Pagina 152 - TELE-satellite - La Più Grande Rivista del Mondo Sul Commercio TV Digitale

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FC Connector
TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 09-10/2012
Connecting the Light
Jacek Pawlowski
Fiber Optics
Table 1.
Wavelength Range
O band
1260 to 1360 nm
E band
1360 to 1460 nm
S band
1460 to 1530 nm
C band
1530 to 1565 nm
L band
1565 to 1625 nm
U band
1625 to 1675 nm
Optical fibers introduced enormous
improvement in data transmission. An-
ybody who is even moderately interest-
ed in technology knows that the most
important reason for that was the very
low signal attenuation of fiber optic
cables. The first cables manufactured
in 1970 had an attenuation of 20 dB/
km. Today, we can achieve even 0.16
dB per kilometer. It is almost nothing
when compared to the classical electri-
cal cables.
However, there is yet another factor
influencing the maximum useful length
of a fiber optic cable. This is chromatic
dispersion. To explain it in simple terms
– various light rays in a fiber have a dif-
ferent propagation time what distorts
the shape of the impulses carrying digi-
tal data. Dispersion puts a limit on the
maximum useful bit rate that can be
transmitted in a given cable. Disper-
sion is less painful in single mode fiber
optic cable (i.e. those with a diameter
less than 12 µm) but even such cables
are not quite free from this phenom-
enon due to material structure non-
Attenuation and dispersion depend
on the light wavelength and optical
fiber material. The first generation of
optical fiber worked with wavelengths
of around 850 nm, the second genera-
tion with 1300 nm and the third one
with 1550 nm. Theoretically, the lowest
attenuation is achievable for 1550 nm
while the lowest dispersion (= the high-
est bit rate) for 1300 nm.
The wavelength used in optical fiber
based systems have been standard-
ized. There are six transmission win-
dows named: O, E, S, C, L and U-band.
The corresponding wavelength ranges
are listed in the table 1.
For your comparison, the visible
range for humans is: 380-780 nm. So,
the light used in fiber optics, no matter
what band, lies in the deep infrared and
you cannot see it.
The optical transmitter generates
(usually) modulated light in one of the
bands specified above. The transmitter
designs are based either on LEDs (Light
Emitting Diodes) or laser diodes. LEDs
A number of connectors for fiber optic cables
have been developed and standardized over the
years. Here are the most popular connectors:
made from gallium arsenide phosphide
(GaAsP) generate light at about 1300
nm while older types made from GaAs
worked at 810-870 nm. That’s why we
still sometimes meet installations using
these wavelengths. LEDs produce inco-
herent light – the signal peak is about
30-60 nm wide. Laser diodes that pro-
duce coherent light are not only much
sharper in spectral view but also more
efficient (50% vs. 1%) and ensuring
higher bit rates. Modern laser diodes
are not that much expensive anymore
and they started superseding LEDs in
many applications.
To receive a signal at the other end of
a fiber optic cable, one needs an optical
receiver. Its main component is a photo
detector converting light into electri-
cal current. It is usually a photodiode.
Depending on the elements and com-
pounds used to create a p-n junction,
we achieve maximum sensitivity for a
different wavelength. For example, to
build a receiver for 1300 nm, one needs
either germanium (Ge), indium phos-
phide (InP), or indium gallium arsenide
(InGaAs) photodiode.
Detailed information on standardized
optical connectors can be found in IEC
61754 norm.
FC Connector
The FC (Fixed Connector) is
a screw on type connector. A
threaded barrel is used to fix
the connection. This connec-
tor type is used with GlobalIn-
vacom’s optical LNBs and dis-
tribution products.