INTRODUCTION
This article is part of the "Understanding
CCTV Series" and are abstracts
from STAM InSight - The Award Winning
CCTV Program on CD ROM, which has many
innovative CCTV tools for skill and
productivity enhancement.
We all know fibre optics is the best
transmission method but it’s workings
are still a big mystery to many people.
This article will demystify some of
these issues and provide an overview,
and explain the advantages and disadvantages
of the fibre optics technology.
WHAT
IS FIBRE OPTICS?
To put it simply, fibre optics is
a technology whereby a signal like
video, data or voice, is modulated
on a light beam and sent down a glass
tube over large distances, with very
little attenuation and loss.
The
principles of fibre optics is simple
and easy to understand. All of us
have seen the "broken straw"
effect in a glass of water.
 When
light travels from air to a denser
medium, like glass for example, the
light slows down by a factor equal
to the optical index of the material
and this slow down in speed results
in the bending of the light beam.
This phenomenon is seen in our daily
life on a regular basis. The beautiful
colours in a diamond occur due to
the same reason.
 As
shown in the example when we see an
object from underwater, the object
is not in the actual position as we
think because light bends travelling
from water to air. When this angle
of entry is increased, there would
come a stage when the light is reflected
back into the same medium, as shown
in ray 3. This angle is called the
angle of Total Reflection. Fibre Optics
uses this simple principle for transmission.
The core of the fibre optics cable,
which is made of glass has an higher
index of refraction than the index
of the cladding, which covers this
core. So when light is injected into
the glass core at the correct angle,
it will reflect back from the surface
and continue doing this in its forward
travel. In other word the light cannot
"escape" from the fibre
optics cable.
 It
is easy to modulate any signal, whether
it be video, data or voice, on to
light and send it into a cable, where
it can not escape and it has to travel
in the forward direction. In the other
word, this light beam carries the
signal through the glass core of the
fibre optics cable.
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COMPONENTS
OF A FIBRE OPTICS SYSTEM
The
use of fibre optics is extensive,
but the components used to make up
a system are usually similar. These
are:
-
Input Modulator: This modulates
the incoming signal with a light
beam
-
Light Emitting Device: This generates
the light for modulation, which
is in the infrared frequency range.
The higher the frequency, larger
the bandwidth of the signal.
-
Fibre Optics Cable: There is a range
of cables which are available to
suit different applications.
-
Light Receiving element: Which receives
the light signal at the other end.
-
Output Demodulator: This separates
the signal from the light beam.
All
fibre optics systems use the above
components. The specification of each
component will vary depending on the
application and use. For example a
LED ( Light Emitting Device) will
be adequate for CCTV as the light
source, but a LD (Laser Diode) is
a must if higher band widths are required.
DIFFERENT
TYPES OF FIBRE OPTICS CABLE
There
are 3 different types of fibre optics
cable available. These are
-
Step Index :
This cable has a distinct index
of refraction for the core and the
cladding. It is the cheapest type
of cable and causes deformation
due to the various path lengths
of the light ray. This is called
modal distortion.
-
Graded index or multimode :
In order to equalise the path lengths
and improve pulse response, the
multimode cable was developed. The
index of refraction of the core
is varied in smooth fashion. This
would mean the angel of reflection
would vary, which helps in equalising
the path lengths and reduce distortion.
- Single
mode :
The single mode has the best pulse
response because the diameter of
the core is very narrow and the
light wave virtually goes straight.
Obviously this the most expensive
cable.
ADVANTAGES
OF FIBRE OPTICS
Fibre Optics signal transmission offers
many advantages over existing metallic
links. Some of these are:
1.
Large Bandwidth
Fibre
optics offers very high bandwidths
as compared to copper wire. The bandwidth
of a fibre optics cable is between100Mhz
to 1000 MHz as compared to a copper
wire, which can handle only 3 to 20
MHz. This large bandwidth allows many
applications and uses of fibre optics.
For example a video signal has a bandwidth
of 5 MHz. It is therefore possible
to send up to 20 - 200 video signals
on a signal fibre optics cable. This
is impossible with a copper wire.
In fact by using digital compression
techniques you could modulate up to
500 - 1000 video signals on a single
cable.
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2.
No Interference
Unlike coaxial cable which uses electric
currents, fibre optics uses light
making it immune to any type of interference
No
EMI interference - As light
is the medium of transmission, fibre
optics cable does not pick up Electro
Magnetic Interference (EMI). EMI
is the biggest source of problems
with co-axial cable. Double images,
moving lines, horizontal bands and
picture snow in a video picture
are due to EMI. All these are eliminated
with the use of fibre optics.
No
ground loop currents - Ground
loop current is caused when the
two ends of the cable are not grounded
to the same potential. This difference
in potential causes a current, which
is usually seen as a moving lines
on a video picture. Fibre optics
does not use electricity and therefore
ground loop currents are impossible.
No
power interference - Voltage
has no effect on light beam. It
is therefore irrelevant whether
the fibre optics passes next to
240VAC or 10000 VAC, or close to
a powerful transmitter. This has
no effect on the light beam in the
fibre optics cable.
Immune
to lightning - Even lightning
that hits a centimetre away from
a fibre optical cable cannot induce
any voltage in it.
Fibre
optics cables are very small and low
in weight. It is therefore much easier
to lay fibre optics cable compared
to metallic cable, which can be heavy
and bulky.
3.
Security of information
It is impossible to tap into a fibre
optics cable without physically intercepting
the signal, in which case it is detectable
at the receiving end.
DISADVANTAGES OF FIBRE OPTICS
-
Cost of fibre optics cable.
Fibre optics is still quite expensive
as compared to the copper wire,
though the prices are coming down
rapidly. This disadvantage could
be made into an advantage by using
innovative cabling and installation
methods. In co-axial cable, a single
cable has to be run from the camera
point to the control room. As fibre
optics has large bandwidths, it
is possible to multiplex many video
signals on a single fibre optics
cable and thereby reduce cabling
and installation cost. For example
in the Ring Method, a single fibre
optics cable is run all over the
site and the cameras are connected
to this cable and the video signals
are multiplexed at different frequencies.
-
Termination
Termination of fibre optics cable
is complex and requires special
tools, better precision of workmanship
and is more time consuming and therefore
more expensive. Fibre optics cable
has a very small diameter and require
specialised tools to align the cables
correctly and then join them. The
glass in the fibre optics core has
to be grounded flat first to ensure
correct alignment for proper and
smooth termination. This looks complex,
but many tools are available to
ensure that the termination etc.
can be achieved more easily.
- Cable
laying
The centre core of a fibre optics
cable is made of glass. Extra precaution
is required during cabling. Stretching
of cable should be avoided as the
glass core may crack. Also the cable
bending is a major problem. Sharp
bends could crack the cable but
also the light beam bouncing of
the glass core may not handle the
bend and the light beam may ‘escape
out’ of the core. Most cable
manufacturers provide permissible
bending angles.
There
is no denying that the advantages
definitely outweigh the disadvantages.
There are more benefits into using
fibre optics. It is because of these
facts the usage of fibre optics is
growing daily.
About the Author
Jayant Kapatker is an international
authority on CCTV and is the brain
behind STAM InSight - The Award Winning
CCTV Program on CD ROM. This interactive
multimedia CD ROM contains over 14
hours of CCTV content. This series
of articles have been based upon the
subjects covered in STAM InSight.
For more info on the CD ROM contact
STAM Multimedia (fax: +1 609 7501936
or email info@stamweb.com) |
