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Optical fiber communication over electrical cable transmission

  1.  23 March 2016

 

An optical fiber is a flexible and transparent fiber made of extruded glass (silica) or plastic, slightly thicker than a human hair. It is able to transmit light between the two ends. Light can only be coupled to a fiber at its entry points.

optical fibre

The main advantages of optical fiber communication over electrical cable transmission is its high bandwidth capability (almost 10Gps) over long distances due to the extremely low loss at same specific wavelengths (e.g. 1.3 um and 1.55 um).  Morover optical fibers are also immune to electromagnetic interference whilst copper wires act as an antenna collecting EM radiation and turning it into current (EM interference).

Here is a brief summary of the multiple advantages of optical fibre communications over electrical cable transmissions.

Fiber optics are:

Safer

The fiber is nonconducting, and is therefore safe in all electromagnetic environments. This means it can safely be used around electrical transmission lines, as well as in high RF and magnetic fields.
A broken or damaged, optical fiber can be detected extremely quickly by monitoring the actual power transmission. With a suitable crowbar circuit, the laser transmission can be shut off within 1-2 microseconds. At a 1 kW laser power level, even a worst-case fiber break releases at most a few mJ of laser power, with no high temperature arc. By contrast, a break in a 1 kV electrical power cable will almost certainly result in an arc discharge and unless great care is taken in the circuit design to detect arcs and minimize energy storage, the arc can persist for milliseconds and deliver joules of energy.

Both high voltages and high power lasers are potential hazards to users or (particulary) service personnel; both hazards can be readily mitigated in most cases by suitable safety interlocks. We do note, however, that high voltage, high current sources can kill if even a small fraction of the power escapes; laser sources can cause eye damage and in extreme circumstances burn skin, but cannot be lethal.

Lighter

For many combinations of power and distance, optical fiber is much lighter than any practical electrical cable.

At voltages below ~1,000 V, electrical conductors are much heavier than optical fiber for similar power delivery capacity. Conductor weight tends to scale as 1/voltage^2 (because power loss varies as V^2/R), so even at 400 volts, copper weighs over twice as much as fiber.
Aluminum conductors are roughly half as heavy as copper for equivalent resistance but still heavier than our best estimate for optical fibers, even with no insulation.  However, aluminum is much lower strength than copper and more difficult to make into fine, highly flexible cables.

No RF interference, emissions , or inadvertent antenna

An optical fiber has no RFI/EMI issue, because it is not a conductor. Copper wire, on the other hand, can leak signals and can be impacted by RFI.

It can act as an antenna, interfering with data being sent along the cable. In extreme cases, copper wire can pick up energy from nearby high power transmitters, not only interfering with data transmission but potentially overheating the wire.



Higher data bandwidth

Multimode optical fiber can readily transmit high bandwidth data over long distances; a typical bandwidth-distance product for multimode fiber is 500 MHz/km, so a 500 m tether can transmit 1 GHz (several Gbits/second, with appropriate modulation).
Signal losses over 500 m are negligible; the bandwidth is limited by dispersion of signals.

Lightweight copper cable, by contrast, has very high losses at high frequencies. Twisted pair optimized for high data rates (Cat 6) can transmit 500 MHz over only 100 meters (bandwidth-distance product of 50 MHz/km); lightweight unshielded cable optimized for power transmission will have even lower bandwidth-distance product.



Both fiber and copper are reasonable in terms of durability

The ultra-pure silica glass used in optical fiber has a higher tensile strength than copper. The jacketing material further strengthens and protects the fiber. The toughness of fiber optic cables is well understood.
The durability of very fine copper wire without thick insulation is not as well understood.



More expensive

Electrical power transmission over copper or aluminum wire is, and will likely remain, cheaper than laser power transmission over fiber. The dominant cost of the laser system is the laser itself, which is not present in the electrical system.


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