Advances in solid-state and photonic technologies
have made the cant be done concept a thing of the
past. Bit rates of 2.5 Gb/s, 10 Gb/s, and 40 Gb/s over many kilometers
of single-mode fiber are a reality. The driver for this bandwidth appetite
was triggered with early optical networks (SONET/ SDH) that demonstrated
that glass fiber is a transmission medium that permits light to travel
through it without amplification for hundreds of kilometers and at incredible
data rates (many Gb/s), two accounts that were not possible with copper-twisted-pair
cable. And this appetite has been accelerated ever since with the proliferation
of transferring additional information over the communications network
with major service contributors such as e-mail, e-commerce, the Internet,
electronic documentation transfer, video, and ubiquitous mobile telephony.
And this is just the beginning as more exotic services are
planned for and contemplated to be offered over the communications network.
This article discusses dense wavelength division multiplexing (DWDM)
photonic technology and the role it will play in shaping future communications
What Is DWDM and How Does It Work?
As bandwidth demand keeps increasing, how can
we be assured that the network is elastic enough to cope with this increase?
How do we assure that systems and networks are able to process and transport
an increasing volume of voice and data (video, high-speed data, interactive
multimedia, etc.) traffic? How do we assure that the network is able
to manage a continuously increasing bandwidth (as new flexible services
become available, such as bandwidth on demand)?
Currently, there are
technological choices to answer these
In all these choices, photonic
technology plays a pivotal role in the
communications network. In particular, network bandwidth elasticity
is better addressed with WDM.
Depending on network application, WDM comes
in two fundamentally different flavors, each with its own complexity,
specifications, and cost structure: dense-WDM (DWDM) with more than
80 wavelength channels per fiber, and coarse-WDM (CWDM) with less than
40 wavelength channels per fiber. However, as time progresses and technology
permits, we will witness a shift of the demarcation points, and DWDM
will have more than 200 (and perhaps 1000) wavelengths, whereas CWDM
will have more than 40.
Thus, DWDM technology exhibits an inherent flexibility:
each wavelength is a huge transporting vehicle and it does not recognize
the type of information it carries, nor does it care . For example,
one wavelength may carry Internet  and another SONET, or ATM [3-5].
The sender and the recipient alone handle the content cargo appropriately.
Moreover, WDM technology with optical devices that provide functionality
such as add-drop multiplexing can
transform any type of network (mesh, ring, or star) into a physical
single-fiber ring network (Fig. 2).
Based on the low-loss behavior of glass fiber (Fig. 3), ITU-T has defined a grid of 81 frequencies in the C-band (196.10-192.10 nm) starting with a frequency at 196.10 THz (l = 1528.77 nm), the remaining standardized frequencies are computed decrementing/incrementing by 50 GHz (0.39 nm). For 40 frequencies on the grid, the starting frequency is the same but the decrement/increment is 100 GHz, and for 20 frequencies it is 200 GHz. Decrementing by 50 GHz beyond the C-band results in 80 more wavelengths defined in the L-band. Similarly, when incrementing by 50 GHz, more wavelengths are defined in a band that requires specialized single-mode fiber with low loss characteristics in the 1.4-nm band where HO- has been causing high absorption. Table 1 lists the wavelength bands used in optical communications.
DWDM Technology Enablers
Pure DWDM systems are supposed to be all-optical.
That is, functionality that was previously implemented with electronics
is now achieved with all-optical devices, such as those listed in the
Filtering is accomplished with passive Fabry-Perot,
Bragg, thin-film, Mach-Zehnder, dielectric, and acousto-optic filters.
Each filter is based on different principles of physics; for example,
the Fabry-Perot is based on interferometry, the Bragg on diffraction,
and the prisms on refraction.
Among the most significant are the Bragg gratings
as they are passive devices, easily manufactured, easily integrated
with other components, and cost effective. Diffraction gratings come
in different flavors: reflected, pass-through, fiber, and so on. However,
the diffraction theory is the same, and it may differ in only some parameters
differentiating the plate from the fiber gratings. In all, the
condition for strong reflection, also known as the
Bragg condition, is:
d = -mlB/2
is an integer, d is the grating
constant, and lB is the wavelength
for which the Bragg resonates and reflects.
Figure 4 illustrates
a fiber Bragg grating. Advanced fiber Bragg gratings, known as chirped
Bragg gratings, have also the additional ability to compensate for chromatic
Multiplexers and Demultiplexers
Wavelength multiplexing and demultiplexing is
accomplished with passive components such as diffraction gratings, thin-films,
and super-prisms. For reflected and pass-through gratings, the angular
separation between wavelengths for a given order m, also known as the
angular dispersion D, is:
is an integer, d is the grating
constant, and b is the angle of diffraction.
Figure 5 illustrates
the principles of a diffraction-based demultiplexer.
In communications, dropping and adding one or
more channels is an important function that permits efficient bandwidth
delivery and distribution over the network. Optical add-drop multiplexing
is accomplished by combining optical demultiplexers and multiplexers,
optical switches, filters, and other components.
Figure 6 illustrates a single wavelength optical add-drop multiplexer
that employs a Bragg fiber grating.
Direct optical amplification  is accomplished
with specialized doped fibers (e.g., erbium-doped fiber amplifiers (EDFA)),
semiconductor optical amplifiers (SOA), and Raman amplification. Each
amplifier has its own characteristics and is usable in a different spectrum
(Fig. 7). In short-haul applications, the deployment of amplification
is limited, but in long-haul, where it is most required, it is not unreasonable
to use more than one amplification method, such as EDFA and Raman combined.
In long-haul optical transmission, the signal
requires periodic regeneration (or repeaters) in order to reach a destination
that may be many hundreds of kilometers afar. Traditional regeneration
requires conversion of the optical signal into electric retiming, reshaping,
and regeneration (amplification), a function known as 3R. Then, the
signal is converted to optical and transmitted to the next repeater,
and so on. However, traditional repeaters are complex devices; they
require maintenance and are costly. Optical technology allows for direct
optical amplification that can stretch the distance between amplifiers
to many kilometers, and thus lower the overall cost and maintenance;
this is known as 1R, since all that the amplifiers accomplish is regeneration.
However, there are optical components and techniques that can also accomplish
reshaping, and more recently retiming, in the optical regime (the latter
is experimental). For example, dispersion compensating fiber removes
the pulse widening due to nonuniform propagation of wavelength in the
glass medium (Fig. 8). Dynamic equalization
restores the amplitude of each channel in the DWDM signal to within
a small fraction of variability (~0.1%). Optical phase-lock-loop techniques
retime the received optical pulses to remove drift and jitter. Thus,
although the current state of the art is 1R or 2R repeaters, all-optical
3Rs are on the research and development bench.
Other Optical Components
The list of optical components and technology
does not stop here. Lasers and laser pumps, detectors, wavelength converters,
couplers and splitters, polarizers, isolators, equalizers, dispersion
compensators, specialty fibers, fiber couplers, pigtails, micro-lens
systems, pulse retimers, pulse reshapers, detectors, fixed or tunable
devices, memories, and more will eventually transform the communications
network landscape to an all-optical DWDM network (Fig.
9). In addition, new artificial materials with new photonic properties
will make new additions to an optical designers tool-box.
In an all-optical network, the signal path between
transmitter (laser) and receiver (photodetector) will be optical
(dominated by optical components), known as the
optical regime, whereas the remainder of the path (before the
laser and after the photodetector) will be fully electrical, known as
the electrical regime, and in it
electronics will be the only game. However, even in the optical regime,
there are optoelectronic sensors to monitor the optical path for performance
and all-electronic devices to process performance data as well as to
control the behavior of (optical) devices and to communicate with the
various units of a system and of a network. Thus, electronics will also
be indispensable players in this all-optical network.
On the technology side, current systems support
fixed wavelength assignment for each node, or they are manually reconfigurable.
However, a wide range of tunable devices is emerging  that will enable
dynamic wavelength assignment, optimized bandwidth allocation, wavelength
and path protection, and better network survivability strategies. Advanced
optical techniques will enable fault monitoring in the optical regime.
Polymers and photonic crystals will provide improved and cost-effective
photonic performance. Finally, nanotechnology, new materials, and integration
of optical functionality will create miniaturized components with complex
functionality and lower power.
In the standards arena, wavelength operation,
administration, management, and provisioning (OAM&P); DWDM fault
management ; DWDM network management ; latency; and quality
of service are just a few examples of current intense activity. Optical
DWDM networks are defined and deployed that are characterized by unprecedented
bandwidth capacity, bandwidth elasticity, reconfigurability, reliability,
and survivability of service of the DWDM system and of the network.
DWDM is here and in its infancy, but it will
quickly grow into a technology for a global communications network that
will allow anyone at anytime and at any place, with the same identification
number, to communicate with voice, with fast data, and with picture.
The beneficiaries of DWDM photonics technology will not be only the
communications field but also fields such as medicine, commerce, home
appliances, and others that will shape future lifestyles and perhaps
Stamatis V. Kartalopoulos,
Ph.D., is president and chief technical officer of PhotonExperts
in Annandale, New Jersey (e-mail: firstname.lastname@example.org).