The ability to achieve high quality periodic poling in lithium niobate (LN) has allowed quasi-phase-matching to be used for second-order nonlinear optics, leading to experimental demonstration of efficient optical frequency generation throughout its wide transparency range (0.35-4.5 microns). Applications of congruent lithium niobate involving visible or ultraviolet wavelengths are limited to low power or high temperature operation due to the effects of photorefractive damage (PRD) and green-induced infrared absorption (GRIIRA). The standard methods of suppressing PRD include doping with 5 mol-% MgO or ZnO and varying crystal stoichiometry. More recent methods employ a combination of lower doping level and near-stoichiometric composition. We use vapor transport equilibration (VTE) and significantly lower MgO doping (<0.5% in the melt) to obtain near-stoichiometric PRD-resistant crystals with improved parameters for periodic poling compared to the commercially available 5% MgO-doped congruent crystals. An efficient process for periodic poling at room temperature using baked photoresist as a patterned dielectric on one crystal surface with LiCl-solution electrodes was developed for periods as short as 8.3 microns for 0.5% and 7 microns for 0.3% MgO-doped VTE:LN. The quality of periodic poling improves as the MgO concentration is lowered. Stable second harmonic generation of 1.3-W continuous-wave 532-nm radiation was observed near room temperature (43 degrees Celsius, as determined by the phase matching condition) with no sign of degradation in a 1.5-cm long crystal of 0.3-% MgO-doped VTE:LN periodically poled with a period of 7.06 microns.
We use a combination of vapor transport equilibration and moderate MgO doping (≤1%) to explore near-stoichiometric damage resistant lithium niobate crystals with improved properties for periodic poling and annealed-proton-exchange waveguide fabrication compared to the commercially available 5-mol% MgO-doped crystals. High damage resistance, measured by the saturated space-charge field generated in the crystal by 514 nm radiation, was obtained for all MgO doping concentrations (0.3, 0.5 and 1%) with appropriate equilibration. Green-induced infrared absorption was also measured in the 0.3-% doped crystal and was below the detection limit. Dispersion in the region 460-1550 nm was measured. Periodic poling was performed using LiCl solution electrodes. Poling quality improves with lowering MgO concentration. Waveguides for frequency doubling of 1550 nm were fabricated in the 1% doped crystal with losses as low as 0.4 dB/cm and normalized efficiency of ~10%/Wcm2.
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