Exact analysis of cascaded second-order nonlinearity in rotated Ti : LiNbO3 couplers

The effects of anisotropic coupling on the nonlinear behavior of different LiNbO3 waveguides and couplers employing cascaded second-order nonlinearity are analyzed and the importance of leaky mode effect is shown. A set of pr actical coupling structures made of both symmetric and asymmetric waveguide s is investigated and their performance is compared. The home-made computer -code, based on coupled wave theory and anisotropic mode propagation in the complex domain, allows an exact electromagnetic investigation of devices, taking into account both guided and leaky propagation in structures which e xhibit losses, the generated second harmonic wave having leaky nature. As a n example, for the well-investigated slab three layer waveguide, by taking into account the leaky nature of the generated second harmonic, the modal m ismatch Delta beta = 333 m(-1) is obtained at the azimuthal angle theta(m) congruent to 61.9 degrees (between the crystal optical c-axis and the propa gation direction) while the angle found via the conventional approach is th eta(m) congruent to 63.5 degrees. Moreover, the calculated second harmonic wave exhibits an attenuation equal to 7.44 x 10(-1) dB/cm that strongly inf luences the whole cascaded second-order phenomenon. The simulation results, for those structures in which it is possible to neglect the hybrid and lea ky nature of the propagation modes, are compared with the literature data a nd an excellent agreement is found.