Photonics: A New Revolution Principles of Optical Fibre Networks with applications … ·  · 2010-01-04Photonics: A New Revolution Principles of Optical Fibre Networks with applications

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Photonics: A New Revolution

Principles of Optical Fibre Networks

with applications to the classroom

Dane Austin

Hong Nguyen

Snjezana Tomljenovic-Hanic

Neil Baker

CUDOS, School of Physics

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Part 1: Optical Fibre Network

Until recently, virtually all communication systems have relied on transmission over electrical

cables or by use of electromagnetic radiation* (radio-frequency and micro-wave) propagating in

free space. The demand for data traffic has initiated the development of optical

telecommunications.

1.1 Historical perspective

1. Since 1960, when the first laser* was built, it has been recognized that the coherence

properties of laser light, and its frequency (of the order of 2 1014Hz) provide enormous

potential bandwidth.

*Light amplification by stimulated emission of radiation: A device that produces a high-intensity,

directional, monochromatic beam of light.

2. The next step was finding an appropriate optical carrier and in 1966 the first glass fibre

with an attenuation below 20dB/km was made. Since then the loss has been reduced and

is now below 0.2dB/km at a wavelength of 1550nm. Optical fibre technology has become

the standard for long-distance data communication.

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Examples of optical fibre cables. The range of fibres produced by Corning.

3. The concept of integrated optics started in 1969 and has led to the the fabrication of

chips* which integrate many components on a single base (substrate) and have a size of

several square centimetres. These devices are now commercially available and offer

reliable and cost-effective solutions to device requirements.

4. With the introduction of Wavelength Division Multiplexing (WDM) *, fibre network links

have become high-capacity advanced telecommunication links.

5. However, the thirst for faster, cheaper, more reliable data communications, with smaller

device features, is still not satisfied. State-of-the-art optics, and optics based on photonic

band gap materials * may, in the future, answer these demands.

* Demand for high-capacity telecommunication links has resulted in an increase in the use of

wavelength-division multiplexing (WDM), which is the technology of combining a number of

wavelengths into the fundamental mode of the same fibre.

* The Vision of CUDOS is to develop the experimental

and theoretical expertise to design and build linear and

nonlinear all-optical signal processing devices, and to

miniaturize these, leading to a photonic chip" believed

to be the building blocks for the next generation of

optical systems.

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1.2 Basic Components

Basic components of a simple

transmission system include:

! a modulated semiconductor laser

pig-tailed to a single-mode fibre to

generate a train of light pulses;

! some 45-50 km of single-mode

fibre cable between repeaters;

! an electronic repeater or optical

amplifier; more lengths of fibre and

! repeaters as required; and

! a semiconductor detector at the

end of the system.

1.3 Optical Fibres

• Optical fibre operates at infrared optical wavelengths (700nm-1600nm).

• The largest category of glasses from which optical fibres are made consists of the oxide

glasses. The most common is silica (SiO2) which has a refractive index of 1.458 at

850nm. The raw material for silica is sand.

• A cylindrical dielectric waveguide made of low-loss materials (silica glass or polymer).

The core of a standard fibre has a refractive index slightly higher than the cladding, so the

light is guided along the fibre axis by total internal reflection (TIR).

•Photonic crystal •Photonic crystal fibre

•Photonic crystals are a new class of materials. Their operation is different from conventional

waveguides. The structure is periodic and regularly repeats on a scale of microns.

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! TIR appears at the interface between the fibre core (nco) and cladding (ncl); nco> ncl

! A ray incident from air into the fibre becomes a guided ray for an incident ray angle, !0,

if:

Advantages of Fibre Optics

! Comparing to conventional metal wire (cooper), optical fibres are:

– Light travels faster than electric signal in the conventional metal wires (100 times faster).

– On using monochromatic light the signal distortion is low.

– Low attenuation, i.e. 0.2 dB/km which is approximately 10 times less than telephone

cables.

! An elastic plastic buffer encapsulates the fibre.

! Actual optical fiber consisting of core,

cladding and buffer coating needs protection

when being installed and it’s the cable that

provides it. Cables may have from one to

hundreds of fibres inside.

! Finally, the jacket is a tough outer covering on the cable.

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0sin

clconn !<"

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– Lower power transmitter instead of the high-voltage electrical transmitter.

– A high band with (data carrying capacity) of the order of GHz which means that they can

carry 100 million times more information.

– Application in the medical field

Small bundles of fibres, not much larger than a hypodermic needle can easily be inserted

into the spot of interest.

! Comparing to conventional metal wire (cooper), optical fibres are further:

– Less expensive

– Thinner

– Lightweight

– Non-flammable (melting point for silica 1900 degree Celsius).

– Non-conductive

– Non-inductive

– Flexible

Terrestrial fibre systems

Optical fibre cable on the drum at the front of the tractor is fed directly into the ground through

the ripper at the back, to be located about 1m deep. Up to 20km of cable can be laid in one day

over open country.

The optical fibre cable is fed from the large drums on the lorry and is hauled through

underground ducting by a winch about 1km away. Each drum holds about 5 km of single-mode

optical fibre cable.

1.5 Submarine Fibre Systems

International submarine optical fibre cable network.

Australia-First submarine optical fibre cable: Tasman 2 in 1991

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This network has replaced satellites for voice circuits, eliminating the 0.3 sec round-trip time

delay due to the distance from earth to the orbiting geo-synchronous satellites above the equator.

Submarine cables are fabricated in continuous lengths of thousands of kms, including repeaters,

and are loaded directly onto a cable ship from the factory.

Repeaters are spaced approximately 50kms apart.

References and further reading

1. Glossary | Communicating with light – fibre optics

http://www.science.org.au/nova/021/021glo.htm

2. Wikipedia, the free encyclopedia: http://en.wikipedia.org/wiki/Optical_fibre

3. CUDOS is an Australian Research Council Centre of Excellence. CUDOS is a

collaborative project combining the established expertise of researchers at the University

of Sydney, Australian National University, Macquarie University, Swinburne University

and the University of Technology, Sydney. It also builds on research links with photonics

research groups at other Australian universities and CRCs, and with international partners

such as Bell Labs and OFS Laboratories in the USA, and CNRS in France.

http://www.physics.usyd.edu.au/cudos/

4. The Optical Fibre Technology Centre is an interdisiplinary research department of the

University of Sydney. OFTC carries out both academic and commercial research over a

wide range of optical fibre and photonic devices: http://www.oftc.usyd.edu.au/

5. J. Hecht, ‘City of Light’, Oxford: Oxford University Press, 1999.

6. H. J. R. Dutton, ‘Understanding optical communication’, New Jersey: Prentice Hall,

1998.

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Part 2: Applications to classroom

2.1. Total internal reflection

Since total internal reflection takes place within the fibres, no incident energy is lost due to the

transmission of light across the boundary.

Task 1

Make a demonstration in class involving the use of the

following materials:

• large jug filled with water

• laser beam (laser pointer).

Remark

You can use a large plastic fruit juice bottle

(the bigger the better) and a torch.

Task 2

Explain the observation.

Task 3 (optional)

Prepare an illustration of total internal reflection, as a mechanism of guidance in optical fibres,

similar to this figure:

Task 4 - another demonstration of total internal reflection

Equipment

• Glass or beaker of water with a few drops of milk in it (you may have to experiment to

optimise the amount of milk)

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• Laser transmitter unit

Laser beam

Laser beam

Schematic of the arrangement for ‘Total internal reflection’.

Response to Task 1

Procedure

1. Direct the beam of laser light into the jug from the opposite side of the hole, through the

water and into the falling stream.

2. Observe.

3. Give an explanation for the observation.

Response to Task 2

Explanation

The laser light which exited the jug through the hole was still in water, due to total internal

reflection. Being in the more optically dense medium (water) and heading towards a boundary

with a less optically dense medium (air), and being at angles of incidence greater than the critical

angle, the light did not leave the stream of water. In fact, the stream of water acts as a light pipe

to channel the laser beam along its trajectory. Reference: The physics classroom

(www.glenbrook.k12.il.us/gbssci/phys/Class/refrn/u14l3c.html)

2.2. Optical fibres as !light pipes"

Light travels in straight lines, but light sent through optical fibres can go around corners.

Task 1

Model the effect of optical fibres in the experiment using the materials listed below using the

procedure you would give to students for the experiment.

Materials

• solid glass rod about 50 to 60 centimetres long, with a bend in the middle

• 2 clamps and stands

• sheet of cardboard (approximately 50 x 50 centimetres)

• torch

• piece of white paper (A4 or smaller)

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Task 2

Prepare three questions for the students.

Task 3 (optional)

A student may notice that the light travelling down the rod is coloured. Explain why this is

happening.

Response to Task 1

Procedure

1. Put the glass rod into the clamp on the stand.

2. Make a hole in the centre of the sheet of cardboard and slide it over the end of the rod. The

cardboard will act as a shield against spurious light that isn’t focused into the rod.

3. Focus the torch on one end of the glass rod, and clamp it into position. (The torch can touch

the glass rod.)

4. Hold a piece of white paper a short distance from the other end of the rod.

5. Observe the light beam on the paper.

Response to Task 2

Questions

1. Given that light usually travels in a straight line, where would you expect the light from the

torch to appear?

2. Explain why the light follows the bend in the glass rod.

3. There is only a weak beam of light transmitted through the glass rod. Why?

Answers to questions

1. Normally the beam would emerge on a spot directly in front of the torch.

2. When light travelling through the glass rod meets the air-glass interface at a small enough

grazing angle, the light is reflected back into the rod.

3. Much of the light from the torch has been absorbed by the glass rod. All glass absorbs light.

For example, when you look through a window pane, only about 90% of the external light

is visible through the pane.

Response to Task 3

Glass does not absorb all the wavelengths of light equally (e.g. Pyrex glass preferentiality absorbs

blue and red wavelengths, transmitting yellow-coloured light). Infrared light is used to send

messages down optical fibres because glass absorbs least in the infrared part of the spectrum.

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Preparation for the class

• Obtain a rod of glass about 3 to 5 millimetres in diameter. Put a 30-40° bend in it by

heating the middle of the rod with a Bunsen burner until the glass softens.

• You can use a small torch with a diameter similar to the diameter of the glass rod. If you

use a larger torch, you may need to use black tape or paper around the space between the

light and the rod to reduce the amount of light that is not focused on the glass rod.

• A small halogen lamp could be used instead of a torch, but this will get too hot to allow

you to wrap the intervening space with tape or paper.

• Try the following variation to focus more light down the rod: remove the glass from the

torch, put the rod as close as possible to the bulb and use a lens to focus the light onto the

end of the rod.

• The results are easier to see if the room is darkened.

• This experiment can be done in small groups if you have enough equipment.

• Use questions from Task 2.

• If the students do not ask about coloured light, use Task 3 as a question.

Reference: www.science.org.au/nova/021/021act.htm

2.3: Light behaves as a Wave — Polarization

A light wave is an electromagnetic wave which is a transverse wave, having both an electric and

a magnetic component. Waves carry energy from one location to another. If the frequency of

those waves can be changed, then they can also carry a complex signal which is capable of

transmitting an idea from one place to another.

As with microwaves, we cannot see individual light waves. How do we know that light behaves

as a wave?

Light emitted by the sun or by a lamp is unpolarised light. Such light waves are created by

electrical charges which vibrate in a variety of directions. The concept of polarization is hard to

visualize. It is much easier to understand concept of polarised light. It is possible to transform

unpolarised light into polarized light by transmission, reflection, refraction and scattering.

Fig. 1 The most common method of polarization using a polaroid filter.

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Polaroid filters are made of a special material which is capable of blocking one of the two planes

of vibration of an electromagnetic wave. When unpolarised light is transmitted through a polaroid

filter, it emerges with vibration in a single plane.

Fig. 2 Polarization by reflection off non-metallic surfaces.

Non-metallic surfaces such as asphalts roads, snow fields and water reflect light such that there is

a large concentration of vibrations in a plane parallel to the reflecting surface. A person viewing

objects by means of light reflected off non-metallic surfaces will often perceive a glare.

Fig. 3 Iceland Spar, a form of the mineral calcite, refracts incident light into two different parts.

The light is split into two beams upon entering the crystal. They are polarized with perpendicular

orientation. If an object is viewed using this crystal, two images will be seen.

Another example comes from photonics:

! In single-mode fibre, we really have not one, but two propagating states (modes), due to

the fact that light can exist in two orthogonal polarisations.

! Polarisation states are not maintained in standard fibre, as light couples from one

polarisation to the other randomly along the fibre.

! As the light propagates along a fibre, it constantly changes in polarisation in response to

variations in the fibre’s composition and geometry.

! Power is not lost, but the axis of polarisation and the orientation of electric and magnetic

fields in relation to them constantly change.

! If there is a polarisation-sensitive device that has significantly higher losses in one

polarisation mode than the other, then the effect of introducing a signal that is constantly

changing in polarisation is to produce changes in total signal power.

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! Polarization-maintaining (PM) fibres are designed to maintain the polarisation state of the

signal.

! This is achieved by making the fibre asymmetric in its profile.

• PM fibres are used primarily in fibre optic gyroscopes, sensors, and in the pig-tailing of

lasers and modulators.

Reference: The physics classroom (www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l1e.htm)

Other Activities

• Five simple activities demonstrating the principle of the 'light pipe' used in optical fibres

can be found in Australasian Science, Spring 1993, pages 46-47.

• 'Fibre optics in the kitchen', activities demonstrating properties of optical fibres, can be

found in The Helix No. 48, June/July 1996, page 23. (Also available at

http://www.publish.csiro.au/helix/cf/issues/th48b.cfm)

• Newton's Apple is a US national science television program. Activities on optical fibres

(together with an introduction to telecommunications) can be found at

http://www.ktca.org/newtons/9/tele.html. And an activity on sending and receiving digital

images can be found at http://www.ktca.org/newtons/10/stllite.html

Cross-sections of polarisation-maintaining fibres: (a) elliptical core, (b) bow tie, and (c) ‘panda’ fibre.

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Part 3: Experiments

Experiment 1 - Modulated light carries information

Aim

To show simple audio information can be encoded onto light by modulating the light

intensity.

Equipment

• Signal generator or PC with supplied signal generator software

• Cable with alligator clips to connect signal generator and LED transmitter unit, or stereo

cable to connect PC headphone socket and LED transmitter unit

• LED transmitter unit, with only one LED turned on

• Receiver unit (check it is on and the volume is turned up)

Light

Detector

Signal

Generator

LED

transmitter

unitLight

Detector

Signal

Generator

LED

transmitter

unit

Schematic of the arrangement for experiment ‘Modulated light carries information’

Method • Turn the frequency of the signal generator to around 1 Hz and turn the output voltage

down to its minimum value.

• Connect the signal generator output to the LED transmitter unit, and ensure the LED

transmitter is on. Check that you have connected the signal generator to the LED that is

on.

• Slowly turn up the output voltage on the signal generator so the LED light starts to fade

and grow strongly.

• Place the photodiode on the receiver unit 10 cm away from the LED. Turn the receiver

unit on.

• Increase the frequency of the signal generator and listen to the change in the audio

frequency

• Record what over what frequency range you can hear a tone (Note that audio frequencies

below around 15 Hz are out of the range of human hearing).

What changing light property contains the information that represents sound?

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What does this demonstrate?

The student observes the flashing light and simultaneously hears a tone. The flashing LED and

the audio frequencies change together when the signal generator frequency is changed. The

student understands that simple audio information can be encoded onto light by modulating the

light intensity.

Extra activity

• Ask the student to point the receiver around the room.

• The student should generate an audio buzz from the fluorescent lights, which flash at 100

Hz.

• The student should then do the same with an incandescent light, which do not flash, and

thus will give no audio buzz.

• Ask the students what can they deduce about these two light sources.

• The students should deduce that fluorescent lights flash and incandescent lights have

continuous output.

On comparison with audible tones from the frequency generator the student should be able to

estimate the frequency of the flashing.

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Experiment 2 - Light can carry complex information

Aim

To demonstrate that complicated information can be encoded onto light.

Equipment

• Radio or CD player

• Stereo cable

• LED transmitter unit, with only one LED on

• Receiver unit (check the volume is turned up)

Light

Detector

Radio

or CD player

LED

transmitter

unit

Light

Detector

Radio

or CD player

Light

Detector

Radio

or CD player

LED

transmitter

unit

Schematic of the arrangement for ‘Light can carry complex information’.

Method • Use the stereo cable to connect the headphone socket on the radio or CD player to the

audio input socket of the to the LED transmitter unit

• Check that you have connected the radio or CD player to the LED that is on.

• Place the photodiode on the receiver unit 10 cm away from the LED. Turn the receiver

unit on.

• The radio station or CD should be audible from the speaker on the receiver unit. If not

check everything is on, the volume knob is turned up and you have connected the correct

LED.

• Block the light from the LED. The sound should stop.

What does this demonstrate?

This demonstrates that complicated information, in this case music, can be encoded onto light.

The student should be able to understand that other types of information, such as internet data,

could also be encoded onto light.

How far can you move the receiver away from the transmitter before you can no longer

hear the sound? What determines this distance?

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Experiment 3 - Signals in optical fibre go further!

Aim To demonstrate that light can be guided.

Equipment

• Radio or CD player

• Stereo cable

• LED transmitter unit, with only one LED on

• Receiver unit (check the volume is tuned up)

• Straight and bent clear plastic rod (to simulate optical fibre)

• Sample of optical fibre

Method • Separate the LED transmitter and receiver just far enough apart so that the clear plastic

rod can fit between the LED and the photodiode.

• Make sure the LED is connected to an audio signal from the radio or CD player

• Turn the transmitter and receiver on. You should hear the music playing softly on the

receiver unit’s speaker.

• Insert the straight plastic rod between the LED and the receiver unit, butting the ends of

the rods against the LED and photodiode. The sound volume should increase

considerably.

• Look at the end of the clear plastic rod with the LED on. It should be clear that a

considerable amount of light is guided by the rod.

• Try substituting the straight piece of clear plastic rod for the bent piece

• Try joining several pieces of clear plastic rod.

• Show a sample of optical fibre. Explain the connection with the clear plastic rod.

Light

DetectorRadio

or CD

player

Approximately 30 cm

Light

DetectorRadio

or CD

player

Approximately 30 cm

Schematic of the arrangement for ‘Signals in optical fibre go further’

What does this demonstrate?

Light can be guided, thus increasing the distance optical signals can be sent, and that light can be

guided around bends.

Can you think of two reasons optical fibre might be used in optical communications?

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Experiment 4 - Scattering Loss in optical fibre

Aim To demonstrate that waveguides have loss.

Equipment • Radio or CD player

• Stereo cable

• LED transmitter unit, with only one LED on

• Receiver unit (check the volume is turned up)

• Long and straight clear plastic rod

Method • Ensure radio or CD player is tuned and on.

• Connect radio or CD player to LED transmitter unit with cable and turn transmitter on.

• Butt the LED against one end of the rod so the light is guided within the rod.

• Put the photodiode very close to the side of the rod, angled away from the transmitter.

• You should be able to hear the radio signal from the receiver speaker.

• Run the receiver along the length of the rod. The sound should fade the further the

receiver is moved away from the transmitter.

Radio

or CD

player

LED Receiver

unitLED

Transmitter

unit

Radio

or CD

player

LED Receiver

unit

LED Receiver

unitLED

Transmitter

unit

Schematic of the arrangement for !Scattering Loss in optical fibre"

What does this demonstrate?

Firstly, it shows that some of the light is scattered out of the rod. This is because imperfections in

the rod scatter the light at large angles. Some of the scattered rays do not satisfy the condition for

total internal reflection and are lost. Also, dust and dirt on the surface can allow the light to

escape. Secondly, this demonstrates that the guided wave becomes weaker along the length of the

waveguide because of scattering and absorption loses. To compensate for loss in real optical fibre

systems devices that amplify light (“optical amplifiers”) are placed after every 80 km of fibre.

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Experiment 5 - Bend loss in optical fibre

Aim To demonstrate that light can escape at sharp bends

Equipment • Radio or CD player

• Stereo cable

• LED transmitter unit, with only one LED on

• Receiver unit (check the volume is turned up)

• Clear plastic rod with sharp bend

Method • Ensure radio or CD player is tuned and on.

• Connect radio or CD player to LED transmitter unit with cable and turn transmitter on.

• Butt the LED against one end of the rod so the light is guided within the rod.

• Put the photodiode very close to the side of the rod, angled away from the transmitter.

• You should be able to hear the radio signal from the receiver speaker.

• Run the receiver along the length of the rod. The sound should become much louder at the bend.

Radio

or CD

player

LED

Receiver

unitLED

Transmitter

unit

Radio

or CD

player

LED

Receiver

unitLED

Transmitter

unit

Schematic of the arrangement for !Bend loss in fibre"

What does this demonstrate?

When the rays get to the bend some of them hit the rod sidewall at an angle that does not

satisfy the condition for total internal reflection and are thus pass through the side wall.

Bend loss is a significant problem in optical fibres.Can you trace rays that travel around

a bend? Can you trace rays that leave a rod at a bend? Assume n=1.5

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Experiment 6 - Wavelength Division Multiplexing

Aim To demonstrate that more than one colour can be sent down an optical fibre, increasing

the information that can be sent.

Equipment • Three radios or CD players.

• Three stereo cables

• LED transmitter unit, with all three LEDs on

• Receiver unit (check the volume is turned up)

• Red, green and blue optical filters

Method • Turn the radios or CD players on one by one, ensuring they are playing different sounds.

• Connect each radio to a different audio socket on the LED transmitter.

• Place the photodiode on the receiver unit about 10 cm from the LEDs.

• You should hear the sound originating from all three sources at once.

• Insert one coloured filter between the LEDs and photodiodes at a time. Only one radio

should be audible per filter.

Light

Detector

Coloured filter

Schematic of the arrangement for !Wavelength Division Multiplexing"

What does this demonstrate?

Optical fibre is transparent over a very wide range of colours or wavelengths. We can increase

the information carried by an optical fibre by launching several different colours of light, each

wavelength carrying a separate signal. We call this wavelength division multiplexing. In real

optical communication systems, prism-like devices are used to combine and separate the different

colours.

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Wavelength division multiplexing increases the information carrying capacity of optical fibre

Channel 1

Channel 2

Channel 3

.

.

.

Channel n

Wavelength division

multiplexing

(mixing)

Wavelength

Po

we

r Usable wavelength band in optical fibre

Channel 1

Channel 2

Channel 3

.

.

.

Channel n

Channel 1

Channel 2

Channel 3

.

.

.

Channel n

Wavelength division

multiplexing

(mixing)

Wavelength

Po

we

r Usable wavelength band in optical fibre

Channel 1

Channel 2

Channel 3

.

.

.

Channel n