A telecom company needs to connect two cities that are hundreds of kilometers apart with a high-speed internet link. The system requires minimal signal loss over the long distance. Which type of cable is most suitable for this scenario?

Question

BlackTom AI · Accepted Answer

Single-mode fiber carries light in a single propagation mode, which minimizes modal dispersion and yields the lowest attenuation per kilometer—making it ideal for hundreds-of-kilometers links requiring minimal signal loss.

1) Relevant knowledge points and concepts
- Attenuation (dB/km): the loss of signal power per kilometer. Lower is better for long links.
- Modal dispersion: in multi-mode fiber, multiple light modes travel at different speeds, broadening pulses and limiting distance and bandwidth.
- Core size and light source: single-mode fiber has a small core (~8–10 µm) and uses laser sources; multi-mode has larger cores (50–62.5 µm) and typically uses LEDs or VCSELs.
- Practical long-haul systems use single-mode fiber plus optical amplifiers (EDFAs) to span hundreds of km.

2) Step-by-step reasoning
- The requirement is minimal signal loss over hundreds of kilometers. Compare typical attenuation:
  - Single-mode fiber: ≈0.2 dB/km (at 1550 nm).
  - Multi-mode fiber: ≈1.0 dB/km (or more, depending on wavelength and type).
  - Copper/Ethernet: designed for tens to hundreds of meters (Cat6 ≈100 m limit); attenuation over km is prohibitive.
- Evaluate loss over distance (examples):
  - For 100 km: single-mode loss ≈0.2 dB/km × 100 km = 20 dB; multi-mode ≈1.0 dB/km × 100 km = 100 dB.
  - For 300 km: single-mode ≈0.2 × 300 = 60 dB; multi-mode ≈1.0 × 300 = 300 dB.
- 20–60 dB of loss can be managed with optical amplifiers and repeaters; 100–300 dB is essentially unrecoverable without impractical equipment.

3) Intermediate calculation steps shown above confirm orders of magnitude difference.

4) Why single-mode is correct
- Single-mode minimizes modal dispersion and has far lower attenuation per km, allowing high-speed signals to travel much farther between amplifiers. It supports higher bandwidth-distance products and is the industry standard for long-haul telecom links.

5) Why other options are incorrect
- Multi-mode fiber: higher attenuation and modal dispersion limit it to short-to-medium distances (typically <2 km for high speeds).
- Ethernet cable (twisted pair/Cat cables): designed for ~100 m; not suitable for hundreds of km.
- Copper cable (coax or twisted pair): very high loss and electromagnetic interference over long distances; impractical and costly for hundreds of km.

Conclusion: Single-Mode Fiber Optic Cable is the most suitable choice, matching the provided correct answer.

A telecom company needs to connect two cities that are hundreds of kilometers apart with a high-speed internet link. The system requires minimal signal loss over the long distance. Which type of cable is most suitable for this scenario?单项选择题

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