## 3 Ion-Exchanged Active Integrated Photonic Devices
In all these cases, the amplifier was realized in a phosphate glass co-doped with 2 wt.% erbium and 4 wt.% ytterbium. In the 2000s, the literature was enriched by several publications in this area, which can be classified into three groups:  
1. Papers dealing with the synthesis and characterization of rare-earth doped glasses suitable for ion-exchange fabrication of active devices [17; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144]. 2. Demonstration and characterization of rare-earth doped ion-exchanged waveguide amplifiers and frequency converters [145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169]. 3. Demonstration and characterization of rare-earth doped ion-exchanged waveguide lasers [166; 167; 168; 169; 161; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169]. An example of a laser device is shown in Figure 7: it is a 3D hybrid structure, with a channel core waveguide and a slab core waveguide which, in a certain volume, interact with each other and with the Bragg grating that provides the optical feedback necessary for laser action. Figure 8 shows the longitudinal and cross sections of the device. The passive glass is a GO14 silicate by Team Photonics, whereas the active glass is a IOG-1 phosphate by Schott, doped with 2.2 wt% Er\({}_{2}\)O\({}_{3}\) and 3.6 wt% Yb\({}_{2}\)O\({}_{3}\). In both glasses, the waveguides are produced by Ag\({}^{+}\)-Na\({}^{+}\) ion exchange. An example of a packaged Er-Yb doped waveguide amplifier (EYDWA) is shown in Figure 9[155]. The substrates were commercially available Schott IOG-1 phosphate glasses co-doped with 2 wt% Er\({}_{2}\)O\({}_{3}\) and 5 wt% Yb\({}_{2}\)O\({}_{3}\). Channel waveguides were fabricated by ion exchange using an Al thin film mask and were buried by using a two-step process, namely, a K\({}^{+}\)-Na\({}^{+}\) thermal exchange (\(T\) = 385 \({}^{\circ}\)C, \(t\) = 60 min), followed by a field-assisted annealing (\(T\) = 380 \({}^{\circ}\)C, \(t\) = 30 min, electric field 120 V/mm) [123]. Figure 8: Longitudinal (**top**) and cross (**bottom**) sections of the hybrid DFB laser structure. Reproduced with permission of Elsevier from [60]. Figure 7: Sketch of a hybrid distributed feedback (DFB) laser constituted by a channel core waveguide, buried in the silicate substrate by and loaded by a slab core waveguide in an Er/Yb-co-doped glass. Both waveguides are made by ion exchange. Reproduced with permission of Elsevier from [60]. of an EYDWA device packaged by using fiber silicon V-groove arrays and a UV curing glue. In a pigtailed 4.0 cm long EYDWA, a net gain of 8.0 dB and an average noise figure of 5.1 dB were obtained in the 1530-1560 nm band. New research topics have appeared in those years; some attention, for instance, was focused on the possibility of using ion-exchange technology in rare-earth doped tellurite glasses [121, 131, 132, 133, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172]. Tellurite glasses, i.e., glasses based on tellurium oxide, doped with rare earth have been long studied since they have the lowest phonon energy (about 780 cm\({}^{-1}\)) among oxide glass formers and, as a consequence, their radiative quantum efficiency is high. Furthermore, they offer good stability, chemical durability, high solubility of rare-earth ions and exhibit high refractive index and a wide transmission range (0.35-6 \(\upmu\)m); these properties make them an excellent host material for amplifiers and lasers to be used in a wider wavelength range, e.g., in S + C + L (1460-1615 nm) or C + L + U (1530-1675 nm) bands of the optical communication systems.