- 2072 Asoj · 4+2 marks
Fibre configurations and single mode advantages
Short answer
Step-index multimode has a uniform core (50–200 µm) and zig-zag rays causing large modal dispersion; graded-index multimode has a parabolic index and curved rays with less dispersion; single mode has a tiny core (8–10 µm) and one axial path, so no modal dispersion. Advantages of single mode: very high bandwidth, lowest attenuation (~0.2 dB/km), long repeater spacing, and supports WDM.
Compare the configuration of different types of fiber. Mention the advantages of single mode fiber.
Optical fibres are classified by the refractive-index profile of the core and the number of modes they carry. The three standard configurations are step-index multimode, graded-index multimode and step-index single mode.
Comparison of fibre configurations
Step-index MM Graded-index MM Single mode
n n n
| ___ | /\ | _
|_| |_ |__/ \__ |__| |__
core 50-200um core 50-85um core 8-10um
zig-zag rays curved rays one axial path
| Feature | Step-index MM | Graded-index MM | Single mode |
|---|---|---|---|
| Core index | Uniform | Falls parabolically from axis | Uniform |
| Core diameter | 50–200 µm | 50–85 µm | 8–10 µm |
| Cladding diameter | 125–400 µm | 125 µm | 125 µm |
| Ray path | Zig-zag, sharp reflections | Smooth curves (sinusoidal or helical) | Nearly straight along axis |
| Number of modes | Many (hundreds) | Many, but equalised delay | One () |
| Modal dispersion | Large | Small | None |
| Bandwidth | ~20–50 MHz·km | ~0.5–1 GHz·km | > 10 GHz·km |
| Source | LED | LED or laser | Laser |
| Cost / handling | Cheapest, easy | Moderate | Fibre cheap, but costly connectors and sources |
| Use | Short links | LANs, campus | Long-haul, high-speed |
In graded-index fibre, rays travelling far from the axis go through lower-index (faster) glass, so they arrive almost together with the axial ray. This is why modal dispersion is much lower than in step-index fibre.
Advantages of single mode fibre
- No modal (intermodal) dispersion, so very high bandwidth (Gb/s, and up to Tb/s with WDM).
- Lowest attenuation (about 0.2–0.35 dB/km at 1550/1310 nm), so repeater spacing of 100 km or more.
- Best for long-haul, undersea and backbone links.
- Supports future upgrades (higher bit rate, DWDM) on the same cable.
- Well-defined output beam, suitable for coherent and high-precision systems.
✓ Answer checked by Claude Opus 5.5 (high effort)
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