Feature optical
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This document discusses fiber optics and optical fiber technology. It covers the following key points: - Optical fibers have significantly improved data transmission by offering very low signal attenuation compared to electrical cables. Attenuation has decreased from 20 dB/km in early fibers to just 0.16 dB/km today. - Dispersion, where different light rays travel at different speeds, limits the maximum bit rate and transmission length in a fiber. Dispersion depends on wavelength and fiber material. - Standardized optical wavelength bands used in fiber systems are O, E, S, C, L and U-band, ranging from 1260 to 1675 nm. Lasers and LEDs are used as optical transmit
Feature optical
- 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 150 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
- 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 152 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 09-10/2012 — www.TELE-satellite.com
- 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