Spatial mode behavior of second harmonic generation in a ridge-type waveguide with a periodically poled MgO-doped lithium niobate crystal

We report on spatial mode behavior of the second harmonic (SH) generated by nonlinear interaction in the a z-cut Mg-doped periodically poled LiNbO3 (MgO:PPLN) ridge-type waveguide. Specific SH modes were selectively generated by mode mixing of the fundamental harmonic (FH) waves. In addition, high-order SH modes TM10 and TM01 were simultaneously generated under similar phase-matching conditions in a quasi-phase matching (QPM) grating structure. The interfered mode profiles of high-order SH modes demonstrated specific mode distribution to have a diagonal angle and the Laguerre–Gaussian (LG) mode at a certain phase-matching condition obtained by temperature control.