Using neat sketches, explain how light is propagated in the following optical fibres. Step index mult- mode Graded index mult-mode Single mode

Step Index Multi-Mode Optical Fiber

- Core: High refractive index glass or plastic
- Cladding: Lower refractive index glass or plastic

Light Propagation:

- Light rays enter the core at various angles.
- Rays with larger angles undergo total internal reflection and propagate in a zigzag manner.
- Rays with smaller angles may escape the core.

[Image of Step Index Multi-Mode Optical Fiber with light rays]

Graded Index Multi-Mode Optical Fiber

- Core: Refractive index decreases gradually from the center to the edge.
- Cladding: Lower refractive index than the core

Light Propagation:

- Light rays enter the core at various angles.
- Rays experience gradual refraction due to the varying refractive index.
- All rays are confined within the core, reducing modal dispersion.

[Image of Graded Index Multi-Mode Optical Fiber with light rays]

Single Mode Optical Fiber

- Core: Very small diameter (typically 8-10 μm)
- Cladding: Slightly lower refractive index than the core

Light Propagation:

- Only one mode of propagation: The fundamental TE01 mode.
- Light is confined to the core by total internal reflection.
- Minimal modal dispersion due to only one mode.

[Image of Single Mode Optical Fiber with light rays]