Specifications

2009
2010
39
FURTHER INFORMATION
▼ GENERAL CHARACTERISTICS
Optical fibre is a transmissive medium which carries photons (light) rather than
electrical current, as is the case for normal copper conductors. The fundamental
requirement for this type of transmission is thus that the material be transparent,
like glass, rather than a good conductor.
Optical fibres are composed of thin fibres of silicon oxide (SiO2), which act as
tunnels to transport the light to distant destinations.
This "tunnel" is extremely small; it is measured in microns. Nonetheless, there
are various types of optical fibre cables, because light has different ways of prop-
agating in such restricted spaces.
The advantages of this technology include:
• Very wide operative bandwidth;
• Total immunity from electromagnetic disturbance
(photons are electrically neutral);
• Absence of electromagnetic emissions and practically zero crosstalk;
• High transmissive capacity;
• Elimination of grounding problems;
• Low attenuation: around 25 times less than that
of equivalent copper conductors;
• Very low weight and size;
• Safety.
▼ CLASSIFICATION
What are singlemode and multimode optical fibres? The fibre is composed of two
coaxial cylinders, one inside the other, in glass fibre, and the difference between
these two layers of glass consists in the fact that they have different indices of
refraction.
Refraction and reflection characterise the mode of propagation of the light wave
in the fibre. From the figure, we can see that when a light ray impinges on the
boundary between the two layers of glass it is reflected so that it stays entirely
within the core. Refraction occurs when the angle of incidence is such that the
ray, while deflected from its original direction, passes through the boundary be-
tween the two layers and propagates in the cladding. In optical fibres the light is
guided by internal reflection, so that refraction is absent.
This is governed by the physical principle that states that if the angle of incidence
is less than a certain critical angle depending on the ratio of the two indices of
refraction, total reflection occurs, in other words, the emerging light ray propa-
gates with an equal but opposite angle relative to the boundary
(Thanks to the
refractive index N, the ratio between the speed of light in vacuum and in the trans-
missive medium, which is different in the core and in the cladding, the light is
confined in the core, while any light which passes from a high to a low refractive
index medium tends to move away from the perpendicular)
.
The value of this critical angle is also the limit (since it is the same) of the angle
of the acceptance cone, at the start of the fibre, relative to its axis. In practice,
this shows that we can exploit as many modes of propagation of light inside the
fibre as we can create angles of incidence below the critical value. It follows that
a multimode fibre is one which can transmit several rays of light simultaneously
with different angles of propagation (since it has a large diameter core), while a
singlemode fibre can only transmit a single ray of light parallel to the fibre’s own
axis. In order to insert the largest number possible of lasers in the transmitter and
receiver, we must use optical coupling methods.
A factor which controls this coupling is the Numerical Aperture (NA) which is the
sine of the angle of the acceptance cone, and is a function of the indices of reac-
tion N1 and N2 of the core and cladding respectively:
Fibre optic cables not only differ in their transmission mode (multimode/singlemode)
but also in the ratio between the indices of refraction of the core and cladding.
Step index multimode fibre is the simplest of fibres. It has a core of diameter in
the range from 100 to 970 micron in glass, PCS (Plastic Clad Silica) or plastic.
In this fibre the core and cladding have very different refractive indices (the core
has the higher value). This fibre is characterised by the fact that with such a large
diameter core, it has many modes of transmission, which results in, among oth-
ers, different distances of transmission.
On the other hand, this fibre is subject to intermode distortion due to the delay
with which signals with high angles of incidence (ray sections) arrive at the far
end, since they have to travel further than those with small angles of incidence.
This type of fibre is used for low speed data transmission over limited distances
(2/3 km).
The most widely used fibres today are of the graded index type. In these fibres
there is a not a sharp difference between the refractive index of the core and
cladding since successive layers of glass of varying refractive indices in the core
ensure that the rays follow a curved path, remaining within the core.
Such fibres are suited to high speed data transmission and provide a limited mar-
gin of error over distances of several km. Finally, for long distance (trunk) high
speed transmission, singlemode step index fibres are the only solution. Indeed,
by using such cables with only passive signal cleaning and regeneration equip-
ment, distances of hundreds of kilometres can be covered.
▼ OPTICAL FIBRE CABLES - STRUCTURE
The main structural difference between different types of optical fibre is that be-
tween loose tube buffer and tight buffer. In loose tube buffer structures, each fi-
bre, along with its various coatings, is loosely packed inside the tube which con-
tains it along with the other fibres.
In turn the tube (generally in Kevlar for greater mechanical strength) is covered
with sheaths of various types depending on the intended installation. In tight
buffer construction, on the other hand, each fibre is contained inside a series of
protective coatings up to the jacket, with no space between the various layers.
Such cables may contain one or many fibres; in the latter case, the fibres are sep-
arate from each other.
The advantages and disadvantages of these constructions depend on the type of
installation. Tight buffer cables can be routed around tighter bends, are more flex-
ible and have better mechanical strength; furthermore, in case of breakage, the
point of failure can be exactly identified.
A loose tube buffer construction, on the other hand, may not enable precise iden-
tification of the point of failure without the aid of special instrumentation, but bet-
ter isolates the individual fibres from mechanical stress and is also more resistant
to thermal excursion and external agents in general.
There are also other types of construction which, however, employ the same cri-
teria discussed above, such as open slot construction, which is similar to loose
buffer, and ribbon cable, which is similar to tight buffer.
OPTICAL FIBRE CABLES