The DWDM Equipment Used in Metropolitan Area Network
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The DWDM Equipment Used in Metropolitan Area Network

Posted on by FS.COM

The long distances made possible by advances in technologies such as optical amplifiers, dispersion compensators, and new fiber types, resulted in the initial deployment of DWDM technology in the long-haul transoceanic and terrestrial networks. Once these technologies became commercially viable in the long-haul market, it was the next logical step to deploy them in the Metropolitan Area Network (MAN) and, eventually, in the access networks using hybrid architectures of fiber and coaxial media.

DWDM is the clear winner in the backbone. It was first deployed on long-haul routes in a time of fiber scarcity. Then the equipment savings made it the solution of choice for new long-haul routes, even when ample fiber was available. While DWDM can relieve fiber exhaust in the MAN, its value in this market extends beyond this single advantage. Alternatives for capacity enhancement exist, such as pulling new cable and SONET overlays, but DWDM can do more. What delivers additional value in the metropolitan market is DWDM’s fast and flexible provisioning of protocol- and bit rate-transparent, data-centric, protected services, along with the ability to offer new and higher-speed services at less cost. There are mainly three types of DWDM equipment used in MAN, as shown in the following parts.

  • DWDM Multiplexer and Demultiplexer

Because DWDM systems send signals from several sources over a single fiber, they must include some means to combine the incoming signals. This is done with a multiplexer (MUX), which takes optical wavelengths from multiple fibers and converges them into one beam. At the receiving end the system must be able to separate out the components of the light so that they can be discreetly detected. Demultiplexers (DEMUX) perform this function by separating the received beam into its wavelength components and coupling them to individual fibers. Demultiplexing must be done before the light is detected, because photodetectors are inherently broadband devices that cannot selectively detect a single wavelength.

In a unidirectional system, there is a MUX at the sending end and a DEMUX at the receiving end. Two system would be required at each end for bidirectional communication, and two separate fibers would be needed.

Unidirectional DWDM

In a bidirectional system, there is a MUX/DEMUX at each end and communication is over a single fiber pair.

Bidirectional DWDM

MUX and DEMUX can be either passive or active in design. Passive designs are based on prisms, diffraction gratings, or filters, while active designs combine passive devices with tunable filters. The primary challenges in these devices is to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.

Related products in Fiberstore: DWDM MUX DEMUX
(three packages: Rack Chassis, LGX Cassette, and ABS Pigtail Module)



  • DWDM Optical Add/Drop Multiplexer

Between multiplexing and demultiplexing points in a DWDM system, there is an area in which multiple wavelengths exist. It is often desirable to be able to remove or insert one or more wavelengths at some point along this span. An Optical Add/Drop Multiplexer (OADM) performs this function. Rather than combining or separating all wavelengths, the OADM can remove some while passing others on. OADMs are a key part of moving toward the goal of all-optical networks.

OADMs are similar in many respects to SONET ADM, except that only optical wavelengths are added and dropped, and no conversion of the signal from optical to electrical takes place. There are two general types of OADMs. The first generation is a fixed device that is physically configured to drop specific predetermined wavelengths while adding others. The second generation is reconfigurable and capable of dynamically selecting which wavelengths are added and dropped.

Thin-film filters have emerged as the technology of choice for OADMs in current metropolitan DWDM systems because of their low cost and stability. For the emerging second generation of OADMs, other technologies, such as tunable fiber gratings and circulators, will come into prominence.

Related products in Fiberstore: DWDM OADM
(three packages: Rack Chassis, LGX Cassette, and ABS Pigtail Module)

Rack Chassis DWDM OADM LGX Cassette DWDM OADM ABS Pigtail Module DWDM OADM


  • DWDM Erbium-Doped Fiber Amplifier

By making it possible to carry the large loads that DWDM is capable of transmitting over long distances, the EDFA was a key enabling technology. At the same time, it has been a driving force in the development of other network elements and technologies.

Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers—the low-loss wavelength for optical fibers used in DWDM. Here is a simplified diagram of an EDFA. A weak signal enters the erbium-doped fiber, into which light at 980 nm or 1480 nm is injected using a pump laser. This injected light stimulates the erbium atoms to release their stored energy as additional 1550-nm light. As this process continues down the fiber, the signal grows stronger. The spontaneous emissions in the EDFA also add noise to the signal; this determines the noise figure of an EDFA.

EDFA Design

The key performance parameters of optical amplifiers are gain, gain flatness, noise level, and output power. EDFAs are typically capable of gains of 30 dB or more and output power of +17 dB or more. The target parameters when selecting an EDFA, however, are low noise and flat gain. Gain should be flat because all signals must be amplified uniformly. While the signal gain provided with EDFA technology is inherently wavelength-dependent, it can be corrected with gain flattening filters. Such filters are often built into modern EDFAs.

Low noise is a requirement because noise, along with signal, is amplified. Because this effect is cumulative, and cannot be filtered out, the signal-to-noise ratio is an ultimate limiting factor in the number of amplifiers that can be concatenated and, therefore, the length of a single fiber link. In practice, signals can travel for up to 120 km (74 miles) between amplifiers. At longer distances of 600 to 1000 km (372 to 620 miles) the signal must be regenerated. That is because the optical amplifier merely amplifies the signals and does not perform the 3R functions (reshape, retime, retransmit). EDFAs are available for the C-band and the L-band.

Related products in Fiberstore: DWDM EDFA

Fiberstore DWDM EDFA

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