00 nm was identified, the first single-mode systems operated at 1300 nm, the second window. When it was possible to reduce the losses at the minimum-loss wavelength, 1550 nm, then single-mode systems started operating at 1550 nm, the third window. Long-haul systems operate in the region of 1550 nm either with a single wavelength or with multiple wavelengths (wavelength division multiplexing-see Chapter 6). The 1300-nm window is still used for short-haul systems. Chapter 5 Optical Fibers Wavelength (nm) Fig. 5.8 Spectral loss of water loss Water loss free PSCF free PSCF. Conventional PSCF Source: Adapted from Chigusa et al. Conventional G.652D fiber (2005). Used with permission. PSCF = Pure Silica Core Fiber Material Characteristics of Fibers-Dispersion The variation of refractive index of a material with wavelength is known in optics as disper- sion, and it is responsible for resolution of white light into its constituent colors. In the context of fiber light propagation, the dispersion can be divided into two parts. The first part is the dispersion induced on the light by the material used in the waveguide, and this is known as material dispersion. The second part is the impact of the actual waveguide structure, and it is known as waveguide dispersion. This section will address material dis- persion only (Senior, 1992; Buck, 2004). The speed of propagation of monochromatic light in an optical fiber is given by the simple equation phase (5.14) This speed is the phase velocity of the light wave, and it is different for each wavelength. In transmitting a pulse of light through the fiber, the pulse can be expressed as the sum- mation of a number of sine and cosine functions, which is known as the spectrum of the pulse. If the spectrum is centered on a frequency, W, and has a small spectral width around W, then a velocity can be associated with this group of frequencies, and this is known as the group velocity. In conformity with the idea of velocity of propagation for monochromatic radiation, a group index can be de