DOI 10.1007/s11082-006-9033-9 Optical and Quantum Electronics (2006) 38:889–901 © Springer 2007 Phase-matched SHG in periodically poled LiNbO 3 waveguides: A novel configuration f.m. pigozzo 1,∗ , e. autizi 1 , a.-d. capobianco 1 , n. argiolas 2 , m. bazzan 2 and c. sada 2 1 Information Engineering Department, University of Padova, Via Gradenigo 6/b, 35131 Padova, Italy 2 Physics Department, University of Padova, Via Marzolo 8, 35131 Padova, Italy (*author for correspondence: E-mail: superg@ray.dei.unipd.it) Received 26 July 2006; accepted 15 November 2006 Abstract. We present a feasiblity study of a new method for enhancing the continuously phase matched second harmonic generation in 2D PPLN optical waveguides fabricated using the off-center Czochralski technique and proton-exchange. We show that a periodic variation of the nonlinear coefficient along the transverse coordinate permits for efficient energetic exchanges. Key words: Czochralski off-center, periodically poled lithium niobate, rotated configuration, second harmonic generation Abbreviations: BPM–beam propagation method; CMT–coupled mode theory; FF–fundamental frequency; PE–proton exchange; PPLN–periodically poled lithium niobate; SH–second harmonic; SHG–second harmonic generation; SVEA–slowly varying envelope approximation; QPM–quasi-phase matching. 1. Introduction Recently we reported on the possibility of exploiting periodically poled lithium niobate (PPLN) crystals grown by the off-center Czochralski tech- nique, for second harmonic generation (SHG) (Autizi et al. 2006). These crystals present the PPLN structure across the whole volume of the grown boules with a tailored period depending on the growth parameters. We showed that in these crystals an efficient SHG can be obtained by the quasi-phase matching (QPM) technique (Autizi et al. 2006). As well known, two electric fields, the first one at the fundamental frequency (FF) and the second one at the second harmonic (SH), which propagate in a nonlinear medium exhibit different phase velocities. However, in guided wave prop- agation, intermodal dispersion can compensate for the material dispersion and very close phase velocities are achievable. The main drawback concern- ing the use of these two modes for continuously phase matched SHG is their negligible modal overlap. An improvement of this method has been proposed (Chowdhury and Mc Caughan 2000) based on continuously phase matched propagation of