Applications of coupled waveguides in optical integration and nonlinear frequency conversion

Coupled waveguides, such as directional couplers and grating-assisted directional couplers, have many applications in integrated optics, optical communication and nonlinear optics. In this thesis, we investigate two important applications of them in optical integration and nonlinear frequency conversion. The work presented in this thesis is composed of two main parts. In the first part, we present a compact double-grating coupler between silicon-on-insulator (SOI) waveguides. This type of waveguide coupler has potential applications in multi-layer photonic integrated circuits. In the second part, we investigate nonlinear frequency conversion, such as second harmonic generation, in directional couplers.; The double-grating coupler consists of two gratings: one acting as an outcoupler to couple the guided light from a waveguide to its radiation mode, and the other acting as an incoupler to couple the light in the radiation mode to the guided mode of another waveguide. The coupling between these two waveguides is therefore achieved. Since the coupling does not require field overlap between the guided modes of the two waveguides, the separation between the two waveguides can be large. This advantage leads to promising applications in optical integration in multi-layer photonic integrated circuits. A transfer efficiency of 29% is achieved between two SOI waveguides, and further optimization is possible by using blazed-gratings and Bragg reflectors.; In the second part of this thesis, we investigate a new theory for nonlinear frequency conversion in directional couplers. As is well known, when a harmonic oscillator is driven at resonance by an external harmonic force, the largest possible oscillation amplitude is obtained. Coupled mode theory shows that similar phenomena occur when the nonlinear effect takes place in a waveguide directional coupler. We show that by considering the amplitude of the second harmonic signal in the coupled waveguides as an oscillator and the nonlinear polarization as an external harmonic force, a resonant condition can be found, resulting in highly efficient power transfer from pump to signal. Similar resonant effects also occur in other nonlinear frequency conversion effects. The proposed phenomena can also be understood by phase-matching between the supermodes of directional couplers. On one hand, the new theory has fundamental significance for nonlinear optics, on the other hand, it also demonstrates that waveguide directional couplers can be found in a new application area besides integrated optics and optical communication. We also report the first experimental observation of continuous-wave second-harmonic generation in waveguide directional couplers. We employ a GaAs/AlGaAs system and observe four resonance peaks in a ∼15nm spectral range, with a maximal conversion efficiency of 1.6%W-1cm-2. This new configuration has the potential to open a new range of applications for nonlinear frequency conversion.