Fig. 1. The basic concept of a DWDM system with many wavelength-channels in the same fiber.
New communication services, e.g., Internet, high-speed data, video, wireless, etc. have triggered a tremendous need for bandwidth that legacy communications networks are not being able to cope with. Presently voice traffic and data are increasing at a rate of 10% and 80%, respectively, per year. Among available choices, increasing the transportable bandwidth of an existing fiber is the most efficient way of meeting the demand. Browse an article on DWDM technology review.
There are two ways to increase bandwidth in a single fiber, either increasing the bit rate or increasing the number of wavelength (channels) in the same fiber. This second method is a viable solution that capitalizes on advances in solid-state and photonic technology. Several wavelengths, each transporting data at 10 or 40 Gbps would increase the transportable bandwidth by a factor as large as the number of wavelengths. Systems with 40, 80, and 128 wavelengths per fiber have been designed, and systems with more wavelengths are in planning or experimental phase.
Wavelength division multiplexing
(WDM) is an optical
technology that couples many wavelengths in the same fiber, thus effectively
increasing the aggregate bandwidth per fiber to the sum of the bit rates of each
wavelength. For example, 16 wavelengths at 10 Gbps per wavelength in the same
fiber raise the aggregate bandwidth to 160 Gbps. An astonishing aggregate
bandwidths at several terabits per second (Tbps) are possible!
Fig. 1. The basic concept of a DWDM system with many wavelength-channels
in the same fiber.
Dense WDM (DWDM) is a technology with a larger (denser) number of wavelengths coupled into a fiber (>40). However, as the number of wavelengths increases, several issues need attention, such as channel width and channel spacing, total optical power launched in fiber, nonlinear effects, cross-talk, span of fiber, amplification, and so on.
DWDM technology was made possible with the realization of several optical components. It is also expected that several optical functions will soon be integrated to offer complex functionality at a cost per function comparable to electronic implementation. The following provides a snapshot of what has enabled the DWDM technology to become reality.
 |
Optical fiber has been produced that exhibits low loss and better optical transmission performance over the wavelength spectrum of 1.3 µm and 1.55 µm. |
|
Optical amplifiers (EDFA) with flat gain over a range of wavelengths and coupled in line with the transmitting fiber boost the optical signal, thus eliminating the need for regenerators. |
|
Integrated solid-state optical filters are compact and can be integrated with other optical components on the same substrate. |
|
Integrated solid-state laser sources and photodetectors offer compact designs. |
|
Optical multiplexers and demultiplexers are based on passive optical diffraction. |
|
Wavelength selectable (tunable) filters can be used as optical add-drop multiplexers. |
|
Optical add-drop multiplexer (OADM) components have made DWDM possible in MAN ring-type and long haul networks. |
|
Optical cross-connect (OXC) components, implemented with a variety of technologies (e.g., lithium-niobate), have made optical switching possible. |
In addition, standards have been developed so that interoperable systems can be offered by many vendors. As DWDM technology evolves, existing standards are updated or new ones are introduced to address emerging issues.
DWDM finds applications in ultra-high bandwidth long haul as well as in ultrahigh-speed metropolitan or inner-city networks and, at the edge of other networks (SONET, Internet protocol [IP], and asynchronous transfer mode [ATM]).
© 1999 Anis Rahman