"Cerenkov" Configuration Second Harmonic Generation

The use of the largest nonlinear coefficient d33 of LiNbO3 for second harmonic generation (SHG), first demonstrated with a guided wave phase-matched interaction suffering from poor field overlap integrals (1), has been considerably improved by the use of a Cerenkov type configuration : nonlinear polarization having a phase velocity greater than that of light in the waveguide, harmonic radiated into the substrate (2,3). The excellent results obtained by T. Taniuchi and K. Yamamoto (1) demonstrate the interest of second harmonic generation (SHG) in the form of Cerenkov radiation (2) In the first part of this paper, we present a new calculation of the efficiency of this process in step index waveguides. In the second part, the validity of this model is checked by studying second harmonic Cerenkov radiation in planar step index waveguides realized by proton exchange (PE) (5) in LiNbO3. The comparison of computed and measured efficiencies show a good relative agreement for multimode waveguides using the usual value of the non-linear coefficient in the exchanged region.