Describe the structure of a step index optical fibre.

It has a very thin central core of glass of high refractive index, surrounded by cladding of glass of slightly lower refractive index, all inside a protective outer sheath.

Explain how light is guided along a step index optical fibre.

Light entering the core at a shallow enough angle strikes the core–cladding boundary at an angle greater than the critical angle, so it is totally internally reflected and travels along the fibre by repeated total internal reflection.

Why is the core surrounded by cladding rather than simply left in air?

The cladding protects the core surface from scratches and dirt that would let light escape, stops light crossing between fibres that touch, and fixes a definite critical angle at a boundary of known refractive indices.

Calculate the speed of light in a fibre core of refractive index 1.52.

v = c/n = 3.00 × 10⁸ / 1.52 = 1.97 × 10⁸ m s⁻¹, and 1.97 × 10⁸ × 1.52 returns 3.00 × 10⁸ m s⁻¹.

Calculate the time taken for light to travel 1.0 km straight along a fibre core in which its speed is 1.97 × 10⁸ m s⁻¹.

t = d/v = 1000 / 1.97 × 10⁸ = 5.1 × 10⁻⁶ s, and 1.97 × 10⁸ m s⁻¹ × 5.1 × 10⁻⁶ s returns about 1000 m as given.

CCEA A-Level Physics 1210

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Refraction

Step index optical fibres

3 things to know
Unit AS 2

Overview

Structure of a step index optical fibre, how light is guided along a step index optical fibre and multipath dispersion in an optical fibre.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

A fibre has a core of refractive index 1.52 and cladding of refractive index 1.48. Calculate the critical angle at the core–cladding boundary.

Key term

Why does pulse spreading limit the rate at which data can be sent along a fibre?

Key term

Why is the core of an optical fibre surrounded by cladding rather than simply left in air?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the structure of a step index optical fibre.
Model answer
It has a very thin central core of glass of high refractive index, surrounded by cladding of glass of slightly lower refractive index, all inside a protective outer sheath.
2
Explain how light is guided along a step index optical fibre.
Model answer
Light entering the core at a shallow enough angle strikes the core–cladding boundary at an angle greater than the critical angle, so it is totally internally reflected and travels along the fibre by repeated total internal reflection.
3
Why is the core surrounded by cladding rather than simply left in air?
Model answer
The cladding protects the core surface from scratches and dirt that would let light escape, stops light crossing between fibres that touch, and fixes a definite critical angle at a boundary of known refractive indices.
4
A fibre has a core of refractive index 1.52 and cladding of refractive index 1.48. Calculate the critical angle at the core–cladding boundary.
Model answer
sin C = n₂/n₁ = 1.48/1.52 = 0.974, so C = 76.9°, and 1.52 × sin 76.8° = 1.48 confirms the boundary condition.
5
Calculate the speed of light in a fibre core of refractive index 1.52.
Model answer
v = c/n = 3.00 × 10⁸ / 1.52 = 1.97 × 10⁸ m s⁻¹, and 1.97 × 10⁸ × 1.52 returns 3.00 × 10⁸ m s⁻¹.
6
Calculate the time taken for light to travel 1.0 km straight along a fibre core in which its speed is 1.97 × 10⁸ m s⁻¹.
Model answer
t = d/v = 1000 / 1.97 × 10⁸ = 5.1 × 10⁻⁶ s, and 1.97 × 10⁸ m s⁻¹ × 5.1 × 10⁻⁶ s returns about 1000 m as given.

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