In one line
A single-mode fibre has a very thin core that lets only one path (mode) of light travel, while a multimode fibre has a thicker core that allows many paths at once.
Explained simply
Imagine a single-lane road versus a wide highway. On a single lane, cars travel one behind another and all arrive in order. On a wide highway, some cars take the inner lane and some weave around, so cars leaving together arrive at slightly different times. In a multimode fibre, different light paths take different times, so a sharp pulse spreads out. A single-mode fibre avoids this.
How it works
- A "mode" is an allowed pattern or path in which light can travel along the fibre.
- The number of modes depends on the V-number \(V = \frac{2\pi a}{\lambda}NA\).
- If \(V < 2.405\), only one mode propagates: single-mode fibre.
- For large V, the number of modes in a step-index fibre is about \(V^2/2\): multimode fibre.
- Different modes travel different path lengths, causing intermodal dispersion, which limits bandwidth in multimode fibres.
| Feature | Single-mode | Multimode |
|---|---|---|
| Core diameter | Small, about 8 to 10 μm | Large, about 50 to 200 μm |
| Modes | One | Many |
| Intermodal dispersion | None | Present |
| Bandwidth and distance | Very high, long distance | Lower, short distance |
| Source | Laser diode | LED or laser |
| Coupling and cost | Difficult, costly | Easier, cheaper |
| Index profile | Step index | Step or graded |
Key formula
\(V = \dfrac{2\pi a}{\lambda}\sqrt{n_1^2 - n_2^2}\); \(a\) = core radius (m), \(\lambda\) = wavelength (m). Number of modes \(N \approx V^2/2\) (step index), \(V^2/4\) (graded index).
Quick example
A step-index fibre has \(a = 25\) μm, \(NA = 0.2\), \(\lambda = 1\) μm. \(V = 2\pi\times25\times0.2/1 = 31.4\), so \(N \approx 31.4^2/2 \approx 493\) modes: clearly multimode. Long-distance and undersea telecom links use single-mode fibre; LANs inside buildings often use multimode.
Common mistakes
- Saying single-mode fibres carry one wavelength; they carry one mode (path).
- Forgetting the cut-off value 2.405 for single-mode operation.
- Thinking multimode means multiple fibres bundled together.
How MU asks it
Asked as "Differentiate between single-mode and multimode fibres" (3 to 5 marks), often combined with step-index and graded-index. Give labelled diagrams (core sizes, ray paths) and a table of five or six points. Numericals ask for V-number and number of modes.