HPAVC/HPAVC CD-ROM.iso/HOMEWORK.ZIP/PHYFIB.TXT

Text File | 1998-07-25 | 16KB | 351 lines

Date sent:        Mon, 13 May 1996 18:55:02 +1300

History of Fibre Optics
Figure 1-Light enters an optical fibre, and some is guided along the fibre.  A fibre can
carry more infomation, faster than a copper wire.



Figure 2 A cross section of a human hair and an optical fibre

When Alexander Graham Bell spoke over a beam of light in the 1880's, he never dreamed of
the possibilities that modern scientists are dreaming up for light.  He used sunlight which
was focused by means of a reflector and a lens to a device which could be made to vibrate in
harmony with speech from a human voice.  The light beam was made to vary the focus in and
out so that the strength on a selenium detector could be made to activate a telephone
receiver and recreate the original voice.  The distance between the transmitter and receiver
were very short, but it was the beginning of communication via light.  The method of
changing the intensity of the light beam is still what we base our communications on, but
now, more often digital communication consisting of "on" and "off" patterns is used.  The
simplest use of optical fibres is that of light pipes.  A light source that gives off heat
and light transmits the light only through the pipes to give "cold" light.  This is how
doctors see inside the body.

What is so great about optical fibre?  It is a piece of glass that allows light to travel
through.  Actually it is a very fine strand of very special glass which might be only 125
microns in diameter.  It is a glass strand that is about the same thickness as a human hair.
Fibre optic technology can simultaneously transmit voice, video, and data over the same wire
several thousand times better than current coaxial cable.  Since the mid 1980's, thousands
of kilometres of optical fibre have been laid in the United States and Japan to carry long
distance telephone communications.  Fibre optics are also used in various medical
instruments designed to examine the interior of the body, since the images transmitted by
these devices can be magnified and rotated for close observation of hollow organs.  Optical
fibres are also used in many laser-based computer printers to produce photo quality copies.

Glass or plastic filaments are spun to diameters between 5 and 100 micrometers and packed
into bundles of several thousand each.  The bundles may be made as rods, ribbons, or sheets.
 Because the bundles keep some of the flexibility of the individual fibres, they can be
twisted and bent to conduct light and images around corners.  In order to protect the
fibres, a protective layer is applied.

Reflection
Figure 3 Reflection of light



When light falls on a medium a percentage is reflected back.  The amount of light reflected
depends on the angle a1 between the incidence ray, and the normal ray.

q1=q2

Refraction

When a ray of light with an angle of incidence a enters an optically denser medium for an
optically less dense one, its direction bends toward the angle of refraction b.

If a medium has identical properties in all directions, then Snell's1  law of refraction
applies: Figure 4 Refraction of light

where the ratio of the angle of incidence and sine of the angle of refraction is equal to
the ratio of  the speed of light in one medium to the speed of light in the other.

sina=    c1
sinb    c2

With two transparent media, the one with the lower speed is considered to be denser.

When light travels in a vacuum at a speed of c0 to a medium with a speed of light c the
following applies sina    =c0    =n sinb    c

The ratio of the speed of light in a vacuum and the speed of light in a medium is called the
refractive index (more precisely the phase refractive index)

For two different mediums with the refractive indexes of n1  and n2 and their speeds of
light c1 and c2, the following applies:

c1=c0
n1

Another form of Snells law is:

sina=    n2
sinb    n1

Critical Angle

It is possible for the difference between in refractive indexes between two mediums to cause
refracted light to have an angle of 90░, or parallel to the medium surface..  This angle is
called the critical angle.  The critical angle can be found by:

sinqc=n2/n1


Figure 5 Total internal reflection of light

Total Internal reflection

When a light ray comes into contact with a medium with a different refractive index, it is
refracted.  If the angle of incidence is less than the critical angle, it will be reflected
inside the medium.  This is called total internal reflection.  It is possible for this ray
to continue on forever in this manner.

Total internal reflection can only occur at an interface where a light ray travels from an
optically denser medium to a optically less dense medium.

Transmitted light through an Optical Fibre

Lets consider a short piece of cable with two rays entering, A and B.

Figure 6 The passage of light through a fibre optic cable

Ray A enters the fibre at an angle of qA.  This ray strikes point C.  Some of the light is
reflected on to point D, and some of the light is refracted outside.  Again at point D, some
light is reflected and some is refracted outside.  This will continue until the all the
energy is lost.

Ray B enters the fibre at the angle qB.  The refracted ray has an angle of 90░, parallel to
the side of the medium.  This ray is therefore the critical angle and forms the slope of a
cone of angles that will be reflected.

qB=sin-1(n1/n2)

Ray  C enters the fibre at an angle less than the qc.  This ray will continue on forever
being totally internally reflected2 .

The skip distance is the distance between two reflections and can be found by:

Ls=dcotq

where d is the core diameter.

Numerical Aperture

In order to launch light from outside into the core glass, the launch angle between light
ray and fibre axis can be found by:

sinq    =    n1
sin(90░-a0)    n2

The greatest launch angle qmax is called the acceptance angle of the fibre.  The sine of the
acceptance angle of the fibre is called the numerical aperture.

NA=sinqmax

This quantity has a major importance in launching light into fibres.

 Characterisation of Several Optical Fibres
Core/cladding
n1
n2
jcritical
qmax
N.A.
1/Ls

Glass/air
1.50
1.0
41.8░
90.0░
1
5944

Plastic/plastic
1.49
1.39
68.9░
32.5░
0.54
3866

Glass/plastic
1.46
1.40
73.5░
24.5░
0.41
2962

Glass/glass
1.48
1.46
80.6░
14.0░
0.24
1657


Bandwidth

The bandwidth is a continuous range of frequencies between a lower and upper limit.  The
more complicated a signal is, the greater the range of frequencies needed to represent it
are.  The output of a FM radio station is much clearer than that of a telephone because a
greater frequency range is given to the FM.  For example, a telephone conversation normally
takes 4 kHz, where as a FM radio takes 200 kHz.  A television station takes 6 MHz of
bandwidth.  The potential of the optical fibres is enormous.  It is possible to calculate
the possible bandwidth of a fibre.  For example, a TV station that uses a 300 MHz carrier,
the ratio is 300 MHz/6 MHz, or 50; for an optical fibre using a carrier of 3x108 MHz to
carry the information, the ratio is 3x108MHZ/6 MHz, or 50,000,000.

Much more information can be sent when pulses can be transmitted.  This is called binary,
and is either on or off.  This is what computers, and CD's use as a means of communication.
Suppose that 8 bits3  are required to represent the amplitude of an analog signal.  A analog
signal is supposed to by sampled at a rate of at least twice as high as its highest
frequency.  In the case of a TV channel with a bandwidth of 6 MHz, this means that 2 x 6
MHz, or 12x106 samples must be taken each second.  Since each sample is described as using 8
bits, the required data rate is 96 Mbps (megabits per second).  The data rates are limited
at the moment by fibre distortions, and equipment to transmit this fast.

Attenuation

When the light is absorbed by the fibres, it is called attenuation.  There is several
reasons for this to occur and they fall into two types, extrinsic and intrinsic loss.
Examples of intrinsic loss are Rayleigh scattering which is caused by microscopic variations
in the index of the refraction of the glass.  This gives a uniform loss over the entire
fibre.  OH- absorption happens when molecules of OH- get into the fibre when it is made.
Metallic ion absorption is caused from trace elements such as gold, magnesium, and iron
being left in the fibre when made.  It is very difficult to get rid of these trace elements
because they are found almost everywhere.  These types of attenuation often only absorbs
light at certain wavelengths.  The other type is when the fibre is bent too much, or from
tiny micro-defects in the fibre.  These are called extrinsic losses, and causes the ray
angle to be greater than the critical angle, and is not reflected.

Attenuation is measured in decibles per kilometer lost, or dB/km.  The early cables produced
had an attenuation of 20 dB/km.  Today the cables are being produced with 0.1 dB/km loss.
Figure 7 (a) sharp bend in fibre and (b) microdefect in fibre



Transmission of Digital Signals

8



9
There are three main components used in fibre optic communications.  The first component is
the transmitter.  It modulates the electrical energy into light energy.  This is the part
that generates the light signals, capable of being switched on and off very quickly.  The
faster that these can be switched on and off, the more information can be sent in a given
time.  The light source is usually a LED (light emitting diode) or a LD (laser diode).  It
is possible to use other laser sources, but these are the cheapest and most reliable.

The second component is the optical fibre which has a high purity, and transparent to the
frequencies being transmitted.  It  must be able to be spliced and repaired when necessary.
To transmit the light a long distance, and to overcome the loss of energy, repeater stations
are set up.  These amplify the signal to avoid loss and distortion.

Finally the last component is the receiver.  This converts back to electrical energy the
light signal.  It is made with a detector, which detects the light and turns it back into
electrical energy.  A signal processor that amplifies the signal, filters and changes the
signal into a useable form, ie analogue sound.

Digital signals are what are used for communication in a telecommunications and data
communications.

Transmission of Analogue Signals


10
Analogue signals are continuous.  That is they have an infinite number of values.  All sound
light we hear and see is also analogue.  Analogue is used when data is not being
communicated.  A endoscope which is used to see inside the body uses analogue.  One fibre
will carry white light inside, and another tube will carry back the reflected image. Figure
11 Basic diagram of the fibre drawing and coating process


Commercial Manufacture of Optical Fibres

The main material of optical fibre is ultrapure silica powder.  This is heated to a high
temperature until it is molten.  A glass rod, or preform, is formed when it is slightly
cooled.  A fibre is then pulled out and stretched, keeping the heat constant to ensure an
even pull..  The next step is to coat this fibre in either glass or a plastic coating to
form the cladding.  It is then put through a test using ultraviolet rays to check for
imperfections.  The fibre is then covered with a plastic coating for protection.

Other trace materials can be put in depending on what type of cable is wanted.  The mixing
of the materials needs to be done extremely carefully.  A single speck of dust can
contaminate an entire batch of fibre.  The glass produced is so pure, that a block one
kilometre thick is as clear as a normal window pane.

Optical Fibres in the Telecommunications Industry

The use of optical fibres for telecommunications is by far the biggest use, and probably the
most potential.  During the next decade or two, almost every house will probably be
connected with optical fibres.  Many experts expect that by the year 2020, nearly all homes
in America will have fibre optic connections.

The first telephone network was tested by Western Electric in 1976.  One year later, Bell
carried the first optical cable in Chicago.  It covered 2.5km.

Figure 12 Some of the possible uses for optical fibres

Today, telecommunication companies in America have replaced nearly all  the major cities
links with optical fibres.  The cables are about as wide as a closed fist, and conduct as
much information as the old copper cables which when put together would be about as wide as
a large tractor tyre in diameter!

In New Zealand, Clear Telecommunications uses fibre optic cables running along side the main
trunk rail track to transmit telephone calls between the cities.  In the last year, Clear
has entered the business district in Wellington, and now connects some buinesses with fibre
optic cables to the local and international network.

Telecom has installed a fibre optic link to carry all telephone messages between the two
islands.  There is also a fibre optic link between Australia and Auckland, and Auckland and
Hawaii.

Kapiti Cable, on the Kapiti Coast, is experimenting with connecting houses with an optical
fibre.  There are about 2000 subscribers to this service.  The facilities at the moment
availably ate a dial up video library, several TV channels, and are experimenting with
access to the global computer network, the Internet.  This is the most advanced network in
New Zealand at the moment.  There are other smaller networks experimenting at the moment in
Auckland and Christchurch too. Medical Uses of Optical Fibres

Figure 13 A tiny probe pierces a cell

There are many new instruments that now use fibre optic cable.  The name Endoscopy is given
to the instruments that look inside the body.  The esophagoscope allows doctors to examine
the oesophagus, the gastroscope is a flexible tube to examine the instruments is used to see
inside the stomach.  Most of these instruments use natural openings in the bodies to get in.
Scientists at the University of Michigan have invented a fibre that is so small, it can slip
between the membrane of cells.  It is the smallest sensor over developed at only
one-thousandth the width of a human hair.

With these aids, it is possible for doctors to perform surgery without having to perform
make a cut.  Lasers can be aimed through a fibre and can "shoot" gall stones in the bladder
or remove blockages in the arteries.

Eye surgeons can fix a wide array if problems using lasers and fibre optics.  The laser can
be directed to exactly where the problems.  Correction of the shape of eyes used to be a
serious problem, now most cases can be done under no anethesic, and no overnight stay at
hospital.

Military Uses of Optical Fibres

The military was the first to use optical fibres and most of the early research with
applications was done my them.  In 1973, the first optical cables were put into operation on
American navy ships

With the new cables, it will be possible to do all communications now possible using radio,
and telephone from one terminal.  It will be possible to have video on demand, with complete
video libraries only a few key presses away.  All the major telecommunications companies in
the USA are currently looking at these options.  IBM is looking at developing technology
that will able computers in the house to replace the phone, TV, radio, and every other kind
of electrical communication.  Eventually it will be possible to have entire libraries of
information "beamed" into the house.

1 Willebrord Snell discovered this important law of refraction in 1621.  It refers to how
much light is bent when moving from one medium to another. 2 Although the energy will
eventually be absorbed into the fibre through attenuation 3  A "bit" is either on or off. ??