--- title: "Construction and types of optical fibre" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:fiber-construction-types" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/fiber-construction-types site: BCT Engineering --- # Construction and types of optical fibre Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2076 Baisakh, 5 marks - Video: [Optical Fibers: Structure, Basics and Types | Step index Optical Fiber | Graded Index Optical Fiber](https://www.youtube.com/watch?v=pavBq7HIoIE) — Engineering Funda - 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-construction-types - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Explain the construction and types of optical fiber with necessary diagrams. ## Short answer An optical fibre consists of a doped-silica core (n1), a pure-silica cladding (n2, n1>n2), an acrylate buffer and an outer jacket. Three principal types exist: step-index multimode (large core, many modes, short links), step-index single-mode (small core, single mode, long haul) and graded-index multimode (parabolic index profile, low modal dispersion, LANs). ## Answer An **optical fibre** is a thin, flexible glass (or plastic) waveguide that carries light by total internal reflection. #### Construction ```text cross-section side view .-------------. jacket ================ / jacket \ buffer ---------------- | .-----------. | clad n2 ________________ | / cladding \ | core n1 ---> light ---> | | .-----. | | clad n2 ________________ | | | core| | | buffer ---------------- | \ '-----' / | jacket ================ \ '-----------' / '-------------' ``` - **Core:** central glass region of higher index n1 (silica doped with germanium) that carries the light. - **Cladding:** surrounding glass of slightly lower index n2 (n1 > n2) that makes TIR possible; standard diameter 125 µm. - **Buffer coating:** acrylate layer(s) (to 250 µm) that protect against moisture, scratches and microbending. - **Jacket and strength members:** outer PVC/PE sheath and Kevlar yarn that give mechanical strength in a cable. #### Types of optical fibre | Type | Core size | Index profile | Modes | Use | |---|---|---|---|---| | Step-index multimode | 50–200 µm | Abrupt step | Many, zig-zag | Short links | | Step-index single mode | 8–10 µm | Abrupt step | One | Long haul | | Graded-index multimode | 50/62.5 µm | Parabolic | Many, curved | LAN | ```text SI multimode: /\/\/\/\/\ large modal dispersion Single mode: ----------> no modal dispersion GI multimode: ~~~~~~~~~~> low modal dispersion ``` Fibres may also be classified by material: all-glass (silica), plastic-clad silica, and all-plastic (POF) fibres. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must the cladding index n2 be lower than the core index n1? A. So the cladding can carry the light B. To make the fibre flexible C. To reduce the fibre's weight D. So that total internal reflection can occur at the boundary Answer: D. TIR requires light to go from a denser to a rarer medium. (Key point: Core and cladding) 2. What is the purpose of the buffer coating? A. Protect against moisture, scratches and microbending B. Guide the light C. Raise the core index D. Act as the optical source Answer: A. Acrylate layers protect the glass; the jacket and Kevlar give mechanical strength. (Key point: Buffer coating) 3. Which core size belongs to single-mode step-index fibre? A. 50-200 um B. 8-10 um C. 50/62.5 um D. 125 um Answer: B. Single mode has 8-10 um core; 50-200 um is step-index multimode, 50/62.5 um graded-index. (Key point: Core sizes) 4. What is the index profile of graded-index multimode fibre? A. Abrupt step B. Uniform C. Parabolic, highest at the axis D. Highest at the cladding Answer: C. The index decreases roughly parabolically from the axis to the cladding. 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