Welcome to the Glass Age

89 quantum efficiency for the excited levels even with a narrow energy gap to the next lower level [20]. According to studies and theories [21,22], the lower the phonon energy of the host is, the smaller are nonradiative losses due to multiphonon relaxation yielding longer fluorescence lifetimes from the excited states. This situation becomes critical when the energy level separation between associated electronic levels is not large, which is true for most excited states of Pr 3+ , Ho 3+ , Er 3+ and Tm 3+ ions [23]. A long fluorescence lifetime gives a higher probability of the second- (sometimes also the third- and fourth-) step excitation to intermediate excited levels and then, a higher radiative quantum efficiency to the luminescent terminal state. Intrinsically, fluoride hosts have a wide optical transmission window, because of their short ultraviolet (UV) absorption edge (band gap) and long infrared absorption (multi-phonon) edge. The UV edge of fluorides is short enough to be a UV laser host for ions like Tm 3+ , and the infrared (IR) edge is long enough for IR pumping or to be an IR light source. One interesting application of up-conversion is a 3D display by dual wavelength pumping schemes for three active centers, which was proposed in management using fibers such as laser cooling and radiation-balanced lasers and amplifiers; and long wavelength (>2.5 μm) infrared fibers and fiber lasers for chemical and biological sensing and power-delivery. In parallel, one can expect further advances related to glass integrated optics, including planar active devices (lasers and amplifiers) and photonic guided-wave components for frequency modulation and multiplexing (mux-demux) and optical switching, based on nonlinear optical properties of high-index and nanostructured glasses. Optical interconnectsfor high integration of optoelectronic components will also exploit the properties of various glasses; in this regard, a wider use of ultrathin glasses (now mainly employed for the cover of cell phones and displays) is envisaged, because of the addition of mechanical flexibility to the pristine optical properties of glass. In summary, we have entered an age that is dominated by light and glass and there is a growing appreciation that glass science and optical waveguide engineering are the best approaches to address current limitations in a wide variety of glass-based photonic systems. 1996 by E. Downing [24]; the scheme of operation and the relevant energy levels are illustrated in Figure 5.7. Fluoride glasses remain one of the best possible hosts due to their stability and efficiency. It is interesting to note that three active lanthanide ions for red/ green/blue (RGB) colors (Pr, Er, Tm) are also active centers for the telecom amplifiers at O-, C- and S-bands, respectively, in the near-IR (NIR). Figure 5.8 shows the photoluminescence spectra of the three elements in relation to the NIR telecom bands [24]. Summary The future of glass is even brighter than the past, given the centrality of data to virtually all sectors of modern life. Infrastructure modernization, autonomous mobility, 5G and quantum communications will all rely on ultra-high capacity and information- secure glasses, whether in bulk, planar or fiber form. Areas deserving special attention relating to the future of optical fiber glasses include: high energy fiber lasers where performance is presently limited by already weak nonlinearities in the glass; thermal