# Wavelength-division multiplexing
**Wavelength-division multiplexing** (WDM) is a fiber-optic transmission technique in which several independently modulated optical carrier signals, each on a distinct wavelength of light, are combined onto a single optical fiber and separated again at the receiving end. It is essentially frequency-division multiplexing applied to the optical domain, with different colors of light serving as independent carriers[1](https://technav.ieee.org/topic/wdm-networks/). A network operator facing growing demand can either add more fiber or use WDM; by carrying multiple service streams on the same fiber, WDM avoids the cost of constructing or leasing new fiber paths[2](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html).
| Key fact | Detail |
|---|---|
| Principle | Multiple laser transmitters at different wavelengths modulate their own data streams; a passive optical multiplexer combines them onto one fiber, and filters demultiplex them at the far end[1](https://technav.ieee.org/topic/wdm-networks/) • [2](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html) |
| DWDM grid | ITU-T G.694.1 defines a frequency grid anchored to 193.1 THz with fixed channel spacings of 12.5, 25, 50, and 100 GHz, plus integer multiples of 100 GHz[3](https://www.itu.int/dms_pubrec/itu-t/rec/g/T-REC-G.694.1-202010-I!!SUM-HTM-E.htm) • [4](https://technav.ieee.org/topic/channel-spacing/) |
| CWDM grid | ITU-T G.694.2 (12/2003) defines a wavelength grid with 20 nm spacing and nominal central wavelengths from 1271 nm to 1611 nm, giving 18 channels[5](https://www.itu.int/rec/T-REC-G.694.2-200312-I/en) • [6](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items) |
| Flexible grid | G.694.1 edition 3.0 (10/2020) adds a flexible DWDM grid built from "frequency slots"; slot width granularity is 12.5 GHz and nominal central frequency granularity is 6.25 GHz[3](https://www.itu.int/dms_pubrec/itu-t/rec/g/T-REC-G.694.1-202010-I!!SUM-HTM-E.htm) • [7](https://www.itu.int/epublications/publication/itu-t-g-876-2021-amd-1-2024-01) |
| Transmitter contrast | DWDM transmitters generally need a control mechanism to meet frequency stability requirements; CWDM transmitters are generally uncontrolled, using uncooled lasers with total wavelength variation of about ±6–7 nm[8](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.1-202010-I!!PDF-E&lang=f&type=items) • [6](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items) |
| Main impairments | Four-wave mixing is the most severe nonlinear penalty at high input power, low chromatic dispersion, or low channel spacing; crosstalk arises mainly at OADM and OXC sites using multiplexer/demultiplexer pairs[9](https://opg.optica.org/ao/abstract.cfm?uri=ao-43-26-5023) • [10](https://www.rfc-editor.org/rfc/rfc4054.html) |
## How it works
Each wavelength of light is a distinct "color" that can be combined for transmission and separated at the receiver end using optical filters; wavelength is usually denoted by the Greek letter lambda (λ), with subscripts distinguishing channels, as in λ₁[11](https://booksite.elsevier.com/samplechapters/9780240804866/9780240804866.PDF). In a basic system, one transmitter per wavelength feeds a WDM device that mixes the signals onto the common fiber; at the other end a matching device separates them, optionally through additional filters, to individual receivers[11](https://booksite.elsevier.com/samplechapters/9780240804866/9780240804866.PDF). Transceivers using narrow-band wavelengths are called "colored" optics, in contrast to the "grey" optics of single-wavelength links[2](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html).
**Filtering technologies.** Multiplexers and demultiplexers can be built with thin-film filters or arrayed waveguide gratings (AWG); the technologies differ in construction but perform the same function of combining and separating wavelengths on a common fiber[2](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html).
## CWDM and DWDM
The two standard flavors of WDM differ in channel density and cost. **Coarse WDM** (CWDM), defined in ITU-T G.694.2, uses 20 nm channel spacing across a broad wavelength range and targets metropolitan and access networks where cost is prioritized over capacity density[4](https://technav.ieee.org/topic/channel-spacing/). The 2003 edition lists 18 nominal central wavelengths from 1271 nm to 1611 nm in 20 nm steps[6](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items). The 20 nm spacing was chosen to maximize the number of channels while remaining compatible with uncooled lasers and wide passband filters: total source wavelength variation of the order of ±6–7 nm is expected to be compatible with current filter technology, and a guardband of one third of the minimum channel spacing is considered sufficient[6](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items). The wavelength variation comes mainly from manufacturing tolerance allowed to the laser maker and from temperature-driven drift in uncooled lasers[6](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items).
**Dense WDM** (DWDM), defined in [ITU-T G.694.1](https://www.edgepedia.org/itu-t-g-694-1), is characterized by narrower channel spacing than CWDM[8](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.1-202010-I!!PDF-E&lang=f&type=items). The fixed grid is anchored to 193.1 THz, and allowed channel frequencies follow the formula 193.1 + n × spacing (in THz), where n is a positive or negative integer including 0; the spacing steps of 12.5, 25, 50, and 100 GHz historically evolved by subdividing the initial 100 GHz grid by successive factors of two[8](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.1-202010-I!!PDF-E&lang=f&type=items). The 50 GHz and 100 GHz spacings have been the most widely deployed in long-haul networks[4](https://technav.ieee.org/topic/channel-spacing/). Because the spacing is far tighter than CWDM's, DWDM transmitters require frequency control, typically temperature-controlled DFB lasers, whereas CWDM operates uncontrolled[8](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.1-202010-I!!PDF-E&lang=f&type=items) • [12](https://www.itu.int/dms_pub/itu-t/oth/06/13/t06130000300003pdfe.pdf).
## History and enabling technology
As [Internet traffic](https://www.edgepedia.org/internet-traffic) grew, WDM systems evolved into dense systems carrying a multitude of wavelengths. [Erbium-doped fiber](https://www.edgepedia.org/erbium-doped-fiber) amplifiers (EDFAs) made DWDM practical, because an EDFA boosts the multitude of wavelengths on the fiber simultaneously, making the systems economical; early systems carried four to eight wavelengths and later sixteen[11](https://booksite.elsevier.com/samplechapters/9780240804866/9780240804866.PDF). WDM networks emerged as the dominant architecture for long-haul and metropolitan telecommunications infrastructure in the 1990s because they let operators multiply the capacity of installed fiber without laying additional cable[1](https://technav.ieee.org/topic/wdm-networks/).
## Flexible grid
In conventional DWDM with a fixed grid, every channel receives a set center frequency and slots of identical width, meaning all transponders have to operate at wavelengths drawn from one common, permitted list.[4](https://technav.ieee.org/topic/channel-spacing/) To allow efficient allocation of optical spectral bandwidth for high-bit-rate systems, the ITU-T extended G.694.1 to define a set of nominal central frequencies, channel spacings, and the concept of the "frequency slot", in which a connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum[13](https://datatracker.ietf.org/doc/rfc7698/). In the flexible grid, a channel occupies contiguous frequency slots whose width is an integer multiple of the 12.5 GHz slot width granularity, with central frequencies on a 6.25 GHz granularity[4](https://technav.ieee.org/topic/channel-spacing/) • [7](https://www.itu.int/epublications/publication/itu-t-g-876-2021-amd-1-2024-01). With this superchannel method, coherent signals running at 400 Gb/s or 800 Gb/s—formats that use higher-order modulation—can fill broader slots while avoiding wasted guard bandwidth.[4](https://technav.ieee.org/topic/channel-spacing/) The IETF has defined GMPLS control-plane frameworks and RSVP-TE signaling extensions for such flexi-grid DWDM networks[13](https://datatracker.ietf.org/doc/rfc7698/) • [14](https://datatracker.ietf.org/doc/rfc7792/).
## Impairments and design limits
WDM capacity is bounded by physical effects that grow with channel count and power. **Four-wave mixing** (FWM) is a nonlinear effect in which waves at different frequencies interact, producing inter-channel crosstalk; its efficiency depends strongly on the phase mismatch between the interacting waves, so it plays its largest role in dispersion-shifted fibers and for densely spaced channels[15](https://opg.optica.org/abstract.cfm?uri=NLGWP-1991-ME7). For high input power, a low chromatic dispersion coefficient, or low channel spacing, the most severe penalties in a WDM network are due to FWM[9](https://opg.optica.org/ao/abstract.cfm?uri=ao-43-26-5023). The nonlinear refractive index of silica also produces self-phase and cross-phase modulation, which broaden spectra and mix signals in wavelength-multiplexed systems[16](https://opg.optica.org/abstract.cfm?uri=CLEO-1994-CThA1).
**Crosstalk and filtering.** Optical crosstalk, the effect of other signals on the desired signal, includes coherent (intrachannel) and incoherent (interchannel) forms; the main contributors are OADM and OXC sites that use a DWDM multiplexer/demultiplexer pair. In sparse networks crosstalk can be handled with a low margin in the required optical signal-to-noise ratio, but in dense networks it can become a binding constraint, with end-to-end path crosstalk kept within limits such as a −25 dB threshold with 1 dB penalty[10](https://www.rfc-editor.org/rfc/rfc4054.html). As more DWDM components are cascaded, the effective passband of the link narrows, which constrains the maximum number of narrow filters and the bit rate a path can carry[10](https://www.rfc-editor.org/rfc/rfc4054.html).
## Applications
Both wireless and wireline networks use WDM, from long-haul to metro and local access, including fiber to the home, mobile backhaul, and fronthaul[2](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html). Each optical wavelength may carry a different client payload at a different data rate and in a different format, for example a mix of SONET signals at OC-48c (about 2.5 Gbps) and OC-192 (about 10 Gbps) over a single fiber[17](https://www.ietf.org/ietf-ftp/rfc/rfc3717.txt.pdf). In access and enterprise settings, CWDM's uncooled lasers and wide filters keep cost low, while DWDM's dense, controlled grid serves ultra-long-distance and high-capacity routes[4](https://technav.ieee.org/topic/channel-spacing/) • [18](https://img-en.fs.com/file/catalog/otn-solutions.pdf). WDM is also used to co-propagate weak quantum signals alongside strong classical data traffic on the same fiber, with crosstalk depending on the classical signal's launch power and the wavelength separation between the two signals[19](https://arxiv.org/abs/2411.16942).
## References
1. [WDM networks, IEEE Technology Navigator](https://technav.ieee.org/topic/wdm-networks/)
2. [WDM 101, Corning Optical Communications](https://www.corning.com/optical-communications/emea/en/home/products/wavelength-division-multiplexers/wdm-101.html)
3. [ITU-T Rec. G.694.1 (10/2020) summary: Spectral grids for WDM applications: DWDM frequency grid](https://www.itu.int/dms_pubrec/itu-t/rec/g/T-REC-G.694.1-202010-I!!SUM-HTM-E.htm)
4. [Channel spacing, IEEE Technology Navigator](https://technav.ieee.org/topic/channel-spacing/)
5. [ITU-T Rec. G.694.2 (12/2003): Spectral grids for WDM applications: CWDM wavelength grid](https://www.itu.int/rec/T-REC-G.694.2-200312-I/en)
6. [ITU-T Rec. G.694.2 (12/2003), PDF](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.2-200312-I!!PDF-E&lang=f&type=items)
7. [Recommendation ITU-T G.876 (2021) Amd. 1 (01/2024)](https://www.itu.int/epublications/publication/itu-t-g-876-2021-amd-1-2024-01)
8. [ITU-T Rec. G.694.1 (10/2020), PDF](https://www.itu.int/rec/dologin_pub.asp?id=T-REC-G.694.1-202010-I!!PDF-E&lang=f&type=items)
9. [Neokosmidis et al., "Evaluation by Monte Carlo simulations of the power limits and bit-error rate degradation in wavelength-division multiplexing networks caused by four-wave mixing," Appl. Opt. 43, 5023 (2004)](https://opg.optica.org/ao/abstract.cfm?uri=ao-43-26-5023)
10. [RFC 4054: Impairments and Other Constraints on Optical Layer Routing](https://www.rfc-editor.org/rfc/rfc4054.html)
11. [Fiber Optic Fundamentals, Chapter 2 (Elsevier/Focal Press, 2002)](https://booksite.elsevier.com/samplechapters/9780240804866/9780240804866.PDF)
12. [Alcatel-Lucent, WDM in PON, ITU-T/IEEE Workshop (2008)](https://www.itu.int/dms_pub/itu-t/oth/06/13/t06130000300003pdfe.pdf)
13. [RFC 7698: Framework and Requirements for GMPLS-Based Control of Flexi-Grid DWDM Networks](https://datatracker.ietf.org/doc/rfc7698/)
14. [RFC 7792: RSVP-TE Signaling Extensions in Support of Flexi-Grid DWDM Networks](https://datatracker.ietf.org/doc/rfc7792/)
15. [Nilsson, Dado, Jaskorzynska, "Four-Wave Mixing Limitation In Multichannel Coherent Optical Communication," NLGWP 1991](https://opg.optica.org/abstract.cfm?uri=NLGWP-1991-ME7)
16. [Tkach and Chraplyvy, "Systems impact of fiber nonlinearities," CLEO 1994](https://opg.optica.org/abstract.cfm?uri=CLEO-1994-CThA1)
17. [RFC 3717: IP over Optical Networks: A Framework](https://www.ietf.org/ietf-ftp/rfc/rfc3717.txt.pdf)
18. [FS, OTN Solutions catalog](https://img-en.fs.com/file/catalog/otn-solutions.pdf)
19. [Robustness of WDM technique for the co-propagation of quantum with classical signals in an optical fiber, arXiv:2411.16942](https://arxiv.org/abs/2411.16942)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Waves and optics › Optical technologies and instruments › Fiber optics*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license