- 2072 Magh · 1+4+5 marks
Fibre losses; multimode vs single mode
Short answer
An optical fibre is a thin glass/plastic strand with a higher-index core and lower-index cladding that guides light by total internal reflection. Losses include absorption, Rayleigh scattering, bending, coupling, and splice/connector losses. Multimode fibre has a larger core and accepts cheap LEDs but suffers higher modal dispersion and attenuation, limiting it to short links; single mode has low loss and no modal dispersion for long-haul.
What is an optical fiber? What are the different types of losses you can visualize in optical fiber communication? What are the advantages and disadvantages of multimode fiber over the single mode fiber?
Optical fibre
An optical fibre is a thin, flexible strand of very pure glass or plastic, with a core of higher refractive index surrounded by a cladding of lower index, that guides light over long distances by total internal reflection.
Types of losses in optical fibre communication
- Absorption loss: intrinsic (UV and IR absorption by silica) and extrinsic (OH⁻ ions, peak near 1380 nm; metal ions such as Fe, Cu).
- Scattering loss: mainly Rayleigh scattering from microscopic density variations, ∝ 1/λ⁴; also Mie scattering from larger defects.
- Bending loss: macrobending (sharp curves of the cable) and microbending (small random axis deviations from cabling stress).
- Coupling loss: at the source-to-fibre and fibre-to-detector interfaces, due to NA and area mismatch.
- Splice and connector loss: lateral, angular and gap misalignment, end-face quality, core/NA mismatch and Fresnel reflection.
Attenuation is measured as α = (10/L)·log(Pin/Pout) dB/km. (Dispersion does not remove power but spreads pulses, which limits bandwidth.)
Multimode fibre compared with single mode fibre
Advantages of multimode fibre:
- Large core (50–200 µm) and large NA make it easy to launch light, so cheap LEDs and VCSELs can be used.
- Connectors, splicing and alignment are easier and cheaper; tolerances are relaxed.
- Lower overall system cost for short links (LANs, buildings, data centres).
- Several fibres can be joined with simple tools in the field.
Disadvantages of multimode fibre:
- Large intermodal dispersion (different modes arrive at different times), so much lower bandwidth–distance product.
- Higher attenuation (about 2.5–3 dB/km at 850 nm compared with 0.2–0.35 dB/km for single mode).
- Limited to short distances (a few hundred metres to ~2 km at high data rates).
- Not suited to optical amplifiers and DWDM long-haul systems.
| Feature | Multimode | Single mode |
|---|---|---|
| Core diameter | 50–200 µm | 8–10 µm |
| Modes | Many | One |
| Source | LED / VCSEL | Laser diode |
| Bandwidth | Low–medium | Very high |
| Loss | Higher | Lowest |
| Distance | Short | Very long |
| Cost of system | Lower | Higher |
✓ Answer checked by Claude Opus 5.5 (high effort)
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