Optical Networks: A Practical Perspective - Part 21. 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. | 170 Components In the corrugated section of the cavity the incident wave undergoes a series of reflections. The contributions of each of these reflected waves to the resulting transmitted wave from the cavity add in phase if the period of the corrugation is an integral multiple of half the wavelength in the cavity. The reasoning for this condition is the same as that used for the Fabry-Perot cavity. This condition is called the Bragg condition and was discussed in Section . The Bragg condition will be satisfied for a number of wavelengths but the strongest transmitted wave occurs for the wavelength for which the corrugation period is equal to half the wavelength rather than some other integer multiple of it. Thus this wavelength gets preferentially amplified at the expense of the other wavelengths. By suitable design of the device this effect can be used to suppress all other longitudinal modes so that the laser oscillates in a single-longitudinal mode whose wavelength is equal to twice the corrugation period. By varying the corrugation period at the time of fabrication different operating wavelengths can be obtained. Any laser that uses a corrugated waveguide to achieve single-longitudinal mode operation can be termed a distributed-feedback laser. However the acronym DFB laser is used only when the corrugation occurs within the gain region of the cavity as shown in Figure a . When the corrugation is outside the gain region as in Figure b the laser is called a distributed Bragg reflector DBR laser. The main advantage of DBR lasers is that the gain region is decoupled from the wavelength selection region. Thus it is possible to control both regions independently. For example by changing the refractive index of the wavelength selection region the laser can be tuned to a different wavelength without affecting its other operating parameters. Indeed this is how many of the tunable lasers that we will study in Section are realized. DFB lasers are .
