Fibre Optic Communication - FAQ

What is it?

It is a communication technology that makes use of light to carry data and information.

How does it work?

The part that carries the light from one place to another is called a fibre-optic cable, sometimes called a 'light-pipe'. This describes its function very well as a fibre optic cable is designed to allow light to pass through it with very little loss.

What is it made of?

A high quality communication cable is made of glass (or silicon dioxide to be precise). Normally you think of glass as being brittle and easily broken, but when it is made thinner than a human hair, it is very flexible and strong.

Of course it is not just simple glass. The outside of the cable is designed to reflect light inwards whilst the middle core is as transparent as possible. So when a beam of light is 'launched' into one end of the cable, it bounces along the cable until it reaches the other end. This type of cable, one that uses reflections to work, is called a 'multi-mode' cable.

There are also 'single-mode' fibre that use a different effect and work for longer distances - but it is more expensive.

You can also get fibre-optic cable made of plastic. This is a good solution for very short distances (a few metres) so you might see it used in hi-fi systems to carry audio information.

Is it just any light?

No, fibre optic is transparent at a very precise wavelength of light i.e. colour. Typically it is very transparent at 1550 nm (nano metre is 1 thousand millionth of a metre). We would see it is a reddish colour.

How is the light created in the first place?

There are two main ways.

1. A Light Emitting Diode (LED).  LEDs are very cheap and do produce reasonably pure light of the right wavelength. So they are used for low power, short distance applications such as local area networks. LEDs are good for a few kilometers and a speed of about 100Mb/sec

Only about a hundredth (1%) of the light it produces is useful for the cable. But even that is good enough for smallish distances.

2. Laser Beam This is more expensive but has a far higher purity than LED. This means that nearly half the light is useful in the cable, so it is very efficient.

Power can be also be much higher, so allowing far greater distances to be covered.

How fast can it transmit data?

Transmission speeds can be extremely high.

For example, recently a 160Km length of cable carried information at 14,000 Gigabits / sec. This kind of performance is used in oceanic cables that connect continents together.

How does it compare to good old copper?

It is very tough - it does not corrode, unlike copper and it is not affected by electrical interference such as lightning or stray radio signals.

But it is more expensive, so copper is still used for most local communications.

Where is fibre-optic communication mainly used?

The heart of the UK telephone system runs on fibre-optic because it is almost the perfect solution - no interference, massive speed, easily carries voice, television, internet, and telephone calls all down the same cable. But it isn't cheap!

Aircraft use fibre-optic cables to transmit data to different systems. They cannot risk any electrical interference interrupting data transmission.


Immunity to Electromagnetic Interference

Although fiber optics can solve data communications problems, they are not needed everywhere. Most computer data goes over ordinary wires. Most data is sent over short distances at low speed. In ordinary environments, it is not practical to use fiber optics to transmit data between personal computers and printers as it's too costly. Electromagnetic Interference is a common type of noise that originates with one of the basic properties of electromagnetism. Magnetic field lines generate an electrical current as they cut across conductors. The flow of electrons in a conductor generates a magnetic field that changes with the current flow. Electromagnetic Interference does occur in coaxial cables, since current does cut across the conductor. Fiber optics are immune to this EMI since signals are transmitted as light instead of current. Thus, they can carry signals through places where EMI would block transmission.

Data Security

Magnetic fields and current induction work in two ways. They don't just generate noise in signal carrying conductors; they also let the information on the conductor to be leaked out. Fluctuations in the induced magnetic field outside a conductor carry the same information as the current passing through the conductor. Shielding the wire, as in coaxial cables can reduce the problem, but sometimes shielding can allow enough signal leak to allow tapping, which is exactly what we wouldn't want.

There are no radiated magnetic fields around optical fibers; the electromagnetic fields are confined within the fiber. That makes it impossible to tap the signal being transmitted through a fiber without cutting into the fiber. Since fiber optics do not radiate electromagnetic energy, emissions cannot be intercepted and physically tapping the fiber takes great skill to do undetected. Thus, the fiber is the most secure medium available for carrying sensitive data.

Non Conductive Cables

Metal cables can encounter other signal transmission problems because of subtle variations in electrical potential. Electronic designers assume that ground is a uniform potential. That is reasonable if ground is a single metal chassis, and it's not too bad if ground is a good conductor that extends through a small building. However, the nominal ground potential can differ by several volts if cables run between different buildings or sometimes even different parts of the same building. Signal levels in semiconductor circuits are just a few volts, creating a problem known as ground loop. When the difference in ground potential at two ends of a wire gets comparable to the signal level, stray currents begin to cause noise. If the differences grow large enough, they can even damage components. Electric utilities have the biggest problems because their switching stations and power plants may have large potential differences.

A serious concern with outdoor cables in certain computer networks is that they can be hit by lightning, causing destruction to wires and other cables that are involved in the network. Certain computer companies are aware of this problem and trying to solve it by having protective devices for wire circuits to block current and voltage surges. Any conductive cables can carry power surges or ground loops. Fiber optic cables can be made non-conductive by avoiding metal in their design. These kinds of cables are economical and standard for many indoor applications. Outdoor versions are more expensive since they require special strength members, but they can still be valuable in eliminating ground loops and protecting electronic equipment from surge damage.

Eliminating Spark Hazards

In some cases, transmitting signals electrically can be extremely dangerous. Most electric potentials create small sparks. The sparks ordinarily pose no danger, but can be really bad in a chemical plant or oil refinery where the air is contaminated with potentially explosive vapours. One tiny spark can create a big explosion. potential spark hazards seriously hinder data and communication in such facilities. Fiber optic cables do not produce sparks since they do not carry current.

Ease Of Installation

Increasing transmission capacity of wire cables generally makes them thicker and more rigid. Such thick cables can be difficult to install in existing buildings where they must go through walls and cable ducts. Fiber cables are easier to install since they are smaller and more flexible. They can also run along the same routes as electric cables without picking up excessive noise. One way to simplify installation in existing buildings is to run cables through ventilation ducts. However, fire codes require that such plenum cables be made of costly fire retardant materials that emit little smoke. The advantage of fiber types is that they are smaller and hence require less of the costly fire retardant materials. The small size, lightweight and flexibility of fiber optic cables also make them easier to be used in temporary or portable installations.

High Bandwidth Over Long Distances

Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables. The information carrying capacity increases with frequency. This however, doesn't mean that optical fiber has infinit bandwidth, but it's certainly greater than coaxial cables. Generally, coaxial cables have a bandwidth parameter of a few MHz/km, where else the fiber optic cable has a bandwidth of 400MHz/km. (These figures are just approximations and do vary from cable to cable!) This is an important factor that leads to the choice of fiber for data communications. Fiber can be added to a wire network so it can reach terminals outside its normal range.


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