## 3 Ion-Exchanged Active Integrated Photonic Devices
More waveguides and devices based on Er-doped glass substrates and ion-exchange technology were reported in the following years [95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107]; besides waveguide lasers and amplifiers, optical upconversion devices were investigated, too [101]. At a certain point, phosphate glasses seemed to allow higher performances to ion-exchanged EDWAs, as they could be doped with larger amounts of rare-earth ions than silicates. Even in phosphate glasses, however, high erbium concentrations may produce quenching of the luminescence; moreover, the absorption cross section of Er\({}^{+}\) ions at \(\sim\)980 nm is small. Thus, the solution was to add another rare-earth ion as a sensitizer for erbium. Glass co-doping with ytterbium ions has proven to be a very efficient solution: it allows the transfer of energy from excited Yb\({}^{3+}\) ions to close Er\({}^{3+}\) ions through a cooperative cross-relaxation process, with the result of significantly enhancing system absorption at 980 nm and making the pumping mechanism more efficient. Moreover, in the case of high concentration, the Er\({}^{3+}\) ions are closer together, so that deleterious non-radiative energy exchanges between neighboring ions can take place; the presence of Yb\({}^{3+}\) ions is also effective in reducing such unwanted Er\({}^{3+}\) - Er\({}^{3+}\) ion energy transfer interactions by increasing the mean inter-atomic distance. A useful spectroscopic investigation of an extensive series of Er\({}^{3+}\)-doped and Er\({}^{3+}\)-Yb\({}^{3+}\)-co-doped soda-time-silicate (SL) and aluminosilicate (AS) glasses was also presented in a paper by [PERSON] et al. [108]. AS glasses showed higher oscillator strengths and larger inhomogeneous broadening of 4\(f\) transitions than SL glasses; it was also suggested that a minimum molar concentration ratio 2:1 of Yb oxide to Er oxide is required to achieve efficient sensitization of Er\({}^{3+}\). In 1995, an integrated laser was produced by K\({}^{+}\)/Na\({}^{+}\) ion exchange in an alkali-rich barium silicate glass containing 17% wt Y\({}_{2}\)O\({}_{3}\) and 1.5% wt Er\({}_{2}\)O\({}_{3}\)[109]. Later, commercial Er-Yb co-doped phosphate glasses became available, too, where single-mode channel waveguides could be produced by K\({}^{+}\)/Na\({}^{+}\) ion exchange; a net gain of 7.3 dB in a 6 mm long waveguide amplifier was claimed [110]. EDWA devices with excellent characteristics were obtained in both silicate and phosphate glasses co-doped with Er and Yb; as an example, a lossless 1 \(\times\) 2 splitter was demonstrated in a buried thallium-exchanged waveguide in a borosilicate glass co-doped with 5% wt Yb\({}_{2}\)O\({}_{3}\) and 3% wt Er\({}_{2}\)O\({}_{3}\)[111]. They measured 2.3 dB/cm gain and 0.07 dB/mW gain efficiency in a 3.9 cm long straight waveguide amplifier with a pump of 130 mW. Almost at the same time, [PERSON] et al. achieved a 7 dB net gain in a 41 mm long amplifier with double-pass configuration (\(\approx\)1.7 dB/cm) using a buried waveguide produced by two-step ion exchange in a phosphate glass doped with 2.5 % by weight of erbium and 3% by weight of ytterbium [112]. Shortly thereafter, however, the same group reported several impressive advances in EDWA's implementation, including: an amplifying four-wavelength combiner, consisting of an all-connectorized 4 \(\times\) 1 glass splitter followed by a 4.5-cm-long Er/Yb-doped waveguide amplifier (\(\approx\)2.6 dB/cm) [98]; a planar amplifier capable of 3.1 dB/cm net gain [113]; a multiplexer/amplifier gain-block constituted by an amplifying section realized in phosphate glass as a 5.5 cm straight waveguide planar waveguide amplifier and a 980/1550 nm multiplexer realized as a 3 cm long directional coupler in a silicate glass [114]. The amplifier and the multiplexer were assembled together via active alignment followed by bonding with UV curing glue; the module had a fiber-to-fiber gain of 15.7 dB and a noise figure of 4.7 dB at 1535 nm.