## 3 Ion-Exchanged Active Integrated Photonic Devices
As already mentioned, other fabrication technologies were also developed, and sol-gel, for instance, has been a very convenient platform for the fabrication of high quality thin glass films, optical planar waveguides and photonic bandgap structures [173, 174, 175, 176]. An interesting research concerned the possibility of integrating the sol-gel and ion-exchange techniques for the fabrication of passive and active channel waveguides in rare-earth doped integrated optic devices [115, 177]. Among the various research lines aiming at improving the characteristics of waveguide optical amplifiers, it is worth mentioning the article by [PERSON] et al. [160], where another 3D hybrid structure is designed with the goal of making possible the use of a low-cost high-power broad area laser diode, instead of an expensive single-mode 980 nm laser, for the amplifier's pumping. Their design, sketched in Figure 10 includes direct butt-coupling of the pump light into a multimode passive waveguide (in silicate glass): an Er-Yb doped single mode waveguide (in phosphate glass) is placed on top of the passive one, and light is progressively coupled from bottom to top waveguide, being captured by Yb ions acting as sensitizers for Er ions. Experimental tests with ion-exchanged waveguides fabricated in the two glasses allowed to measure amplified spontaneous emission (ASE); Figure 11 shows the upconversion green light in the upper single mode waveguide, a proof of the effectiveness of the pumping scheme. According to their numerical simulation, gain values of over 3 dB/cm should be achievable. As already mentioned in Section 2, too, another way of fabricating hybrid active/passive glass structures is to butt join two glass pieces, an undoped one and a rare-earth-doped one, in order to optimize the optical characteristics and minimizing losses. Such a structure was used especially to fabricate lossless splitters and combiners, widely used in telecom systems; to mention some examples, [PERSON] et al. separately designed and optimized a passive section in silicate glass and an active section in phosphate glass before joining  
Figure 9: Photo of a packaged EYDWA device. Standard single mode fibers are pigtailed to both the input and output ends of the channel waveguide. The glass chip is then mounted to a metal box to provide mechanical stability and easy handling. Reproduced from [155] with permission by Elsevier. them by using a UV curing glue, so to produce a lossless 1x8 splitter/combiner [178]. [PERSON] et al. showed that it was possible to join two aluminophosphate glasses (undoped and doped, respectively) by sandwiching an aqueous phosphate solution between the polished and accurately cleaned surfaces of the two blocks. A rigid joint is formed in 24 h at room temperature. A thermal treatment for 5 h at 375 \({}^{\circ}\)C provides a greater chemical and mechanical robustness [58]. Further, [PERSON] et al. reported a low-cost technique for industrial mass production, which was based on vacuum hot press assisted direct bonding applied to custom made multicomponent silicate glasses [179]. A thermal treatment at 605 \(\pm\) 20 \({}^{\circ}\)C for 240 min under a pressure of 28 kPa was sufficient to achieve a very good bonding. Figure 12 shows the first bonded sample. The two glasses were germanoborosilicates with sodium oxide for the ion-exchange process; the doped glass contained 0.5 mol% Er\({}_{2}\)O\({}_{3}\) and 1.3 mol% Yb\({}_{2}\)O\({}_{3}\). A broad review on Er-Yb laser and amplifiers, covering both the photoluminescence issues and the various materials and technologies, was published in 2011 by [PERSON] and  
Figure 11: Photo of the green light propagating in the single mode (**upper**) waveguide in the structure of Figure 11. Pump wavelength is around 980 nm, and the green color, due to upconversion emission from excited Er\({}^{+}\) ions, proves that the transfer from the pump to the active layer is effective. Figure 12: Photograph of the bonded glass sample, with the active (**pink**) and passive (**transparent**) parts.