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Encyclopedia > Wavelength division multiplexing

In telecommunications wavelength division multiplexing (WDM) is a technology which multiplexes several optical carrier signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals. This allows for a n-fold increase in capacity, in addition to making it possible to perform bidirectional communications over one strand of fibre. Telecommunication is the extension of communication over a distance. ... Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ... The wavelength is the distance between repeating units of a wave pattern. ... A laser (Light Amplification by Stimulated Emission of Radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light from a lasing medium of controlled purity, size, and shape. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... The term bidirectional can refer to: Bidirectional script support Duplex This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...


Note that this term applies to an optical carrier (which is typically described by its wavelength), whereas frequency division multiplexing typically applies to a radio carrier (which is more often described by frequency). However, since wavelength and frequency are inversely proportional, and since radio and light are both forms of electromagnetic radiation, the distinction is somewhat arbitrary. Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. ...


The device that joins the signals together is known as a multiplexer, and the one that splits them apart is a demultiplexer. With the right type of fibre you can have a device that does both at once, and can function as an optical add-drop multiplexer. The optical filtering devices used in the modems are usually etalons, stable solid-state single-frequency Fabry-Perot interferometers. The term multiplexer has uses in several fields of application: Electronics In electronics, a multiplexer or mux is a device that combines several electrical signals into a single signal. ... A main element of optical fiber networks. ... In optics, a Fabry-Perot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly-reflecting mirrors. ...


The first WDM systems combined two signals and appeared around 1985. Modern systems can handle up to 160 signals and can expand a basic 10 Gbit/s fibre system to a theoretical total capacity of over 1.6 Tbit/s over a single fiber pair.


WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fibre. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. All they have to do is to upgrade the multiplexers and demultiplexers at each end. TATA Engineering and Locomotive Company A telephone company (or telco) provides telecommunications services such as telephony and data communications. ... In fiber optics, an optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal, then amplify it electrically, and finally reconvert it to an optical signal. ...


This is often done by using optical-to-electrical-to-optical translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.


Dense and coarse WDM

WDM systems primarily operate on single mode fibre optical cables, which have a core diameter of 9 µm. Certain forms of WDM can also be used in multi-mode fibre cables (also known as premises cables) which have core diameters of 50 or 62.5 µm.


Early WDM systems were expensive and complicated to run. However, recent standardization and better understanding of the dynamics of WDM systems have made WDM much cheaper to deploy. The market has segmented into two parts, "dense" and "coarse" WDM.


Dense WDM (DWDM) is generally held to be WDM with more than 8 active wavelengths per fibre, with systems with fewer active wavelengths being classed as coarse WDM (CWDM).


CWDM technology is based on the same WDM concept as DWDM technology. The two technologies differ primarily in the spacing of the wavelengths, number of channels, and the ability to amplify signals in the optical space.


As of 2003, CWDM devices have dropped in price to the point where they are similar in price to end-user equipment such as Ethernet switches. 2003 is a common year starting on Wednesday of the Gregorian calendar. ... Ethernet (this name comes from the physical concept of ether) is a frame-based computer networking technology for local area networks (LANs). ...


The introduction of the ITU-T G.694.1 frequency grid in 2002 has made it easier to integrate WDM with older but more standard SONET systems. (I don't have the details to hand, but I believe it specifies a 200 GHz frequency grid, with 100 GHz channel spacing as a refinement). Today's DWDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation. The introduction of this article does not provide enough context for readers unfamiliar with the subject. ... Synchronous Optical Networking, commonly known as SONET, is a standard for communicating digital information over optical fiber. ...


Recently the ITU has standardized a 20 nanometre channel spacing grid for use with CWDM, using the wavelengths between 1310 nm and 1610 nm. Many CWDM wavelengths below 1470 nm are considered "unusable" on older G.652 spec fibres, due to the increased attenuation in the 1310-1470 nm bands. Newer fibres which conform to the G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Widepass nearly eliminate the "water peak" attenuation peak and allow for full operation of all twenty ITU CWDM channels in metropolitan networks. For more information on G.652.C and .D compliant fibres please see:

  • Google Search: G.652.C (http://www.google.com/search?num=100&hl=en&q=G.652.C)
  • Google Search: G.652.D (http://www.google.com/search?num=100&hl=en&q=G.652.D)

DWDM systems are significantly more expensive than CWDM because the laser transmitters need to be significantly more stable than those needed for CWDM. Precision temperature control of transmitter lasers is required in DWDM systems to prevent "drift" off a very narrow centre wavelength. In addition, DWDM tends to be used at a higher level in the communications hierarchy, and is therefore associated with higher modulation rates, thus creating a smaller market for DWDM devices with very high performance levels, and corresponding high prices.


Optical receivers, on the other hand, tend to be wideband devices, with the wavelength selectivity at the receive end provided as part of the optical demultiplexer.


See also


 
 

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