Short note: Optical fiber loss

Optical fibre loss (attenuation) is the reduction of optical power as light travels along a fibre. It decides the maximum distance between repeaters.

α (dB/km) = (10/L) · log10(Pin/Pout)

Typical values: ~2.5 dB/km at 850 nm, ~0.35 dB/km at 1310 nm, ~0.2 dB/km at 1550 nm.

Main causes

1. Absorption: intrinsic (by silica in UV and IR bands) and extrinsic (by impurities, especially OH⁻ ions with a peak near 1380 nm, and metal ions like Fe and Cu).
2. Scattering: mainly Rayleigh scattering from tiny density variations; loss ∝ 1/λ⁴, so it falls at longer wavelengths. Mie scattering from larger defects.
3. Bending losses: macrobends (sharp curves of the cable) and microbends (tiny random axis deviations) let light escape into the cladding.
4. Coupling/joint losses: at source coupling, splices and connectors due to misalignment and Fresnel reflection.

The combination gives the low-loss "windows" at 850, 1310 and 1550 nm used in practice.