Parameters of single-mode fiber

Single-mode fiber is characterized by a small core (~9 µm), low attenuation, minimal dispersion, and precise geometrical and optical specifications for long-distance, high-bandwidth transmission.

Core and Geometrical Parameters

• Core Diameter: Typically around 8–10 µm, designed to support only the fundamental transverse mode of light .
• Cladding Diameter: Standardized at 125 µm for compatibility with connectors and splicing .
• Core Concentricity Error: Measures the offset between core and cladding centers, critical for minimizing splice loss .
• Cladding Non-Circularity: Ensures uniform light propagation and reduces mode distortion .
• Cut-off Wavelength: The wavelength above which the fiber supports only a single mode, typically around 1260–1320 nm .

Optical and Transmission Parameters

• Mode Field Diameter (MFD): Defines the effective width of the light-carrying region, usually 8–10 µm at 1310 nm, affecting bending loss and splice performance .
• Attenuation Coefficient: Low loss is essential for long-distance transmission; OS1 fibers have ~0.4 dB/km at 1310 nm, OS2 fibers ~0.3 dB/km at 1550 nm .
• Chromatic Dispersion: Typically 17 ps/(nm·km) at 1550 nm for G.652 fibers, influencing pulse broadening over distance .
• Polarization Mode Dispersion (PMD): Usually <0.5 ps/√km, important for high-speed digital transmission .
• Macrobending Loss: Loss due to fiber bending, minimized by proper MFD and cladding design .

Mechanical and Material Properties

• Refractive Index Profile: Step-index design with a slightly higher core index than cladding to confine light .
• Longitudinal Uniformity: Ensures consistent chromatic dispersion and attenuation along the fiber length .
• Material Composition: High-purity silica with controlled dopants to reduce attenuation and maintain mechanical strength .

Wavelength and Application

• Operating Wavelengths: Primarily 1310 nm and 1550 nm, with some fibers supporting 1625 nm for monitoring .
• Bandwidth and Distance: Single-mode fibers support high bandwidth over long distances, suitable for backbone networks, inter-building links, and submarine cables .

Fiber Categories

• OS1: Indoor, tight-buffered, suitable for short links up to ~2 km at 1310 nm .
• OS2: Outdoor, loose-tube, optimized for long-distance transmission, up to 40 km at 1550 nm . These parameters collectively ensure that single-mode fibers maintain signal integrity, minimize dispersion, and support high-speed, long-distance optical communication.