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Optical Fibre - Single Mode, Multimode, Step Index, Graded Index, and Bandwidth

 

Optical Fibre Communication Concepts

1. Single Mode Fibre

Single mode fibre allows only one light ray (mode) to propagate through the core. It has a very small core diameter (about 8–10 ยตm), which reduces dispersion significantly.

  • Very high bandwidth
  • Low signal loss
  • Used for long-distance communication

2. Multimode Fibre

Multimode fibre allows multiple light rays to travel through the core simultaneously. It has a larger core diameter (50–100 ยตm).

  • Higher dispersion
  • Lower bandwidth compared to single mode
  • Used for short-distance communication

3. Step Index Fibre

In step index fibre, the refractive index of the core is constant and sharply decreases at the cladding.

  • Light travels in a zig-zag path
  • High dispersion
  • Low bandwidth

4. Graded Index Fibre

In graded index fibre, the refractive index gradually decreases from the center of the core to the edges.

  • Light follows a curved path
  • Reduced dispersion
  • Higher bandwidth than step index fibre

5. Bandwidth

Bandwidth refers to the data-carrying capacity of the fibre. It is affected by dispersion.

  • Higher dispersion → Lower bandwidth
  • Lower dispersion → Higher bandwidth

6. Numerical Aperture (NA)

Numerical Aperture indicates the light-gathering ability of an optical fibre.

Formula: NA = √(n₁² - n₂²)

  • Higher NA → More light enters fibre
  • Lower NA → Less light enters fibre

7. Comparison Table

Property Single Mode Multimode Step Index Multimode Graded Index
Core Size Small Large Large
Dispersion Very Low High Low
Bandwidth Very High Low High
Light Path Straight Zig-zag Curved
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