---
title: "Short note: Optical fiber loss"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:optical-fiber-loss-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/optical-fiber-loss-short-note
site: BCT Engineering
---

# Short note: Optical fiber loss

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2079 Chaitra, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/optical-fiber-loss-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

Write a short note on optical fiber loss.

## Short answer

Optical fibre loss α (dB/km) results from absorption (impurities like OH⁻), Rayleigh scattering (∝ 1/λ⁴), and bending. Low-loss windows are at 850 nm (~2.5 dB/km), 1310 nm (~0.35 dB/km), and 1550 nm (~0.2 dB/km). Attenuation limits repeater spacing.

## Answer

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

```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is fibre attenuation in dB/km calculated?
   A. (10/L) log10(Pin/Pout)
   B. (10/L) log10(Pout/Pin)
   C. L log10(Pin/Pout)
   D. (L/10) log10(Pin/Pout)
   Answer: A. Attenuation is the power ratio in dB per unit length, alpha = (10/L) log10(Pin/Pout). (Key point: Attenuation formula)

2. How does Rayleigh scattering loss vary with wavelength?
   A. It is proportional to lambda^4, so it rises at longer wavelengths
   B. It is proportional to 1/lambda^4, so it falls at longer wavelengths
   C. It is independent of wavelength
   D. It is proportional to 1/lambda, so it falls slowly
   Answer: B. Rayleigh loss follows 1/lambda^4, which is why 1310 and 1550 nm are preferred to 850 nm. (Key point: Rayleigh scattering)

3. Which extrinsic impurity causes a strong absorption peak near 1380 nm?
   A. Germanium ions
   B. Silica itself
   C. OH- (water) ions
   D. Kevlar fibres
   Answer: C. OH- ions give the water peak near 1380 nm, separating the 1310 and 1550 nm windows. (Key point: OH- absorption)

## Related in Propagation and Antenna

- [Types of optical fibre: construction and propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-fiber-types-construction) — Propagation and Antenna, chapter 6, asked 2×
- [Optical sources and fibre losses](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-sources-fiber-losses) — Propagation and Antenna, chapter 6, asked 2×
- [Optical fibre system vs wireless](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-working-vs-wireless) — Propagation and Antenna, chapter 6
- [Construction and types of optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-construction-types) — Propagation and Antenna, chapter 6
- [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6
- [Absorption, scattering and bending losses](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aabsorption-scattering-bending-losses) — Propagation and Antenna, chapter 6

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- [Optical fiber, NA, single vs multimode](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aoptical-fiber-na-acceptance-mode-comparison) — Engineering Physics, chapter 6