wavelength-division multiplexing

Shine red, green, and blue laser pointers down the same glass rod at once and they pass through together without mixing — at the far end a prism can split them back into separate colours. Wavelength-division multiplexing does exactly this in an optical fiber: it sends many independent data streams down one fiber at the same time, each carried by light of a slightly different wavelength (colour), so a single strand of glass does the work of many.

It is really frequency-division multiplexing wearing optical clothes — for light, wavelength and frequency are two sides of the same coin. Each data channel is given its own laser tuned to a distinct wavelength; the streams are combined into one fiber by an optical multiplexer (think a reverse prism), travel together, and are pulled back apart at the far end by a demultiplexer that routes each wavelength to its own receiver. Dense WDM (DWDM) packs the wavelengths very tightly, fitting dozens to a hundred-plus channels into one fiber, each running at 10, 100, or more gigabits per second, for terabits per second on a single hair-thin strand.

WDM is why the global internet backbone and undersea cables can carry such staggering capacity without laying new fiber for every upgrade: you just light up more wavelengths on the glass already in the ground. It multiplies a fiber's capacity many-fold while the physical cable stays the same, and it pairs naturally with optical amplifiers that boost all wavelengths together. The cost is the precise, temperature-stable lasers and optical filters needed to keep the colours from blurring into one another.