "You see the Fibre Channel community, the Gigabit Ethernet community, and perhaps the atm community at 1244- and 2488-Mbit/sec rates looking very seriously at short-wavelength laser technology for the most cost-effective multimode interface--and looking at 1300-nm singlemode lasers as a kicker to get you the extra distance when you need it in multimode fiber, as well as for use in singlemode fiber at those high speeds." q