Fiber Optic Cable Basic Values

Fiber optic cables transmit light through a core surrounded by cladding, with key values including core diameter, cladding diameter, numerical aperture, attenuation, and bandwidth.

Core and Cladding

The core is the central part of the fiber where light propagates, typically made of silica or plastic. Singlemode fibers have a core diameter of about 9 µm, while multimode fibers have larger cores, usually 50 µm or 62.5 µm. The cladding surrounds the core and has a slightly lower refractive index, enabling total internal reflection to confine light within the core .

Numerical Aperture (NA)

The numerical aperture (NA) defines the light acceptance angle of the fiber and is a measure of how much light can be captured and guided. Typical NA values are 0.20–0.29 for multimode fibers and 0.10–0.14 for singlemode fibers. A higher NA allows more light to enter but can increase modal dispersion in multimode fibers .

Attenuation

Attenuation measures signal loss along the fiber, expressed in decibels per kilometer (dB/km). Typical values are:

• Singlemode fiber: ~0.35 dB/km at 1310 nm, ~0.20 dB/km at 1550 nm
• Multimode fiber: ~3 dB/km at 850 nm, ~1 dB/km at 1300 nm Lower attenuation allows longer transmission distances without repeaters .

Bandwidth

Bandwidth indicates the data-carrying capacity of the fiber. Typical values are:

• Multimode fibers: 200–600 MHz·km at 850 nm
• Singlemode fibers: >10 GHz·km Higher bandwidth supports faster data rates and longer distances .

Wavelengths

Fiber optic systems commonly operate at 850 nm, 1300 nm, and 1550 nm, which correspond to low-loss windows in silica fibers. Singlemode fibers are optimized for 1310 nm and 1550 nm, while multimode fibers are often used at 850 nm and 1300 nm .

Additional Considerations

• Mode Field Diameter (MFD): For singlemode fibers, typically ~9 µm, affecting coupling efficiency and splice loss.
• Fiber Coating: Protective layers (acrylate or polyimide) prevent mechanical damage and microbending losses.
• Connector Losses: Typical insertion loss for connectors ranges from 0.3 dB (ST, SC, LC) to >1 dB for large-core or high-power applications . These basic values are essential for selecting the appropriate fiber type for a given application, whether for telecommunications, data centers, or industrial laser delivery systems.