1. FEATURE Fiber Optics
Wavelengths
& Connector
Types
Optical
• 6 different wavelength bands
• what dispersion is doing to a signal
• the advantages of Laser diodes
• finding the lowest attenuation
Fibers
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2. FEATURE Fiber Optics
Connecting the Light
Jacek Pawlowski
Optical fibers introduced enormous made from gallium arsenide phosphide Band Wavelength Range
improvement in data transmission. An- (GaAsP) generate light at about 1300 O band 1260 to 1360 nm
ybody who is even moderately interest- nm while older types made from GaAs E band 1360 to 1460 nm
ed in technology knows that the most worked at 810-870 nm. That’s why we S band 1460 to 1530 nm
important reason for that was the very still sometimes meet installations using C band 1530 to 1565 nm
low signal attenuation of fiber optic these wavelengths. LEDs produce inco- L band 1565 to 1625 nm
cables. The first cables manufactured herent light – the signal peak is about U band 1625 to 1675 nm
in 1970 had an attenuation of 20 dB/ ■ Table 1.
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 30-60 nm wide. Laser diodes that pro- cal current. It is usually a photodiode.
optic cable (i.e. those with a diameter duce coherent light are not only much Depending on the elements and com-
less than 12 µm) but even such cables sharper in spectral view but also more pounds used to create a p-n junction,
are not quite free from this phenom- efficient (50% vs. 1%) and ensuring we achieve maximum sensitivity for a
enon due to material structure non- higher bit rates. Modern laser diodes different wavelength. For example, to
homogeneity. are not that much expensive anymore build a receiver for 1300 nm, one needs
Attenuation and dispersion depend and they started superseding LEDs in either germanium (Ge), indium phos-
on the light wavelength and optical many applications. phide (InP), or indium gallium arsenide
fiber material. The first generation of To receive a signal at the other end of (InGaAs) photodiode.
optical fiber worked with wavelengths a fiber optic cable, one needs an optical Detailed information on standardized
of around 850 nm, the second genera- receiver. Its main component is a photo optical connectors can be found in IEC
tion with 1300 nm and the third one detector converting light into electri- 61754 norm.
with 1550 nm. Theoretically, the lowest
attenuation is achievable for 1550 nm A number of connectors for fiber optic cables
while the lowest dispersion (= the high-
est bit rate) for 1300 nm.
have been developed and standardized over the
The wavelength used in optical fiber years. Here are the most popular connectors:
based systems have been standard-
ized. There are six transmission win- FC Connector
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, FC Connector
the light used in fiber optics, no matter The FC (Fixed Connector) is
what band, lies in the deep infrared and a screw on type connector. A
you cannot see it. threaded barrel is used to fix
The optical transmitter generates the connection. This connec-
(usually) modulated light in one of the tor type is used with GlobalIn-
bands specified above. The transmitter vacom’s optical LNBs and dis-
designs are based either on LEDs (Light tribution products.
Emitting Diodes) or laser diodes. LEDs
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3. SC Connector ST Connector
LC Connector MT-RJ Connector
SMA Connector CLICK!
SC Connector LC Connector SMA Connector
SC (Subscriber Connector) is a LC (Lucent Connector) is a smaller SMA (SubMiniature version A) was the first
snap-in connector. It is frequent- version of SC connector (the ferrule standardized connector for optical fibers. It
ly used for newer network instal- is half the size). uses a threaded connection to keep the plug
lations. intact in the socket
MT-RJ Connector
ST Connector MT-RJ (Mechanical Transfer Regis- CLICK!
ST (Straight Tip) is a slotted tered Jack) is a small size connector This is a proprietary design – a push-pull low
bayonet style connector very commonly used for networking ap- profile connector dedicated for the miniature
popular in late 80s and early 90s. plications. H+S fiber optic distribution system.
154 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 09-10/2012 — www.TELE-satellite.com