ant mole percentages on refractive index of silica glass. Reprinted from Ref. [1], p. 22, Fig. 6.1-12 Transmission loss in silica-based fibers. Reprinted courtesy of Academic Press. from Ref. [12], p. 474, courtesy of Irwin. CHAPTER 6.1 Optical fiber, cable and connectors Attenuation (dB/km) The material and waveguide dispersion are often foh.85 referred to as chromatic dispersion, measured in units ps/nm . km, which is defined as (6.1.27) Graded-index Fiber where L is the fiber length and tg is the time required to propagate a distance L. The subscript g refers to group velocity. When a pulse propagates in a dispersive medium it propagates with the group velocity, Single-Mode Fiber (6.1.28) Wavelength (nm) The phase travels with the phase velocity given by Fig. 6.1-13 Wavelength dependence of fiber attenuation. Re- printed from Ref. [14], p. 8. Copyright © 1989 by Hewlett Packard Company. Reproduced with permission. (6.1.29) The dispersion properties are completely determined by the group velocity. 6.1.2.3 Dispersion Material dispersion originates from the fact that the refractive index is a function of wavelength, n(1): Dispersion is due to the fact that different wavelengths experience different propagation constants, B(2), and therefore travel with different velocities causing a longer (6.1.30) temporal pulse at the end of the fiber. Dispersion does not alter the wavelength (frequency) content of the light The refractive index can be calculated from the pulse. From a communications point of view dispersion is Sellmeier equation: a very important factor because it directly affects the bit rate. The following are three major components con- tributing to the dispersion: (6.1.31) Material Waveguide where the constants ai and bi may vary with the partic- Intermodal ular fiber composition. These constants have been accu- rately measured and tabulated for different Ge, B and P concentrations [16]. Material Total Microbend Multimode Macrobend Initial Waveguide Microbend Single mode Initial A. nm Fig. 6.1-14 Bend-indu