7.7 DIGITAL COMMUNICATIONS OVER FIBERS

One of the more important uses of fiber communications is the transmission of digital data in the form of high-speed light pulses. The ability to pulse light at extremely high rates (images 1 Gbps) leads to an obvious potential for high-bit-rate fiberoptic channels. In this section, we examine the parameters needed to determine the extent to which this potential is achieved.

We have seen that the fiber channel can cause transmitted light pulses to be attenuated and spread during propagation. This spreading can be interpreted as an effective channel filtering, as discussed in Section 7.3. This means that if an idealized square pulse of light of width τ is launched into the fiber, it will appear at the output with the intensity spread to a width w, as shown in Figure 7.11 with the amount of spreading dependent on the fiber length. If this pulse corresponded to a digitally modulated OOK pulse, and if a sequence of such pulses were sent, the pulse spreading will cause (a) a pulse energy loss, because a portion of the pulse is spread outside its original width, and (b) interpulse distortion, because the tails of a given spread pulse will overlap onto adjacent pulses. To reduce these effects, it is necessary to use transmitted pulse intervals greater than the expected spreading. Thus the minimal transmitted pulse width equals the spread pulse width, and ...

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