12 Optical Waveguides Fabricated by Ion Implantation/Irradiation: A Review Ovidio Peña-Rodríguez 1,2 , José Olivares 1,2 , Mercedes Carrascosa 3 , Ángel García-Cabañes 3 , Antonio Rivera 4 and Fernando Agulló-López 1 1 Centro de Microanálisis de Materiales (CMAM), Universidad Autónoma de Madrid (UAM), Cantoblanco, Madrid 2 Instituto de Óptica, Consejo Superior de Investigaciones Científicas (IO-CSIC), Madrid 3 Departamento de Física de Materiales C-IV, Universidad Autónoma de Madrid, Madrid 4 Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, Madrid Spain 1. Introduction Optical waveguides are key elements of many photonic devices; for this reason many materials and methods have been intensively studied to fabricate them. Ion implantation provides a general and flexible method to achieve this goal, with several advantages over the alternative techniques (Townsend et al., 1994). Most optical waveguides and integrated optical devices are manufactured using low-energy light ions (H and He), taking advantage of the effects caused by nuclear damage. However, this is achieved at the expense of using very high fluences (10 16 -10 17 cm -2 ), which reduces the practical utility of the method (Olivares et al., 2007c). The study of irradiations with heavier ions (A > 12) and higher energies (4-100 MeV), where the electronic stopping power dominates over the nuclear, has increased recently, as a way to overcome this limitation (Olivares et al., 2005a). The characteristics associated with these processes differ significantly from those applied to nuclear collisions. In particular, the complete amorphization of the lattice can only be achieved when the electronic stopping power is above a certain threshold (Olivares et al., 2005a). In this respect, it has been shown that a controlled damage can be generated by selecting the type of ion and its mass, energy and fluence, obtaining a micro-processing of crystals with a degree of accuracy, flexibility and efficiency well beyond the current state of the art. The optical waveguides produced with this method are both an end in itself (Caballero et al., 2005; Olivares et al., 2005b, 2007c; García-Navarro et al., 2006; Manzano et al., 2010) and a mean that allows the precise study of several fundamental aspects of the generation and accumulation of electronic damage (Agulló-López et al., 2006; García-Navarro et al., 2007; Rivera et al., 2010a; b; García et al., 2011). For example, by irradiating LiNbO 3 with moderate ion fluences (2x10 14 cm -2 ) of F were obtained waveguides with nonlinear properties comparable to those of other LiNbO 3 guides