Optical Networks - _5_7 Dispersion_62

Optical Networks - _5_7 Dispersion_62 - 314 Transmission...

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314 Transmission System Engineering are concerned only with one channel, we could align the center wavelengths exactly by temperature-tuning the individual mux/demuxes. However, other channels could become even more misaligned in the process (tuning one channel tunes the others as well). In addition, the lasers themselves will have a tolerance regarding their center wavelength. In a cascaded system, wavelength inaccuracies cause additional power penalties due to added signal loss and crosstalk (see Problems 5.19 and 5.20). 5.7 Dispersion Dispersion is the name given to any effect wherein different components of the transmitted signal travel at different velocities in the fiber, arriving at different times at the receiver. A signal pulse launched into a fiber arrives smeared at the other end as a consequence of this effect. This smearing causes intersymbol interference, which in turn leads to power penalties. Dispersion is a cumulative effect: the longer the link, the greater the amount of dispersion. Several forms of dispersion arise in optical communication systems. The impor- tant ones are intermodal dispersion , polarization-mode dispersion ,and chromatic dispersion . Of these, we have already studied intermodal dispersion and chromatic dispersion in Chapter 2 and quantified the limitations that they impose on the link length and/or bit rate. Intermodal dispersion arises only in multimode fiber, where the different modes travel with different velocities. Intermodal dispersion was discussed in Section 2.2. The link length in a multimode system is usually limited by intermodal dispersion and not by the loss. Clearly, intermodal dispersion is not a problem with single-mode fiber. Polarization-mode dispersion (PMD) arises because the fiber core is not perfectly circular, particularly in older installations. Thus different polarizations of the signal travel with different group velocities. PMD is proving to be a serious impediment in very high-speed systems operating at 10 Gb/s bit rates and beyond. We discuss PMD in Section 5.7.4. The main form of dispersion that we are concerned with is chromatic dispersion , which has a profound impact on the design of single-mode transmission systems (so much so that we often use the term dispersion to mean “chromatic dispersion”). Chromatic dispersion arises because different frequency components of a pulse (and also signals at different wavelengths) travel with different group velocities in the fiber and thus arrive at different times at the other end. We discussed the origin of chro- matic dispersion in Section 2.4. Chromatic dispersion is a characteristic of the fiber, and different fibers have different chromatic dispersion profiles. We discussed the
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5.7 Dispersion 315 chromatic dispersion profiles of many different fibers in Section 2.5.9. As with other kinds of dispersion, the accumulated chromatic dispersion increases with the link length. Chromatic dispersion and the system limitations imposed by it are discussed in detail in the next two sections.
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Optical Networks - _5_7 Dispersion_62 - 314 Transmission...

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