Attenuation of Single-mode and Multimode Optical Cables

Single-mode fibers have lower attenuation and support longer distances than multimode fibers, which experience higher signal loss due to modal dispersion.

Single-Mode Fiber (SMF)

Single-mode fibers are designed with a small core diameter of about 9 µm, allowing only a single light mode to propagate. This minimizes modal dispersion, enabling long-distance, high-bandwidth transmission. Typical attenuation values for single-mode fibers are 0.4 dB/km at 1310 nm and 0.3 dB/km at 1550 nm for OS1 cables, and around 0.3 dB/km at 1310–1550 nm for OS2 cables, supporting distances up to 40 km without repeaters at speeds up to 10 Gbps . Single-mode fibers use laser light sources, which produce coherent, single-wavelength light that travels in a straight path, further reducing signal loss .

Multimode Fiber (MMF)

Multimode fibers have a larger core diameter, typically 50 µm or 62.5 µm, allowing multiple light modes to propagate simultaneously. This causes modal dispersion, where light rays travel different paths and arrive at the receiver at slightly different times, increasing signal attenuation and limiting transmission distance . Typical multimode fibers operate at 850 nm and 1300 nm, with LEDs or VCSELs as light sources. Attenuation is generally higher than single-mode fibers, and practical link lengths are shorter, often up to 2 km for OM1/OM2 fibers and longer for laser-optimized OM3/OM4 fibers .

Factors Affecting Attenuation

1. Wavelength: Longer wavelengths (e.g., 1550 nm) reduce attenuation in single-mode fibers, while shorter wavelengths (850 nm) are common for multimode fibers .
2. Fiber Quality: Impurities, microbends, and splices increase attenuation.
3. Dispersion: Modal dispersion dominates in multimode fibers, while chromatic dispersion can affect single-mode fibers at high bit rates over long distances .
4. Connectors and Splices: Each connection introduces additional loss, which must be included in the optical power budget .

Summary

• Single-mode fibers: Low attenuation, long-distance transmission, small core (9 µm), laser sources, minimal modal dispersion.
• Multimode fibers: Higher attenuation, shorter distances, larger core (50–62.5 µm), LED/VCSEL sources, significant modal dispersion.
• Design Consideration: Single-mode fibers are preferred for long-haul, high-speed networks, while multimode fibers are cost-effective for short-distance applications like data centers and LANs . Understanding these attenuation characteristics is crucial for network planning, ensuring sufficient optical power reaches the receiver while maintaining signal integrity.