Research On Supercontinuum Generation In Silicon Nitride Optical Waveguide

Supercontinuum generation（SCG）refers to the phenomenon that the frequency spectrum of the ultrashort pulse is greatly broadened after the linear and nonlinear effects.SCGs that expand in different bands have important applications in different fields:SCGs in the visible light band can improve the clarity of biomedical imaging and achieve accurate frequency measurement,and SCG in the C band can be used as a multi-wavelength light source to achieve TDM-WDM-TDM conversion and SCG in the mid-infrared band are widely used in sensing and spectroscopy detection.With the continuous development of optical chips,the method of generating SCG based on optical fiber requires a long transmission distance and high pump energy,which can no longer meet the requirements of high integration and small size of the current integrated optical platform.By rationally designing the structure of the silicon-based optical waveguide,it has a strong confinement to light and has a nonlinear coefficient far greater than that of the optical fiber,and it can flexibly adjust the dispersion,so that,octave SCG can be achieved in millimeter size waveguide by pumping of picojoule energy,and it’s compatible with the CMOS,which makes it easier to combine optical with electricity together.These advantages make silicon-based optical waveguides become a new platform for the study on SCG.At present,SCG is mainly researched in integrated optical platforms based on materials such as silicon,lithium niobate,silicon nitride（Si₃N₄）,and nitrogen-rich silicon nitride.In particular,silicon nitride has a wide transparent window,there is no nolinear absorption in the visible light and communication bands.At the same time,the waveguide formed with silicon dioxide can achieve strong light confinement in a smaller core layer due to the high refractive index difference,and has a nonlinear coefficient tens of times that of silicon,which increases the effective nonlinear effect of the waveguide,making it possible to achieve strong nonlinear effects with lower energy,therefore,Si₃N₄ has great potential in the application of communication band SCG.This paper mainly studies the generation of supercontinuum from visible light to communication band in Si₃N₄ waveguides of different sizes.First,the linear and nonlinear effects involved in the generation of SCG and the theoretical formula describing this process are analyzed,and then analyze its dispersion by adjusting the Si₃N₄ waveguide structure to obtain a waveguide structure that meets the dispersion requirements.Then analyze the impact of the pulse on the SCG production and the coherence of the SCG,so as to obtain the parameters of the waveguide and pulse of the SCG that can cover the visible light to the communication band.Then design the mask according to the analysis results:in the process of designing the mask,not only the waveguide that generates visible light to the communication band SCG,but also other waveguides of different widths,in order to select different widths waveguides in the same device and different pulse parameters to flexibly achieve SCG in different ranges.Finally,the waveguide device was fabricated in the laboratory and tested for light transmission and loss.