Quasi-Phasematching for Guided-Wave Nonlinear Optics in Lithium Niobate

This thesis reports the invention and demonstration of a method for implementing quasi-phasematching (QPM) for nonlinear optical frequency conversion in a lithium niobate (LiNbO_3) waveguide, and the invention of a way to design waveguides for frequency conversion that are insensitive to fabrication errors. To accomplish QPM, a grating of inverted ferroelectric domains was fabricated at the surface of a LiNbO _3 wafer by indiffusion of a lithographically -defined grating of titanium. QPM enabled the use of the largest LiNbO_3 nonlinear coefficient, d_{33}, which cannot be birefringently phasematched. An optical waveguide was then created using the annealed proton exchange process. In a quasi-phasematched second harmonic generation waveguide, more than a milliwatt of blue light at 420 nm was generated from an infrared input power of about 200 mW at 840 nm. In a difference-frequency generation device, 2 mu W of power at 2.1 μm was generated by mixing 160 mW of 0.81 μm radiation and 1 mW of 1.32 μm radiation. These devices operated at room temperature, using wavelengths compatible with those provided by diode lasers. Had birefringent phasematching been used instead of QPM, these interactions would have been impossible or required inconvenient operating temperatures. Waveguide designs providing phasematching conditions that are insensitive to small variations in the guide dimension were investigated. Use of this "noncritical" waveguide design technique can increase fabrication tolerances by an order of magnitude and should facilitate the production of devices with long interaction lengths. The techniques of guided-wave quasi-phasematching and noncritical phasematching explored in this thesis will contribute to the practical implementation of nonlinear optics in waveguides.