Coming soon... dispersion-managed soliton networks
A new transmission technology that keeps its
shape over longer distances is expected to leave
the lab and reach commercial systems this year.
BY JEFF STERN, Marconi Solstis
You may be hearing some talk about an emerging
technology that holds tremendous promise for
service providers jockeying for leadership in
today's dynamic, cutthroat marketplace.
Dispersion-managed solitons, also known as
return-to-zero (RZ) modulation, can dramatically
increase the transmission speeds of optical
networks to rates in excess of 1 Tbit/sec (1,000
Gbits/sec) over thousands of kilometers, using
WDM techniques.
For service providers seeking to transmit data
further, faster, and wider; dispersion-managed
soliton-based networks may be the answer. Not
only will this technology allow service providers
to significantly lower transmission and bandwidth
costs, it will make networks easier to provision
and manage and can incorporate add/drop
functionality that will offer the flexibility to tailor
bandwidth capacity to demand at intermediate
nodes along an ultra-long-haul route.
19th century roots, 21st century demand
Although the dispersion-managed soliton is a
relatively new technology, its origins date back to
the 19th century, when Scottish engineer John
Scott Russell discovered solitary waves while
conducting experiments to determine the most
efficient design for canal boats. But not until the
mid-1960s, when digital computers were used to
study nonlinear wave propagation, were Russell's
discoveries truly appreciated.
The physical properties of the soliton wave, which
is an optical pulse that doesn't break up or spread
out over distances, make it an ideal technology for
optical-fiber communications networks. A soliton
is designed to change shape in a periodic,
controlled manner so it arrives at its destination
unaltered. Therefore, dispersion-managed
soliton-based infrastructures reduce the need to
regenerate channels, making these networks a
cost-effective step up from traditional fiber
networks.
Lower transmission costs
With conventional optical transmission, optical
amplifiers are used to boost the optical signals,
typically every 60 to 100 km. Every few hundred
kilometers, however, it is necessary to fully
regenerate the optical signal to remove the effects
of noise and other transmission impairments. That
involves a lot of very expensive equipment to turn
the wavelengths of light back into an electrical
signal so that each can be individually processed.
In contrast, solitons change the way in which data
is encoded in the optical network, allowing much
greater distances to be achieved before
regeneration is necessary, which in turn enables
service providers to slash their transmission costs.
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The tremendous competitive challenges faced
today by service providers mean that the market
is certainly ripe for the benefits of this
revolutionary technology. Commercial
deployments are expected to get underway this
year.
In the meantime, the technology continues to be
developed, refined, and tested in trials by several
leading service providers. To date, world-record
data-transmission speeds have been demonstrated
during laboratory trials. For example, using this
technology, a standard optical fiber carried 10
Gbits/sec of data across a distance of 16,000 km,
and 40 Gbits/sec of information was transmitted
for more than 1,000 km.
Dispersion-managed solitons in action
For service providers contemplating a move from
a conventional fiber network to a
dispersion-managed soliton-based system, the
transition is quick, easy, and virtually transparent.
The network architecture will look much the
same as it does today. The only change is a drastic
reduction in the number of regeneration or
terminal sites required. Figure 1 illustrates the
type of system anticipated for deployment this
year. Initial dispersion-managed soliton systems
will operate at 10 Gbits/sec, with later products
moving to 40-Gbit/sec operation. Transmission
systems carrying up to 160 wavelengths are
planned. These systems will support data
transmission over thousands of kilometers without
the need for electrical regeneration and offer
optical add/drop capability to allow optical
wavelengths to be added or dropped at
intermediate amplifier nodes.
The ability to turn up capacity quickly on a route
to meet new demands from customers will be a
major benefit of these dispersion-managed soliton
systems for service providers. Currently, if new
capacity is needed on a route thousands of
kilometers long, the service provider must
intervene at multiple regenerator sites along the
route to install or activate new equipment.
Dispersion-managed soliton technology will allow
service providers to remotely activate
wavelengths on an end-to-end basis using
network-management systems, which will
dramatically shorten provisioning times.
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On a technical level, the main differences between
the soliton equipment and conventional DWDM
systems is a new type of transponder to generate
the soliton pulses and improved optical amplifiers
to handle the higher optical signal levels. (A
transponder converts a signal from electrical to
optical and back to electrical in the fiber.) Soliton
products will also feature the use of Raman
amplification as well as conventional
erbium-based optical-amplifier technology.
Fiber-optic system strengths, limits
Before deciding whether to implement a
dispersion-managed soliton network, it's
worthwhile to examine the nuts and bolts of an
optical-fiber transmission system, its strengths and
limitations, to understand the reasons for these
new soliton developments.
The simplest form of optical transmission is a
point-to-point optical-fiber cable with a
transmitter at one end and a receiver at the other.
A laser light shone down the fiber carries data
along the length of the fiber. The ultimate goal is
for the data to reach the end of the line, identical
to how it began. Optical-fiber networks are
subject to a number of impairments that affect
how far data can travel down the fiber, however.
The principle impairments are signal loss,
chromatic dispersion, and nonlinearity.
Loss refers to the reduction in power that
takes place as light travels along the fiber. As
a result, long-distance cables require optical
amplifiers at periodic intervals to restore the
strength of the signal. But noise on the optical
signal increases with the number of amplifier
spans, eventually limiting overall system
range before electrical regeneration is
necessary.
Chromatic dispersion arises because light
travels down the fiber at different speeds
depending on the wavelength. Since all
optical signals consist of a finite spread of
wavelengths, dispersion leads to optical pulse
broadening over a long distance, thereby
limiting bandwidth. This effect is well known
in conventional optical transmission systems
and can be countered by the use of dispersion
compensation modules (DCMs) at
optical-amplifier and regenerator nodes.
Nonlinear effects, which are only now
becoming important as fiber-optic systems
reach for still greater performance, arise due
to the very high light-intensity levels needed
to go very long distances. The intensity of the
light actually changes the refractive index of
the fiber causing a phase modulation of the
light as it is transmitted. That leads to a
change in optical frequency along the pulse of
light, which in turn leads to pulse broadening,
limiting system bandwidth.
Balancing the effects
The dilemma for the ultra-long-system designer
therefore is whether to raise the optical signal
power to overcome noise effects or reduce it to
prevent nonlinear effects. That is where solitons
offer an advantage. It is possible to balance the
effects of dispersion and nonlinearity to create
soliton pulses that transmit through the fiber with
near-ideal properties, allowing much higher
optical power levels to be used than would
otherwise be the case.
A conventional system uses non-return-to-zero
modulation (see Figure 2). Solitons, however, use
a return-to-zero format which is more robust to
nonlinear impairments and allows the optical
pulse to be shaped so that the nonlinear effects are
offset by the accumulated dispersion effects in the
fiber. The dispersion characteristics of the route
still have to be correctly managed as part of the
overall system design, hence the term
"dispersion-managed solitons." Although, a
different set of design rules is used, dispersion
management in soliton networks is done in a
similar manner to conventional long-haul,
high-bit-rate DWDM systems-by using passive
compensation elements (DCMs) at the
optical-amplifier nodes.
Capacity for distance
Some skeptics argue that when it comes to
ultra-long-haul transmission, service providers
may have to sacrifice bandwidth capacity for
distance. However, dispersion-managed soliton
vendors are working to refine the technology to
achieve transmission distances of several thousand
kilometers without sacrificing capacity.
Once dispersion-managed soliton technology is
introduced in the marketplace, it is anticipated
that the technology will continue to evolve,
lowering its cost. Lower cost will facilitate its use
throughout service providers' networks, not just in
ultra-long-haul backbones.
Dispersion-managed soliton-based networks are
expected to set new standards in optics
transmission. This new transmission technology
will put service providers on the fast track to
success by equipping their networks with systems
that can beat a bandwidth crunch that is expected
to continue in the foreseeable future.
Jeff Stern is the product strategy director at
Marconi Solstis, based in Stratford-on-Avon,
UK. He can be reached via e-mail at
jeff.stern@marconi.com.
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