Optics Telecom

As the most popular WDM technology, DWDM (Dense wavelength-division multiplexing) 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.

A typical DWDM system is composed of transmitter, receiver, optical amplifier, transponder, DWDM multiplexer and DWDM demultiplexer. They allow DWDM system to interface with other equipment and to implement optical solutions throughout the network, complying with the ITU channel standards.

• Optical transmitters/receivers
  Transmitters in DWDM systems provide the source signals which are then multiplexed. Multiple optical transmitters are used as the light sources in a DWDM system. We can also utilize a transceiver to replace transmitters and receivers to achieve the same purpose. Usually a DWDM transceiver applied in DWDM network can reach a transmission distance of up to 120km.
• 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.
• Transponders
  Transponders are designed to convert optical signals from one incoming wavelength to another outgoing wavelength suitable for DWDM applications. Transponders are optical-electricaloptical (O-E-O) wavelength converters. Within a 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.
  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 DWDM demultiplexers are capable of distinguishing each wavelength without crosstalk. 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.
  

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.

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 metropolitan area, 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 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.