Waveguide Structures Design Based On Nonlinear Materials For Optical Wavelength Conversion

Nonlinear wavelength conversion had played a key role in all-optical signal processing,optical data storage, biomedicine, color display, biotechnology, environmental monitoringand process analysis and so on. The third-harmonic generation (THG), four wave mixing(FWM) based on the third-order nonlinear interactions and the difference frequency gen-eration (DFG), sum-frequency generation (SFG), second-harmonic generation (SHG) fromthe second-order nonlinear effects, may convert the wavelength of semiconductor lasers tothe ultraviolet (UV), visible light, mid-infrared (IR) light and terahertz regions. On theother hand, the FWM, DFG, cascaded second-harmonic generation and difference frequen-cy generation (cSHG/DFG), cascaded sum-frequency generation and difference frequencygeneration (cSFG/DFG) can move the signal from one wavelength to another, and this maybe applied in the dense wavelength division multiplexing (DWDM) optical communicationnetwork. In this thesis, we design some appropriate waveguide structures to achieve thesenonlinear processes, and theoretically investigate the characters of wavelength conversion.The detailed research contents can be found as follows.Firstly, we study the wavelength conversion in the lossy quasi-phase-matched (QPM)AlGaAs waveguide. We obtain the analytical expressions of converted wave power for DFG,cSHG/DFG processes under the non-depletion approximation in lossy QPM waveguides. Itis shown that the analytical results and the numerical simulation with depletion agree verywell for lossy waveguides. Employing the analytical solutions, the formulas of optimizedwaveguide lengths are obtained for lossy DFG and cSHG/DFG processes. After designingan AlGaAs QPM ridge waveguide, we detailly investigate and compare the characteristicsof the second-order nonlinear effects with and without loss, such as conversion efficiency,conversion bandwidth, pump wavelength tolerance and temperature stability.Secondly, we study the SHG in AlGaAs microring resonator. In order to avoid theloss of QPM structures, we investigate the principle of curved AlGaAs waveguide for phasematch, and analyze the effective nonlinear coefficient of microring resonator. It is foundthat microring resonator can be employed to satisfy the phase matching condition of SHG. We design a high-index-contrast AlGaAs microring resonator with the wavelengths of pumpand second-harmonic in resonator wavelength. Under the non-depletion approximation, theanalytical solution of conversion efficiency is obtained. Based on the analytical solutions,it is clearly that the enhanced field in the microring resonator can improved the conversionefficiency. Considering the depletion situation, we numerically simulate the SHG processusing the fragment method. The results show that the conversion efficiency may be reach1in lossless case.Thirdly, we study the THz wave DFG form photonic crystal waveguides (PCWs). Weplace the near-infrared light waveguide in the line defect of THz PCW to achieve THzwave DFG from near-infrared light sources. In this structure，PCW structure provides atight confinement of THz-wave field, resulting in a good mode field overlap of three waves.The unique phase matching condition between two pump waves and THz Bloch wave canbe satisfied through choosing appropriate waveguide parameters and pump wavelengths.The couple-mode equations and the effective interaction area formula for DFG processesis obtained from the modal theory. Firstly, we design a AlGaAs PCW, the high power-normalized conversion efficiency of0.7632×104W1for3THz generation is obtainedthrough simulating the continuous THz DFG process. Secondly, we design a LiNbO3PCWfor efficient1THz DFG. Since the photonic crystal structures have been widely exploitedfor THz researches, these approaches have the potential to open a new window for THztechnology.Fourthly, we study the slow light enhanced FWM in PCW. By employing the modaltheory, we derive the couple-mode equations for third-order nonlinear effects in PCWs.These nonlinear interactions include self-phase modulation, cross-phase modulation anddegenerate FWM. The equations similar to that in nonlinear fiber optics could be expandedand applied for third-order nonlinear processes in other periodic waveguides. Based onthe equations, we systematically analyze the group-velocity dispersion, optical propagationloss, effective interaction area, slow light enhanced factor and phase mismatch for a slowlight engineered silicon PCW. Considering the two-photon absorption and free-carrier ef-fects, the wavelength conversion efficiencies in two low-dispersion regions are numerically simulated by utilizing finite difference method. Finally, we investigate the influence of slowlight enhanced multiple FWM process on the conversion efficiency.