150 151TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 09-10/2012 — www.TELE-satellite.com www.TELE-satellite.com — 09-10/2012 — TELE-satellite International — 全球发行量最大的数字电视杂志

•6differentwavelengthbands•whatdispersionisdoingtoasignal

•theadvantagesofLaserdiodes•findingthelowestattenuation

Optical Fibers

Wavelengths & Connector

Types

FEATURE Fiber Optics

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FC Connector

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Connecting the LightJacek Pawlowski

FEATURE Fiber Optics

Table 1.

Band Wavelength RangeO band 1260 to 1360 nmE band 1360 to 1460 nmS band 1460 to 1530 nmC band 1530 to 1565 nmL band 1565 to 1625 nmU 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-homogeneity.

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 ConnectorThe 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.

SC Connector ST Connector

LC Connector MT-RJ Connector

SMA Connector CLICK!

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SC ConnectorSC (Subscriber Connector) is a

snap-in connector. It is frequent-ly used for newer network instal-lations.

ST ConnectorST (Straight Tip) is a slotted

bayonet style connector very popular in late 80s and early 90s.

LC ConnectorLC (Lucent Connector) is a smaller

version of SC connector (the ferrule is half the size).

MT-RJ ConnectorMT-RJ (Mechanical Transfer Regis-

tered Jack) is a small size connector commonly used for networking ap-plications.

SMA ConnectorSMA (SubMiniature version A) was the first

standardized connector for optical fibers. It uses a threaded connection to keep the plug intact in the socket

CLICK!This is a proprietary design – a push-pull low

profile connector dedicated for the miniature H+S fiber optic distribution system.