## 3 Ion-Exchanged Active Integrated Photonic Devices  
As already briefly mentioned in the previous sections, ion exchange may occur in a variety of substrates and using many different ions. The process, however, can be easily performed especially when using univalent ions and glasses whose composition includes alkali ions as glass modifiers. Similarly, in crystalline materials such as lithium niobate, only Li\({}^{+}\) ions are effective and they exchange with H\({}^{+}\) ions. These limitations, however, are not very critical, as testified by the large number of photonic devices that have been successfully implemented. Among those devices, a very important class is constituted by the active ones, i.e., integrated optical frequency converters, amplifiers and lasers. Let us here briefly overview some of the excellent results already achieved, limiting ourselves to devices produced by ion exchange in rare-earth-doped (RED) glasses. It has already been mentioned that RED glasses constitute an excellent physical medium for generating and amplifying light. Indeed, oxide glasses are well-known as excellent hosts for rare-earth ions: not by chance, one of the very first solid-state lasers was demonstrated in 1961 by [PERSON] in American Optical Company using a Nd\({}^{3^{+}}\)-doped glass [74]. In that same year [PERSON] and colleagues constructed and operated the world's first optical fiber laser and three years later the first optical fiber amplifier [75]. Even the first thin-film waveguide glass amplifier, in 1972 [76], and the first integrated optical glass laser, in 1974 [77] were implemented in the same material, Nd\({}^{3^{+}}\)-doped glass and operated at \(\sim\)1.06 \(\upmu\)m wavelength. It has to be noted that, whereas the amplifier was fabricated by RF-sputtering deposition of the doped guiding film, the waveguide laser was indeed realized in a Nd-doped borosilicate glass by using the double-diffusion process with an electric field already reported by [PERSON] and [PERSON] [4]. The channel waveguides had an almost circular profile, and the pump threshold was around 18 uJ, namely about one half of the threshold of the corresponding bulk laser [77]. The excellent properties of ion-exchanged waveguide lasers had therefore been demonstrated, but a strong interest in these devices emerged only much later, after the implementation, around 1985, of the second generation of fiber communication systems, which used single-mode fibers. Thus, for instance, in 1989 [PERSON] et al. studied the fabrication of optical waveguides in commercial Kigre Nd-doped lithium-silicate glass by means of Ag-Li exchange and verified that the process did not influence the emission properties of Nd [78]. In 1990, a waveguide laser emitting at 1054 nm was reported by [PERSON] et al. [79]; the laser, end-fire pumped by a diode-laser operating at 802 nm, was capable of a maximum output power higher than 150 mW. It was fabricated in a Nd\({}^{3^{+}}\)-doped glass (Hoya LHG-5 phosphate glass, 3.3 wt% Nd) by using field-enhanced silver ion exchange. The same group also demonstrated a gain of 3.4 dB at 1.054 um and an efficiency of 0.027 dB/mW in a 10 mm long waveguide amplifier pumped with an argon-ion laser [80]. Several other works were performed in the early 1990s concerning Nd-doped waveguide lasers and amplifiers, still using ion-exchange fabrication technology [81; 82; 83; 84; 85]; it was also proven that distributed Bragg reflector (DBR) integrated lasers could be fabricated by realizing holographically an OG in photoresist and then etching it into the channel guide by argon ion milling [83]. By using ion exchange in a semiconductor microcrystallite-doped glass second harmonic generation of Nd:YAG laser radiation was also achieved [36]. In the meantime, the evolution of fiber communication systems continued and saw the introduction of 1.5 um laser diode sources, owing to the fact that silica fibers have the lowest propagation losses in that wavelength window.