TAILIEUCHUNG - Optical Networks: A Practical Perspective - Part 35

Optical Networks: A Practical Perspective - Part 35. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 310 Transmission System Engineering Figure Chromatic dispersion limits on the distance and bit rate for transmission over standard single-mode fiber with a chromatic dispersion value of D 17 ps nm-km. A chromatic dispersion penalty of 2 dB has been assumed in the NRZ case this implies that the rms width of the dispersion-broadened pulse must lie within a fraction of the bit period. For sources with narrow spectral width the spectral width of the modulated signal in GHz is assumed to be times the bit rate in Gb s. For RZ transmission the rms output pulse width is assumed to be less than the bit interval. directly modulated or whether an external modulator is used. SLM DFB lasers have unmodulated spectral widths of typically less than 50 MHz. Directly modulating a DFB laser would ideally cause its spectral width to correspond to the modulation bandwidth for example about GHz for a Gb s on-off modulated signal . In practice however the spectral width can increase owing to chirp. As the modulation current and thus optical power varies it is accompanied by changes in carrier density within the laser cavity which in turn changes the refractive index of the cavity causing frequency variations in its output. The magnitude of the effect depends on the variation in current or power but it is not uncommon to observe spectral widths over 10 GHz as a consequence of chirp. Chirp can be reduced by decreasing the extinction ratio. The spectral width can also be increased because of back-reflections from connectors splices and other elements in the optical path. To prevent this effect high-speed lasers are typically packaged with built-in isolators. For externally modulated sources the spectral width is proportional to the bit rate. Assuming the spectral width is approximately equal to the bit rate a 10 Gb s externally modulated signal has a spectral width of 10 GHz which is a practical number today. Dispersion 311 At jum this corresponds to a .

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