Optical Frequency Comb Based On Normal Dispersion LiNbO₃ Thin Film Waveguide

Optical frequency comb(OFC)has a wide application prospect in WDM optical communication system,arbitrary waveform generation,spectral measurement,precision ranging and other fields,and is one of the current research hotspots.In this paper,we mainly focus on the broadband coherent flat comb(frequency interval is10GHz-100GHz)in 1550nm band generated by normal dispersion Li Nb O₃ thin film optical waveguide.It includes using the electro-optic modulation pulse or two phase-locked laser to pump the normal dispersion Li Nb O₃ waveguide to produce flat optical frequency comb,and using the two phase-locked CW laser to pump the micro ring resonator to produce flat optical frequency comb.(1)The dispersion of quasi TE₀ mode in multimode Li Nb O₃ thin film waveguide is optimized.A coherent flat optical frequency comb with 3d B bandwidth of about 32nm is produced by using hyperbolic secant pulse pumping in the normal dispersion Li Nb O₃optical waveguide.The effects of pulse peak power,width,shape,initial chirp,second-order dispersion,third-order dispersion and loss on the performance of optical frequency comb are studied.An optical frequency comb with 2.6d B bandwidth of about32nm is generated by using anomalous dispersion Li Nb O₃ optical waveguide for soliton pulse compression cascaded with normal dispersion Li Nb O₃ optical waveguide,and the flatness of the optical frequency comb is optimized.The influence of pulse overlap on optical frequency comb is analyzed.(2)Two phase-locked CW lasers are input into normal dispersion Li Nb O₃ optical waveguide after two-stage pulse compression to generate an optical frequency comb with 20 d B bandwidth of about 100 nm.The effects of the initial power difference,the third-order dispersion and the loss of the waveguide on the performance of optical frequency comb are studied.By shaping the compressed pulse into a fifth order super Gaussian pulse,a flat optical frequency comb with 3d B bandwidth of about 90nm is produced.The influence of bending radius on the dispersion is studied.By designing the Spiral structure,the meters long Li Nb O₃ waveguide is designed into 5mm×5mm Spiral cells,which greatly saves the chip area.(3)An optical frequency comb with 10d B flatness in 96nm bandwidth is produced by using two phase-locked CW laser pumping a normal dispersion Li Nb O₃ microring resonator.The effects of pump power,loss and dispersion on the effective red detuning region,tuning range and performance of optical frequency comb are studied.The structure of the microring resonator is optimized.The loss of the microring resonator can be reduced and the excitation of higher-order modes can be avoided by designing the circular bending to Euler U-shaped bending.