Nonlinear characterization and modeling of periodically poled lithium niobate waveguides for 1.5-lm-band cascaded wavelength conversion Ilaria Cristiani a , Carlo Liberale a , Vittorio Degiorgio a, * , Giovanni Tartarini b , Paolo Bassi b a Dipartimento di Elettronica, Istituto Nazionale per la Fisica della Materia, Universit  a di Pavia, Via Ferrata 1, I-27100 Pavia, Italy b Dipartimento di Elettronica Informatica e Sistemistica, Universit  a di Bologna, Viale Risorgimento 2, I-40136 Bologna, Italy Received 17 August 2000; accepted 2 November 2000 Abstract We present a nonlinear characterization of annealed-proton-exchange lithium niobate channel waveguides in view of their application for wavelength shifting in wavelength division multiplexing systems. We exploit second harmonic generation as a very sensitive technique to test waveguide uniformity and optical quality. The experimental results are successfully compared with those obtained using numerical simulations based on the ®nite element method. The analysis suggests useful criteria for the optimization of the fabrication parameters in order to enhance the eciency of wavelength conversion processes. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear optics; Cascading; Wavelength conversion; Periodically poled lithium niobate; Channel waveguides 1. Introduction The increasing demand for high capacity net- works has led to a fast development of wavelength division multiplexing (WDM) techniques for op- tical telecommunication systems. In WDM sys- tems the signal wavelength can be used to perform complex functions, such as all-optical routing and space switching, that can make the network com- pletely transparent to signal format and bit rate. Wavelength conversion represents a key issue for optical cross-connect development [1]. Devices based on cross-phase modulation in semiconduc- tor optical ampli®ers have already been success- fully used to this purpose with good performance and reliability [2]. Recent experiments of nonlinear wave mixing in an organic crystal [3] and in a periodically poled lithium niobate (PPLN) crystal [4] suggest a competitive approach. The operating principle consists in the cascade of two second- order processes. In order to convert a signal from the frequency x s to the new frequency x f  x s  Dx, a strong pump beam at frequency x p  x s  Dx=2 is used. In the ®rst step the second harmonic (SH) of x p is generated. Then, a dierence fre- quency process between the generated 2x p and the input signal x s takes place leading to the desired 1 January 2001 Optics Communications 187 (2001) 263±270 www.elsevier.com/locate/optcom * Corresponding author. Tel.: +39-0382-505208; fax: +39- 0382-422583. E-mail address: v.degiorgio@ele.unipv.it (V. Degiorgio). 0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0030-4018(00)01099-3