Editing Wavelength-division multiplexing (section)

==Systems==
[[Image:WDM operating principle.svg|400px|thumb|WDM operating principle]]
[[File:Dwdm equipment.jpg|alt=|thumb|373x373px|WDM/DWDM System in rack 19/21<nowiki>''</nowiki>]]
A WDM system uses a [[multiplexer]] at the [[transmitter]] to join the several signals together and a [[demultiplexer]] at the [[Receiver (radio)|receiver]] to split them apart.<ref name=":0" /> With the right type of fiber, it is possible to have a device that does both simultaneously and can function as an [[optical add-drop multiplexer]]. The optical filtering devices used have conventionally been [[etalon]]s (stable solid-state single-frequency [[Fabry–Pérot interferometer]]s in the form of thin-film-coated optical glass). As there are three different WDM types, whereof one is called ''WDM'', the notation ''xWDM'' is normally used when discussing the technology as such.<ref name=":1">{{Cite journal|last1=Li|first1=Hongqin|last2=Zhong|first2=Zhicheng|year=2019|title=Analysis and Simulation of Morphology Algorithm for Fiber Optic Hydrophone Array in Marine Seismic Exploration|jstor=26853921|journal=Journal of Coastal Research|volume=94|pages=145–148|doi=10.2112/SI94-029.1 |s2cid=202549795 }}</ref>

The concept was first published in 1970 by Delange,<ref>O. E. Delange, "Wideband optical communication systems, Part 11-Frequency division multiplexing". hoc. IEEE, vol. 58, p.&nbsp;1683, October 1970.</ref> and by 1980 WDM systems were being realized in the laboratory. The first WDM systems combined only two signals. Modern systems can handle 160 signals and can thus expand a basic {{val|100|ul=Gbit/s}} system over a single fiber pair to over {{val|16|ul=Tbit/s}}. A system of 320 channels is also present (12.5&nbsp;GHz channel spacing, see below.)

WDM systems are popular with [[telecommunications companies]] because they allow them to expand the capacity of the network without laying more fiber. By using WDM and [[optical amplifier]]s, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. The capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

This is often done by the use of optical-to-electrical-to-optical (O/E/O) translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.<ref name=":1" />

Most WDM systems operate on [[single-mode optical fiber]] cables which have a core diameter of 9&nbsp;μm. Certain forms of WDM can also be used in [[multi-mode optical fiber]] cables (also known as premises cables) which have core diameters of 50 or 62.5&nbsp;μm.

Early WDM systems were expensive and complicated to run. However, recent standardization and a better understanding of the dynamics of WDM systems have made WDM less expensive 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.

WDM systems are divided into three different wavelength patterns: '''normal''' (WDM), '''coarse''' (CWDM) and '''dense''' (DWDM). Normal WDM (sometimes called BWDM) uses the two normal wavelengths 1310 and 1550&nbsp;nm on one fiber. Coarse WDM provides up to 16 channels across multiple [[Fiber-optic communication#Transmission windows|transmission windows]] of silica fibers. ''Dense WDM'' (DWDM) uses the C-Band (1530&nbsp;nm-1565&nbsp;nm) transmission window but with denser channel spacing. Channel plans vary, but a typical DWDM system would use 40 channels at 100&nbsp;GHz spacing or 80 channels with 50&nbsp;GHz spacing. Some technologies are capable of 12.5&nbsp;GHz spacing (sometimes called ultra-dense WDM). New amplification options ([[Raman amplification]]) enable the extension of the usable wavelengths to the L-band (1565–1625&nbsp;nm), more or less doubling these numbers.

Coarse wavelength-division multiplexing (CWDM), in contrast to DWDM, uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To provide 16 channels on a single fiber, CWDM uses the entire frequency band spanning the second and third transmission windows (1310/1550&nbsp;nm respectively) including the critical frequencies where OH scattering may occur. OH-free silica fibers are recommended if the wavelengths between the second and third transmission windows are to be used{{Citation needed|date=February 2019}}. Avoiding this region, the channels 47, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used. With OS2 fibers the water peak problem is overcome, and all possible 18 channels can be used.

WDM, CWDM and DWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. [[EDFA]] provide an efficient wideband amplification for the [[C band (infrared)|C-band]], Raman amplification adds a mechanism for amplification in the L-band. For CWDM, wideband optical amplification is not available, limiting the optical spans to several tens of kilometers.