Quasi-phase-matched parametric interactions in proton-exchanged lithium niobate waveguides

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We present a review of parametric fluorescence with bulk and guided geometries in quasi-phase matched lithium niobate. Whereas bulk experiments have yielded results close to theoretical predictions, waveguided versions have shown strongly reduced efficiencies. Attributing the observed conversion efficiency reductions to a loss of the material nonlinearity, to a destruction of the inverted domains during the waveguide fabrication, or to both, we carefully studied the influence of the proton-exchange process on the nonlinear and structural properties of the periodically poled lithium niobate. We found that an annealed proton-exchange process can essentially conserve the nonlinearity but will erase the periodic domain structure. This erasure can be avoided by use of a highly diluted proton-exchange melt. This direct proton-exchange process perfectly preserves all the nonlinear optical and structural properties of periodically poled LiNbO3.