Single Mode vs Multimode Fiber

Single-mode and multimode fiber are the two kinds of optical fiber, and the difference between them decides which transceivers a network can use. The difference is physical: it is the size of the core, the glass centre of the fiber that carries the light. Single-mode fiber has a narrow core, roughly 9 microns across, so narrow that light travels down it in a single path, or mode. Multimode fiber has a wider core, 50 or 62.5 microns, so wide that light travels down it in many paths at once. That one physical difference explains every practical difference between the two.

Distance is the first practical difference. Because light in single-mode fiber travels in a single path, it stays coherent over long distances: tens of kilometres at common data rates, and further with the right optics. Because light in multimode fiber travels in many paths, the paths arrive at slightly different times, and that spreading, called modal dispersion, limits the distance: hundreds of metres at gigabit and 10G speeds, less at higher speeds. A campus backbone, a metro link or a connection between buildings is therefore a single-mode job, while a link inside a building or across a data-centre floor is usually a multimode job.

Wavelength follows the same split. Single-mode transmission commonly uses 1310nm and 1550nm, the windows where single-mode fiber loses least light. Multimode transmission commonly uses 850nm, the window where short-wavelength sources are cheapest and where multimode fiber performs best. A transceiver built for one window will not work in the other, and a transceiver built for one fiber type will not work over the other. The fiber type and the wavelength must match together, and a buyer who knows one but not the other has only half the answer.

Cost runs in the opposite direction to distance. Multimode transceivers are cheaper, because their light sources are simpler, and multimode fiber is cheaper to terminate, because its wider core is more forgiving of alignment. Single-mode transceivers cost more, because their sources must be precise enough to launch light into a 9-micron core. For a short link, multimode wins on cost twice over. For a long link, single-mode wins outright, because multimode cannot reach at any price. The right choice is therefore not the cheaper medium in general, but the cheaper medium that reaches.

How do you tell which fiber a building already has? The jacket colour is the first clue: yellow usually means single-mode, aqua or orange usually means multimode, though colour codes vary and should not be trusted alone. The printing on the jacket is more reliable: it usually states the fiber type and the core size. The connector and the installed transceivers are the most reliable evidence of all: if the existing modules are single-mode modules, the plant is single-mode, and the replacement modules must match. When in doubt, state the jacket printing and the existing module types during the inquiry, and the answer can be specific.

On this site, the single-mode lines with full published specifications are the QSFP28 LR4 and the XFP LR, and the multimode lines with full published specifications are the 40G QSFP+ SR4 and the SFP+ SR. The series pages for the other families state which fiber type each family belongs to at category level. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.