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Quasi-phase-matched (QPM) wavelength conversions based on the second-order nonlinear interaction, such as second-harmonic generation (SHG), difference-frequency generation (DFG) and sum-frequency generation (SFG), in periodically poled lithium niobate (PPLN)) have attracted much attention due to their excellent conversion properties such as broad bandwidth, high efficiency, and low noise. To achieve an efficient conversion, high-power pump light is always desired. However, PPLN crystals (either bulk or waveguide) are somehow vulnerable to high-power irradiating light, especially at a short wavelength due to the photorefractive effect (PRE), known as a refractive index change induced by an intense light illumination. The PRE may deteriorate device performance significantly. To suppress the PRE, PPLN crystals usually have to be operated at high temperatures (over one hundred degrees Celsius) or to be specially doped. Despite the significant impacts of the PRE on device performance, to date, there is limited information in the literature on the PRE mechanism in QPM PPLN. In this work, we adopt the pump-probe method and characterize the PRE in undoped and 5-mol% MgO-doped PPLN crystals. Especially, we compare the PRE in bulk and annealed proton-exchanged (APE) waveguides of PPLN. A broadband light source at 1.55 &mgr;m was used as the probe, and narrowband light at two wavelengths of 0.5 and 1.1 &mgr;m was alternatively taken as the pump. The period of crystals were selected to meet the QPM condition of SHG. It is shown that the decay property and temperature dependence of PRE, the wavelength and amplitude changes of the SHG tuning curve are distinct for the undoped and MgO-doped PPLN, as well as for the bulk and waveguide, which implies a few competing interactions in the crystals, such as the PRE, thermal-optic, photogalvanic and two-photon absorption effect, etc.
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