Properties for selecting optical fibre

Choosing an optical fibre for a link means matching the fibre's optical, mechanical and cost properties to the needs of the system: distance, bit rate, wavelength, source type and installation environment.

1. Attenuation (loss)

• Loss in dB/km sets the maximum distance between repeaters.
• Silica fibre has low-loss windows at about 850 nm (2–3 dB/km), 1310 nm (~0.35 dB/km) and 1550 nm (~0.2 dB/km).
• Long-haul links need low-loss single mode fibre at 1310/1550 nm.

2. Bandwidth and dispersion

• Dispersion spreads pulses and limits the bit rate × distance product.
• Step-index multimode has large modal dispersion (about 20–50 MHz·km); graded-index is better (about 0.5–1 GHz·km); single mode has no modal dispersion (tens of GHz·km and more).
• For high data rates, choose single mode fibre, and dispersion-shifted fibre if operating at 1550 nm.

3. Numerical aperture and core size

• A large NA and core (50–62.5 µm, multimode) collect light easily from cheap LEDs and make splicing easier.
• A small core (8–10 µm, single mode) needs a laser and precise connectors.

4. Operating wavelength and source/detector match

• The fibre must have low loss and low dispersion at the wavelength of the available LED/laser and photodiode.

5. Mechanical properties

• Tensile strength, bend radius (macro- and micro-bending loss), and cable protection (buffer, strength member, jacket).
• The cable type depends on the route: duct, aerial, direct burial or undersea.

6. Environmental properties

• Temperature range, humidity, water ingress, and resistance to radiation and chemicals.

7. Ease of handling and cost

• Cost of fibre, connectors and splicing, and of matching transmitters.
• Multimode plus LED is cheap for LANs and short links; single mode plus laser is used for long-haul and high-capacity links.

8. Future upgrade

• Single mode fibre allows later upgrade to higher bit rates and WDM without laying new cable.