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 µm), 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 µm).
- 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 |