Đang chuẩn bị nút TẢI XUỐNG, xin hãy chờ
Tải xuống
Optical Networks: A Practical Perspective - Part 77. 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. | This Page Intentionally Left Blank appendix Pulse Propagation in Optical Fiber In mathematical terms chromatic dispersion arises because the propagation constant 3 is not proportional to the angular frequency a that is dft da constant independent of z . dp da is denoted by 3 and Bj-1 is called the group velocity. As we will see this is the velocity with which a pulse propagates through the fiber in the absence of chromatic dispersion . Chromatic dispersion is also called group velocity dispersion. If we were to launch a pure monochromatic wave at frequency uo into a length of optical fiber the magnitude of the real electric field vector associated with the wave would be given by E r t J x y cos cuot - P coq z . E.l Here the z. coordinate is taken to be along the fiber axis and J x y is the distribution of the electric field along the fiber cross section and is determined by solving the wave equation. This equation can be derived as follows. For the fundamental mode the longitudinal component is of the form E 2tt Ji x y exp iflz . Here J x y is a function only of p y x2 y2 due to the cylindrical symmetry of the fiber and is expressible in terms of Bessel functions. The transverse component of the fundamental mode is of the form Ex Ey 2ir Jt x y ex.p iflz where again Jt x y depends only on y x2 y2 and can be expressed in terms of Bessel functions. Thus for each of the solutions corresponding to the fundamental mode we can write E r. w 2 r J x y e f m ze x y E.2 731 732 Pulse Propagation in Optical Fiber where J x y Ji x y 2 Jt x y 2 and the e is the unit vector along the direction of E r w . In this equation we have explicitly written p as a function of co to emphasize this dependence. In general . and e are also functions of co but this dependence can be neglected for pulses whose spectral width is much smaller than their center frequency. This condition is satisfied by pulses used in optical communication systems. Equation E.l now follows from E.2 by taking the .
