Describe the use of optical fibres, particularly in telecommunications

3.2.2 Refraction of Light – Use of Optical Fibres in Telecommunications

1. Why Light is Guided in an Optical Fibre

When light passes from a medium of higher refractive index to one of lower refractive index, it is bent away from the normal. If the angle of incidence exceeds the critical angle, the light is reflected back into the original medium – a phenomenon called total internal reflection (TIR).

Snell’s law describes the relationship at the interface:

\$n1 \sin \theta1 = n2 \sin \theta2\$

For TIR the incident angle \$\theta_1\$ must satisfy:

\$\theta1 > \thetac \quad\text{where}\quad \thetac = \sin^{-1}\!\left(\frac{n2}{n_1}\right)\$

In an optical fibre the core has a higher refractive index (\$n{\text{core}}\$) than the surrounding cladding (\$n{\text{clad}}\$), ensuring that light entering the core at a suitable angle is continually reflected down the length of the fibre.

2. Structure of an Optical Fibre

• Core – central glass or plastic cylinder with refractive index \$n_{\text{core}}\$.
• Cladding – outer layer with slightly lower refractive index \$n_{\text{clad}}\$, providing the condition for TIR.
• Jacket – protective polymer coating that shields the fibre from mechanical damage.

3. Types of Optical Fibre

1. Single‑mode fibre – core diameter ≈ 8–10 µm; allows only one light mode to propagate, suitable for long‑distance, high‑bandwidth links.
2. Multi‑mode fibre – core diameter ≈ 50–62.5 µm; supports many modes, used for shorter distances such as within buildings.

4. Role in Telecommunications

Optical fibres form the backbone of modern communication networks because they can transmit large amounts of data quickly and with minimal loss.

• Data is carried as pulses of light generated by lasers or LEDs.
• Signals can travel hundreds of kilometres without the need for repeaters, thanks to very low attenuation (≈ 0.2 dB/km for modern fibres).
• Bandwidth is extremely high – a single fibre can carry terabits per second (Tb/s) using wavelength‑division multiplexing (WDM).
• Immunity to electromagnetic interference (EMI) makes fibres ideal for environments with high electrical noise.

5. Comparison with Copper Cables

6. Key Points to Remember

• Optical fibres rely on total internal reflection, which requires \$n{\text{core}} > n{\text{clad}}\$.
• The critical angle \$\theta_c\$ determines the range of angles that will be guided down the fibre.
• Single‑mode fibres are used for long‑distance, high‑capacity links; multi‑mode fibres for shorter, lower‑capacity links.
• Advantages in telecommunications include very low attenuation, extremely high bandwidth, immunity to EMI, and light weight.
• Although the initial cost is higher, the long‑term savings and performance make optical fibres the preferred medium for modern data transmission.