elength of the propagating light. Fibers with a core diameter much larger than the wavelength of the launched light will support many propagating modes, and the propagating conditions can be analyzed with geometric optics. A fiber with a core diameter similar to that of the light wavelength supports only one propagation mode. Figure 5.5 illustrates these concepts. Material Characteristics of Fibers-Losses The basic material used in the manufacture of optical fiber for optical transmission is silica glass. A large number of glasses have been developed and studied with the objective of improving fiber transmission properties. There are two parameters of glass that have substantial impact on its performance: the losses and the changes of refractive index with Chapter 5 Optical Fibers 850-nm 1300-nm OH Losses 1550-nm Window Window Window Experimental Infrared Absorption Rayleigh Scattering Ultraviolet Absorption Waveguide Imperfections Wavelength (um) Fig. 5.6 Measured loss versus wavelength for SiO2 fibers. Source: Adapted from Hecht (1999). Used with permission. wavelength. The basic material used in the manufacture of optical fibers is vitreous silica dioxide (SiO2), but to achieve the properties required from a fiber, various dopants are also used, (Al2O3, B2O3, GeO2, P2O5). Their task is to slightly increase and decrease the refractive index of pure silica (SiO2). Initially the fiber losses were high, but through improvements in the quality of the materials and the actual production process, the losses have been reduced SO as to be close to the theoretical expected losses. In the part of the electromagnetic spectrum where optical fiber transmission takes place, the losses are bracketed between two asymptotes: ultraviolet absorption and infrared absorp- tion. The measured loss-versus-wavelength curve is shown in Figure 5.6. The actual loss in the window between 0.8 and 1.6 um is dictated by the Rayleigh scat- tering losses. The physical origin of the Rayleigh scattering losses is the excitation and re- radiation 