Study On Flat Optical Frequency Comb Based On Normal Dispersion Silicon Nitride Nonlinear Integrated Optical Waveguide

The wide-band coherent flat optical frequency comb with high repetition has important applications in the fields of wavelength division multiplexing coherent optical communication systems,optical arbitrary waveform generation,optical precision measurement,etc.It is a research hotspot in the field of nonlinear optics.The optical frequency comb consists of a high-power optical pulse through a highly nonlinear optical waveguide that produces a large spectral broadening by a combination of dispersion and nonlinear effects.It consists of a series of frequency components that are evenly spaced and have a fixed phase relationship with each other.When the high-power input light pulse is located in the normal dispersion region of the silicon nitride waveguide,the time-domain pulse width and the frequency-domain spectram are broadened and flattened by self-phase modulation(SPM)and optical wave splitting(OWB)effects,and good coherence is maintained.In this paper,the dispersion engineering of silicon nitride optical waveguides and the optimization design of tunable high repetition rate and broadband flat coherent optical frequency comb based on silicon nitride are studied.The main research content and innovative work are:(1)The time domain and frequency domain evolution of the pulses in different dispersion regions of nonlinear optical waveguides due to the interaction of dispersion and nonlinearity are analyzed theoretically.By using COMSOL and MATLAB,the TE fundamental mode dispersion curves of three different silicon nitride multimode waveguide structures,Strip waveguide,single Slot waveguide and double Slot waveguide,are optimized by adjusting the structural parameters of silicon nitride waveguide.The effect of bending radius on the dispersion characteristic curve is also discussed.The simulation results provide a high power confinement factor silicon nitride waveguide with ideal normal dispersion and non-linear parameters for the generation of optical frequency combs.(2)An asymmetric silicon nitride double Strip waveguide based on odd-even supermode theory is proposed to achieve large normal or anomalous dispersion at 1550nm band.The effects of waveguide height,Strip waveguide width and Strip waveguide spacing on the supermode dispersion characteristic curve are investigated.By means of dispersion control,a large abnormal dispersion value is obtained at 1550 nm band,which can be used for pulse compression in optical frequency combs.(3)A scheme of wide-band coherent flat optical frequency comb based on silicon nitride nonlinear integrated optical waveguide is proposed.The adjustable high repetition rate pulse source is amplified by a high-power EDFA and fed into a three-stage optimized design of a silicon nitride integrated optical waveguide.Based on the nonlinear effects such as chirped pulse compression,self-phase modulation and optical wave breaking,a broadband coherent flat optical frequency comb with a 3dB flatness bandwidth range of 40nm to 60nm is achieved.The effects of different input pulse shape,initial chirp,pulse width,pulse peak power,second-order dispersion and third-order dispersion of waveguide on optical frequency comb are investigated.