---
title: "Short note: Multi mode optical fiber"
subject: "Data Communication"
chapter: 4
key: "semester_5/data_communication:multimode-fiber-short-note"
source: https://www.bctengineer.com/q/semester_5/data_communication/multimode-fiber-short-note
site: BCT Engineering
---

# Short note: Multi mode optical fiber

Data Communication · Chapter 4: Transmission Media

- Asked in: 2076 Chaitra, 4 marks
- Video: [Single-mode Optical fiber and Multi-mode Optical Fiber in Optical Fiber Communication](https://www.youtube.com/watch?v=KW3HSJthFws) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/multimode-fiber-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-4

## Question

Write a short note on multi mode optical fiber.

## Short answer

Multimode fibre has a wide core (50 or 62.5 µm) that supports many light paths, using LED sources at 850/1300 nm. Step-index causes high modal dispersion; graded-index reduces it by varying the refractive index. It is cheaper than single-mode but limited to a few hundred metres and up to 10 Gbps.

## Answer

**Multimode optical fibre** has a core wide enough (50 or 62.5 µm, with cladding of 125 µm) to allow many light rays (modes) to travel along different paths at the same time. A cheap LED source is typically used, with wavelengths of 850 nm and 1300 nm.

```text
Step-index:   cladding ____________
              core    \/\/\/\/  rays zig-zag by
              cladding ‾‾‾‾‾‾‾‾‾‾‾‾ total reflection
Graded-index: refractive index falls smoothly
              from centre to edge, so rays bend
              gently and arrive nearly together.
```

#### Types
- **Step-index multimode**: the core has uniform refractive index, with an abrupt drop at the cladding. Rays travelling at steep angles take longer paths, so the pulse spreads (modal dispersion). Bandwidth is lowest.
- **Graded-index multimode**: the refractive index decreases gradually from the centre outwards. Rays at the edge move faster, which compensates for their longer path, so dispersion is much smaller.

#### Features
- Cheaper than single-mode (LED sources, easy connectors, easy alignment).
- Higher modal dispersion, so limited distance (typically 300 m to 2 km) and bandwidth (up to 10 Gbps over short links).
- Used in LANs, campus backbones and data centres.

## Check yourself

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

1. Why is it called multimode fibre?
   A. It carries many wavelengths using several cladding layers
   B. It uses many cores in one cable
   C. Its wide core (50 or 62.5 µm) lets many light rays travel different paths at once
   D. It can switch between LED and laser sources
   Answer: C. The wide core supports many modes (paths) simultaneously. (Key point: Multimode core)

2. Why does step-index multimode fibre have the lowest bandwidth?
   A. Its cladding absorbs most of the light
   B. Rays at steep angles take longer paths, so the pulse spreads (modal dispersion)
   C. The refractive index changes gradually so rays slow down
   D. It uses a laser with a wide spectrum
   Answer: B. Different path lengths make rays arrive at different times, spreading the pulse. (Key point: Modal dispersion)

3. How does graded-index fibre reduce dispersion?
   A. The core is made narrower than in single mode
   B. Index rises toward the edge so rays are pushed back to the centre
   C. A uniform index with an abrupt drop at the cladding is used
   D. Index falls from centre to edge so edge rays move faster and compensate for their longer path
   Answer: D. Faster edge rays make up for the longer path, so rays arrive nearly together. (Key point: Graded index)

4. Which set describes multimode fibre use?
   A. Laser source, very costly, hundreds of km, undersea links
   B. LED source, 40 km repeater spacing, telephone trunks
   C. Laser source, 8-10 µm core, long haul backbones
   D. LED source, cheaper, 300 m to 2 km, LANs and data centres
   Answer: D. Multimode is cheap with LEDs and limited to short distances. (Key point: Typical features)

## Related in Data Communication

- [Short note: Optical fiber versus STP](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aoptical-fiber-vs-stp) — Data Communication, chapter 4
- [Optical fiber vs satellite transmission](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aoptical-fiber-vs-satellite) — Data Communication, chapter 4
- [Coaxial cable vs optical fiber](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acoax-vs-fiber) — Data Communication, chapter 4
- [Optical fiber advantages over coax and twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afiber-advantages) — Data Communication, chapter 4, asked 3×

## Related in other subjects

- [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
- [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×
- [Derive numerical aperture of optical fiber](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Ana-derivation-optical-fiber) — Engineering Physics, chapter 6, asked 4×
- [Advantages of optical fiber over copper](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-advantages) — Instrumentation II, chapter 5, asked 3×