Mode behavior of second harmonic wave in a ridge-type periodically poled lithium niobate waveguide

The mode behavior of Second harmonic (SH) generated by nonlinear interaction was investigated in this paper. We fabricated a z-cut 12-mm-long ridge-type Mg doped periodically poled LiNbO3 (MgO:PPLN) waveguide with a QPM period of 4.3 mu m, which satisfies the quasi-phase-matching condition for blue light generation about 460 nm region. The physical area of the ridge waveguide was measured by microscope to be 7.5 mu m x 5.5 mu m, which is large enough to guide multi-mode waves. A fundamental wave from the continuous-wave diode pumped solid state Ti: Sapphire Tunable Ring Laser was coupled into the MgO: PPLN waveguide with the polarization of transverse-to-magnetic field (TM). We could observe not only fundamental mode but also the high-order modes distribution without any disturbance from the others owing to their unique phase-matching condition. The conversion efficiency of SH mode was measured by changing the wavelength of first harmonic (FH) and sample temperature. The maximum conversion efficiency for fundamental mode was obtained at the wavelength of 921.5 nm where both fundamental modes of FH and SH satisfied the quasi-phase-matching condition. On the other hand, high-order modes were generated with maximum efficiency at 923 nm, 923.35nm, and 924.5nm respectively, where they were phase-matched to the odd-even mixed mode of FH. We also investigated the effect of overlap factor between FH and SH modes on conversion efficiency and the interference between high-order modes, TM(10)(2 omega) and TM(01)(2 omega), whose phase-matching conditions were close to each other and overlapped which contributes to generating a specific modes such as diagonally declined mode and Laguerre-Gaussian (LG) mode. And the experiment showed a good agreement with simulation results.