Design And Numerical Calculation Of Mid-Infrared Nonlinear Silicon Waveguides

The mid-infrared region has attracted a lot of interests due to its potential applications in military,industrial process control,environmental monitoring and free space communication.The traditional mid-infrared devices are limited by the response speed,operating temperature and device processing,etc.,and cannot fulfill the urgent requirements of high speed,integration and industrialization.In this thesis,we study the nonlinear process inside mid-infrared silicon waveguide,and develop new working mechanisms for second harmonic generation and four wave mixing respectively,which may pave a way for better control mid-infrared signals..Silicon material is the cornerstone of the microelectronics industry.In recent years,the discovery of its strong third-order-nonlinearity opens a door for all-optical and high-speed silicon-based information processing.Compared with other materials,silicon also shows unprecedented advantages in micro-nanofabrication and integration.Silicon waveguides are the basic building block of silicon integrated optics.The guided signals inside the waveguide interact with silicon,enabling a nonlinear process generation.The ultra-small mode size(on the order of ?m2)of a silicon photonic waveguide,making feasible a high optical pump intensity at low input powers.Coupled with its large third order nonlinear susceptibility,the silicon photonic waveguide has enabled researchers to investigate various nonlinear effects using ultrafast lasers[In this thesis,we design two types of nonlinear silicon waveguides in mid-infrared by analyzing the phase-matching condition and propagation process of nonlinear effects:(1)Bulk silicon possesses no second-order susceptibility(?(2)),inhibiting second-order nonlinear processes in the emerging silicon photonic platform.Here,we propose a method to overcome this limitation by enabling a third-order(?(3))nonlinear mixing scheme between optical waves and an externally applied static electric field inside a silicon waveguide.We show in theory that facilitated by a modal phase-matching scheme efficient second-harmonic generation can be realized under an applied voltage of 65V,giving rise to an equivalent ?(2)=4.7pm/V.We also show that unlike the classical second-harmonic generation the wavelengths of phase-matched pump and second-harmonic waves are pump-power dependent due to the ?(3)nature of this process.(2)In the four-wave mixing process inside a silicon waveguide,one major factor limits the effective length is the propagation loss.To address this issue,we introduce a gradient structure to compensate the loss induced pump power attenuation,which may destroy the phase-matching condition.In this way,the phase matching conditions are satisfied even after a much longer propagation distance,thereby increasing the effective length and improving the four-wave mixing conversion efficiency.In the meantime,by increasing the degree of design freedom,we design an achievable four-wave mixing silicon waveguide,to convert 3789nm mid-infrared signal to 1550nm communication band.Numerical calculation proves that the four-wave mixing conversion efficiency of this gradient waveguide is more than 30dB higher than that of the conventional waveguide.