Double grating design of 3D phase matched waveguide for second harmonic χ (2) process

Theoretical analysis on second harmonic (SH) generation with phase matched grating in waveguide is presented from the viewpoint of device design. Usually high intensity sources are necessary in order to observe a SH in a χ (2) nonlinear structure. For this purpose, the novel proposed design takes into account a double grating effect which enhances the guided SH signal along the waveguide. In the presented structure two grating are considered: the first grating, considered at the interface between air and core, is designed in order to obtain an efficient SH conversion process by considering the quasi phase matching (QPM) condition; the second grating, placed at the interface between the core and the substrate region, increases the SH power along the propagation direction through the coupling with the substrate modes generated by the diffraction effect. The novelty of this work is in the combined effect of the two gratings. The grating lengths and periods are designed by considering the nonlinear coupled mode theory with the effective dielectric constant (EDC) assumption. The analysis includes three dimensional (3D) cases where phase matching is involved, in particular the model is applied to a GaAs/AlGAs waveguides with fundamental wavelength at λ FU =1.55 μm and SH signal at λ SH =0.775 μm.