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FEATURE Fiber Optics Wavelengths & Connector TypesOptical • 6 different wavelength bands • what dispersion is doing to a signal • the advantages of Laser diodes • finding the lowest attenuationFibers150 TELE-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 151
FEATURE Fiber OpticsConnecting the LightJacek Pawlowski Optical fibers introduced enormous made from gallium arsenide phosphide Band Wavelength Rangeimprovement in data transmission. An- (GaAsP) generate light at about 1300 O band 1260 to 1360 nmybody who is even moderately interest- nm while older types made from GaAs E band 1360 to 1460 nmed in technology knows that the most worked at 810-870 nm. That’s why we S band 1460 to 1530 nmimportant reason for that was the very still sometimes meet installations using C band 1530 to 1565 nmlow signal attenuation of fiber optic these wavelengths. LEDs produce inco- L band 1565 to 1625 nmcables. The first cables manufactured herent light – the signal peak is about U band 1625 to 1675 nmin 1970 had an attenuation of 20 dB/ ■ Table 1.km. Today, we can achieve even 0.16dB per kilometer. It is almost nothingwhen compared to the classical electri-cal cables. However, there is yet another factorinfluencing the maximum useful lengthof a fiber optic cable. This is chromaticdispersion. To explain it in simple terms– various light rays in a fiber have a dif-ferent propagation time what distortsthe shape of the impulses carrying digi-tal data. Dispersion puts a limit on themaximum useful bit rate that can betransmitted 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 aenon due to material structure non- higher bit rates. Modern laser diodes different wavelength. For example, tohomogeneity. 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 arsenidefiber 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 standardizedof around 850 nm, the second genera- receiver. Its main component is a photo optical connectors can be found in IECtion with 1300 nm and the third one detector converting light into electri- 61754 norm.with 1550 nm. Theoretically, the lowestattenuation is achievable for 1550 nm A number of connectors for fiber optic cableswhile 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 Connectordows named: O, E, S, C, L and U-band.The corresponding wavelength rangesare listed in the table 1. For your comparison, the visiblerange for humans is: 380-780 nm. So, FC Connectorthe light used in fiber optics, no matter The FC (Fixed Connector) iswhat band, lies in the deep infrared and a screw on type connector. Ayou 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. LEDs152 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 09-10/2012 — www.TELE-satellite.com
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 firstsnap-in connector. It is frequent- version of SC connector (the ferrule standardized connector for optical fibers. Itly used for newer network instal- is half the size). uses a threaded connection to keep the pluglations. 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 lowbayonet style connector very commonly used for networking ap- profile connector dedicated for the miniaturepopular 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