Monolithic integration for nonlinear optical frequency conversion in semiconductor waveguides.
PhD thesis, University of Glasgow.
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This thesis presents an investigation into the feasibility of tunable, monolithically integrated, nonlinear optical frequency conversion sources which work under the principles of an optical parametric oscillator (OPO). The room-temperature continuous wave (CW) operation of these devices produces narrow line-width, near- and mid-infrared wavelengths, primarily used in chemical sensing applications. The devices detailed here, based on the GaAs–AlGaAs superlattice material system, benefit from post growth, ion implantation induced, quantum well intermixing, to achieve 1st order phase matching. The experiments, which have been performed to optimize the second-order nonlinear processes in our GaAs–AlGaAs superlattice waveguides, have demonstrated improved conversion efficiencies when compared to the performance achieved previously in similar superlattice nonlinear waveguides. We have achieved pulsed type-I phase matched second harmonic generation (SHG) with powers up to 3.65 μW (average pulse power), CW type-I phase matched SHG up to 1.6 μW for the first time, and pulsed type-II phase matched SHG up to 2 μW (average pulse power), again for the first time. Moreover, we have been able to achieve both CW type-I and CW type-II phase matched difference frequency generation, which converts C-band wavelengths into L- and U-band wavelengths, over at least a 20 nm conversion bandwidth. These results have been made possible through the systematic optimization of processes developed to fabricate nonlinear optical waveguides. Fabrication processes have also been developed to facilitate the incorporation of on-chip lasers and optical routing components, required to achieve a fully integrated OPO and nonlinear optical frequency converter. The optical routing in these devices has been demonstrated using a frequency selective multi-mode interference (MMI) coupler. The superlattice laser material has been designed by optimizing the material structure and employing different growth technologies. Room-temperature CW laser action has been achieved in 100 nm thick, superlattice core, half-ring lasers. The laser excitation is measured at 801 nm, and the internal power of the on-chip pump is estimated to be in excess of 200 mW in a full-ring, after accounting for optical routing, linear, bending and nonlinear losses. We have been able to conclude that our designed OPO and frequency converter is just feasible with the performance achieved in different components.
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