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Unformatted text preview: 2.7 Other Fiber Technologies 103 transmission in a 25-channel WDM system at a bit rate of 40 Gb/s per channel, over a distance of 1500 km, has been demonstrated in the laboratory [SKN01]. 2.7 Other Fiber Technologies We will discuss two fiber types that are not traditional glass fibers. The first is designed by having periodic structures, and the second uses plastic material. 2.7.1 Photonic Crystal Fiber In previous sections we have seen how dopants and fiber profile can be engineered to reduce loss, dispersion and nonlinearity, for better transmission. There is another category of fiber designs that is not limited by bulk material properties. As in semi- conductors, engineers can create sometimes startling properties that do not exist in bulk materials by playing with periodic structures and defects in periodic structures within the fiber. These fiber designs are called photonic crystal fibers (PCFs). PCFs were first demonsrated in 1996 and have been an active area of research since then. Some of the properties that can be created are dispersion, nonlinearity, and even negative refractive index (e.g., according to Snells law, as illustrated in Figure 2.4, if the refractive index is positive, the rays are refracted on the opposite side of the normal on entering the material, but negative refractive index means rays will be refracted on the same side). PCF enables a number of functions in fiber, some of which are relevant to disper- sion compensation, amplification, and wavelength conversion by nonlinear optics. The PCF structures for fiber have been in two dimensions. We should note that the associated science and fabrication of PCF has extended beyond fiber to materials for other devices and that structures in three dimensions are being explored as well....
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- Spring '09