Absorption, scattering and bending losses

Attenuation (signal loss) in an optical fibre is the decrease of optical power with distance. It limits the repeater spacing and is expressed in dB/km:

α (dB/km) = (10/L) · log10(Pin/Pout)

 loss
 dB/km
  |\
  | \  Rayleigh        OH peak
  |  \  (1/λ⁴)          /\         IR
  |   \                /  \        absorption
  |    \____  ________/    \_____    /
  |         \/               \___\__/
  +----+--------+--------+--------+---> λ (nm)
      850     1310     1380     1550

The low-loss windows at 850, 1310 and 1550 nm come from the combination of the losses below.

1. Absorption losses

Light energy is absorbed by the glass and converted to heat.

• Intrinsic absorption: by the pure silica itself; electronic absorption in the ultraviolet and molecular vibration absorption in the infrared (beyond ~1.6 µm). It sets the long-wavelength limit.
• Extrinsic absorption: by impurities such as transition metal ions (Fe, Cu, Cr) and especially OH⁻ (water) ions, which give absorption peaks near 950, 1240 and 1380 nm.
• Atomic defect absorption: defects in the glass structure, e.g. caused by radiation.

2. Scattering losses

Light is scattered out of the guided path by small non-uniformities in the glass.

• Rayleigh scattering: by density and composition fluctuations much smaller than λ; loss ∝ 1/λ⁴. It is the dominant loss at short wavelengths and the main reason long links use 1310 and 1550 nm.
• Mie scattering: by larger imperfections (core–cladding irregularities, bubbles), reduced by careful manufacture.
• Non-linear scattering (stimulated Brillouin and Raman) appears only at high optical power.

3. Bending (radiation) losses

• Macrobending: bends with radius much larger than the fibre diameter (e.g. cable round a corner). On the outer side of a sharp bend the ray angle falls below the critical angle, so light leaks into the cladding. Loss rises steeply once the bend radius goes below a critical value.
• Microbending: small random bends of the fibre axis (µm scale) caused by uneven cabling pressure or temperature changes. They couple light from guided to radiating modes.

Bending loss rises exponentially when the bend radius falls below a critical radius (for multimode fibre Rc ≈ 3n1²λ / (4π(n1² − n2²)^(3/2))). Microbending is reduced with soft-plus-hard dual coatings and loose-tube cables, macrobending by keeping cables above their minimum bend radius.

Summary