High Figure-of-merit And Broad Bandwidth Wavelength Conversion In Silicon-based Waveguides At 2?m Band

With the increasing requirements of communication speed and capacity,widening the communication band to mid-infrared band is expected to effectively solve the problem of limited communication bandwidth.In a 2?m mid-infrared communication system,all-optical signal processing still play a crucial role in the operation of the system.Silicon-based waveguides have high integration and mature fabrication processes and also show good nonlinear performance in the mid-infrared band.Silicon-based waveguides with high figure-of-merit and large conversion bandwidth that combined with polymers,metals and other materials are designed and fabricated in this paper.Moreover,wavelength conversions based on four-wave mixing effect for the fabricated waveguides are experimentally realized in the mid-infrared band.For hybrid waveguides and strongly confined waveguides,the scalar coupled wave equations describing nonlinear optical effects are no longer applicable.In this paper,when the incident pump light and signal light are continuous waves,the full-vectorial coupled wave equations for degenerate four-wave mixing is introduced.The full-vectorial calculation formulas of degenerate and non-degenerate two-photon absorption coefficients and free carrier absorption loss are given.The relationship between phase matching and dispersion is analyzed.When the group velocity dispersion at the wavelength of pump light is small and flat,a broad conversion bandwidth can be achieved.The calculation formula of figure-of-merit with two-photon absorption coefficient is introduced,which can be used to analyze and compare waveguides affected by both linear loss and two-photon absorption loss.A new type of silicon-based metal plasmonic hybrid waveguide is proposed and its size is optimized for best figure-of-merit and broad conversion bandwidth.When the incident pump power is 500 mW and the waveguide length is 6.6 mm,a conversion bandwidth of 512 nm and a conversion efficiency of up to-6.65 dB are predicted by simulation.As comparisons,optimized designs are made for suspended silicon waveguides and silicon-rich silicon nitride waveguides.The nonlinear performance improvement of the suspended silicon waveguide is limited by the nonlinear loss caused by the two-photon absorption of the silicon material,while the nonlinear performance improvement of the silicon-rich silicon nitride waveguide is limited by the low nonlinear refractive index of the silicon-rich silicon nitride material.The overall fabrication processes of silicon-based waveguides are optimized,including chemical vapor deposition,metal sputtering and evaporation deposition,electron beam lithography and inductively coupled plasma dry etching.Using the optimized fabrication processes,suspended silicon waveguide,silicon-rich silicon nitride waveguide and silicon/silicon-rich silicon nitride hybrid waveguide are fabricated.After scanning electron microscopy test,the actual dimensions of fabricated waveguides are in accordance with the theoretically designed dimensions and the side walls are smooth.In addition,the feasible fabrication processes of silicon-based metal plasmonic hybrid waveguide are also given.Using the end-coupling method,the designed and fabricated suspended silicon waveguide,silicon-rich silicon nitride waveguide and silicon/silicon-rich silicon nitride hybrid waveguide are tested for linear loss and wavelength conversion efficiency of four-wave mixing.The conversion efficiency curves obtained by experiments are in good agreement with the theoretical predictions,which verifies that the designed and fabricated silicon-based waveguides have high conversion efficiency and broad conversion bandwidth.Under the experimental condition of low incident pump power,the overall nonlinear performance of the suspended silicon waveguide is much better than that of the silicon-rich silicon nitride waveguide and the silicon/silicon-rich silicon nitride hybrid waveguide.After analysis,the overall nonlinear performance of the silicon/silicon-rich silicon nitride hybrid waveguide is between that of suspended silicon waveguide and the silicon-rich silicon nitride waveguide.The experimental results are consistent with theoretical analyses.