Efficient wavelength conversion is an attractive approach for obtaining coherent radiation in regions of the spectrum where lasers are unavailable or impractical. Optical signal processing in WDM networks, optical-CDMA communications, and quantum communication are examples of applications that can utilize efficient nonlinear frequency conversion at low power levels. Lithium niobate (LN) is a very promising material for the purpose, because it has a mature crystal-growth process, wide transparency range, large second-order nonlinear coefficient, and allows quasi-phasematching via periodic poling (PP). Waveguides enable efficient conversion at low powers and can be formed via reverse proton-exchange. Precise modeling of both the fabrication process and the properties of the resulting waveguides is thus necessary for the demonstration of high-density optical integrated circuits.
This dissertation presents a complete fabrication model that accurately predicts the nonlinear diffusion of protons in PPLN as well as the dispersion of the waveguides between 450 and 4000 nm. Using this model, waveguides are fabricated for two experiments: efficient generation of 3-4-um radiation for spectroscopy via difference frequency generation using two near-IR lasers; and parametric amplification of 1.57-um seed signal radiation for remote wind sensing using a 1.064-um pump laser.
The waveguides are fabricated in conventional congruent-composition LN. Photorefractive damage (PRD) and green-induced infrared absorption (GRIIRA) limit the generated output power in these devices at room temperature due to the presence of high-intensity visible light. Resistance to PRD and GRIIRA can be achieved by heavy doping with Mg2+, or by using crystals with stoichiometric composition.
PRD-resistant, bulk near-stoichiometric lithium niobate (SLN) was fabricated by vapor-transport equilibration (VTE) of originally congruent lithium niobate wafers with light MgO (0.3-1 mol%) doping. Details of the poling process and the dependence of photorefractive properties on crystal composition are presented. We obtained periodic poling down to a period of 7 um and achieved 2 W at 532 nm via second harmonic generation in a 0.3 mol-% VTE-MgO:LiNbO3 bulk crystal at room-temperature. These breakthroughs will enable efficient tunable radiation from the visible to the mid-IR for a variety of applications.