Tag Archives: DWDM demultiplexer

Capacity Expansion and Flexibility—DWDM Network


DWDM increases the bandwidth of an optical fiber by multiplexing several wavelengths onto it. Even though it costs more than CWDM, it is currently the most popular WDM technology because it offers the most capacity. This article provides an overview of DWDM networks and its current applications.

Introduction of DWDM Technology
Dense wavelength-division multiplexing (DWDM) revolutionized data transmission technology by increasing the capacity signal of embedded fiber. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, then multiplexed onto one fiber. By providing channel spacings of 50 GHz (0.4 nm), 100 GHz (0.8 nm) or 200 GHz (1.6 nm), several hundreds of wavelengths can be placed on a single fiber. DWDM takes advantage of the operating window of the Erbium Doped Fibre Amplifier (EDFA) to amplify the optical channels and extend the operating range of the system to over 1500 kilometers. The following picture shows the operation of a DWDM system.


Components of DWDM System
Important components for DWDM systems are transmitters, receivers, optical amplifiers, transponders, DWDM multiplexers, and DWDM demultiplexer. These components, along with conforming to ITU channel standards, allow a DWDM system to interface with other equipment and to implement optical solutions throughout the network.

  • Optical transmitters/receivers

Transmitters are described as DWDM components since they provide the source signals which are then multiplexed. The characteristics of optical transmitters used in DWDM systems are highly important to system design. Multiple optical transmitters are used as the light sources in a DWDM system. Here we can ues a transceiver to replace transmitters and receivers, since it is the combiantion of them. Transceivers applied in DWDM network are often called the DWDM transceiver, of which the transmission distances can reach up to 120 km. The following picture shows the receivers and transmitters in DWDM systems.

Optical transmitters/receivers

  • Optical amplifiers

Optical amplifiers (OAs) boost the amplitude or add gain to optical signals passing on a fiber by directly stimulating the photons of the signal with extra energy. They are “in-fiber” devices. OAs amplify optical signals across a broad range of wavelengths. This is very important for DWDM system application. Erbium-doped fiber amplifiers (EDFAs) are the most commonly used type of in-fiber optical fibre. Following picture shows the operation of OA.

Optical amplifiers

  • Transponders

Transponders convert optical signals from one incoming wavelength to another outgoing wavelength suitable for DWDM applications. Transponders are optical-electricaloptical (O-E-O) wavelength converters. A transponder performs an O-E-O operation to convert wavelengths of light. Within the DWDM system a transponder converts the client optical signal back to an electrical signal (O-E) and then performs either 2R (reamplify, reshape) or 3R (reamplify, reshape, and retime) functions. The following picture shows the operation of bidirectional transponder.


A transponder is located between a client device and a DWDM system. From left to right, the transponder receives an optical bit stream operating at one particular wavelength (1310 nm). The transponder converts the operating wavelength of the incoming bitstream to an ITU-compliant wavelength. It transmits its output into a DWDM system. On the receive side (right to left), the process is reversed. The transponder receives an ITU-compliant bit stream and converts the signals back to the wavelength used by the client device.

  • DWDM Multiplexers and Demultiplexers

Multiple wavelengths (all within the 1550 nm band) created by multiple transmitters and operating on different fibers are combined onto one fiber by way of an optical multiplexer. The output signal of an optical multiplexer is referred to as a composite signal. At the receiving end, a demultiplexer separates all of the individual wavelengths of the composite signal out to individual fibers. The individual fibers pass the demultiplexed wavelengths to as many optical receivers. Typically, mux and demux (transmit and receive) components are contained in a single enclosure. Optical mux/demux devices can be passive. Component signals are multiplexed and demultiplexed optically, not electronically, therefore no external power source is required. Following picture shows the operation of DWDM multiplexers and demultiplexers.

DWDM Multiplex and Demultiplex

Applications for DWDM
As occurs with many new technologies, the potential ways in which DWDM can be used are only beginning to be explored. Already, however, the technology has proven to be particularly well suited for several vital applications.

  • DWDM is ready made for long-distance telecommunications operators that use either point–to–point or ring topologies. The sudden availability of 16 new transmission channels where there used to be one dramatically improves an operator’s ability to expand capacity and simultaneously set aside backup bandwidth without installing new fiber.
  • This large amount of capacity is critical to the development of self-healing rings, which characterize today’s most sophisticated telecom networks. By deploying DWDM terminals, an operator can construct a 100% protected, 40 Gb/s ring, with 16 separate communication signals using only two fibers.
  • Operators that are building or expanding their networks will also find DWDM to be an economical way to incrementally increase capacity, rapidly provision new equipment for needed expansion, and future–proof their infrastructure against unforeseen bandwidth demands.

Wavelength Selective Couplers and Splitters


Wavelength Selective Couplers (or Splitters) are used to either combine or split light of different wavelengths with minimal loss. Light of two different wavelengths on different input fibers can be merged (combined) onto the same output fiber. In the reverse direction light of two different wavelengths on the same fiber can be split so that one wavelength goes to one output fiber and the other wavelength is output onto the other output fiber. The process can be performed with very little loss.

As the coupling length is wavelength dependent, the shifting of power between the two parallel waveguides will take place at different places along the coupler for different wavelengths. All we need to do is choose the coupling length carefully and we can arrange for loss free wavelength combining or splitting. These functions are shown in the figure below. The graph of power transfer shows how power input on one of the fibers shifts back and forth between the two waveguides. The period of the shift is different for the two different wavelengths. Thus in the left-hand section of the diagram (combining wavelengths) there will be a place down the coupler where all of the light is in only one waveguide. If we make the coupler exactly this length then the signals have been combined. On the right-hand side of the diagram the reverse process is shown where two different wavelengths arrive on the same input fiber. At a particular point down the coupler the wavelengths will be in different waveguides so if we make this the coupling length then we have separated the wavelengths exactly. In fact both the processes described above are performed in the same coupler—the process is Bi-Directional (BiDi). Thus the coupler on the left can operate in the opposite direction and become a splitter and the splitter on the right can operate in the opposite direction and become a coupler (combiner). Note that each coupler or splitter must be designed for the particular wavelengths to be used.

Wavelength Selective Coupling and Splitting

Commercial devices of this kind are commonly available and are very efficient. The quoted insertion loss is usually between 1.2 and 1.5 dB and the channel separation is quoted as better than 40 dB. “Wavelength flattened” couplers or splitters of this kind operate over quite a wide band of wavelengths. That is a given device may allow input over a range of wavelengths in the 1310 nm band up to 50 nm wide and a range of wavelengths in the 1550 nm band also up to 50 nm wide.

Power Input to an EDFA

On the left-hand side of the figure we see an example of coupling two different wavelengths into the same output fiber. At the input of an EDFA you want to mix the (low level) incoming signal light with (high level) light from the pump. Typically the signal light will be around 1550 nm and the pump will be 980 nm. In this case it is possible to choose a coupling length such that 100% of the signal light and 100% of the pump light leaves on the same fiber. A major advantage of this is that there is very little loss of signal power in this process.

Splitting Wavelengths for CWDM Systems

On the right-hand side of the figure we show an example of CWDM demultiplexing. A mixed wavelength stream with one signal in each of the 1300 and 1550 nm bands is separated into its two component wavelengths. A CWDM system like this might be used in a system for distributing CATV and advanced VOD services to people in their homes. One signal stream might be carried at 1310 nm and the other at 1550 nm. A resonant coupler is shown here operating as a splitter separating the two wavelengths. Note that an identical splitter could also be used to combine the two wavelengths with very little loss.

Adding the Management Channel in DWDM Systems

In DWDM systems where many channels are carried in the 1550 nm band there is often a requirement to carry an additional relatively slow rate channel for management purposes. A convenient way to do this is to send the management information in the 1310 nm band and the mixed DWDM stream in the 1550 band. Wavelength selective couplers are commonly used for this purpose. A management signal (a single wavelength) in the 1310 band is coupled onto a fiber carrying many wavelengths between 1540 nm and 1560 nm. Another similar device (wavelength selective splitter) is used to separate the signals at the other end of the link.

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