Optical Wavelength Guide

Optical transmission uses three wavelength windows, and each window belongs to a fiber type and a distance range. The windows are 850nm, 1310nm and 1550nm, measured in nanometres, and they are not arbitrary: they are the wavelengths at which fiber loses least light, or at which light sources are cheapest, or both. Understanding the three windows is understanding most of optical link design, because the wavelength, the fiber type and the reach must all match together.

The 850nm window is the short-wavelength window, and it belongs to multimode fiber. Light sources at 850nm are inexpensive and simple, which is why multimode transceivers cost less than single-mode ones. The trade-off is distance: 850nm over multimode fiber reaches hundreds of metres, not kilometres, because modal dispersion in the wider multimode core spreads the signal. The published SFP+ SR model on this site, for example, states an 850nm wavelength and a 300m reach over multimode fiber. That combination — 850nm, multimode, short reach — is the standard pattern for in-building links.

The 1310nm window is the middle window, and it belongs to single-mode fiber at moderate distances. Single-mode fiber loses relatively little light at 1310nm, and the sources are well established, which makes 1310nm the standard choice for campus, metro and medium-haul links. The published QSFP28 LR4 model states a 1310nm wavelength with 10km and 20km reach options, and the published XFP LR model states a 1310nm wavelength with a 20km reach. Those are the classic 1310nm applications: links that run beyond a building but not across a country.

The 1550nm window is the long-wavelength window, and it belongs to single-mode fiber at long distances. Single-mode fiber loses least light at 1550nm, which is why the longest links use it, and it is also the window used by wavelength-division systems that carry many channels over one fiber. The published 1.25G SFP series on this site lists 1550nm combinations at 80km, 100km, 120km and 160km over single-mode fiber, stated at series level. Those distances are only achievable because the fiber loss at 1550nm is low enough to allow them.

The practical rule is simple: match the wavelength to the fiber, and match both to the distance. A multimode plant calls for 850nm. A single-mode plant at moderate distance calls for 1310nm. A single-mode plant at long distance calls for 1550nm. A module at the wrong wavelength for the plant will not link, however good the module is, and a module at the right wavelength but the wrong reach will fail on distance. State the fiber type and the link length during the inquiry, and the wavelength follows from those two answers.

Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.