tailieunhanh - Optical Networks: A Practical Perspective - Part 18

Optical Networks: A Practical Perspective - Part 18. 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. | 140 Components couplers interconnected by an array of waveguides. The MZI can be viewed as a device where two copies of the same signal but shifted in phase by different amounts are added together. The AWG is a device where several copies of the same signal but shifted in phase by different amounts are added together. The AWG has several uses. It can be used as an n x 1 wavelength multiplexer. In this capacity it is an n-input 1-output device where the n inputs are signals at different wavelengths that are combined onto the single output. The inverse of this function namely 1 x n wavelength demultiplexing can also be performed using an AWG. Although these wavelength multiplexers and demultiplexers can also be built using MZIs interconnected in a suitable fashion it is preferable to use an AWG. Relative to an MZI chain an AWG has lower loss flatter passband and is easier to realize on an integrated-optic substrate. The input and output waveguides the multiport couplers and the arrayed waveguides are all fabricated on a single substrate. The substrate material is usually silicon and the waveguides are silica Ge-doped silica or SiOi-TazOs. Thirty-two-channel AWGs are commercially available and smaller AWGs are being used in WDM transmission systems. Their temperature coefficient nm C is not as low as those of some other competing technologies such as fiber gratings and multilayer thin-film filters. So we will need to use active temperature control for these devices. Another way to understand the working of the AWG as a demultiplexer is to think of the multiport couplers as lenses and the array of waveguides as a prism. The input coupler collimates the light from an input waveguide to the array of waveguides. The array of waveguides acts like a prism providing a wavelength-dependent phase shift and the output coupler focuses different wavelengths on different output waveguides. The AWG can also be used as a static wavelength crossconnect. However this wavelength .