Total Loss Rayleigh Scattering Infrared Ultraviolet Absorption Fig. 5.7 Theoretical intrinsic Absorption loss characteristics of germania- doped silica (GeO2-SiO2) glass. Source: Adapted from Senior (1992). Used with permission. Photon Energy (eV) The theoretical intrinsic loss has a minimum value of 0.185 dB/km at a wavelength close to 1550 nm. Figure 5.7 summarizes the intrinsic theoretical losses of a silica glass fiber. In addition to the intrinsic losses, the fiber has additional losses that are referred to as extrinsic. These are associated with the presence of various substances in the glass, the quality of the glass, the processing, and the mechanical imperfections associated with the fiber structure. These losses can be removed with refinements in the fabrication process and in the quality of glass. The presence of metallic and rare-earth impurities contributes to the extrin- sic fiber loss as does the presence of the hydroxyl group OH that enters the glass through water vapors. These contributors to the extrinsic loss are the most difficult to remove (see Figure 5.6). Another contributor to the extrinsic losses is the micro-bending and macro-bending of the fiber that arises from periodic microbends as a result of the spooling or cabling of the fiber and the bending of the fiber for cabling and deployment (Senior, 1992; Buck, 2004). When the intrinsic and extrinsic losses are combined, the total fiber loss is as shown in Figure 5.6. The experimental results shown there are from the early 1980s, and the fiber of today has a loss that is very close to the intrinsic loss of the material. This was achieved by removing the OH extrinsic loss and improving the quality of the glass. The total loss of a modern fiber is shown in Figure 5.8. The evolution of fiber in terms of loss generates three optical windows. The first window was around 850 nm with multimode fiber. The reason for operating at 850 nm was the availability of semiconductor lasers at this wavelength. When the silica zero-dispersion wavelength at 13