Cerenkov second harmonic generation for full characterization of single-mode planar nonlinear waveguides

A non conventional method is presented for the determination of the refractive-index profile parameters (index change Delta n and optical depth d) of single-mode second-order nonlinear planar waveguides, exploiting second harmonic generation in the Cerenkov configuration. The method is based on the dependence of the output angle of the second-harmonic Cerenkov radiation-mode on the propagation constant of the guided-mode at the fundamental frequency; this is in turn related to the refractive-index profile parameters of the waveguide and to the boundary conditions, thus in particular to the waveguide-cover refractive, index n(c). If two different liquids of known refractive index are placed as the waveguide cover, two different Cerenkov output-angles are obtained that can be expressed as a function of Delta n and d by means of two independent equations, thus allowing the determination of both waveguide parameters. It is worth noting that the waveguide parameters at the fundamental frequency, typically in the near infrared are obtained by means of measurements performed at the second harmonic wavelength, in the visible, with evident advantages in terms of easiness of operation. The experimental results confirm the reliability of the method.