Wavelength division multiplexing (WDM)
Carrying several independent signals on one optical fiber, each on its own wavelength, combined by a multiplexer and separated by a demultiplexer. CWDM spaces up to 18 channels 20 nm apart; DWDM packs 40 to 96 channels 100 or 50 GHz apart in the C-band.
Wavelength division multiplexing (WDM) sends several independent signals through one optical fiber at once, each on its own wavelength. A multiplexer combines the channels at one end and a demultiplexer separates them at the other; because signals at different wavelengths pass through the fiber independently, apart from nonlinear effects at high power, a fiber carrying channels carries times the capacity of one. Two ITU-T grids fix the channel wavelengths so that lasers and filters from different makers work together. Coarse WDM (CWDM, G.694.2) places 18 channels 20 nm apart from 1271 to 1611 nm, wide enough for uncooled lasers. Dense WDM (DWDM, G.694.1) places channels on a frequency grid anchored at 193.1 THz (1552.52 nm), usually 100 GHz (0.8 nm) or 50 GHz (0.4 nm) apart, so that 40 to 96 channels fit in the C-band and share one erbium-doped fiber amplifier. CWDM vs DWDM compares the two in detail.
How a WDM link works
Each transmitter emits at its assigned grid wavelength: a temperature-controlled DFB laser or a tunable laser for DWDM, an uncooled DFB laser for CWDM. The multiplexer combines the channels onto one fiber. Along the route, amplifiers boost all the channels together, and ROADMs or fixed add-drop filters take individual channels out and put new ones in. At the far end the demultiplexer routes each wavelength to its own receiver, which detects it directly or coherently.
The channel spacing is a compromise between density and cost. Each laser has to stay inside its filter's passband through temperature changes and aging, and each filter has to reject the neighboring channels, so a closer spacing needs more stable lasers and sharper filters.
The DWDM frequency grid
G.694.1 defines the channels by frequency rather than wavelength:
where is a positive or negative integer and the fixed-grid spacing is 12.5, 25, 50 or 100 GHz, or a multiple of 100 GHz. The wavelength is in vacuum, with m/s, and a frequency step corresponds to a wavelength step : 0.80 nm per 100 GHz near 1550 nm, 0.40 nm per 50 GHz. Equal steps in frequency are slightly unequal in wavelength, which is one reason the grid is defined in frequency.
| Frequency | Wavelength (vacuum) | Common 100 GHz channel number |
|---|---|---|
| 196.10 THz | 1528.77 nm | 61 |
| 195.90 THz | 1530.33 nm | 59 |
| 193.10 THz | 1552.52 nm | 31 |
| 192.10 THz | 1560.61 nm | 21 |
| 191.70 THz | 1563.86 nm | 17 |
Transceiver makers commonly number 100 GHz channels by the frequency's digits above 190 THz, so channel 31 is 193.10 THz; the recommendation itself counts by , and 193.10 THz is . The DWDM Channel Grid Calculator lists every channel of a grid across a band, with its frequency and wavelength, and finds the channel nearest a given wavelength.
Flexible grid
Since 2012, G.694.1 also defines a flexible grid for coherent systems whose channels differ in width. Center frequencies sit at 193.1 THz + GHz, and each channel is given a slot GHz wide, chosen to fit its spectrum, which is roughly the symbol rate times one plus the pulse-shaping roll-off. A 100 Gb/s channel at 32 GBd fits a 37.5 GHz slot, and a 400 Gb/s channel near 60 GBd, such as 400ZR, fits 75 GHz.
The CWDM wavelength grid
G.694.2 places 18 channels 20 nm apart:
| Band | CWDM channels |
|---|---|
| O (1260 to 1360 nm) | 1271, 1291, 1311, 1331, 1351 nm |
| E (1360 to 1460 nm) | 1371, 1391, 1411, 1431, 1451 nm |
| S (1460 to 1530 nm) | 1471, 1491, 1511 nm |
| C (1530 to 1565 nm) | 1531, 1551 nm |
| L (1565 to 1625 nm) | 1571, 1591, 1611 nm |
The wide spacing exists for the uncooled laser. A DFB laser's wavelength drifts about 0.1 nm/°C, or about 7 nm over a 0 to 70 °C operating range, and 20 nm leaves room for that drift, the lasers' manufacturing spread and the edges of the filters. On older fiber the water absorption peak near 1383 nm adds so much loss to the nearby channels that many CWDM systems use only the eight channels from 1471 to 1611 nm; low-water-peak fiber, G.652.D among the single-mode fiber families, carries all 18. Most CWDM channels lie outside the erbium amplifier's band, so CWDM links are unamplified and limited to tens of kilometers.
Four of the O-band channels, 1271 to 1331 nm, are also the lanes of 100G CWDM4 and 400G-FR4 datacenter transceivers, where the fiber's low chromatic dispersion near 1310 nm matters more than amplification. 100GBASE-LR4 uses a narrower grid, LAN-WDM: four lanes 800 GHz apart at 1295.56, 1300.05, 1304.58 and 1309.14 nm, all close to the fiber's zero-dispersion wavelength.
WDM multiplexers and demultiplexers
The same passive device usually works in either direction, as a multiplexer or as a demultiplexer. The common types:
| Type | How it separates the wavelengths | Typical use |
|---|---|---|
| Thin-film filter module | A cascade of dielectric bandpass filters, each passing one channel and reflecting the rest to the next | CWDM; DWDM and LAN-WDM up to about 16 channels |
| Arrayed waveguide grating | A planar chip whose waveguides of stepped lengths focus each wavelength onto its own output | DWDM with 40 to 96 channels |
| Fiber Bragg grating with a circulator | Each grating reflects one narrow channel to a drop port and passes the rest | Single-channel add-drop |
| Interleaver | An unbalanced Mach-Zehnder interferometer or etalon that sends alternate channels to alternate ports | Splitting a 50 GHz comb into two 100 GHz combs |
| Wavelength-selective switch | A free-space grating and a liquid-crystal or MEMS array that steer each channel to any port | ROADMs |
| Fused fiber WDM coupler | Wavelength-dependent coupling between two fused fibers | 980/1550 nm pump combiners in EDFAs; 1310/1550 nm splitters |
A thin-film filter loses a few tenths of a dB at each stage, so the last channel of a long cascade sees much more loss than the first, while an arrayed waveguide grating's loss, a few dB, is about the same at every port. WDM mux/demux technologies compares the main types on loss, isolation and temperature drift. In silicon photonic transceivers the multiplexer sits on the chip: an arrayed waveguide grating, an echelle grating, a cascade of Mach-Zehnder filters or a row of ring resonators.
Channel count and capacity
The C-band, 1530 to 1565 nm, spans 4.38 THz and holds 44 channels of the 100 GHz grid or 87 of the 50 GHz grid; systems commonly carry 40 or 80 channels, or 96 at 50 GHz in an extended C-band. The capacity per fiber is the channel count times the rate per channel. Eighty 100 Gb/s channels at 50 GHz carry 8 Tb/s, a spectral efficiency of 2 b/s/Hz; 400 Gb/s channels in 75 GHz slots reach 5.3 b/s/Hz, and 58 of them fill the C-band with about 23 Tb/s. Adding the L-band roughly doubles the usable spectrum, and parallel fibers or multi-core fibers multiply it again.
Interaction between channels
Channels that share a fiber can disturb one another. Filters leak a little of each neighbor into a channel, specified as adjacent-channel isolation, typically 25 dB or more. At high power the fiber's nonlinearity couples the channels: four-wave mixing creates new frequencies that land on other channels of an equally spaced grid, worst on dispersion-shifted fiber, where the channels stay in phase; cross-phase modulation turns one channel's intensity changes into another channel's phase noise; and stimulated Raman scattering moves power from the shorter-wavelength channels to the longer ones, a tilt that grows with the total bandwidth and matters most in C+L systems. Over long amplified links the per-channel OSNR, set by amplifier noise, usually limits the reach.
Common questions
What is the difference between CWDM and DWDM?
CWDM spaces up to 18 channels 20 nm apart from 1271 to 1611 nm, so uncooled lasers and simple filters work, but most of its channels lie outside the erbium amplifier's band and links stay unamplified and short. DWDM spaces channels 100 GHz (0.8 nm) or closer in the C- and L-bands, which needs temperature-controlled lasers but lets 40 to 96 channels share each amplifier over metro and long-haul distances.
What is a WDM multiplexer?
A passive optical device that combines wavelengths arriving on separate fibers into one fiber; used in reverse, it is a demultiplexer that separates them. Thin-film filter modules, arrayed waveguide gratings and fiber Bragg gratings are the common types, and a fused 980/1550 nm or 1310/1550 nm coupler is the simplest two-wavelength case.
How many channels can one fiber carry?
CWDM has 18 channels. In DWDM the count is set by the usable band and the spacing: the 4.38 THz C-band holds 44 channels of the 100 GHz grid and 87 of the 50 GHz grid, and commercial systems carry 40, 80 or 96. Adding the L-band roughly doubles the count.
Is WDM the same as frequency division multiplexing?
Physically, yes: each channel is an optical carrier at its own frequency, near 193 THz for 1550 nm. Optical components have long been specified by wavelength, hence the name, but the DWDM grid itself is defined in frequency.
Can WDM carry both directions on one fiber?
Yes. Bidirectional (BiDi) transceivers transmit on one wavelength and receive on another over a single fiber, and passive optical networks work the same way: GPON sends downstream at 1490 nm and upstream at 1310 nm, and XGS-PON uses 1577 nm and 1270 nm, so the two generations can share one fiber.
References: ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM frequency grid (2020). ITU-T Recommendation G.694.2, Spectral grids for WDM applications: CWDM wavelength grid (2003). R. Ramaswami, K. N. Sivarajan and G. H. Sasaki, Optical Networks: A Practical Perspective, 3rd ed. (Morgan Kaufmann, 2010). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010).