rea. The larger the effective area, the greater the amount of optical power that a fiber becomes capable of supporting without becoming susceptible to the nonlinear effects mentioned above. Thus, LEAF fiber's large A provides higher power handling capability as well as a degree of immunity to those nonlinear effects. For this reason, LEAF and similar fiber from other optical fiber manufacturers permits extended transmission dis- tances before light signal amplification is required. Because LEAF reduces the above-mentioned nonlinear effects, it also provides a medi- um more conducive to the development of additional channels on a DWDM system. Now that we have an appreciation for the role of improved fiber in the development of optical systems with greater trans- mission capacity, let's study a second area that makes DWDM a reality. That area is fiber Bragg gratings. FIBER Bragg GRATINGS Fiber Bragg gratings represent an important component that enables modern optical multiplexing to occur. A fiber Bragg grating represents a small section of a fiber that is modified to create periodic changes in the index of refraction. Because it is possible to vary the space between the changes in the index of refraction, it becomes possible to create an optical filter. Thus, by varying the space between the different index of refraction created in a fiber, a certain fre- quency of light, which is referred to as the Bragg resonance wavelength, is reflected back while all other wavelengths pass through the fiber. With fiber Bragg gratings it becomes possible to develop optical add/drop multiplexers. This, in turn, allows specific wavelengths to be added or removed from an optical fiber. To understand why this capability repre- sents another technological development that enhances the growth in DWDM systems, assume that a communications carrier intends to install a multiplexed optical transmission system between New York and Miami. Through the use of fiber Bragg gratings, it becomes possible to drop a channel in Baltimore, another 