Solitons continued:
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French Researchers Form Transmission Start-Up.
Network Briefing, July 28, 1999 pNA
Full Text
By Phil Jones
The research team which this year broke the world terabit transmission distance record has upped sticks
from CNET, the research and development unit of France Telecom (6/7/99), and formed a start-up
company which will go head to head with Alcatel, Lucent Technologies, and Nortel Networks in the
burgeoning optical transmission gear market.
Algety Telecom, based in Lannion, France, has received first round funding of 4.4m euros ($4.7m) from
US and European venture capitalists, including Banexi Venture Partners, Crescendo Ventures, Newbury
Ventures and Technocom Ventures. It has also received 2.7m euros ($2.9m) from French government
sources, including Agence National de L'Innovation (ANVAR).
So far the funding has paid for Algety's exclusive acquisition of all technology rights and patents
connected to the soliton transmission that the company's founders were instrumental in developing at
CNET. It has also seen Algety recruit a staff of 25, including a team of software engineers which is now
working to produce the first commercial soliton transmission system in time for launch in mid-2000,
said Jerome Faul, Algety's managing director.
Soliton technology threatens to be the next big thing in optical transmission systems, and promises to
both boost the capacity of today's dense wave division multiplexed (DWDM) backbone networks and,
critically, cut their construction and maintenance costs by reducing the need for signal amplification and
regeneration. In a regular commercial fiber network, claims Faul, operators typically require to amplify
optical signals every 50km to 100km, and to completely regenerate the signal (refreshing the traffic by
converting it from optical, to electronic and back) at distances of 400km or less.
Algety's technology means amplification is only needed after each 100km. But it is its ability to stretch a
terabit per second signal to 1,000km which will be the market differentiator says Faul. So far,
prospective rivals, such as Lucent, have said they can extend unregenerated DWDM transmission to
400km. But over a 1,000km link, this would still require regeneration plants costing millions of dollars
in three locations, at 200km, 600km and 1,000km, Faul argues. "We only need to regenerate once. So
we can reduce the costs by a factor of three," he said.
The company plans to jealously guard its soliton technology, which uses very short, but very stable
bursts of lightwaves to carry data. There are no plans to license the technology to other vendors. Instead,
the company is planning to take on much bigger competitors in a market which, according to Faul, is
conservatively expected to generate equipment revenues of $3bn by 2001, and which some observers,
struck by the apparently insatiable global appetite for bandwidth, says may be worth nearer $20bn soon
afterwards.
However, Faul is confident that Algety, with its roots in the R&D labs of a national telco, will be able to
carve a niche for itself with pan-continental carriers in Europe, Asia and North America. Having already
demonstrated 1Tbps over 1,000km, Faul says his company already has the edge over the likes of Nortel,
which is talking of 1.6Tbps sometime in the future, but not talking about the distance it can span at this
speed. "No one is talking about distance except us. Distance is a very bad for our competitors," Faul
said. By early 2002, Faul added, Algety expects to push its capacity barrier out to 3Tbps.
Full Text COPYRIGHT 1999 ComputerWire Inc.
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SWITCHING & TRANSMISSION; Soliton OC-192 stretches DWDM's limits.
(Technology Information)
Telephony, June 7, 1999 pNA
Full Text
Over the past four to five years, great strides have occurred in dense wave division multiplexing
technology, with systems growing and gaining advanced feature sets. During 1998 the number of OC-48
Sonet interfaces connected to DWDM systems increased dramatically. And some of these interfaces are
direct connections from asynchronous transfer mode switches and Internet protocol routers.
Many more switches and routers will be connected to DWDM systems this year, and carriers will begin to
connect broadband cross-connects directly to DWDM systems as these devices deploy OC-48 ports.
Some next generation terabit switch routers can support more than 100 OC-48 ports. With this in mind,
network planners are carefully choosing DWDM systems with special emphasis on channel count and
wavelength bandwidth capacity. During the past few years, DWDM systems have dramatically increased
channel capacity. But most of these systems have not been challenged with carrying both high channel
count and high bandwidth on each channel.
Vendors have already announced the availability of OC-192 ports on network switches and routers.
OC-192 Sonet systems are being deployed today by forward-looking carriers that want to maximize
their fiber usage. Using OC-192 over a DWDM system, instead of four OC-48 signals, saves three
wavelengths for future use. Or, viewed another way, it provides three wavelengths that can be sold to
other carriers for additional revenue. However, with the increased deployment of OC-192 line rates, new
problems arise.
The optical layer
DWDM systems carry electrical signals modulated on optical wavelengths within a defined spectral
range. Often referred to as the "15xx" nm range, the industry has identified the 1500 nm range as
optimal for transferring wavelengths because it offers the best performance and the least attenuation,
and it can be optically amplified.
The International Telecommunication Union defines a standard grid of frequencies to be used by DWDM
manufacturers, which provides 100 GHz spacing between each wavelength. DWDM manufacturers that
offer high channel counts have already surpassed the ITU grid by spacing wavelengths at 25 and 50 GHz
intervals. Because of the limited bandwidth range and the close channel spacing, DWDM systems must
offer robust mechanisms to overcome potential transmission problems in the optical layer. OC-48
signals operating at 2.5 Gb/s are easily managed by available DWDM systems. However, the OC-192
line operating at 10 Gb/s is more difficult to accommodate because it is more sensitive to fiber
dispersion and nonlinearity.
Each Sonet signal input to a DWDM system must be converted to a specific wavelength for transmission
over the fiber. Short-range DWDM systems use directly modulated laser transponders for this function.
For longer distances, systems use an externally modulated laser transponder. The externally modulated
transponder incorporates a lithium niobate crystal in the modulator that narrows pulse width, thus
allowing for greater distances. However, even with external modulation, dispersion is a problem for a
high line rate such as OC-192.
The traditional approach to the DWDM system design has been to employ a non-return to zero (NRZ)
signal encoding technique for each wavelength. However, at the very high line rate of 10 Gb/s, an NRZ
signal is highly susceptible to polarization mode dispersion. The dispersion causes pulse spreading,
which limits the effective distance of signal transmission over the fiber. As transmission rates increase,
so do the fiber non-linearity distortions, primarily self-phase modulation and cross-phase modulation.
Also, for DWDM systems, intermodulation products are generated between the DWDM channels, which is
called four-wave mixing. All these impairments result in restricted fiber lengths for NRZ encoded
signals.
Carriers can address self-phase modulation and cross-phase modulation by using dispersion
compensation. Depending on the DWDM system design, dispersion compensationcan be inserted at the
terminal ends or at line amplifier sites. Distributing the dispersion compensation at each line site
effectively reduces self-phase and cross-phase modulation effects. It is important to note that these
transmission limitations are due to the nonlinear effects of the fiber, which are different depending on
the type of fiber deployed. As a result, the carrier must evaluate each fiber type for its performance
based on the specifications of the DWDM system it wants to deploy.
A new DWDM design
The answer to the technical challenges in high bit-rate transmission over fiber may lie in a coding format
called Soliton. This unique signal coding takes advantage of fiber non-linearity to increase the
propagation distance. A Soliton is a type of wave or, in the case of optical fiber, a narrow pulse of light
that retains its shape as it travels long distances along the fiber. The Soliton's ability to keep its shape
helps overcome chromatic dispersion, which results in loss of data integrity (Figure 1).
Modern Soliton technology is based on a scientific discovery first documented in 1834 by Scottish
engineer John Scott Russell. Russell, a boat builder, observed the Soliton wave phenomena in the water
while studying the movement of canal boats.
Russell's fundamental research was not appreciated until the mid-1960s when scientists started using
computers to study nonlinear wave propagation. In the 1970s, photonics researchers proposed Soliton
as a solution to counteracting dispersion and nonlinearity in optical fibers. For the next 20 years
research continued on Soliton light pulses. However, until the development of erbium-doped optical
amplifiers, there was no commercial application for the Soliton. In 1991 a laboratory demonstration at
AT&T Bell Labs transmitted Solitons error-free for 14,000 kilometers using erbium-doped amplifiers.
Since the early '90s, numerous scientific papers have been presented on Soliton; conferences are held
on the subject around the world. Pirelli designed the WaveMux DWDM product in 1997 using Soliton
technology.
Soliton vs. conventional NRZ
In 1998 Pirelli participated in the first field trial of a commercial Soliton transmission product. The field
trial involved a four-wavelength system and demonstrated doubled transmission length when compared
with the already installed OC-192 NRZ systems (Figure 2).
The field trial was conducted on standard SMF-28 fiber on a route characterized by high polarization
mode dispersion. The high-performance line amplifiers used in this field trial incorporated dispersion
compensating gratings.
The "chirped" Bragg gratings ensured pulse restoration in addition to amplification of the Soliton signal.
Very little pulse distortion occurred when the Soliton signal was displayed as an electrically filtered eye
diagram.
In addition to the greater distances achieved using Soliton pulses to carry OC-192 signals, Soliton
reduces the amount of electrical equipment required. Sonet regenerators were eliminated at the
mid-route site in the field trial, saving both equipment and cost. This is a major benefit for two reasons.
First, eliminating a regenerator may also remove a clock source. Second, it is a significant step in the
move toward an all-optical network.
While the field trial was conducted on SMF-28 fiber, the Soliton solution can be implemented on
multiple fiber types, including TrueWave and dispersion-shifted fiber. The Soliton solution also is
compatible with optical add/drop and optical cross-connect systems.
The use of a Soliton signal for OC-192 transmission is important because it overcomes the most severe
transmission problems encountered with the traditional NRZ signal. Dispersion and nonlinear effects
cancel out, giving stable pulse shape and width for an almost infinite propagation distance. Soliton
return-to-zero pulses have higher peak power than NRZ, and the nonlinear cross-phase modulation
threshold is higher for Soliton than NRZ signals. Therefore, the power can be increased.
Typical signal-to-noise ratio requirements for an NRZ signal impose a lower limit for the launch power,
or per channel power, and fiber nonlinearity imposes an upper limit to the launch power. This results in
shorter spans for NRZ signals. Soliton pulses are robust to any small perturbations. Therefore, small
changes in dispersion, power, pulse shape or polarization mode dispersion do not affect the Soliton
pulse. The overall result is longer spans, higher span loss and lower bit-rate error (Figure 3).
Technology for the future
Looking toward the future, Soliton pulses have applications at higher bit-rates than OC-192. Upgrading
to 20 Gb/s and higher can be accomplished via optical time domain multiplexing/demultiplexing using
Solitons. Conventional NRZ transmission is not compatible with optical time domain multiplexing and its
speed is therefore limited by the electronics to 10 Gb/s with current technology.
Using Soliton technology to upgrade the channel rate, network designers are not limited by the speed of
the electronics. Because the system margins at this high bit-rate are much tighter, additional design
issues must be addressed in the system architecture.
Another area where Soliton provides a technical advantage is in the design of an all-optical regenerator.
A Soliton 3R optical regenerator - or synchronized phase modulator - can be used to clean up noisy
signals without electro-optics, which convert signals from optical to electrical.
Current NRZ signals cannot be regenerated in the optical domain without the use of costly
electro-optics. An all-optical regenerator is a key enabling subsystem for the all-optical network. This
will be required on the output ports of optical cross-connects before optical transmission to digitize the
network design.
As telecommunications continues to change in real time and high-speed data services require flexible
networking options, DWDM plays a critical role. Future DWDM network architectures will support flexible
add/drop functions and protected ring configurations similar to those in Sonet networks today. New
methods for performance measurement in the optical layer are being developed and incorporated into
optical multiplexing equipment. Soliton technology is a scalable platform fully capable of exploiting
future technology advances in WDM.
Full Text COPYRIGHT 1999 Intertec Publishing Corporation, a PRIMEDIA Company. All rights reserved.
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Soliton savior: MCI feels the technology will save its customers money
Global Telephony, Oct 1998 pNA
Author
Shaw, Russell
Full Text
By tripling the distance that data, voice and video traffic can be sent over a fiber optic cable, MCI
Communications Corp. believes its new soliton-based technology will increase the efficiency of its
telephone network, while helping it save money on its planned network capital budget.
"Solitons offer the prospect of lowering transmission costs," says Niall Robinson, senior engineer for MCI
Engineering. "In return, this will reduce the cost of providing network capacity to our customers."
In tests earlier this year, MCI achieved a 10-Gb/s (gigabit per second) transmission rate over 900
kilometers of installed fiber without using regeneration amplifiers. On a typical route, being able to use
soliton technology means that up to eight regeneration amplifiers can be removed. Savings earned by
the reduction in electrical power bills and eventual replacement of many of the units, priced at "well over
six figures" apiece, can help save more than 20% of capital expenses, MCI believes. These savings, the
company says, can then be passed on to customers.
The target date for systemwide deployment of soliton technology in MCI's network is about two years.
Total conversion costs cannot be specifically earmarked yet because the soliton system is still
undergoing extensive testing. Once approved for widespread use, it won't be rolled out on a
region-by-region basis, but installed in the carrier's entire network.
Although MCI's particular application of soliton technology is new, soliton theory itself is not. First
discovered in 1834, the concept is a wave propagation phenomenon, based on the principle that pulses
of specific amplitude and duration can be sent over long distances without dispersion, or loss of signal
shape. Dispersion-the curse of fiber optic engineering-has been the main reason why signals can only
be sent short distances without numerous regeneration amplifiers of the type that MCI believes it can
replace.
MCI's new design is based on wavelength division multiplexing technologies. It uses converters made by
Pirelli Cables & Systems North America to generate light pulses in channels of different frequencies with
optimum power and shape. Loosely based on the concept of preventive maintenance, the plan calls for a
series of small signal corrections along the data path, rather than large corrections undertaken at longer
intervals when the signal has already begun to disperse and deteriorate.
"Pulse dispersion is a property of the optical fiber used for communication," Robinson explains. "Due to
dispersion, the transmitted pulses are, in time, spread out. This pulse spreading causes errors in the
transmission signal."
MCI uses electrical regenerators to reshape the pulse, allowing the signal to continue further down the
fiber, adds Robinson. But "soliton pulses do not suffer from the pulse-broadening effects of fiber
dispersion," he says. "In fact, they utilize the dispersion to maintain their pulse shape."
The regenerators operate only at a specific bit rate and format, says Robinson. "But [if] the soliton pulse
does not require reshaping, we can use optical amplifiers along the transmission line to boost the signal.
Plus, one of these amplifiers can be used to boost many soliton signals."
Although soliton transmissions work best at OC-192 bit rates, the fact that Pirelli's amplifiers are not
bit-rate or format-specific (such as being operable only at OC-48, OC-192 or OC-768) introduces a
combination of flexibility and distance performance greater than that offered by competing vendors such
as Lucent Technologies Inc. and Ciena Corp., MCI says.
"In fact," Robinson says, "the system MCI demonstrated in February took a commercial OC-192 system,
converted the OC-192 pulses into solitons and then transmitted them across the network. At the
receiving end, the solitons were converted back to the original OC-192 pulses before being detected by
the commercial OC-192 equipment."
Upon completion of its soliton-based network, MCI expects to develop a 32-channel system. Each
channel will transmit up to 40 Gb/s, producing a total throughput capacity of 1.28 Tb/s (terabit a
second).
Full Text COPYRIGHT 1998 Intertec Publishing Corporation, a PRIMEDIA Company. All rights reserved.
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Solitons work in real network.
Photonics Spectra, July 1998 p57
Author
Fontana, Flavio
Summary
MCI Telecommunications Corp. and Pirelli Cables and Systems have utilized a soliton transmission
system to undertake very long distance unregenerated connections in a high-speed telecommunications
network. MCI assessed the use of 10-Gb/s channels as fundamental components of a wavelength
division multiplexing system (WDM) based on step-index fiber, and the core component of the link is a
soliton transmitter that enables the generation of chirp-free pulses. In the first field trial, the system
successfully operated at 10 Gb/s over a distance of more than 900 km, while maintaining a repeater
spacing of at least 90 km. The system also showed upgradability to a WDM configuration in which 16 or
32 channels will be transmitted concurrently.
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