Attenuation in optical fibres

Attenuation in optical fibres

1. What Attenuation Is

Attenuation = loss of optical power along fibre due to absorption, scattering, bending, etc., measured in dB/km:

Typical modern fibre: 0.2 dB/km at 1550 nm (ultralow), ~0.35 dB/km at 1310 nm, ~2.5 dB/km at 850 nm. 20 dB = 100× loss.

2. Loss Mechanisms (exam list — 4)

Mechanism	Cause	Wavelength Dependence
Absorption — intrinsic	Silica UV/IR electronic/vibrational absorption tails	minima at 1550 nm
Absorption — extrinsic (impurities)	OH⁻ ions (water peak at 1383 nm), transition metals	peaks; mitigated by dehydration
Scattering — Rayleigh	Density fluctuations, inhomogeneities ≪λ:

| dominates at short $λ$ (why 1550 <1310 <850 loss) |
| Bending / microbending | Macrobend (tight radius) leaks evanescent, microbend (press) couples modes to cladding | increases at longer $λ$ for small radius |

• Scattering is fundamental lower limit (~0.15 dB/km at 1550 nm theoretically).
• Bending loss: critical radius $R_c≈ a/Δ$? Actually $R_c∼ λ/(NA²)$ order; avoid <30 mm bend for SMF.

3. Total Loss Curve

Attenuation vs λ: high at UV (electronic), falls as 1/λ⁴ Rayleigh to minimum ~1.55 µm, rises beyond ~1.7 µm due to IR absorption. Two windows: 1.31 µm (zero dispersion) and 1.55 µm (low loss). OH peak at 1.38 µm is removed in modern low-water-peak fibre.

4. Other Losses

• Splicing/connector loss: 0.1–0.5 dB per splice (misalignment).
• Radiation at bends: e.g., 10 mm radius SMF bends add ~0.5 dB/turn at 1550 nm.

5. Worked Example

20 km link at 0.2 dB/km: loss=4 dB ⇒ P_out/P_in=10^{-0.4}=0.398 (≈40% remains). At 0.35 dB/km 1310 nm: 7 dB ⇒ 20% remains.

Power after 50 km at 0.2 dB/km: 10 dB ⇒ 10% remains.

If bending adds 3 dB, length equivalent extra 15 km at 0.2 dB/km.

6. Exam Points

• Define attenuation in dB/km — 3 marks, very frequent.
• List mechanisms: absorption (intrinsic/extrinsic), Rayleigh $∝1/λ⁴$, bending — 5-7 marks, very frequent.
• Why 1550 nm window has lowest loss (Rayleigh low + IR not yet high, OH removed) — 5 marks, frequent.
• Convert $P_{in}/P_{out}$ to dB — 3-5 marks, frequent.