## 3 Ion-Exchanged Active Integrated Photonic Devices
Fiber lasers and amplifiers had therefore to use another rare earth than Nd, with photoluminescence emission in the near infrared band: this was Erbium, which is characterized by a strong emission at 1.532 um. Er-doped fiber amplifiers (EDFAs) soon became a very important component of fiber transmission systems, that needed regeneration and re-amplification of the guided light after a certain distance, of the order of tens of kilometers. Most of the pioneering research on EDFAs was performed at Southampton University by the [PERSON]'s group and at AT&T Bell Labs by [PERSON] and colleagues: the two groups published the first successful results almost at the same time, in 1987 [86; 87]. A strong limitation to the practical use of these early fiber amplifiers was related to the pump signal: [PERSON]'s group was using pump wavelengths in the range 655-675 nm, whereas [PERSON] used a pump at 514.5 nm. In both cases, bulk pump lasers were necessary. A substantial step forward market diffusion of EDFAs was made with the availability of powerful laser diodes. Then, the demand of ever increasing system transmission capacity and of a mechanism for creating multipoint networks led to the use of the wavelength division multiplexing (WDM) technology. As integrated optical devices could offer a cheap and small-footprint approach to the fabrication of WDM devices, a renewed R&D interest was focused on Er-doped glass devices and to the integration of passive and active functions on a same glass substrate. An issue, however, was related to material requirements, much more stringent for integrated optics than for fibers, due to the different fabrication technologies and to the much higher rare-earth concentration required in short-length planar devices. An interesting effort to assess the relative merits of different compositions of the host glass was published by Miniscalco in 1991 [88]. He concluded that 'in most situations' Al-doped silica was the preferred glass for Er\({}^{3^{+}}\)-doped fiber amplifiers at 1500 nm. In planar short-length devices, silicate glasses may suffer even more from the lower solubility of rare-earth-ions with respect to phosphate glasses, since it may lead to clustering and reducing the conversion efficiency. In spite of this, the first ion-exchanged waveguide laser emitting at 1540 nm used a silicate BK7 glass containing 0.5 wt% Er\({}_{2}\)O\({}_{3}\)[89]. The laser cavity was formed by bonding dielectric mirrors to the chip end facets, and a thermal ion exchange in a KNO\({}_{3}\) bath was employed in waveguide fabrication. In the same year 1992, an Er-doped waveguide amplifier with 0.65 dB/cm net gain with 100 mW pump power was demonstrated [90]. This device was constituted by a phosphorus-doped silica core doped with 0.55% wt Er and silica cladding, deposited by flame hydrolysis on a silicon substrate. Quite obviously, owing to the appeal of erbium-doped waveguide amplifiers (EDWAs), various technological approaches to waveguide fabrication were pursued, with deposition techniques of doped glass films (e.g., RF-sputtering, sol-gel, PECVD, flame hydrolysis) competing with diffusion techniques (ion exchange, ion implantation) in bulk doped glasses [20]. As an example of RF-sputtered amplifying waveguides, a net optical gain of 4.1 dB/cm at 1.535 \(\upmu\)m was achieved by [PERSON] et al. by depositing Er-doped multicomponent phosphate glass films [91]; the same group also reported about the possibility of using MeV ion implantation to introduce erbium in a variety of thin films, from oxide glasses to ceramics and to amorphous and crystalline silicon [92]. A commercial interest was also emerging. Among the glass suppliers, the Corning company was particularly interested in the development of ion-exchangeable Er-doped glasses and of amplifiers, as testified also by two patents [93; 94]; in the latter one, an optimized composition of a boron-free silicate glass that could be doped with up to 5% wt erbium oxide and ion exchanged with thallium is described [94].