Propagation speed of optical fiber and cable
Signals in copper cables typically travel at 60–90% of the speed of light, while optical fibers carry light at about 70% of light's speed in vacuum.
Copper Cables
The propagation speed of electrical signals in copper cables is determined by the velocity factor (VF), which is the ratio of the signal speed to the speed of light in vacuum . This factor depends mainly on the dielectric material used for insulation. Typical values include:
- Plenum-rated cables: VF ≈ 0.42–0.72 (42–72% of the speed of light)
- Riser cables: VF ≈ 0.70 (~210,000,000 m/s)
- Coaxial cables with foam dielectric: VF ≈ 90%
- Solid Teflon® insulation: VF ≈ 70% The propagation delay is inversely proportional to the VF, meaning lower VF results in longer delays. For example, twisted pair Ethernet cables have a maximum propagation delay of about 5.7 ns/m, corresponding to a VF of 0.585c .
Optical Fibers
Optical fibers transmit data as pulses of light, and their speed is determined by the refractive index (n) of the fiber material. The velocity factor for optical fibers is the reciprocal of the refractive index, typically around 0.67–0.70 . This means light travels at roughly 200,000 km/s, or about 70% of the speed of light in vacuum.
- Single-mode fibers: Light travels along a single path, minimizing dispersion and maintaining high signal integrity over long distances .
- Multimode fibers: Light travels via multiple paths, causing modal dispersion and slightly reducing effective propagation speed over long distances . Fiber optic cables can support extremely high data rates, commercially up to 100 Gbps, with laboratory experiments exceeding 1 petabit per second, due to the high speed of light transmission and low signal loss .
Summary
| Medium | Typical Velocity Factor | Approximate Speed |
|---|---|---|
| Copper cables (twisted pair, coax) | 0.42–0.90 | 125,000–270,000 km/s |
| Optical fiber (single/multimode) | 0.67–0.70 | ~200,000 km/s |
In conclusion, optical fibers generally provide faster signal propagation than copper cables, with less signal degradation over long distances, making them ideal for high-speed data transmission. Copper cables are slower due to dielectric effects but remain widely used for shorter distances and cost-effective installations.
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