Specifications

2009
2010
40
FURTHER INFORMATION
▼ ATTENUATION
As it travels along the fibre’s core, the light will attenuate under the action of four
factors:
1) ABSORPTION
This is caused by impurities in the fibre which absorb the light; such impuri-
ties in the glassy matrix of the fibre are due to the presence of hydroxide
compounds (OH).
2) SCATTERING
This is caused by variable density of the material or at the molecular level in
the fibre’s glassy matrix. Scattering is at the basis of the OTDR concept (Op-
tical Time Domain Reflectometer).
3) MACROBENDING
Lack of circularity of the core, micro-bends, micro-fractures. Bending the fi-
bre beyond its maximum bending radius during installation (this is a typical
problem in installation) creates problems of refraction.
4) MICROBENDING
Micro-distortions of the cable during its fabrication can cause the same prob-
lems as macrobending.
As well as depending on these factors, which can be considered accidental, atten-
uation in fibre optic transmission is also a function of wavelength. This varies in
a non-uniform manner with the emission wavelength; this is why, at the lowest
values of attenuation where losses are minimal, three standard windows have
been defined (see figure) for light emission:
• First window 820-850 nm
• Second window around 1300 nm
• Third window around 1500 nm
As can be seen, rather than using the frequency it is preferable to use the wave-
length to define bandwidth in fibre optic transmission, since the numbers are more
convenient to use. Attenuation is the loss in power of the light pulse travelling
along a fibre. Dispersion is the diffusion of the light pulse travelling along a fibre.
If we had only attenuation losses on each pulse, we would simply lose power,
but unfortunately there are also dispersion effects which correspond physically to
the capacity of diffusion of light in an environment.
In reality, the useful bandwidth is limited by the combination of dispersion/diffusion
(although the resulting bandwidth is still vastly superior to that available with
electrical transmission methods). Connection losses and reflections are the pri-
mary causes of attenuation in fibres; connections must always be made with the
greatest attention to detail and skill.
▼ BANDWIDTH OF OPTICAL FIBRES
Measured in MHz x km: since the pulse spreads out
in proportion with the length of the cable run (dispersion).
• Window I 800 - 900 nm MM 150 MHz x Km
• Window II 1250 - 1350 nm MM 500 MHz x Km
• MM laser window II 1 GHz x Km
• SM laser window II 10 GHz x Km
• Window III 1500 - 1550 nm SM 100 GHz x Km
▼ TYPES OF OPTICAL FIBRES
The ISO/IEC 11801 standard defines four types of optical fibres:
• OM1 (50 or 62.5/125 μ): bandwidth ≥ 200 MHz·km at 850 nm
• OM2 (50 or 62.5/125 μ): bandwidth ≥ 500 MHz·km at 850 nm
• OM3 (50/125 μ): bandwidth ≥ 1500 MHz·km at 850 nm
• OS1 singlemode: ≥ 100 GHz x Km
N.B.for classes OM1, OM2, OM3 at 1300 nm banda ≥ 500 MHz·km
▼ PRESENTATION OF OPTICAL FIBRE CONNECTIONS
The ISO/IEC 11801 standard defines 3 classes of connection:
• OF-300: channels which support applications over a minimum distance
of 300 metres;
• OF-500: channels which support applications over a minimum distance
of 500 metres
• OF 2000:channels which support applications over a minimum distance
of 2000 metres.
The corresponding type of fibre must be evaluated in each case in terms of the
length of the connection and the type of application in question.