Optical fibres are classified primarily based on their refractive index profile and the number of modes they support. This classification results in three main types: single-mode, multimode step-index, and multimode graded-index.
Classification Based on Mode
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Single-Mode Fiber (SMF): This type allows only one mode of light to propagate. It has a small core diameter (around 8-10 µm) and a relatively small difference in refractive index between the core and cladding. SMF is used for long-distance, high-bandwidth applications due to its minimal modal dispersion.
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Multimode Fiber (MMF): This type allows multiple modes of light to propagate simultaneously. MMF has a larger core diameter (typically 50 or 62.5 µm), leading to higher modal dispersion compared to SMF, thus limiting its bandwidth and distance capabilities.
Classification Based on Refractive Index Profile
Within multimode fibers, a further distinction is made based on the refractive index profile:
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Step-Index Fiber: This type has a uniform refractive index within the core, with a sharp decrease at the core-cladding boundary. This abrupt change in refractive index causes significant modal dispersion in multimode fibers.
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Graded-Index Fiber: This type features a refractive index that gradually decreases from the center of the core towards the cladding. This graded profile helps to reduce modal dispersion, as different modes travel at slightly different speeds, compensating for their differing path lengths. This results in a higher bandwidth compared to step-index multimode fibers.
Summary Table
| Feature | Single-Mode Fiber (SMF) | Multimode Step-Index Fiber | Multimode Graded-Index Fiber |
|---|---|---|---|
| Number of Modes | 1 | Many | Many |
| Core Diameter | Small (8-10 µm) | Large (50-62.5 µm) | Large (50-62.5 µm) |
| Refractive Index Profile | Step | Step | Graded |
| Modal Dispersion | Minimal | High | Reduced |
| Bandwidth | High | Low | Medium |
| Application | Long-distance, High-bandwidth | Short-distance, Low-bandwidth | Medium-distance, Medium-bandwidth |
In conclusion, optical fibers are classified based on the number of modes they support and their refractive index profile, leading to single-mode, multimode step-index, and multimode graded-index fiber types. These classifications determine the fiber's performance characteristics, such as bandwidth, dispersion, and suitability for different applications.
