Tuning in to tunable lasers
Each December, Lightwave looks at some of the technologies that we think will play a major role in the way fiber-optic technology will be applied to communications networks in the next few years. Last year, we ran our first feature-length article on tunable-laser technology. As you can see by the news story that begins on page 1 of this issue, the promise extended by last year's feature is nearing fruition. But there remains considerable debate on the technologies that will most adequately meet user needs--just as there remains disagreement on what those needs might be.
The technology debate centers on which of three competing approaches will make the most useful tunable laser. As the news story describes, distributed feedback (DFB) lasers are well understood and supply adequate output power for long-haul applications. Sampled grating-laser designs, of which the grating coupler sampled reflector lasers offered by Altitun are perhaps currently the best known examples, provide very wide tuning but tend to suffer from comparatively low output power. Finally, distributed Bragg reflector (DBR) lasers occupy a space between the two, where they generally provide more power than a sampled-grating laser and wider tuning than DFB lasers.
Each technology has its proponents. Nortel Networks, Lucent, and Fujitsu have hitched themselves to DFB technology--although Lucent plans to switch to DBR technology in the future. Alcatel Optronics and JDS Uniphase have already selected DBR as their technology choice, while Altitun and Marconi use the sampled-grating approach.
Marconi is well aware of the current technical limitations of sampled-grating technology and is working hard on overcoming them. Marconi's laser current provides an output power that peaks at around 2mW, a total significantly less than the 10mW DFB lasers customarily provide. The company's Brian Denby reports, however, that Marconi should have the power problem licked by next year, perhaps as early as the second quarter. While admitting that the four-section nature of sampled-grating lasers represents an increase in complexity over DFB and DBR devices, he believes the company is close to solving the manufacturing issues associated with such a laser design.
As interesting as the technology battles will prove to be, the debate over the initial applications for these lasers may be of more immediate concern. Observers frequently cite the use of tunable devices as spare parts for carriers looking to maintain fielded equipment as the most obvious first application of these new devices. Certainly, Lucent appears to have focused on this application with its new tunable-transmitter module. But representatives from other firms, principally Altitun, Nortel Networks, and Marconi, see other uses for tunable lasers taking precedence. For example, Marconi plans to focus on the development of an all-optical crossconnect product the company plans to introduce by the middle of 2000, according to Phil Griffin, the company's photonics development director. Marconi sees a need to be able to hop wavelengths across rings--even if that means moving wavelength 3 on one ring into the wavelength 8 slot on another. Tunable-laser sources promise this kind of flexibility, Griffin says. The company also plans to incorporate tunable technology into its line of optical add/drop multiplexers.
Systems vendors who also see tunable lasers as key components in a new class of products shouldn't look to the components division of Marconi for help, however. The company has decided to keep its tunable technology in-house. The same appears to be the case for Fujitsu. Fortunately, while companies such as Alcatel, Lucent, and Nortel Networks also have systems businesses, they have generally kept distribution channels open to other manufacturers. Along with such traditional third-party vendors as JDS Uniphase and newcomers such as Altitun in the market, it would appear that there should be at least some choice of suppliers for tunable lasers in general. With Marconi captive, however, Altitun may prove to be the only independent supplier of sampled-grating lasers in the market.
Stephen M. Hardy
Editorial Director and Associate Publisher