Welcome to the Glass Age

86 Active glasses and 3D displays The first glass laser, using a neodymium- doped silicate glass, was developed in 1961 [10], just after the invention of the first laser (ruby) [11] for humankind in 1960. Subsequently, several important developments in glass-based active devices followed, before the global installation of optical fiber telecommunication [12, 13]. Most active devices utilize the luminescence and/or its stimulated emission of lanthanoids’ 4f electronic transitions. Figure 5.5 shows the principle of optical amplification with electronic energy levels and representative emission bands of active lanthanoid ions in comparison with the (silica) optical fiber loss spectrum, which has the minimum loss at 1.55 μm. As noted in Section 2, relating to telecommunication systems, the invention of the EDFA [14] can be likened to that of the transistors in electronics in terms of its technological impact. The technology to amplify the light signal directly without the conversion of light/electricity/light is achieved by stimulated emission of a 4f optical transition in rare-earth-doped silica glass fibers and realizes ideal amplification with high gain and low noise. The technological development of optical telecommunication is based on the growth of technologies of fiber fabrication and those of laser diodes (LD). In fact, the invention of efficient III-V LDs and their fiber coupling has also enabled efficient pumping of Er 3+ with its three-level system. In addition, the history of the technological Figure 5.6. Energy level diagrams and important electronic transitions of Tm 3+ and Er 3+ ions for optical amplifications and infrared-to-visible upconversion by excited-state absorption (ESA) mechanisms. Blue and green luminescent fibers in two photos are Tm-doped and Er-doped ones excited by the corresponding NIR LD for telecom amplification. Source: S. Tanabe.