Wavelength division multiplexing (WDM)

WDM multiplexes multiple optical channels onto one fiber by separating them in wavelength. At the transmitter, a wavelength multiplexer (typically an array waveguide grating or thin-film filter cascade) combines the channels into a single guided beam. At the receiver, a matching demultiplexer separates them back into individual channels for direct detection.

Two industry-standard wavelength grids:

Coarse WDM (CWDM) — ITU-T G.694.2:

• 20 nm channel spacing
• Up to 18 channels spanning 1271 – 1611 nm
• Wide channels accommodate uncooled DFB lasers (no temperature stabilization required)
• Low-cost, used for short-reach metro and enterprise links

Dense WDM (DWDM) — ITU-T G.694.1:

• Frequency-anchored grid at 50 GHz, 100 GHz, or 200 GHz spacing
• 50 GHz $\approx 0.4$ nm at 1550 nm; 100 GHz $\approx 0.8$ nm
• Reference frequency $f_{0} = 193.1$ THz (corresponds to $\sim 1552.52$ nm)
• $N$th channel: $f_N = f_0 + N \cdot \Delta f$
• Channels at fixed grid positions to enable interoperability and standardized filters

Typical DWDM channel counts:

The C-band (1530 – 1565 nm) is the dominant DWDM operating range because EDFAs provide flat amplification across this band. L-band extension (1565 – 1625 nm) approximately doubles capacity at the cost of additional amplification stages. See telecom wavelength bands for the full band designations.

Modern coherent transponders operate on flexible-grid wavelength assignments rather than fixed 50/100 GHz slots, dynamically allocating spectral width per channel based on the bit rate and modulation format. A 400 Gb/s coherent channel might occupy 75 – 100 GHz of spectrum; a 100 Gb/s channel might occupy 37.5 GHz.

WDM combined with space division multiplexing (multiple parallel fibers or multi-core fibers) is the principal capacity-scaling mechanism for submarine cables and high-density terrestrial networks.