Mid-Infrared Highly Nonlinear Waveguide Design And Wavelength Conversion

Mid-infrared techniques have shown great potential for their application in optical communication,industrial and environmental monitoring,medical diagnostics and infrared spectroscopy.Mid-infrared wavelength conversion is one of the very important techniques in these applications.Compared with the conventional nonlinear materials for mid-infrared wavelength conversion,silicon waveguide has a larger nonlinear coefficient and is more convenient for integration.In this dissertation,two types of mid-infrared highly nonlinear waveguide is designed and the corresponding wavelength conversion performances are investigated.The main results are:I.A generalized full vectorial model of nonlinear wave propagation in graphene-silicon hybrid waveguide is developed.For the graphene-silicon hybrid waveguides where the weak guidance approximation is violated,the wave propagation analysis based on scalar theory does not give a good approximation.The induced polarisation is considered both in time domain and Fourier frequency domain depending on factors such as the bandwidth or pulse duration of the applied light and the speed of response of the nonlinear medium.The influences of the linear and nonlinear losses,free carrier effects and nonlinear phase shift on the propagating lightwave have been taken into account,and the perturbed fields are expressed according to the unperturbed fields.Using the reciprocal theorem to relate the perturbed and unperturbed fields are then related.By expanding the induced polarization into linear and nonlinear perturbation terms,different guiding modes are coupled by their non-zero longitudinal polarization through dispersion and nonlinear processes.Finally,the amplitude coupled equations are expressed in frequency and time domain respectively,according to the practical input field.?.The tunability of graphene's linear and nonlinear properties in near-and mid-infrared is studied.The Kerr nonlinearity of graphene achieve its maximum at the three-photon absorption-edge resonanceh??2|?c|?in accordance with graphene's low-linear-loss region.Graphene's chemical potential(Fermi level),which is determined by carrier density,can be changed through chemical doping or applying gate votage.Therefore,the best nonlinear performance and the least linear loss of graphene in the mid-infrared band can be realized by tuning graphene's chemical potential.?.Two types of highly nonlinear graphene hybrid waveguide are proposed:graphene-covered tapered fiber and graphene-silicon hybrid waveguide.The dimensions of the proposed waveguide are optimized to achieve the highest nonlinearity and the least absorption simultaneously in the near-and mid-infrared band.The bandwidth of the wavelength conversion in these two types of waveguide is numerically investigated.The proposed figure-of-merit(FOM)is applied to the situation when both the linear and nonlinear losses are obvious.Then using such a graphene-covered tapered fiber with an optimal diameter of 0.736 ?m and 34.4-?m long graphene,a maximum conversion efficiency of-38.07 dB is numerically obtained for the 1.55 ?m pump with a peak pump power of 10 W.Moreover,a 3 dB bandwidth as broad as 430 nm can be realized in the 1.55 ?m telecommunication band.As for the graphene-silicon hybrid waveguide,the conversion efficiency for a degenerate four-wave mixing process reaches-18.5 dB only with a pump power of 0.5 W and a waveguide length of only tens of microns.In the wavelength range of 1.3-2.3 ?m,the conversion efficiency can be kept flattened by adopting suitable waveguide dimensions.The corresponding 3-dB bandwidth can reach 40-110 nm for each fixed pump.IV.Based on the degenerated four-wave mixing,the mid-infrared wavelength conversion is experimentally demonstrated in a silicon waveguide.In mid-infrared band near 1.95?m,the wavelength conversion efficiency is measured under different pump wavelength and power conditions,and the conversion efficiency is about-51 dB.