Investigation Of Four-wave Mixing In Silicon Waveguides And Corresponding Applications

Silicon waveguides take the merits of strong beam-guidance,ultra-small footprint,mature fabrication procedures,low cost,and high compatibility with the Complementary Metal-Oxide-Semiconductor technology,based on which various photonic components become significant building blocks of the integrated optical circuits,and make possible on-chip functionalities including optical signal processing,optical logic gate and all-optical communication,etc.Specifically,with high nonlinear coefficient and controllable dispersion property,silicon waveguides can be used as the nonlinear medium of four-wave mixing in realizing various nonlinear optical functionalities.The nonlinear wavelength conversion/multicasting via stimulated four-wave mixing are key technologies of the dense wavelength division multiplexing systems,meanwhile the correlated photon-pair sources via spontaneous four-wave mixing are key components of quantum optics systems.The combination of two types of functionalities enables high-dimensional quantum key distribution,which is the research emphasis of high-security,robust,on-chip integrated quantum communication systems.In this thesis,we theoretically and experimentally present the comprehensive and deep investigation of four-wave mixing in silicon waveguides and its applications.We methodically analysis the impact of nonlinear coefficient and dispersion coefficient on four-wave mixing,the accurate modeling of fourwave mixing,the technical strategy of achieving broadband four-wave mixing,the noisesuppression of correlated photon-pair sources,and the promotion of pair rate and pair brightness.In this thesis,we theoretically and experimentally present the comprehensive and deep investigation of four-wave mixing in silicon waveguides and its applications.Methodical analysis is given to five key sections including the impact of dispersion/nonlinear coefficient on four-wave mixing,the accurate modeling of four-wave mixing,the technical strategy of achieving broadband four-wave mixing,the noise-suppression of the correlated photon-pair sources,and pair rate/spectral brightness scaling of the correlated photon pairs.Firstly,the research significance of integrated optical circuits is introduced,the standard structures and fabrication procedures of silicon waveguides are described,the fundamental and key parameters of four-wave mixing are analyzed,the advances of four-wave mixing applications in classical regime(nonlinear wavelength conversion/multicasting)and quantum regime(correlated photon-pair sources)are emphatically reviewed,the advantages of silicon waveguides in achieving four-wave mixing is demonstrated.For the first time,we set up the full-vectorial nonlinear propagation model valid in silicon waveguides,which is well validated by nonlinear wavelength conversion experiments;we derive the full-vectorial coupled-mode equations of multiple-pumped fourwave mixing,which is well validated by nonlinear wavelength multicasting experiments.The results demonstrate that the widely-applied scalar-approach model leads to the misestimating of nonlinear coefficient,meanwhile the full-vectorial-modified effective mode area definition ensures accurately theoretical study of four-wave mixing in silicon waveguides.We theoretically investigate the impact of the cross-sectional dimension/refractive index distribution of silicon strip waveguides on dispersion to achieve broadband wavelength conversion and multicasting.For the first time,we fabricate the alumina-coated silica-cladded and the silica-cladded vertical-dual-slots silicon strip waveguide samples,through nonlinear wavelength conversion experiments demonstrating that the proposed waveguide structures are valid to achieve spectrally-flattened near-zero anomalous dispersion within the telecom wavelengths.The results present that by carefully designing the cross-sectional structure of silicon strip waveguides,one can not only broaden the nonlinear wavelength conversion bandwidth with respect to classical optical communication signals or quantum keys,but also enlarge the tuning range of continuous-wave pumped correlated photon pair sources.We analyze the noise contribution/suppression of correlated photon-pair sources via spontaneous four-wave mixing in silicon waveguides,and for the first time put forward that the photonic crystal grating couplers can be used to suppress the silicon-induced spontaneous Raman scattering noise.We experimentally demonstrate the continuous-wavepumped/pulsed-pumped silicon waveguide sources to generate temporal-random/temporaldistinguishable correlated photon pairs,and achieve record-high coincidence-to-accidental ratio of 673/1220,respectively.We experimentally measure the heralded second-order correlation to characterize the single-photon regime of the resulting photon pairs.The results demonstrate that efficient noise suppression using band-pass filters makes possible high coincidence-to-accidental ratio;the bottleneck of pair rate scaling is mainly due to the nonlinear loss in silicon waveguides and the detector saturation;it is invalid to promote the spectral brightness of correlated photon pairs by narrow-bandwidth filtering only.Thanks to the narrow-bandwidth filtering and nonlinear enhancement of silicon microring resonators,we demonstrate continuous-wave correlated photon-pair sources with high spectral brightness.For the first time,we experimentally validate the non-classical optical bistability and propose the resonance-locked method by active cooling and temperaturedifference-compensation;we optimize the quality factor of silicon micro-ring resonators for pair rate and spectral brightness scaling;we fabricate silicon micro-ring resonator samples matching the International telecommunication union frequency grid standard and generate correlated photon pairs in multiply wavelength channels;we characterize the spectral-correlation purity by comparing the joint spectral intensity and the unheralded second-order correlation.The results present that active cooling ensures long-term stable correlated photon-pair generation;it is efficient to achieve pair rate and spectral brightness scaling by using higher intrinsic quality factor and suitable external quality factor;continuous-wave-pumped spontaneous four-wave mixing in silicon micro-resonators can generate correlated photon pairs behaving as optical frequency combs.