By KATHLEEN RICHARDS
As carriers implement 10-Gbit/sec technology in their networks, systems and component vendors are continuing to unveil the 40-Gbit/sec products designed to take these networks to the next level. This month at the European Conference of Optical Communications (ECOC) in Munich, Germany, Corning Inc. (Corning, NY) and Siemens AG (Munich) will jointly demonstrate several new technologies by running eight 40-Gbit/sec channels over 400 km of nonzero dispersion-shifted fiber.
This long-haul system demonstration will incorporate four spans of 100 km of Corning's LEAF fiber with Raman amplifier modules and erbium-doped fiber amplifiers (EDFAs) on each span at four repeater sites. Siemens's 40-Gbit/sec DWDM systems will reside at each end of the fiber. Corning's slope-compensating dispersion-compensating modules (DCMs) will be used to shape the signal as it travels down the LEAF fiber-which has a large effective area compared to other fibers.
"What we are trying to do here is to manage all of the factors that would normally limit 40-Gbit/sec transmission," explains George Wildeman, product-line manager for optical amplifiers at Corning. "For example, noise is a big limitation in 40-Gbit/sec systems, so we use Raman amplification, which allows us to reduce the overall noise figure of the system. We also reduce [fiber] non- linearities that are created and can affect 40-Gbit/sec systems pretty quickly by managing dispersion with the dispersion- compensating modules and then also Raman amplification. Raman amplification is a more gentle form of amplification because you're amplifying the signal as it is going through the span gradually."
Siemens AG has been working on 40-Gbit/sec electrical time-division multiplexing (ETDM) since 1995. This demonstration shows the company's progress toward this product and employs several new components to reach transmission distances not achieved previously. This is the first public demonstration of these technologies, although earlier versions were shown to some of Siemens's customers at Telecom Geneva 1999 and made available for trials in their laboratories.
"This demonstrates the key technology in our FOX product, a 4x10 to 40-Gbit/sec multiplexer, and the OCR40G, a 40-Gbit/sec transponder and regenerator," explains Dr. Harald Geiger, head of hardware design, optical networks, at Siemens. These products are expected to be commercially available in 2001.
In the ECOC demonstration, a unidirectional 40-Gbit/sec signal is created by bit multiplexing four 10-Gbit/sec pseudo-random bit sequences up to 40 Gbits/sec. For demonstration purposes, only a single transceiver and receiver are used, modulating eight channels in parallel. The full system will support 80 channels in the C- and L-bands, according to Geiger.
"With this system, we have achieved 8 x 40-Gbit/sec transmission over 400 km without forward error correction," says Dr. Erich Gottwald, project manager at Siemens. "The clock recovery and demultiplexing circuit is based on a parallel anti-correlation principle-inside the ASIC only 20-Gbit/sec circuitry is required." The clock recovery still works at bit-error rates of more than 0.01, allowing powerful forward error correction.
The receiver is a pure digital receiver; no analog amplifier is used (see Figure). "The photodiode directly drives the clock recovery and demultiplexer circuit unlike the gain-controlled electrical amplifier, typically used, which is at the limit of technology," explains Gottwald. "Electrical amplifiers deteriorate the signal, therefore, it is replaced by an optical distortion-free amplifier. Hence the requirements on the optical signal-to-noise ratio can be reduced, resulting in longer regenerator-free links."
After the signal is given an initial boost by an EDFA, it travels over four 100-km spans with Raman and EDFAs from Corning at repeater sites on each span.
"The Raman amplifier module combines Raman pumps in the 1,400-nm region of the window," says Wildeman. "It puts that pump power out onto the transmission fiber in a counter-propagating direction. So the signal comes into the repeater site, and it is seeing Raman gain as it is going down the transmission fiber.
"So you sort of preamplify the 40-Gbit/sec signal with the Raman amplifier, and the EDFA provides the booster power to go to the next span," he explains.
For this demonstration, Corning is using EDFAs developed primarily for 10-Gbit/sec systems. The Raman amplifier module is added to the front end of the EDFA, which makes it 40-Gbit/sec capable. The Raman amplifier used in the demonstration is the PureGain 5000 first introduced by Corning at this year's SUPERCOMM. It is being demonstrated at ECOC for the first time. These amplifiers will be available in small quantities in the fourth quarter of this year, according to Wildeman. Corning is also developing "40-Gbit optimized" Raman EDFAs, but declined to comment on the development time frame.
The ECOC long-haul system testbed will also offer the first demonstration of Corning's slope-compensating DCMs optimized for LEAF fiber. These DCMs were introduced by the company at the Optical Fiber Communications Conference earlier this year and are expected to become commercially available by the fourth quarter.
"The 40-Gbit/sec systems take a lot more management of the signal and shaping of the pulse than 10-Gbit/sec systems do," says Wildeman. "I think one of the keys here that we are trying to demonstrate is that the backbone networks that include LEAF are 40-Gbit/sec ready. It's one of the big questions that our end-customers are wondering about. They are installing a lot of 10-Gbit/sec systems now. We're going to be running 40-Gbit/sec demonstrations over the next year, and this is the first one where we are showing this capability."
Despite a lot of progress, some technical work remains. Polarization-mode dispersion compensators for 40-Gbit/sec signal transmission is another technology that Corning is developing. "It's not vital for 40 Gbits/sec, but it can enhance the distance that the signal can go on LEAF fiber and on other types of fiber in the field," says Wildeman.
Meanwhile, Siemens is preparing to implement a bidirectional 40-Gbit/sec DWDM link based on ETDM between Darmstadt and Mannheim on the fiber network of Deutsche Telekom in Germany. This field trail is part of the KomNet research project, started in 1998 and supported by the German Ministry of Research and Technology.
The 40-Gbit/sec DWDM link will be tested on two different transport systems, first supporting eight channels and then upgrading to 32 channels. The decision of whether to incorporate live traffic is currently under discussion with other KomNet partners, to which Siemens interfaces over 2.488-Gbit/sec STM-16. The 40-Gbit/sec trial is scheduled for January through April 2001.