Welcome to the Glass Age

81 EDFAs operated at 1.55 μm, the wavelength of minimum loss for silica, and are highly efficient. Other wavelengths of interest, such as 1.3 μm where silica glass exhibits zero chromatic dispersion, were expected to afford greater information carrying capacities than operation at 1.55 μm. However, the emissions from Pr and Dy at 1.3 μm are fully quenched given the relatively high vibrational energies in SiO 2 glasses and, so, such amplifiers required low phonon energy glasses, for example the aforementioned fluoride and chalcogenide glass systems. Though much progress was made, including operational networks, in the end, optical fiber systems moved instead to all-silica based components using EDFAs as the amplifiers and dispersion-shifted or compensated fiber designs, originally developed in the late 1970s, to control dispersion and bring together lowest loss and low dispersion at a single wavelength. Optical fibers, and their ability to confine and guide light, are not only useful for transmission and amplification, but also for nonlinear processes, such as frequency conversion and switching. Whereas transparent fibers usually benefit from low nonlinearity, optical switching and frequency generation require high nonlinearities, where glasses generally gain from components that are weakly bound and heavy relative to, for example, silica. During the 1990s to 2000s, much focus on nonlinear optical fibers centered on chalcogenide and heavy-metal oxide glasses, such as tellurites and germanates [3]. In these glasses, because the nonlinear coefficients can be orders of magnitude larger than those of silica, fiber device lengths are short (cm to meter) and, accordingly, losses are not as critical. Following the “dot-com” boom of the late 1990s to early 2000s, optical fibers have enjoyed considerable growth Figure 5.2. A simple sketch of the many application areas of photonic devices implementing the ion-exchange technique. Source: Courtesy of S. Berneschi.