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Fiber Optic Telecommunication phần 4

Tham khảo tài liệu 'fiber optic telecommunication phần 4', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fiber Optic Telecommunication makes light coupling into single-mode fiber difficult due to the fiber s small N.A. and core diameter. For this reason LEDs are most often used with multimode fiber. LEDs are used in lower-data-rate shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. The output spectrum of a typical LED is about 40 nm which limits its performance because of severe chromatic dispersion. LEDs operate in a more linear fashion than do laser diodes. This makes them more suitable for analog modulation. Figure 8-22 shows a graph of typical output power versus drive current for LEDs and laser diodes. Notice that the LED has a more linear output power which makes it more suitable for analog modulation. Often these devices are pigtailed having a fiber attached during the manufacturing process. Some LEDs are available with connector-ready housings that allow a connectorized fiber to be directly attached. They are also relatively inexpensive. Typical applications are local area networks closed-circuit TV and transmitting information in areas where EMI may be a problem. Figure 8-22 Drive current versus output power for LED and laser Courtesy of AMP Inc. Laser diodes LD are used in applications in which longer distances and higher data rates are required. Because an LD has a much higher output power than an LED it is capable of transmitting information over longer distances. Consequently and given the fact that the LD has a much narrower spectral width it can provide high-bandwidth communication over long distances. The LD s smaller N.A. also allows it to be more effectively coupled with single-mode fiber. The difficulty with LDs is that they are inherently nonlinear which makes analog transmission more difficult. They are also very sensitive to fluctuations in temperature and drive current which causes their output wavelength to drift. In applications such as wavelengthdivision multiplexing in which several .