Long-wavelength vcsels near the communications
By ROBERT PEASE
horizonW.L. Gore & Associates Inc. (Newark, DE) has announced a breakthrough in upgrading the performance of an alternative light source for fiber-optic system developers. Thanks to advances in 1300-nm operation, vertical-cavity surface-emitting lasers (vcsels) may soon be marketed for large bandwidth, long-distance applications using both singlemode and multimode fiber.
vcsels are a promising alternative to edge-emitting lasers and light-emitting diodes (see Lightwave, April 1997, page 1). However, these devices currently are limited to 850-nm operation. Although vcsels are less expensive to produce, are easier to couple to optical fiber, and offer less complex packaging than edge-emitting lasers, their present wavelength ceiling reduces the number of applications in which they can be applied.
For example, 850-nm vcsels are unable to meet the power requirements of the long-haul networks now served by 1300-nm conventional lasers and light-emitting diodes (leds). Therefore, they currently are limited to multimode fiber applications. vcsels also pose significant eye-safety challenges in parallel arrays, and some potential customers have found it difficult to accept the restricted access conditions and additional liability associated with them.
Seeking a solution to these problems, Gore has demonstrated what it claims to be the first long-wavelength vcsel that will meet the performance requirements for high-speed data communications applications. The 1300-nm product, expected to be commercially available by late next year, successfully overcame the barriers faced by its short-wavelength predecessors, according to the company. By nature, 1300-nm light is potentially less damaging to the eye than 850-nm sources, so operations can be achieved without compromising performance, allowing more power to be launched into the fiber.
The 1300-nm capability makes the prototype device more compatible with today`s fiber-optic communications systems than 850-nm vcsels. In demonstrations, the 1300-nm vcsel provided useful output power from 0 to 70C and 2.5V operating voltages. The device also offers 5-milliampere threshold currents and can be modulated at data rates greater than 1 Gbit/sec.
Stepping up
Short-wavelength vcsels are currently used as a cost-effective alternative light source in certain types of gigabit networks. For example, high-speed applications that include transmission distances of only a few hundred meters over multimode fiber, such as Gigabit Ethernet and Fibre Channel, have made vcsels the emitter of choice because light-emitting diodes cannot meet the speed requirements and edge-emitting lasers are more costly. The introduction of long-wavelength vcsels will provide greater bandwidth and distance in multimode fiber, while offering a new, lower-cost light source for singlemode fiber applications as well.
"Long-bandwidth vcsels are compatible with singlemode fiber at lower costs," says Dave Welch, business leader of Gore Photonics. "They can be easily coupled to singlemode fiber and used with very simple drive circuits. We expect 1300-nm vcsels will be the lowest-cost singlemode laser available."
To overcome the barriers of temperature and operating voltage, says Welch, Gore developed a unique structure in which an integrated 850-nm laser optically pumps the 1300-nm active region instead of directly injecting electrical current into the 1300-nm vcsel cavity. The design includes an undoped 1300-nm optical cavity that greatly reduces free-carrier losses. Resistive heating is reduced in the 1300-nm cavity. The absence of current crowding, coupled with reduced thermal lensing, allows higher singlemode powers.
Gore is one of a handful of microelectronics manufacturers that have been working toward a long-bandwidth vcsel product. Others include, but are not limited to, Hewlett-Packard Co., Honeywell Microswitch Systems, emcore Corp.`s MicroOptical Devices Div., Picolight, and Mitel Semiconductor. Although impressed with Gore`s announcement, these companies point out the technology is extremely complicated and there are many scientific approaches in developing a 1300-nm product. While they are all developing a similar product, none are ready to commit to when they might achieve a marketable unit. There also seems to be a mix of praise and skepticism for Gore`s claims of success.
Other voices
With several ongoing government-funded projects for the production of 1300-nm vcsels, Picolight is considered by some a top contender in the battle to be first to produce a commercially available product. Since late 1996, Picolight has been under contract by the U.S. Department of Defense`s Ballistic Missile Defense Organization to fabricate a long-wavelength vcsel; establish its manufacturability; test its performance, reliability, and uniformity over wafers; and evaluate its use in wavelength-division multiplexing (wdm) applications. Although Picolight declines to comment on the status of its 1300-nm vcsel project, performance, reliability, and manufacturability highlight its approach. With many ways to demonstrate such a device`s performance, making a manufacturable product is another issue.
"Gore`s press release hints at one of the key performance features--operation over temperature--that has been missing in the technology up to this point," says Jack Jewell, chief technical officer at Picolight. "It`s a major challenge to achieve those performance windows while simultaneously meeting reliability and manufacturability goals." He says that taking a research approach that generates certain performance features, yet doesn`t meet reliability tests requires much more development before labeling it an actual product. By the same token, an approach that gives performance and reliability but isn`t manufacturable doesn`t provide usable technology. Jewell adds, "If it`s missing any one of the three parameters--performance, reliability, or manufacturability--it doesn`t go."
Jewell also points out that advances in 1300-nm do not mean the end of the short-wavelength vcsel market, which he believes will continue to thrive despite the eventual introduction of long-wavelength products. "[Short-wavelength vcsels] are a solid business that`s supported by numerous companies and customers," says Jewell.
Hewlett-Packard has its own 1300-nm vcsel project underway and applauds Gore for its success in demonstrating a product at 0 to 70C. That alone, says a Hewlett-Packard source, is a significant breakthrough in capability. Meanwhile, Honeywell plans to develop what it calls a "building block" family of vcsel products in various wavelengths that will service all segments of the market. The company is looking at 1300-nm vcsels, as well as other wavelengths that make the most sense for meeting its customer needs.
At MicroOptical Devices (mode--Albuquerque, NM), chief scientist Kevin Lear says 1300-nm operation is a critical step for vcsels to take in order to continue to expand into the telecommunications and longer-distance local area network markets. "We have a product in development and, because of mode`s merger with emcore Corp. last December, we`re able to strongly leverage emcore`s material and equipment expertise to develop a manufacturable long-wavelength vcsel product," says Lear.
The manufacturing approach that Gore is taking includes wafer bonding, which was successfully used by Hewlett-Packard in producing their high-brightness leds. Applying the same technique to surface-emitting laser diodes is more complicated. Hewlett-Packard attempted to fabricate vcsels with the wafer-bonding technique for a number of years but, according to Lear, abandoned the wafer-fusing technique because of concerns about the ability to manufacture the process.
"Nonetheless, this type of preliminary demonstration by Gore certainly helps build the momentum toward a 1300-nm vcsel," says Lear, "and we`re glad to see there are technical accomplishments taking place to move the marketplace forward."
"Curbing" expenses
As technology pushes fiber-optics applications closer to the curb and home, Gore believes the 1300-nm vcsel will be the most cost-effective singlemode light source available. "High-speed multimode applications such as Fibre Channel, Gigabit Ethernet, and Asynchronous Transfer Mode will make good use of the 1300-nm vcsel," says Gore`s Welch. "They may also be used in wide area networks and local-loop services where distances are greater than a few hundred meters. We also expect to impact the emerging market for parallel optical modules, where the combination of eye safety and greater transmission distance may have significant value."
Although not ready to discuss details, Gore is also dabbling in preliminary technology toward vcsels and vcsel arrays operating at 1550-nm, which could be useful in future wdm applications. q