A single fiber can carry many wavelengths simultaneously, each acting as an independent channel. This is WDM, and it is what turns a few hundred terahertz of fiber bandwidth into hundreds of 100 Gbps lanes that can be addressed individually.
9.1 The basic idea
Combine different lasers, each at a slightly different wavelength, into one fiber at the transmit end. Demultiplex them at the receive end with a wavelength-selective filter (a thin-film filter, a fiber Bragg grating, or an arrayed waveguide grating). Each wavelength is detected and decoded independently.
9.2 CWDM and DWDM grids
Two standardized channel grids:
CWDM (Coarse WDM): ITU-T G.694.2. Channels spaced 20 nm apart, 18 channels from 1271 to 1611 nm. Wide spacing relaxes laser wavelength tolerances, so uncooled DFBs are fine. Cheap. Used in metro networks, mobile fronthaul, enterprise links. Typical aggregate rate: 8-18 channels at 10-25 Gbps each.
DWDM (Dense WDM): ITU-T G.694.1. A 50 or 100 GHz frequency grid centered on 193.10 THz (1552.52 nm), giving channel spacings of 0.4 or 0.8 nm. 80 to 96 channels in the C-band, more if you also use L-band. Each channel runs at 100 to 800+ Gbps. Used in long-haul, submarine, and high-capacity metro networks.
Modern DWDM transponders use flex-grid allocation: they negotiate variable-width channels (e.g., 75 GHz wide for a 600 Gbps signal, 150 GHz wide for an 800 Gbps signal) instead of fixed slots. This packs the spectrum more efficiently as data rates grow.
9.3 The arrayed waveguide grating (AWG)
The AWG is the most common WDM mux/demux. A planar lightwave circuit etched in silica-on-silicon: an input slab waveguide fans out into an array of parallel waveguides of slightly different length, then refocuses through a second slab into a row of output ports. Different wavelengths interfere constructively at different output ports, producing a precise wavelength-to-port mapping.
OUT 1 (λ1)
slab waveguide array OUT 2 (λ2)
┌──┐ ────┐ ┌──────────────┐ ────┐ ──── OUT 3 (λ3)
IN ─┤ ├── ├──┤ ████████ ├── ├────── OUT 4 (λ4)
└──┘ ────┘ └──────────────┘ ────┘ ──── OUT 5 (λ5)
OUT 6 (λ6)
(waveguide lengths increase by ΔL between adjacent guides)AWGs route 40 to 96 channels with a few tenths of a dB insertion loss per port and adjacent-channel crosstalk under -25 dB. They are a marvel of integrated photonic engineering and the workhorse of every DWDM system.
9.4 Optical add-drop multiplexers (OADM)
A pure WDM line is point-to-point: launch all wavelengths at A, demux all at B. A real network has many drops. The OADM is a wavelength-selective device that takes a multi-wavelength incoming line, drops a few wavelengths to local transponders, adds new traffic on those (or other) wavelengths, and lets the rest pass through unmodified.
A reconfigurable OADM (ROADM) can drop and add any wavelength by command, using wavelength-selective switches based on micro-mirror arrays or liquid-crystal-on-silicon. ROADMs make it possible to provision new circuits across a national-scale fiber backbone in seconds, without sending a technician to a building. They are why your Netflix stream sometimes shifts routing mid-evening without you noticing.