Editing Wavelength-division multiplexing (section)

===DWDM systems===
At this stage, a basic DWDM system contains several main components:

[[File:WDM modules 3.jpg|thumb|WDM multiplexer for DWDM communications]]
# A DWDM '''terminal multiplexer'''. The terminal multiplexer contains a wavelength-converting transponder for each data signal, an optical multiplexer and where necessary an optical amplifier (EDFA). Each wavelength-converting transponder receives an optical data signal from the client layer, such as SONET/SDH or another type of data signal, converts this signal into the electrical domain, and re-transmits the signal at a specific wavelength using a 1,550&nbsp;nm band laser. These data signals are then combined into a multi-wavelength optical signal using an optical multiplexer, for transmission over a single fiber (e.g., SMF-28 fiber). The terminal multiplexer may or may not also include a local transmit EDFA for power amplification of the multi-wavelength optical signal. In the mid-1990s DWDM systems contained 4 or 8 wavelength-converting transponders; by 2000 or so, commercial systems capable of carrying 128 signals were available. 
# An '''intermediate line repeater''' is placed approximately every 80–100&nbsp;km to compensate for the loss of optical power as the signal travels along the fiber. The 'multi-wavelength optical signal' is amplified by an EDFA, which usually consists of several amplifier stages.
# An '''intermediate optical terminal''', or '''optical add-drop multiplexer (OADM)'''. This is a remote amplification site that amplifies the multi-wavelength signal that may have traversed up to 140&nbsp;km or more before reaching the remote site. Optical diagnostics and telemetry are often extracted or inserted at such a site, to allow for localization of any fiber breaks or signal impairments. In more sophisticated systems (which are no longer point-to-point), several signals out of the multi-wavelength optical signal may be removed and dropped locally.
# A DWDM '''terminal demultiplexer'''. At the remote site, the terminal de-multiplexer consisting of an optical de-multiplexer and one or more wavelength-converting transponders separates the multi-wavelength optical signal back into individual data signals and outputs them on separate fibers for client-layer systems (such as SONET/SDH). Originally, this de-multiplexing was performed entirely passively, except for some telemetry, as most SONET systems can receive 1,550&nbsp;nm signals. However, in order to allow for transmission to remote client-layer systems (and to allow for digital domain signal integrity determination) such de-multiplexed signals are usually sent to O/E/O output transponders prior to being relayed to their client-layer systems. Often, the functionality of output transponder has been integrated into that of input transponder, so that most commercial systems have transponders that support bi-directional interfaces on both their 1,550&nbsp;nm (i.e., internal) side, and external (i.e., client-facing) side. Transponders in some systems supporting 40&nbsp;GHz nominal operation may also perform [[forward error correction]] (FEC) via [[Optical Transport Network|digital wrapper]] technology, as described in the [[ITU-T]] [[G.709]] standard.
# '''Optical Supervisory Channel (OSC)'''. This is data channel that uses an additional wavelength usually outside the EDFA amplification band (at 1,510&nbsp;nm, 1,620&nbsp;nm, 1,310&nbsp;nm or another proprietary wavelength). The OSC carries information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or EDFA site. It is also normally used for remote software upgrades and user (i.e., network operator) Network Management information. It is the multi-wavelength analog to SONET's DCC (or supervisory channel). ITU standards suggest that the OSC should utilize an OC-3 signal structure, though some vendors have opted to use [[Fast Ethernet]] or another signal format. Unlike the 1550&nbsp;nm multi-wavelength signal containing client data, the OSC is always terminated at intermediate amplifier sites, where it receives local information before re-transmission.

The introduction of the ITU-T G.694.1<ref>{{cite web|title=ITU-T G.694.1, Spectral grids for WDM applications: DWDM frequency grid|url=http://www.itu.int/rec/T-REC-G.694.1/en|archive-url=https://web.archive.org/web/20121110070855/http://www.itu.int/rec/T-REC-G.694.1/en |archive-date=2012-11-10 }}</ref> [[frequency grid]] in 2002 has made it easier to integrate WDM with older but more standard SONET/SDH systems. WDM wavelengths are positioned in a grid having exactly 100&nbsp;GHz (about 0.8&nbsp;nm) spacing in optical frequency, with a reference frequency fixed at 193.10&nbsp;THz (1,552.52&nbsp;nm).<ref>DWDM ITU Table, 100&nbsp;Ghz spacing" [http://www.telecomengineering.com/downloads/DWDM%20ITU%20Table%20-%20100%20GHz.pdf telecomengineering.com] {{webarchive|url=https://web.archive.org/web/20080704124655/http://www.telecomengineering.com/downloads/DWDM%20ITU%20Table%20-%20100%20GHz.pdf |date=2008-07-04 }}</ref> The main grid is placed inside the optical fiber amplifier bandwidth, but can be extended to wider bandwidths. The first commercial deployment of DWDM was made by Ciena Corporation on the Sprint network in June 1996.<ref>{{cite news|last=Markoff|first=John|title=Fiber-Optic Technology Draws Record Stock Value|newspaper=The New York Times|date=March 3, 1997|url=https://www.nytimes.com/1997/03/03/business/fiber-optic-technology-draws-record-stock-value.html}}</ref><ref>{{cite journal|last=Hecht|first=Jeff|title=Boom, Bubble, Bust: The Fiber Optic Mania|journal=Optics and Photonics News|publisher=The Optical Society|page=47|date=October 2016|url=https://internethistory.org/wp-content/uploads/2020/01/OSA_Boom.Bubble.Bust_Fiber.Optic_.Mania_.pdf}}</ref><ref>{{cite web|title=New Technology Allows 1,600% Capacity Boost on Sprint's Fiber-Optic Network; Ciena Corp. System Installed; Greatly Increases Bandwidth|website=Sprint|date=June 12, 1996|url=https://www.thefreelibrary.com/NEW+TECHNOLOGY+ALLOWS+1,600+PERCENT+CAPACITY+BOOST+ON+SPRINT'S...-a018380396}}</ref> Today's DWDM systems use 50&nbsp;GHz or even 25&nbsp;GHz channel spacing for up to 160 channel operation.{{update-inline|date=January 2023}}<ref>{{cite web |url=http://www.infinera.com/products/ils2.html |title=Infinera Corporation &#124; Products &#124; Infinera Line System 1 |access-date=2012-03-19 |url-status=dead |archive-url=https://web.archive.org/web/20120327022231/http://www.infinera.com/products/ILS2.html |archive-date=2012-03-27 }}</ref>

DWDM systems have to maintain more stable wavelength or frequency than those needed for CWDM because of the closer spacing of the wavelengths. Precision temperature control of the laser transmitter is required in DWDM systems to prevent drift off a very narrow frequency window of the order of a few GHz. In addition, since DWDM provides greater maximum capacity it tends to be used at a higher level in the communications hierarchy than CWDM, 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. These factors of smaller volume and higher performance result in DWDM systems typically being more expensive than CWDM.

Recent innovations in DWDM transport systems include pluggable and software-tunable transceiver modules capable of operating on 40 or 80 channels. This dramatically reduces the need for discrete spare pluggable modules, when a handful of pluggable devices can handle the full range of wavelengths.