Advances in solid-state and photonic technologies have made the “can’t 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 networks.

What Is DWDM and How Does It Work?
Bandwidth Elasticity

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 questions:

• Install more and better fiber, as the need arises. Although this is currently pursued, nevertheless, it requires substantial planning and investment, and it may not be possible in all cases.
• Use higher speed photonic technology to increase the bit rate (to 10 Gb/s, 40 Gb/s, > 100 Gb/s). This implies that to stay at the forefront of data rate, one has to always use the most advanced technology that is neither mature or cost attractive.
• Use optical components instead of electronic components (e.g., amplifiers, filters, etc.). This is a design choice that depends on the availability of a range of components and how well their specifications match. However, because most optic components are data-rate independent, the cost per bit turns out to be a tiny fraction compared with electronic implementation.
• Increase the number of optical carriers (wavelengths) per single fiber, a technology known as wavelength division multiplexing (WDM). This is a successful technology as it takes advantage of existing fiber, and the only changes required are at its termination points; in some cases, it greatly simplifies the traditional regeneration as optical amplification is much simpler and more cost effective.
  

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.

Wavelength Division Multiplexing
Early optical transmission in long-haul single-mode fiber applications used a single wavelength at 1310 nm, whereas wavelengths in the 800-nm band have been used in short-haul multimode fiber applications. However, photonic devices performing in the low-loss spectral band of about 1.55 µm have enabled more than a single wavelength in the same fiber (Fig. 1). Having put a number of wavelengths in the same fiber, the aggregate bandwidth per fiber is multiplied by this number. For example, 40 wavelengths at 40 Gb/s each yield an aggregate bandwidth of 1.6 trillion bits per second per fiber (1.6 Tb/s). In SONET terms, this is equivalent to 20 million simultaneous conversations per fiber.

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 [1]. For example, one wavelength may carry Internet [2] 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.

The scalability of DWDM does not stop here; research has demonstrated that the number of wavelengths per fiber could increase to more than 1000, and this clearly is not a limit. Bandwidth at this level would enable every single person in the United States to be continuously connected with anyone else—not only with voice but also with video and data. Moreover, considering that a cable could have more than 200 fibers lighted, one realizes that DWDM enables not only every single person in the United States to be continuously connected but also, perhaps, every single person in the world.

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 following sections.

Filters

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 = -ml_B/2

where m is an integer, d is the grating constant, and l_B 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 dispersion.

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:

db/dl = m/(dcosb)

where m 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.

Optical Switching
Switching is accomplished with solid-state technology (lithium-niobate), micro-electro-mechanical mirror systems (MEMS), tiny bubbles, liquid crystals, electro-holographic methods, and other technologies that are still in the experimental phase. However, each technology has its own merits and each one finds its own niche in the applications space. For example, MEMS make large optical cross-connecting fabrics (1000 x 1000) but are slow switching (ms) compared to lithium niobate, which makes small fabrics (32 x 32) but is faster (ns). Table 2 lists a comparative sample of optical switching technologies.

Optical Add-Drop Multiplexing

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.
Employing tunable components, dynamic add-drop wavelength multiplexing enables dynamic wavelength assignment, dynamic bandwidth allocation, service and network protection and survivability, and overall great flexibility in cost-efficient bandwidth management.

Optical Amplification

Direct optical amplification [6] 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.

Optical Regenerators

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 designer’s tool-box.

What About Electronics?
At this point, we are compelled to answer an important question. As the communications network is transformed to an all-optical network, what will the role of electronics be?

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.

Network “Nodes”
Although we use the term “node” [7, 8] in a habitual manner (emanating from traditional networks), data networks employ “routers.” However, in DWDM applications an advanced router performs DSn and OC-n grooming, optical multiplexing, and switching, and it also provides quality of service (QoS); that is, all attributes and functions of a traditional communications node. Similarly, traditional nodes have data (and packet) store- and-forward routing capability. In some configurations, traditional nodes and routers work side-by-side to provide traditional synchronous and asynchronous service, voice, and data. Therefore, although nodes and routers may be conceptually different, as the network evolves, there is a convergence of functionality, and therefore, here we do not discriminate between the two.

Future Direction
DWDM continues to evolve, and work continues feverishly on many fronts: technology, standards, and network architectures.

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 [9] 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 [10]; DWDM network management [11]; 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 the world.

Stamatis V. Kartalopoulos, Ph.D., is president and chief technical officer of PhotonExperts in Annandale, New Jersey (e-mail: kartalopoulos@sprintmail.com).

References

1. S.V. Kartalopoulos, Introduction to DWDM Technology: Data in a Rainbow. Piscataway, NJ: IEEE Press, 2000.
2. B. Furcht, Handbook of Internet and Multimedia: Systems and Applications. Piscataway, NJ: IEEE Press, 1999.
3. S.V. Kartalopoulos, Understanding SONET/SDH and ATM: Communications Networks for the Next Millennium. Piscataway, NJ: IEEE Press, 1999.
4. Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, Telcordia GR-1377, Issue 2, Dec. 1995.
5. SONET OC-192 Transport Systems Generic Criteria, Telcordia GR-253, Issue 3, Aug. 1996.
6. Optical Interfaces for Multi-Channel Systems with Optical Amplifiers, ITU-T Draft Rec. G.692, Oct. 1998.
7. Network Node Interface for the Optical Transport Network (OTN), ITU-T Draft Rec. G.709, Oct. 1998.
8. Characteristics of Optical Transport Networks (OTN) Equipment Functional Blocks, ITU-T Draft Rec. G.798, Oct. 1998.
9. S.V. Kartalopoulos, “Emerging technologies at the dawn of the millennium,” IEEE Commun. Mag., vol. 39, pp. 22-26, Nov. 2001.
10. S.V. Kartalopoulos, Fault Detectability in DWDM: Toward Higher Signal Quality and System Reliability. Piscataway, NJ: IEEE Press, 2001.
11. Management Aspects of the Optical Transport Network Elements, ITU-T Draft Rec. G.874, Oct. 1998.