Many Colors, One Fiber
A single strand of glass can carry dozens or even hundreds of independent data channels simultaneously, each encoded on a slightly different wavelength of infrared light. Wavelength-division multiplexing treats each wavelength as a separate lane on an optical highway, multiplying capacity without additional fiber. This technology transformed telecommunications in the late 1990s and continues to scale.
The ITU Grid
The International Telecommunication Union defines a standardized frequency grid anchored at 193.1 THz (1552.52 nm). DWDM systems place channels at integer multiples of 100 GHz, 50 GHz, or even 12.5 GHz spacing. Each laser must be frequency-locked to its assigned grid slot with precision better than 1 GHz to prevent inter-channel crosstalk.
Capacity and Spectral Efficiency
Total capacity is simply the number of channels times the per-channel data rate. But spectral efficiency — bits per second per hertz — reveals how close the system is to fundamental limits. Simple on-off keying wastes spectrum; coherent detection with dual-polarization 16-QAM modulation pushes efficiency to 4 bit/s/Hz or beyond, approaching the nonlinear Shannon limit of optical fiber.
Visualising the Spectrum
This simulation renders the WDM channel plan as a spectral diagram. Each channel appears as a colored peak on the wavelength axis. As you add channels or narrow the spacing, watch how the spectrum fills up. The total capacity readout updates in real time. You can explore the tradeoff between channel count, spacing, and modulation complexity that drives modern optical network design.