---
title: "Properties for selecting optical fibre"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:fiber-selection-properties"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/fiber-selection-properties
site: BCT Engineering
---

# Properties for selecting optical fibre

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2072 Magh, 6 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/fiber-selection-properties
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

What properties do you think to be considered when an optical fiber is to be selected for any communication system?

## Short answer

Select fibre by matching attenuation (lowest at 1310/1550 nm), dispersion/bandwidth, numerical aperture and core size to the source and detector, plus mechanical strength, bend radius, environment and cost. Single mode gives the lowest loss and highest bandwidth for long-haul; multimode with large NA suits short, low-cost links. Also consider future WDM upgrades.

## Answer

Choosing an optical fibre for a link means matching the fibre's optical, mechanical and cost properties to the needs of the system: distance, bit rate, wavelength, source type and installation environment.

#### 1. Attenuation (loss)
- Loss in dB/km sets the maximum distance between repeaters.
- Silica fibre has low-loss windows at about 850 nm (2–3 dB/km), 1310 nm (~0.35 dB/km) and 1550 nm (~0.2 dB/km).
- Long-haul links need low-loss single mode fibre at 1310/1550 nm.

#### 2. Bandwidth and dispersion
- Dispersion spreads pulses and limits the bit rate × distance product.
- Step-index multimode has large modal dispersion (about 20–50 MHz·km); graded-index is better (about 0.5–1 GHz·km); single mode has no modal dispersion (tens of GHz·km and more).
- For high data rates, choose single mode fibre, and dispersion-shifted fibre if operating at 1550 nm.

#### 3. Numerical aperture and core size
- A large NA and core (50–62.5 µm, multimode) collect light easily from cheap LEDs and make splicing easier.
- A small core (8–10 µm, single mode) needs a laser and precise connectors.

#### 4. Operating wavelength and source/detector match
- The fibre must have low loss and low dispersion at the wavelength of the available LED/laser and photodiode.

#### 5. Mechanical properties
- Tensile strength, bend radius (macro- and micro-bending loss), and cable protection (buffer, strength member, jacket).
- The cable type depends on the route: duct, aerial, direct burial or undersea.

#### 6. Environmental properties
- Temperature range, humidity, water ingress, and resistance to radiation and chemicals.

#### 7. Ease of handling and cost
- Cost of fibre, connectors and splicing, and of matching transmitters.
- Multimode plus LED is cheap for LANs and short links; single mode plus laser is used for long-haul and high-capacity links.

#### 8. Future upgrade
- Single mode fibre allows later upgrade to higher bit rates and WDM without laying new cable.

| Application | Usual choice |
|---|---|
| Building LAN, < 2 km | Graded-index multimode, 850/1310 nm |
| Metro / long-haul | Single mode, 1310/1550 nm |
| Very short, low cost | Step-index multimode or plastic fibre |

## Check yourself

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

1. What does loss in dB/km determine?
   A. Core size
   B. Source cost
   C. Bend radius
   D. Maximum distance between repeaters
   Answer: D. Lower loss gives longer spans. (Key point: Attenuation)

2. For high data rates, which fibre is chosen?
   A. Single mode
   B. Step-index multimode
   C. Plastic fibre
   D. Graded-index multimode
   Answer: A. No modal dispersion. (Key point: Dispersion)

3. Why must the fibre match the source wavelength?
   A. To increase NA
   B. It must have low loss and dispersion at the available LED or laser wavelength
   C. To reduce cost of jacket
   D. To raise bend radius
   Answer: B. Source, fibre and detector must match. (Key point: Source match)

4. Best fibre for a building LAN under 2 km?
   A. Single mode at 1550 nm
   B. Plastic fibre
   C. Graded-index multimode at 850/1310 nm
   D. Step-index multimode only
   Answer: C. Cheaper sources and easy handling. (Key point: Application choice)

## Related in Propagation and Antenna

- [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
- [Dispersion and attenuation in optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Adispersion-attenuation) — Propagation and Antenna, chapter 6
- [Elements of optical communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-communication-elements) — Propagation and Antenna, chapter 6, asked 3×
- [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×
- [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

## Related in other subjects

- [Communication system; optical fiber merits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-optical-merits) — Basic Electronics Engineering, chapter 5
- [Optical fiber advantages and labelled diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aoptical-fiber-adv-diagram) — Basic Electronics Engineering, chapter 5
- [Role of antenna; optical fiber pros and cons](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aantenna-role-optical) — Basic Electronics Engineering, chapter 5
- [Optical fiber and its advantages](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aoptical-fiber-advantages) — Basic Electronics Engineering, chapter 5, asked 5×