Study On Broadband Flat Optical Frequency Comb Based On CMOS-compatible Integrated Nonlinear Optical Waveguide

Broadband coherent flat optical frequency comb with high repetition rate is suitable for microwave photonic,wavelength division multiplexing and optical arbitrary waveform generation.The optical frequency comb consists of ultrashort laser pulse trains with the same time interval,and its frequency spectrum is shaped like a comb.Each frequency component in the frequency domain has a fixed interval and corresponds to the repetition rate of the pulse in the time-domain.The Fourier transform of a single pulse in the time domain corresponds to the frequency envelope of the optical frequency comb.When a high-power optical pulse is injected into the normal dispersion region of a highly nonlinear optical waveguide,due to the effects of self-phase modulation and optical wave-breaking effects,the time-domain pulse and frequency-domain spectrum of the optical pulse will become broad and flat,and the spectrum maintains good coherence.In this thesis,we have conducted in-depth research on the generation of high repetition rate broadband flat optical frequency comb based on tantalum pentoxide optical waveguide and aluminum gallium arsenide optical waveguide at 1550nm band.The main research contents are as follows:?1?The spectrum and pulse evolution of optical pulses transmitted in normal and anomalous dispersion nonlinear waveguides are described respectively.The dispersion of the quasi-TE₀ fundamental mode of the multimode tantalum pentoxide waveguide is optimized.Based on the numerical simulation of pulse-pumped normal dispersion tantalum pentoxide optical waveguide,a flat optical frequency comb with a bandwidth of nearly 50 nm was realized.The effects of different pulse peak power,pulse half width,pulse initial chirp,pulse shape,waveguide dispersion and loss on the optical frequency comb are analyzed.At the same time,pulse soliton compression technology is used to increase the peak power of the pulse.Based on the numerical simulation of the pulse-pumped anomalous dispersion cascade normal dispersion tantalum pentoxide optical waveguide,a flat optical frequency comb with a bandwidth of nearly 60 nm is realized,and the performance of the optical frequency comb is improved.?2?The dispersion of the quasi-TE₀ fundamental mode of the multimode aluminum gallium arsenide optical waveguide is optimized.In addition,the flatness and coherence of the optical frequency comb generated with the pluse by passing through the anomalous dispersion aluminum gallium arsenide optical waveguide was studied.Based on the pulse-pumped normal dispersion arsenic gallium aluminum optical waveguide,the numerical simulation realizes a flat optical frequency comb with a bandwidth of nearly 200nm.The influence of various parameters of the pulse,dispersion and loss on the generated flat optical frequency comb is also analyzed,and the characteristics of the optical frequency comb spectrogram generated with different input pulse shapes are analysed.The performance of flat optical frequency combs produced by different material platforms is compared.?3?The inverse taper waveguide and Euler-bend waveguide in Spiral long waveguide is simulated and analyzed.Through using the FDTD method,the edge coupling between optical fiber and strip multimode tantalum pentoxide optical waveguide was studied by using the inverse taper waveguide.The influence of the initial width of the inverse taper waveguide on the coupling efficiency is analyzed,and nearly 85%quasi-TE₀ mode coupling efficiency is achieved in the simulation.The electric field intensity distribution and the energy ratio of quasi-TE modes are compared and analyzed when electric field intensity distribution of each order quasi-TE modes propagate through the arc-bend waveguide and the Euler-bend waveguide respectively.It is verified that Euler-bend optical waveguide in Spiral long waveguide can effectively solve the problems of bending loss and high-order mode excitation in multimode wide waveguide.