光纤通信系统Optical Fibermunications英文及中文翻译.pdfVIP

光纤通信系统 Optical Fiber Co unications 英文资料及中文翻译 Co unication may be broadly defined as the transfer of information from one point to another .When the information is to be conveyed over any distance a co unication syste is usually required .Within a co unication syste the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal .This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation .Sophisticated techniques have been developed for this process by using electromagnetic carrier waves operating at radio requites as well as microwave and millimeter wave frequencies. The carrier maybe modulated by using either optical an analog digital information signal.. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the length intensity are obtained (i.e. on-off pulses). Although often simpler to implement, analog modulation with an optical fiber co unication syste is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is mot always provided by semiconductor optical source, especially at high modulation frequencies .For these reasons ,analog optical fiber co unications link are generally limited to shorter distances and lower bandwidths than digital links . Initially, the input digital signal from the information source is suitably encoded for optical transmission