Optical Networks: A Practical Perspective - Part 15. 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. | 110 Components also used to combine 980 nm or 1480 nm pump signals along with a 1550 nm signal into an erbium-doped fiber amplifier see Figures 3 34 and . In addition to the coupling ratio a we need to look at a few other parameters while selecting couplers for network applications. The excess loss is the loss of the device above the fundamental loss introduced by the coupling ratio a. For example a 3 dB coupler has a nominal loss of 3 dB but may introduce additional losses of say dB. The other parameter is the variation of the coupling ratio a compared to its nominal value due to tolerances in manufacturing as well as wavelength dependence. In addition we also need to maintain low polarization-dependent loss PDL for most applications. Principle of Operation When two waveguides are placed in proximity to each other as shown in Figure light couples from one waveguide to the other. This is because the propagation modes of the combined waveguide are quite different from the propagation modes of a single waveguide due to the presence of the other waveguide. When the two waveguides are identical which is the only case we consider in this book light launched into one waveguide couples to the other waveguide completely and then back to the first waveguide in a periodic manner. A quantitative analysis of this coupling phenomenon must be made using coupled mode theory Yar97 and is beyond the scope of this book. The net result of this analysis is that the electric fields EO1 and Eo2 at the outputs of a directional coupler may be expressed in terms of the electric fields at the inputs En and E 2 as follows Eoif lA _ - pi cos cZ i sin xZ En f Eoilfl e yzsin xZ cos d J Ei2 f Here I denotes the coupling length see Figure and 3 is the propagation constant in each of the two waveguides of the directional coupler. The quantity k is called the coupling coefficient and is a function of the width of the waveguides the refractive indices of the waveguiding region
