Optical fibre tutorial
- an overview or tutorial covering fibre optic cabling - the construction of
the fibre optic cables, how they work, their applications and specifications.
In recent years, optical fibres, and fibre optic cabling has
fallen in cost, making it fall within the economic reach of many more
telecommunications and data networking applications. As a result fibre optics
are now in widespread use, and form the backbone of most telecommunications
networks and many local area data networks.
While there are many components used in building up a fibre
optic link, the fibre optic cabling is obviously the key element.
Optical fibre construction
Fibre optic technology relies on the fact that it is possible
to send a light beam along a thin fibre suitably constructed. A fibre optic
cable consists of a glass or silica core. The core of the optical fibre is
surrounded by a similar material, i.e. glass or silica, called the cladding,
that has a refractive index that is slightly lower than that of the core. It is
found that even when the cladding has a slightly higher refractive index, the
light passing down the core undergoes total internal reflection, and it is
thereby contained within the core of the optical fibre.
The Outside the cladding there is placed a plastic jacket.
This is used to provide protection to the optical fibre itself. In addition to
this, optical fibres are usually grouped together in bundles and these are
protected by an overall outer sheath. This not only provides further protection
but also serves to keep the optical fibres together.
Optical fibre types
There is a variety of different types of fibre optic cable
that can be used, and there are a number of ways in which types may be
differentiated. There are two major categories:
- Step index fibre optic cabling
- Graded index fibre optic cabling
The step index cable refers to cable in which there is a step
change in the refractive index between the core and the cladding. This type is
the more commonly used. The other type, as indicated by the name, changes more
gradually over the diameter of the fibre. Using this type of cable, the light is
refracted towards the centre of the cable.
Optical fibres or optical fibers can also be split into
single mode fibre, and multimode fibre. Mention of both single mode fiber and
multi-mode fiber is often seen in the literature.
Single mode fiber This form of optical
fibre is the type that is virtually exclusively used these days. It is found
that if the diameter of the optical fibre is reduced to a few wavelengths of
light, then the light can only propagate in a straight line and does not bounce
from side to side of the fibre. As the light can only travel in this single
mode, this type of cable is called a single mode fibre. Typically single mode
fibre core are around eight to ten microns in diameter, much smaller than a
hair.
Single mode fiber does not suffer from multi-modal dispersion
and this means that it has a much wider bandwidth. The main limitation to the
bandwidth is what is termed chromatic dispersion where different colours, i.e.
Wavelengths propagate at different speeds. Chromatic dispersion of the optical
fibre cable occurs within the centre of the fibre itself. It is found that it is
negative for short wavelengths and changes to become positive at longer
wavelengths. As a result there is a wavelength for single mode fiber where the
dispersions is zero. This generally occurs at a wavelength of around 1310 nm and
this is the reason why this wavelength is widely used.
The disadvantage of single mode fibre is that it requires
high tolerance to be manufactured and this increases its cost. Against this the
fact that it offers superior performance, especially for long runs means that
much development of single mode fiber has been undertaken to reduce the costs.
Multimode fiber This form of fibre has a
greater diameter than single mode fibre, being typically around 50 microns in
diameter, and this makes them easier to manufacture than the single mode fibres.
Multimode optical fiber has a number of advantages. As it has
a wider diameter than single mode fibre it can capture light from the light
source and pass it to the receiver with a high level of efficiency. As a result
it can be used with low cost light emitting diodes. In addition to this the
greater diameter means that high precision connectors are not required. However
this form of optical fibre cabling suffers from a higher level of loss than
single mode fibre and in view of this its use is more costly than might be
expected at first sight. It also suffers from multi-mode modal dispersion and
this severely limits the usable bandwidth. As a result it has not been widely
used since the mid 1980s. Single mode fiber cable is the preferred type.
Attenuation within an optical fibre
Although fibre optic cables offer a far superior performance
to that which can be achieved with other forms of cable, they nevertheless
suffer from some levels of attenuation. This is caused by several effects:
- Loss associated with the impurities There will always
be some level of impurity in the core of the optical fibre. This will cause
some absorption of the light within the fibre. One major impurity is water
that remains in the fibre.
- Loss associated with the cladding When light reflects
off the interface between the cladding and the core, the light will actually
travel into the core a small distance before being reflected back. This
process causes a small but significant level of loss and is one of the main
contributors to the overall attenuation of a signal along an fibre optic
cable.
- Loss associated with the wavelength It is found that
the level of signal attenuation in the optical fibre depends the wavelength
used. The level increases at certain wavelengths as a result of certain
impurities.
Despite the fact that attenuation is an issue, it is
nevertheless possible to transmit data along single mode fibres for considerable
distances. Lines carrying data rates up to 50 Gbps are able to cover distances
of 100 km without the need for amplification.
Materials used for optical fibres
There are two main types of material used for optical fibres.
These are glass and plastic. They offer widely different characteristics and
therefore fibres made from the two different substances find uses in very
different applications.
Optical fibre sizes
One of the major ways of specifying optical fibre cables is
by the diameters of the inner core and the external cladding. As may be expected
there are industry standards for these and this helps in reducing the variety of
fittings needed for connectors, splices and the tools needed for fitting.
The standard for most optical fibres is 125 microns (um) for
the cladding and 245 microns (um) for the outer protective coating. Multimode
optical fibres have core sizes of either 50 or 62.5 microns whereas the
standards for single mode fibres is approximately 8 to 10 microns.
When specifying optical fibre cables, the diameters usually
form the major part of the cable specification. A multimode fibre with a core
diameter of 50 microns and a cladding diameter of 125 microns would be referred
to as a 50/125 fibre.
In addition to the specification of the diameter, other
parameters such as the loss, etc are also required, but these elements do not
form part of the cable type in the same way as the diameter.
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