Second and third harmonic generation by cascading in a QPM LiNbO3 waveguide

Quasi-phase-matched (QPM) LiNbO3 waveguides are at present the most efficient device for second-harmonic-generation (SHG). They require the periodic inversion of the ferroelectric domains and the fabrication of annealed-proton-exchanged (APE) waveguides. In order to work at a wavelength of interest for telecommunications, 1300nm, we realized an APE waveguide, single-mode at such wavelength, on a periodically-poled substrate with a period of 11.7 mu m. Domain inversion was obtained by Ti-indiffusion. The sample was linearly characterized. For the nonlinear experiment, a tunable Nd:YAG-pumped OPA (25ps-pulses, 10 Hz-repetition rate) was used as the light source. The maximum efficiency for SHG was found around 1300nm, within a few nanometers from the expected wavelength. The ratio among the output power of the second harmonic and of the fundamental guided mode was 56%. Together with the SH, the third harmonic (at 433nm) was generated as the result of a chi(2) cascading process. On the basis of the results obtained, the problems connected with the design and fabrication of QPM devices will be discussed, with particular attention to the tolerances in waveguide fabrication. It is worth noting that the two wavelengths were generated simultaneously, using a single laser source. This opens the possibility of generating multicolour output in diode-pumped QPM-waveguides.