SMF

Single-mode fibre (SMF) is an optical cable with a core diameter of roughly 8 to 10 micrometres, small enough that light travels through it in essentially one path, or mode. This contrasts sharply with multimode fibre, which has a core of 50 or 62.5 micrometres and allows dozens of light paths to propagate simultaneously. The tight confinement in SMF eliminates modal dispersion, the spreading of signal pulses that occurs when different paths arrive at different times.

SMF operates at wavelengths in the near-infrared, typically 1310 nanometres or 1550 nanometres. The 1550 nm band is standard for long-distance telecom because it matches the minimum loss window of silica glass and allows amplification using erbium-doped fibre amplifiers (EDFAs). Single-mode cables are always terminated with narrow connectors, most commonly the LC or SC type, and require precision polishing and alignment of the fibre end faces.

Where it works and why it matters

SMF dominates telecommunications infrastructure: intercontinental submarine cables, metropolitan backbone networks, and long-haul terrestrial routes all rely on it. A single strand can carry data over 100 kilometres without regeneration. The narrow core means less light scatters into the cladding, and chromatic dispersion becomes the main signal-limiting factor rather than modal effects. For this reason, long-distance SMF systems require careful attention to pulse shape, launch power, and fibre dispersion management.

Installation of SMF demands more care than multimode systems. Splicing requires fusion splicers or high-precision mechanical couplers. Bending the fibre tightly causes macrobending loss; coiling a spool to less than a 30 mm radius can introduce losses of several decibels. Field termination is also less forgiving, and tight tolerances on connector end-face geometry are necessary to keep return loss below required limits.

The term single-mode describes the propagation behaviour precisely: in an SMF waveguide, only the fundamental mode (the lowest-order spatial pattern of the electric field) propagates efficiently at the operating wavelength. Higher-order modes are cut off and do not transmit. This fundamental constraint makes SMF ideal for high-speed, point-to-point links where the cost of precision installation and test equipment is justified by the performance gain.