L-band vendors are ready to open another window
By KATHLEEN RICHARDS
Service providers` ongoing quest for greater bandwidth has vendors unleashing a slew of optical components designed to operate at longer wavelengths in the 1565- to 1610-nm L-band. Several companies discussed amplifiers, laser sources, and test equipment developed for L-band usage at OFC `99 in San Diego, CA.
For example, Corning Inc. introduced the PureGain 4100 erbium-doped fiber amplifier (EDFA) module, which is slated for production in the fourth quarter for availability to original equipment manufacturers (OEMs). "The demand is still not quite there, but beginning fourth quarter, there will be demand for L-band modules and we would like to lead this," states George Wildeman, product line manager for optical amplifiers at Corning, based in Corning, NY. Other companies announcing L-band EDFAs included INO and Nortel Networks.
Lab fodder for years, extended-band components started to appear last year as several service providers began pushing the outer limits of conventional-band, 1530- to 1560-nm capacity on their long-haul wavelength-division multiplexed networks.
"In some networks on some fibers, C-band is already being exhausted," says Wildeman. "So the natural place to go is to longer wavelengths where the attenuation of the fiber is still low, which it is in the L-band. OEMs are now beginning to design EDFA modules that can be put in parallel with the C-band amp. With a C-band amp and an L-band amp operating over the same fiber type, you can add traffic modularly."
From all accounts, combining C-band with the L-band will allow network providers to double the transmission window for wavelength-division multiplexing, from about 32 to 64 nm. Service providers can operate 40 to 80 channels at OC-48 (2.5 Gbits/sec) or OC-192 (10 Gbits/sec) in each band, a total of 800 Gbits/sec.
"People are really excited about this," reports Mark Thomas, director of optical components at market researcher RHK (South San Francisco), "because the channel spacings on the C-band are getting pretty tight at 50 GHz, especially when you`re starting to modulate OC-192."
"You`re able to put more data, voice, or whatever services through the amplifier," confirms William Ulmschneider, vice president of business development at Optigain Inc. (Peace Dale, RI). "The next trend is going to be coming up with L-band lasers like distributed-feedback lasers and transmitters to extend up to those wavelengths."
Optigain builds optical-fiber amplifiers and fiber lasers. The company offers C-band and L-band amplifier components and recently announced amplified spontaneous emission sources (ASEs) at OFC. The L-Band ASE source covers the 1570- to 1607-nm band.
Applications for the L-band technology will range from long-haul, dense wavelength-division multiplexing (DWDM) networks to some metropolitan infrastructures, say component manufacturers. The first customers may be telecommunications service providers that utilized the first dispersion-shifted fiber, which started to be installed in 1989. C-band operation on the first dispersion-shifted fiber is limited by a nonlinear interference effect called 4-wave mixing; which mixes channels after four to 12 channels.
"The dispersion-shifted fiber will likely be a good fiber or at least an adequate fiber in the L-band," says Corning`s Wildeman. "There are several long-distance providers around the world--many of them international but some of them domestic--that have dispersion-shifted fiber, so L-band will be used first in those networks because they are so capacity limited."
Major deployment of L-band components, however, is still a year away, say vendors and analysts. "I don`t think anyone has a system right now that`s deployed," explains RHK`s Thomas. "Operators need to exhaust C-band, or get close to it, before we`ll start seeing stuff deployed in the field."
Optigain`s Ulmschneider agrees: "We have shipped L-band EDFA amplifiers to some customers over the last couple of months. Initially, it`s for test equipment for the L-band, to allow the major systems integrators to determine if their systems can utilize it."
Among the many L-band products displayed or announced at OFC is Anritsu`s showcased DFB-LD light sources designed to measure L-band amplifiers. Alcatel highlighted its1905 LMI laser modules for long-haul DWDM applications and the 1915 LMI laser modules for 2.5-Gbit/sec applications. Tektronix introduced a new LPB series of tunable lasers, including the LPB 1550 for 1520- to 1620-nm wavelengths.
Despite the growing momentum, there is still some uncertainty regarding the possibility of higher attenuation on standard singlemode fiber in the L-band. "Many telecommunications providers are starting to gather that data," says Corning`s Wildeman. "We`re not expecting there to be a lot of problems in the L-band with attenuation based on our knowledge of optical fiber. There could be bend loss in splice trays and some cables, but I`m expecting that if there is, it`s going to be an intermittent problem."
Which window is next when service providers begin to fill the L-band? "C-band is a lot of capacity. L-band is even more, but I have no doubt the capacity will be used, so we`ve got research going on in other windows," says Wildeman. "The 1400- to 1500-nm window is a likely candidate because it still has low attenuation in that region." q