Welcome to the Glass Age

80 significant efforts focused on three thematic areas, predominantly driven by the growing needs for longer distance and higher capacity communication links: (i) ultra-low loss glasses, (ii) broadband optical amplifiers, and (iii) nonlinear glasses for low power switching and wavelength conversion. Regarding ultra-low loss glasses and optical fibers, considerable effort focused on the development of non-oxide glasses, specifically chalcogenides and fluorides. Relative to SiO 2 , weaker bonding and heavier atomic constituents in both these glass families lead to reduced vibrational energies. This causes absorption due to multiphonon processes to shift to longer wavelengths (lower frequencies), which leads to lower minimum intrinsic attenuation values than silica when Rayleigh scattering is considered (decreasing as (wavelength) -4 ). A great many glasses and fibers in the ZrF 4 - and InF 3 -based fluoride systems, and As 2 S 3 , As 2 Se 3 , and Ga-La-S chalcogenide systems, were explored. However, while intrinsic ultra-low loss was possible, extrinsic losses, dominated by impurities in these melt-quenched glass systems, ultimately ended their consideration for amplifier-less long- haul systems. The near-intrinsic purity of SiO 2 , enabled by chemical vapor deposition manufacturing methods, coupled with the remarkable strength of silica and scalability of CVD, has led to the present condition where nearly 2 meters of fiber is manufactured every day for each person on Earth. Figure 5.1 shows a phase of the CVD process of manufacturing a glass preform from which an optical fiber will be drawn. Though the first optical fiber amplifier dates to 1964, the 1990s and early 2000s saw a frenzied focus on optical fiber amplifiers research, initially focused on erbium doped fiber amplifiers (EDFAs), then on praseodymium (Pr) and dysprosium (Dy) doped analogs. The EDFA enabled long haul communications since weakened signals could be all-optically amplified without electro-optical conversion and regeneration. Further, Figure 5.1. The deposition section of a modified chemical vapor deposition (MCVD) lathe. The white glass is the porous silica soot deposited inside of a pure silica substrate tube prior to sintering and consolidation. Source: Clemson University.