Wavelength-division multiplexing

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In fibre optic telecommunications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to making it possible to perform bidirectional communications over one strand of fibre.

The term wavelength-division multiplexing is commonly applied 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 two terms are closely analogous.

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WDM systems

A WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. 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 concept was first published in 1970, and by 1978 WDM systems were being realized in the labratory. The first WDM systems only combined two signals. Modern systems can handle up to 160 signals and can expand a basic 10 Gbit/s fiber system to a theoretical total capacity of over 1.6 Tbit/s over a single fibre 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. Capacity of a given link can be expanded by simply upgrading the multiplexers and demultiplexers at each end.

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.

Most WDM systems 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.

Optical receivers, in contrast to laser sources, tend to be wideband devices. Therefore the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system.

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Coarse WDM

WDM systems are divided into two market segments, dense and coarse WDM. Systems with more than 8 active wavelengths per fibre are generally considered Dense WDM (DWDM) systems, while those with fewer than eight active wavelengths are classed as coarse WDM (CWDM).

CWDM and DWDM technology are based on the same concept of using multiple wavelengths of light on a single fibre, but the two technologies differ in the spacing of the wavelengths, number of channels, and the ability to amplify signals in the optical space.

The Ethernet LX-4 physical layer standard is an example of a CWDM system in which four wavelengths near 1310 nm, each carrying a 3.125 gigabit-per-second data stream, are used to carry 10 gigabits per second of aggregate data.

CWDM is also been used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.

An interesting and relatively recent development relating Coarse WDM is the creation of Small Form Factor Pluggable (SFP) transceivers utilizing standardized CWDM wavelengths. SFP Optics allow for something very close to a seamless upgrade in even legacy systems that support SFP interfaces. Thus, a legacy ethernet switch can be easily "converted" into a multiwavelength switch simply by judicious choice of transceiver wavelengths, combined with an inexpensive passive optical multiplexing device. This is in contrast to Dense WDM systems which, though Optically Amplifiable and far more efficient (in terms of bandwidth) are orders of magnitude more expensive. Thus it would seem that CWDM would be poised to find a solid market share in metropolitan systems as well as high-end enterprise.

Coarse WDM (CWDM) Coarse Wave Division Multiplexing (CWDM) combines up to 16 wavelengths onto a single fiber. CWDM technology uses an ITU standard 20nm spacing between the wavelengths, from 1310nm to 1610nm. With CWDM technology, since the wavelengths are relatively far apart (compared to DWDM), the transponders are generally not very expensive.

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Dense WDM

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. WDM wavelengths are positioned in a grid having exactly 100 GHz (about 0.8nm) spacing in optical frequency, with a reference frequency fixed at 193.10 THz (1552.52nm). The main grid is placed inside the optical fiber amplifier bandwidth, but can be extended to wider bandwidths. Today's DWDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation.

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 the links at the bottom of the article:

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 laser transmitter 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, for example on the Internet backbone 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. In other words, they are needed only in small numbers and it is therefore not possible to amortize their development cost amongst a large number of transmitters.

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See also

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References

The original version of this article was based on FOLDOC, with permission
  1. Tomlinson, W. J.; Lin, C., "Optical wavelength-division multiplexer for the 1-1.4-micron spectral region", Electronics Letters, vol. 14, May 25, 1978, p. 345-347. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1978ElL....14..345T&db_key=PHY&data_type=HTML&format=
  2. Ishio, H. Minowa, J. Nosu, K., "Review and status of wavelength-division-multiplexing technology and its application", Journal of Lightwave Technology, Volume: 2, Issue: 4, Aug 1984, p.448- 463
  3. First discusion: O. E. Delange, ‘Wideband optical communication systems, Part 11-Frequency division multiplexing,” hoc. IEEE, vol. 58, p. 1683, Oct. 1970.
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External links

es:DWDM fr:Multiplexage en longueur d'onde nl:Golflengtemultiplexing

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