For an optical fiber in which the index of refraction decreases uniformly and monotonically from N1 (core refractive index) to N2 (cladding refractive index), the theoretical NA is given by the following: NA = sin a = Vn22 When the core is made smaller, there are fewer modes that carry energy. If the core is made small enough, but not too small, then the fiber only car- ries one mode. When the core dimension is only a few times larger than the wavelength of the signal to be transmitted, only one of the originating rays (modes) will be transmitted; this results in low loss (the other rays are attenuated in the proximity of the source). For wavelengths of 1.3 to 1.55 um, the core needs to be less than 10 um in diameter for single-mode trans- mission properties to hold. The problem of modal dispersion that affects multimode fiber then disappears. With a single-mode fiber, the information capacity is much higher; however, because the core is so small, more expen- sive light sources and connectors are required. Currently, many forward-looking providers install a fiber infrastructure based on cables with a very high fiber count in the order of 500 or even 1,000. Following different service demands, these cables contain different fiber types such as standard single-mode fiber according to International Telecommunication Union Standardization Sector (ITU-T) G.652, nonzero dispersion-shifted single-mode fiber (discussed in the following section) Optics Primer according to ITU-T G.655, or fibers without OH-peak (such as Lucent All- Wave). The main transmission fiber types are standard fiber (nondisper- sion-shifted fiber [NDSF], such as Corning SMF-28), dispersion-shifted fiber (DSF), and the various types of nonzero dispersion-shifted fiber (NZDSF) (such as Corning LEAF or LS and Lucent Truewave). New trans- mission fiber types with lower loss in the 1.4-micron region are beginning to emerge together with fibers specialized for the metro space. In this space, however, NDSF is still the most widely used fiber. Corning SMF-