tailieunhanh - Optical Networks: A Practical Perspective - Part 19

Optical Networks: A Practical Perspective - Part 19. 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. | 150 Components Figure A two-stage multiplexing approach using interleaving. In this 32-channel demultiplexer the first stage picks out every alternate wavelength and the second stage extracts the individual wavelength. A significant benefit of this approach is that the filters in the last stage can be much wider than the channel width. As an example suppose we want to demultiplex a set of 32 channels spaced 50 GHz apart. After the first stage of demultiplexing the channels are spaced 100 GHz apart as shown in Figure . So demultiplexers with a broader passband suitable for demultiplexing 100 GHz spaced channels can be used in the second stage. In contrast the single-stage or serial approach would require the use of demultiplexers capable of demultiplexing 50 GHz spaced channels which are much more difficult to build. Carrying this example further the second stage itself can in turn be made up of two stages. The first stage extracts every other 100 GHz channel leading to a 200 GHz interchannel spacing after this stage. The final stage can then use even broader filters to extract the individual channels. Another advantage of this approach is that no guard bands are required in the channel plan. The challenges with the interleaving approach lie in realizing the demultiplexers that perform the interleaving at all the levels except the last level. In principle Optical Amplifiers 151 any periodic filter can be used as an interleaver by matching its period to the desired channel spacing. For example a fiber-based Mach-Zehnder interferometer is a common choice. These devices are now commercially available and interleaving is becoming a popular approach toward realizing high channel count multiplexers and demultiplexers. Optical Amplifiers In an optical communication system the optical signals from the transmitter are attenuated by the optical fiber as they propagate through it. Other optical components such as multiplexers and couplers also add loss. After