Senior Science

9.4Information Systems

Section 6

Optic

Fibres

9.4Section 6:::Optic Fibres

Electrical energy can be converted to light energy for use in optical fibre communication systems

9.4.6.a / Outline properties of optical fibres as communication carriers
9.4.6.b / Outline the principle of total internal reflection and relate this to the advantages of fibre optics over more conventional carriers of information
9.4.6.c / Outline the differences and relative merits in the use of fibre optic cables and metal cables to transmit and receive information
9.4.6.i / Perform a first-hand investigation to demonstrate the transmission of light through an optical fibre
9.4.6.ii / Process and analyse information from secondary sources to compare and contrast copper cables with fibre optic cables in relation to
-carrying capacity
-cost
-rate of information transfer
-security

Introduction:: Using light to communicate

All early light wave communication systems had two major problems

  1. The transmission path needed an unobstructed line of sight between transmitters and receivers. This is because light cannot go around corners by itself.
  2. The transmission path was affected by weather conditions.

These problems were overcome by developing waveguides. The early designs were expensive and could not compete with electrons travelling through copper wire.

The use of glass as a waveguide seemed to be a good idea. Light passes easily through glass. But, there are problems using glass as a waveguide. One problem is the quick loss of light within the fibre (attenuation of light).

A quick activity:
Turn a piece of glass on its end
Can you see through it?
What colour do you see?

In fact, a few centimetres of ordinary glass is opaque. By looking at the end of a glass sheet this can be easily seen. The glass is an opaque green colour. The colour is due to the presence of impurities such as copper, manganese and other minerals.

Several technological breakthroughs have resulted in a communications revolution using glass as the transmission medium.

  • The development of the transistor and computer chips.
  • The development of laser
  • The development of optical fibres – made from very pure glass..
  • The development of light detectors

Putting together a lightwave system

It is common to talk about using visible light in the optic fibres of the telephone system. However, most optic fibre communication does not use light. Rather, infra red wavelengths are actually used. Light refers to the portion of the electromagnetic spectrum that is visible to the eye – 770 to 330 nanometers (770 x 10-9 meters to 330 x 10-9 meters). Infra red radiation is used because it loses less energy and can travel greater distances through the optic fibre. Only certain sections of the infra red spectrum are used in optical fibres for communication – these are bands around 850nm, 1300nm, and 1550nm.

Visible light in optic fibres has a large number of uses. A fibrescope is a particular example. A device in which a number of fibres are combined in order to transmit an image is called a fibrescope. A large number of fibres need to be combined in one bundle to provide a satisfactory image. In the average fiberscope there are 750 000 fibres, each being 0.001 centimetres in diameter. These instruments have a wide range of applications. For example, they are used to view internal organs such as the stomach. They also have found wide range of uses in industry. For example, it can be used for inspection in inaccessible areas to examine turbines and in parts of a nuclear reactor.

Optical demonstrationsLaser light

A.Reflection

ActivityShine a laser across the room onto a wall.

QuestionWhy do we see a red spot?

ResponseThe laser is reflected off the wall. Our eyes detect the light energy as it in the visible region of the electromagnetic spectrum.

ActivityShine a laser across the room. Tap a blackboard duster above the beam.

QuestionWhy do we see the beam?

ResponseThe laser beam is reflected off the chalk particles suspended in the air.

ActivityReflect the laser beam using a normal mirror. Measure the angle of

incidence and the angle of reflection

QuestionHow do these angles compare

ResponseI = R; The angle if incidence equals the angle of reflection.

  1. Refraction

ActivityShine the laser through an empty beaker and note where the beam exits.

Repeat while pouring water into the beaker.

QuestionWhat happens?

ResponseAs the water fills the beaker, the light passes through a different medium. This causes the laser to bend more.

ActivityShine the laser onto a solid block of ice

QuestionWhat happens?

ResponseThe block of ice glows as the light is totally internally reflected due to the flaws in the ice structure.

ActivityShine the laser onto the round side of a semicircular prism.

QuestionWhat happens?

ResponseAs the angle of incidence increases past the critical angle the laser beam will no longer be transmitted but will be internally reflected.

ActivityShine the laser horizontally into the side of a

plastic bottle at a small hole on the opposite side of the bottle. Slowly fill the bottle with water.

QuestionWhat happens?

ResponseThe laser light will be totally internally reflected within the water spout and will appear in the bottom of the beaker used to collect the overflow. Try this on an OHP!

9.4.6.a / Outline the properties of optical fibres as communication carriers

What is an Optic Fibre?

An optic fibre consists of two sections with a covering.

  • The glass core, which runs through the center of the strand, and
  • The glass cladding, that surrounds the core.

The glass in the cladding is optically less dense than the core glass. As such the cladding glass has a lower refractive index. This causes the cladding to act as a mirror for light travelling in the core. As a result, the light (infra red) travels through the core by a series of continuous reflections.


DiagramA Single Mode Optical Fibre


Diagram:Size of an optical fiber compared to some common objects.

The Properties of Optical Fibres as Communication Carriers

The main job of an optical fibre is to guide light with a minimum loss of signal.

Optical fibres have a number of properties that allow them to act as communication carriers.

An optical fibre is made from very pure, almost mineral free glass. This purity means that light can travel through an optic fibre for hundreds of kilometres.

An optic fibre cable is flexible and can be easily bent. This flexibility makes the fibre very useful. It means that the cable can be laid so that it bends around obstacles without the need for special relay (reflection) devices.

The structure of an optic fibre results in light rays undergoing total internal reflection. This is an important property of the fibre as it means light is transmitted with very little loss of energy over long distances.

Optical fibres are capable of transmitting light at about 2/3 the speed of light in a vacuum. This means that communication using optic fibre is very fast.

When an optical fibre transmits light there must be some loss of signal strength. Attenuation is the loss of signal strength. The amount of light coming out at the end of the fibre is less than the amount of light originally transmitted.

Certain properties of the fibre cause a loss of signal strength. (ie attenuation).

  • Impurities in the glass absorb some of the light. The impurities are mostly heavy metals such as iron, copper, vanadium, cobalt, nickel, manganese and Chromium. OH- ions that come from water are also impurities that absorb signal strength. Certain wavelengths must be avoided 1.37, 1.23, 0.95m because of absorption by the OH- ions.
  • Changes in density of the glass can cause the light to scatter.
  • Light is lost at bends. At a bend, the angle at which the light hits the core/cladding interface results in transmission, not reflection. Light entering the cladding glass is lost.

The Total Attenuation graph below shows the wavelength bands that are transmitted with the greatest efficiency (ie 850nm, 1300nm, and 1550nm).

There are two basic kinds of optical fibres.

  • Single-mode fibres
  • Multi-mode fibres

Not all rays entering a fibre will be guided through the fibre. Only some directions are allowed. These allowed directions are called modes. A multimode fibre supports more than one propagating mode.

The signal loss (attenuation) in a single mode fibre is lower than that in a multi mode fibre.

A single mode fibre

Cladding
------Core
Cladding

A multi-mode fibre

Cladding
Core
Cladding

Single-mode fibres are used for long distance transmissions. They have extremely small cores, and they accept light only along the axis of the fibres. As a result, single–mode fibres require the use of laser light. They also need to be precisely connected to the laser, to the other fibres and to the detector.

Multi-mode fibres have cores larger than those of single mode fibres. They accept light from a variety of angles. Multi-mode fibres can use more types of light sources and cheaper connectors than single-mode fibres, but they cannot be used over long distances.

Several optical fibres are usually packed together to make a cable.

DiagramFibre Optic Cable

Notes Questions

  1. Why can’t ordinary glass be used to make optical fibre?
  2. What is special about the glass used to make optical fibres?
  3. What is the source of light that passes through optic fibres?
  4. For telephone communication,
  5. What part of the electromagnetic spectrum is used in optic fibres?
  6. What is the advantage of using this part of the electromagnetic spectrum?
  7. Outline one situation in which visible light is used in optical fibres.
  8. Name the important two sections of an optic fibre.
  9. How does the infra red radiation travel through the optical fibre core?
  10. An optic fibre core has a diameter of 10m – how many millimetres is this?
  11. Which has the bigger diameter – an optic fibre or a human hair?
  12. How far can infra-red travel through an optical fibre?
  13. Why is it important that optical fibres are very flexible?
  14. How fast does an infra red signal travel inside an optic fibre (answer in ms-1)
  15. What is attenuation?
  16. Name three impurities in the glass used for optical fibres?
  17. Explain why it is important to remove impurities from the glass used for optical fibres.
  18. What wavelength bands are used to transmit telephone signals in optical fibres?
  19. What is an optical fibre cable?

Notes Questions (for next page)

  1. CompleteLight travels (faster/slower) through air than through glass.
  2. Draw a diagram showing a ray of light that is transmitted at a glass/air boundary.
  3. Describe what is meant by total internal reflection (Draw a diagram).
  4. Outline what is meant by the term “wave guide”.

9.4.6.b / Outline the principle of total internal reflection and relate this to the advantages of fibre optics over more conventional carriers of information

How does fibre optics work?

Light travels faster through air, than it does through glass. This is because glass is more optically dense than air. Light travels at different speeds through different types of glass or plastic. The optical density is not the same in different types of glass and plastic.

When a ray of light reaches a boundary it can be both reflected and transmitted.

  • The reflected ray is reflected such that the incident angle = the reflected angle.
  • The transmitted ray is refracted (ie bent because of the change in speed in the different mediums)

Diagram – reflection, transmission / Diagram – Total internal reflection

Sometimes, the ray of light undergoes total internal reflection. No transmission of the ray of light occurs (ie there is no loss of light from the glass).

This only happens when the light is travelling from an optically dense medium to an optically less dense medium.

Also it only happens when the ray hits the boundary at a sharp angle.

It is total internal reflection that allows an optical fibre to act as a wave guide for light.

DiagramA fibre optic wave guide showing total internal reflection

Cladding
Core
Cladding
In physics, angles are measured from the normal. The normal is a line drawn at right angles to the boundary surface. Therefore, total internal reflection occurs when a light ray hits the boundary at an angle greater than the critical angle.
9.4.6.c / Outline the differences and relative merits in the use of fibre optic cables and metal cables to transmit and receive information

Optical Fibres versus Metal Cables?

There already exists a large network of metal cables to transmit telephone messages. It is very expensive to replace this existing network of cables, particularly in remote areas. For this reason telecommunication companies are using both the “old” metal cables as well as gradually introducing new optic fibre technology. In remote areas use of satellite and microwave technology is cheaper than developing a cable link.

Optical fibres have many advantages over conventional electrical circuits. Fibre optics technology is new but because of its high efficiency, optical fibre cables are steadily replacing the traditional metal lines. Currently all undersea cables are made of optical fibres.

  • Greater information carrying capacityThe wide bandwidth of optical fibre allows transmissions of very large amounts of data in very short times. With new technology it is possible to transmit 240 000 telephone conversations on a pair of optical fibres. Lucent technologies has recently developed a new fibre that is capable of carrying the current per-second traffic of the entire worldwide Internet traffic on a single strand of optical fibre. This is the same as transmitting over 90 000 volumes of an encyclopedia in one second.

By comparison the best electrical or microwave links can carry around 100 000 telephone conversations.

  • Lower energy lossWith the very low energy loss of the latest optical fibres, spans of over 100km are possible. Generally, optical fibre signals need less amplification than signals sent down copper wire.

Transmission of microwaves requires relay towers every 40 km. This is because microwaves need line of sight relays. Copper cables, like optical fibres, can be laid virtually anywhere. The signals in copper wires loose energy relatively quickly. Therefore, they require more regular boosting and larger energy input, than light signals.

  • Not subject to electrical or electromagnetic interferenceBecause glass is an insulator, there are no problems from contacts with electrical circuits or other fibres. For the same reason, there are also no problems such as cross talk and surge currents due to lightning strikes. Consequently, there is no need for cable transposition schemes or high voltage protection equipment. Optic fibre wave guides are not affected by inclement weather

Lightning strikes can severely effect metal telephone lines – high voltage protection equipment needs to be installed.

  • WeightAn optical fibre is thinner than a human hair and takes less room than an ordinary metal cable. Individual fibres are not particularly strong, but a cable can be very strong by having a Kevlar ‘outer jacket’ and a central strength member.

Metal cables are quite strong. However, to transmit the same volume of data as an optical fibre, the equivalent metal cable would be much larger and therefore very heavy.

  • SecurityIt is difficult to cut into an optic fibre to listen to another persons telephone conversation. The digital nature of the signal and a number of other factors means very sophisticated and expensive electronic equipment would be needed. For this reason, security of telephone conversations and other transmissions of data is quite high.

Phone tapping of metal cables is relatively easy and cheap. Since mobile phones broadcast their signal to nearby transmission towers, other people can intercept the signal. Concerns about privacy are therefore an issue with these telephones.

  • Jointing optical fibresSignal losses occur at joints in optical fibres. It needs to be a precise procedure. The usual method is called fusion splicing. The two fibres are fused or welded together. The ends of each fibre need to be squared, cleaned and then butted together under a microscope. An electric arc is used to weld the ends together. Most fusion splicing sets now have automatic control of both arcing time and fibre alignment.

Joining electrical cables is relatively easy and can be performed with very little specialised equipment.

  • Signal boostingIn fibre optic communication, the signal can become distorted. The distortions need to be corrected. Signal compensation and regeneration overcome this problem. Signal compensation is a cheap process needing little energy input and relatively simple technology. Regeneration is much more expensive and needs complicated, electronic technology. Regeneration also results in a energy input.

In fibre optic communication the signals are digital – ie small, separate pulses of light.

Light travels through the core at different angles. This means that different parts of the same signal (pulses) will travel different distances and will arrive at different times.

Compensator

Original pulseDispersed pulse /

Compensated pulse

As well, the individual pulses also spread out over the long distances travelled.

The result is the receiver can no longer distinguish where one pulse begins and another ends.

For electric signals, compensation and regeneration is not needed. Regeneration of signals does occur for microwave links.

  • Cost

.

Notes Questions (for next page)

  1. Why do companies like Telstra still use metal cables to carry telephone signals?
  2. Give one reason why companies like Telstra might use microwave links rather than laying a cable system?
  3. Name two advantages of fibre optic cables over metal wire cables.
  4. How many telephone conversations can be transmitted on the “current best” optic fibre technology?
  5. Compare the energy losses in optic fibres compared to metal wires.
  6. Why do metal telephone cables need high voltage protection?
  7. Name the two features of an optic fibre cable that makes it very strong?
  8. Why is privacy an issue for mobile phones and telephones using metal cables?
  9. Which cable is easier to join – optic fibre or copper wires?

9.4.6.ii / Process and analyse information from secondary sources to compare and contrast copper cables with fibre optic cables in relation to
-carrying capacity
-cost
-rate of information transfer
-security
9.4.6.i / Perform a first-hand investigation to demonstrate the transmission of light through an optical fibre
ACTIVITYTotal Internal Reflection

AimTo demonstrate total internal reflection