---
title: "Dispersion and attenuation in optical fibre"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:dispersion-attenuation"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/dispersion-attenuation
site: BCT Engineering
---

# Dispersion and attenuation in optical fibre

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2069 Bhadra, 6 marks
- Video: [Signal Attenuation in Optical Fibers | L 27 | Optical Communication](https://www.youtube.com/watch?v=iYshMDsD_Tk) — Dopamine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/dispersion-attenuation
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

Explain the dispersion and attenuation properties of an optical fibre.

## Short answer

Attenuation is loss in dB/km from absorption, Rayleigh scattering, and bending; typical windows are 850, 1310, and 1550 nm, with 0.2 dB/km at 1550 nm. Dispersion is pulse broadening from intermodal, chromatic, and polarization mode dispersion, limiting bit rate; reduced by single mode fibre and narrow-linewidth lasers.

## Answer

Attenuation and dispersion are the two main transmission impairments of an optical fibre. **Attenuation** reduces the signal power and limits distance; **dispersion** spreads the pulses and limits bandwidth (bit rate).

#### Attenuation
Attenuation is the loss of optical power along the fibre, given in dB/km:

$$
\alpha\ (\text{dB/km}) = \frac{10}{L}\log_{10}\frac{P_{in}}{P_{out}}
$$

Causes:
1. **Absorption**
   - *Intrinsic*: absorption by silica itself, ultraviolet (electronic) and infrared (molecular vibration) bands.
   - *Extrinsic*: impurities such as transition metal ions (Fe, Cu) and OH⁻ ions (water peak near 1383 nm).
2. **Scattering**
   - *Rayleigh scattering* from tiny density variations in glass; varies as $1/\lambda^4$, so it is the main loss at short wavelengths.
   - Mie scattering from larger defects.
3. **Bending losses**
   - *Macrobending*: large bends let light escape.
   - *Microbending*: tiny random bends from cabling pressure.
4. Splice and connector losses (in a link).

Low-loss windows: 850 nm (~2.5 dB/km), 1310 nm (~0.35 dB/km), 1550 nm (~0.2 dB/km).

#### Dispersion
Dispersion is the time spreading of a light pulse as it travels. Spread pulses overlap (inter-symbol interference), limiting the maximum bit rate.

```text
 in: _|^|_|^|_    out: _/^^\/^^\_  (pulses overlap)
```

1. **Intermodal dispersion**: different modes in multimode fibre take different paths and arrive at different times. Large in step-index fibre, reduced in graded-index, zero in single mode fibre.
2. **Chromatic (intramodal) dispersion**: different wavelengths in the source spectrum travel at different speeds.
   - *Material dispersion*: index of glass varies with wavelength.
   - *Waveguide dispersion*: propagation depends on core size relative to wavelength.
   - Zero near 1310 nm in standard fibre; shifted to 1550 nm in dispersion-shifted fibre.
3. **Polarization mode dispersion**: the two polarizations travel at slightly different speeds.

| Point | Attenuation | Dispersion |
|---|---|---|
| Effect | Reduces power | Broadens pulse |
| Limits | Link distance | Bit rate / bandwidth |
| Unit | dB/km | ns/km or ps/(nm·km) |
| Remedy | Amplifiers, low-loss window | Single mode fibre, narrow laser |

## Check yourself

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

1. Which pairing is correct?
   A. Attenuation limits bit rate; dispersion limits distance only
   B. Attenuation limits link distance; dispersion limits bit rate/bandwidth
   C. Both limit only bandwidth
   D. Neither limits performance
   Answer: B. Attenuation reduces power over distance, while dispersion spreads pulses so they overlap (ISI), limiting bit rate. (Key point: Attenuation vs dispersion limits)

2. Rayleigh scattering loss varies as:
   A. lambda^4
   B. 1/lambda^4, so it dominates at short wavelengths
   C. It is independent of wavelength
   D. 1/lambda
   Answer: B. It arises from tiny density variations in glass and falls steeply as wavelength increases. (Key point: Rayleigh scattering)

3. Which window has the lowest loss in silica fibre?
   A. 850 nm (about 2.5 dB/km)
   B. 1310 nm (about 0.35 dB/km)
   C. 1383 nm (water peak)
   D. 1550 nm (about 0.2 dB/km)
   Answer: D. 1550 nm has the minimum attenuation of about 0.2 dB/km. (Key point: Low-loss windows)

4. Chromatic (intramodal) dispersion in standard fibre is near zero at about:
   A. 850 nm
   B. 1310 nm
   C. 1550 nm
   D. 1383 nm
   Answer: B. Material and waveguide dispersion cancel near 1310 nm; dispersion-shifted fibre moves the zero to 1550 nm. (Key point: Chromatic dispersion)

## Related in Propagation and Antenna

- [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
- [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
- [Numerical aperture and dispersion](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ana-and-dispersion) — 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