The structural characteristics of single-mode optical fiber are

Single-mode optical fibers have a small core diameter, precise refractive index profile, and a cladding that guides a single light mode over long distances with minimal dispersion.

Core and Cladding

The core of a single-mode fiber is very small, typically 8 to 10.5 microns in diameter, which allows only a single transverse mode of light to propagate, minimizing intermodal dispersion . The cladding surrounds the core and has a standard diameter of 125 microns, providing total internal reflection to confine light within the core . The core-to-cladding ratio is therefore approximately 9/125, which is standard for most single-mode fibers .

Mode Field Diameter and Numerical Aperture

The mode field diameter (MFD) defines the effective area over which the light is distributed in the core, typically around 9 microns. This parameter is crucial for coupling efficiency with light sources like laser diodes and photodetectors . The numerical aperture (NA), usually 0.09 to 0.15, measures the fiber's light-gathering ability and ensures that only a single mode is transmitted, reducing dispersion and enabling long-distance transmission .

Refractive Index Profile

Single-mode fibers have a precisely controlled refractive index profile, often step-index, where the core has a slightly higher refractive index than the cladding. This difference allows total internal reflection and defines the cut-off wavelength, below which only the fundamental mode propagates . The longitudinal uniformity of the refractive index is critical for maintaining low chromatic dispersion and consistent signal quality over long distances .

Attenuation and Dispersion

Single-mode fibers are designed for low attenuation, typically less than 0.35 dB/km at 1310 nm and 0.22 dB/km at 1550 nm, allowing long-haul transmission . Chromatic dispersion and polarization mode dispersion are key factors limiting data rates, and specialized fibers like dispersion-shifted or nonzero dispersion-shifted fibers are engineered to optimize performance for specific wavelengths .

Mechanical and Geometrical Attributes

Other structural characteristics include core concentricity error, cladding non-circularity, and macrobending loss, which affect fiber reliability and installation performance . The fiber must maintain longitudinal uniformity and precise geometry to ensure minimal signal loss and high-quality transmission over long distances.

Standards

The ITU-T G.652 standard defines the most widely used single-mode fibers, specifying geometrical, optical, and mechanical parameters to ensure compatibility with both 1310 nm and 1550 nm transmission windows . Compliance with these standards ensures predictable performance in telecommunications networks. In summary, the structural characteristics of single-mode optical fiber—small core, standard cladding, controlled refractive index, low NA, and precise geometrical tolerances—enable the propagation of a single light mode with minimal dispersion, making it ideal for long-distance, high-bandwidth optical communication .