Editing Wavelength-division multiplexing (section)

==Coarse WDM==
[[File:SFP WDM 2.jpg|thumb|Series of SFP+ transceivers for {{nowrap|10 Gbit/s}} WDM communications]]
Originally, the term ''coarse wavelength-division multiplexing'' (CWDM) was fairly generic and described a number of different channel configurations. In general, the choice of channel spacings and frequency in these configurations precluded the use of [[erbium doped fiber amplifier]]s (EDFAs). Prior to the relatively recent ITU standardization of the term, one common definition for CWDM was two or more signals multiplexed onto a single fiber, with one signal in the 1550&nbsp;nm band and the other in the 1310&nbsp;nm band.

In 2002, the ITU standardized a channel spacing grid for CWDM (ITU-T G.694.2) using the wavelengths from 1270&nbsp;nm through 1610&nbsp;nm with a channel spacing of 20&nbsp;nm. ITU G.694.2 was revised in 2003 to shift the channel centers by 1&nbsp;nm so, strictly speaking, the center wavelengths are 1271 to 1611&nbsp;nm.<ref>{{cite web|title=ITU-T G.694.2, WDM applications: CWDM wavelength grid|url=http://www.itu.int/rec/T-REC-G.694.2/en|archive-url=https://web.archive.org/web/20121110070837/http://www.itu.int/rec/T-REC-G.694.2/en |archive-date=2012-11-10}}</ref> Many CWDM wavelengths below 1470&nbsp;nm are considered unusable on older [[G.652]] specification fibers, due to the increased attenuation in the 1270–1470&nbsp;nm bands. Newer fibers which conform to the G.652.C and G.652.D<ref>{{cite web|title=ITU-T G.652, Transmission media and optical systems characteristics – Optical fibre cables|url=http://www.itu.int/rec/T-REC-G.652/en|archive-url=https://web.archive.org/web/20121110111349/http://www.itu.int/rec/T-REC-G.652/en |archive-date=2012-11-10 }}</ref> standards, such as Corning SMF-28e and Samsung Widepass, nearly eliminate the water-related attenuation peak at 1383&nbsp;nm and allow for full operation of all 18 ITU CWDM channels in metropolitan networks.

The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This limits the total CWDM optical span to somewhere near 60&nbsp;km for a {{nowrap|2.5 Gbit/s}} signal, suitable for use in metropolitan applications. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.

===CWDM Applications===
CWDM is being 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&nbsp;nm while the upstream signal is at 1550&nbsp;nm.{{Citation needed|date=February 2019}}

The [[10GBASE-LX4]] {{nowrap|10 Gbit/s}} [[physical layer]] standard is an example of a CWDM system in which four wavelengths near 1310&nbsp;nm, each carrying a {{val|3.125|u=Gbit/s}} data stream, are used to carry {{nowrap|10 Gbit/s}} of aggregate data.<ref name=":2">{{Cite journal|last=Hornes|first=Rudy. L|year=2008|title=The Suppression of Four-Wave Mixing by Random Dispersion|jstor=40233639|journal=SIAM Journal on Applied Mathematics|volume=69|issue=3 |pages=690–703|doi=10.1137/070680539 }}</ref>

Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as bandpass filters and prisms. Many manufacturers are promoting passive CWDM to deploy fiber to the home.{{Citation needed|date=February 2019}}