Welcome to the Glass Age

87 transition from passive fibers to active fibers also demonstrates the interesting relationship between active ions and the host glasses. Although fiber amplifiers are already playing crucial roles in the optical networks both at the 1.55 μm and 1.3 μm bands, further requirements exist to fully utilize the windows of optical fibers with superior performance. The requirements are a wide and flat gain spectrum around 1.53~1.65 μm (C+L band) in a novel EDFA and around 1.4~1.51 μm (S-band) in Tm 3+ (TDFA) for wavelength division multiplexing (WDM) systems [15], and greater gain per pump-power at 1.31 μm in Pr 3+ [16]. In the research history of rare-earth doped glasses, also noteworthy are various efforts to develop blue and green lasers by up-conversion luminescence of Pr 3+ , Ho 3+ , Er 3+ and Tm 3+ ions, mainly in fluoride glass hosts, during the late 1980s and early 1990s, before the invention of blue LED or LD based on GaN [17,18]. Some of these transitions are shown in Figure 5.6. High-power LDs in the NIR developed for fiber telecommunication in the late 1980s have stimulated research because of the possibility of a visible laser by pumping with the LDs. Fluoride glasses developed so far [19] are an ideal host to give the lanthanide dopant ions a low-phonon-energy environment and thus long excited state lifetimes, enabling efficient multi-step pumping and high luminescence Figure 5.8. Left: Energy levels of RGB luminescent center ions, Pr 3+ , Er 3+ , Tm 3+ , showing NIR amplification transitions at the O-, S-, and C+L-bands of optical fiber telecommunication. Right: NIR PL spectra of these same ions [25]. Source: S. Tanabe. Figure 5.7. Principle of 3D color display utilizing the dual-NIR-pumped visible upconversion and the energy levels of RGB luminescent ions; Pr 3+ , Er 3+ , Tm 3+ [23]. Some intermediate excited levels of lesser importance are omitted for simplification. Source: S. Tanabe.