Singlemode

Single-mode fiber, commonly abbreviated SMF, is an optical fiber designed to carry light predominantly in one transverse propagation mode. This single-mode operation minimizes modal dispersion and enables high bandwidth over long distances, compared with multimode fibers.

SMF has a very small core diameter, typically about 8 to 10 micrometers, with a surrounding cladding of about 125 micrometers. The refractive-index profile is usually step-index, and the numerical aperture is modest to keep light confined to the core. The fiber supports only the fundamental LP01 mode when the normalized frequency V is below 2.405, where V = (2πa/λ) NA, with a the core radius and NA the numerical aperture.

Operating wavelengths in telecommunications are centered around 1310 nanometers (the O-band) and 1550 nanometers (the C-band). Modern SMF offers relatively low attenuation in these windows, typically about 0.2 dB per kilometer at 1550 nm, with slightly higher values near 1310 nm. In addition to low attenuation, SMF is chosen for its favorable dispersion characteristics, which enable long-haul transmission with high data rates.

Standard single-mode fiber is specified by ITU-T recommendations, with ITU-T G.652 defining the basic class and several variants (such as G.652.D and G.652.E) tailored for dispersion and attenuation performance in contemporary networks.

Applications include long-haul and metro optical networks, fiber-to-the-home deployments, data-center interconnects, and various sensing systems. Handling typically requires precise splicing or fusion, careful connectorization, and testing with optical time-domain reflectometry to characterize link quality. SMF is more sensitive to bending losses than multimode fiber, making proper routing and protection important in installation.