Optical Frequency Comb Based On Aluminum Nitride And Lithium Niobate Microcavities

The integrated,highly-coherent and self-referenced optical frequency comb has shown great application prospects in the fields of ultra-fast precision ranging,optical frequency synthesis and timing.To achieve low-noise and octave-spanning frequency comb,the generation of Kerr frequency comb based on single crystal aluminum nitride（AlN）is studied,and low-noise,highly-coherent and octave-spanning frequency comb,also known as dissipative Kerr soliton（DKS）,is realized.The electro-optical frequency comb based on an electrical and optical dual-resonance structure of thin-film lithium niobate has also been generated,which can be used to determine the free spectrum range（FSR）of the Kerr frequency comb.Wurtzite AlN,with high linear and nonlinear refractive index and large direct bandgap（～6.2 eV）,has been considered as a promising candidate for integrated nonlinear optical devices.In this paper,a 1.2-μm-thick AlN film is selected to reduce the scattering loss caused by lattice mismatch at the interface between the epitaxial AlN and the sapphire substrate.Simple photolithography and ICP dry etching process,with three masks for transferring the pattern,are applied to fully etch AlN layer,which can help to improve the nonlinearity of the microresonators.A 0.5μm gap can be achieved to realize nearly critical coupling for TM and TE modes.High intrinsic Qs(Q_int)of 1.5×10⁶ and 2.1×10⁶ for the TM₀₀ and TE₀₀ mode of the AlN microring resonator with a bending radius of 60μm are obtained,corresponding to the propagation loss of 0.26 and 0.17 d B/cm,respectively.Then,for the first time,an octave-spanning Kerr frequency comb is generated from 1100to 2400 nm at a low pump power of 250 mW.The competition and coexistence between Raman and Kerr effect is also observed.In addition,taking advantage of the existence of Raman line,a broadband Kerr-Raman dual-comb for TE₀₀ mode with normal dispersion is also realize.However,the measured broadband beat note noise indicates that the Kerr frequency comb has not reached the soliton state yet.To easily access the octave-spanning DKS,a simple method is proposed in this paper.In the scheme,an adjacent mode near the red-detuned side of the pump mode is used to help to mitigate the cavity thermo-optical effects,thereby allowing the stable access to DKS with a single pump.At a slow pump tuning speed of 1 nm/s,a stable single soliton state with an octave-spanning spectral bandwidth from130 to 273 THz（1100-2300 nm）can be attained.The temporal characteristics are also carried out through the second-harmonic generation based autocorrelation measurement.A measured30-fs pulse trace confirms high coherence of the soliton microcomb.The broad soliton step（80 pm,10 GHz）allow to directly create the DKS with high probability by simply switching the pump wavelength from an off-to an on-resonance state.Finally,this paper introduces the generation of dual-resonance enhanced electro-optical frequency comb based on thin film lithium niobate wafers.The electro-optical frequency comb has a FSR of 20 GHz which can be used to determine the FSR of the Kerr frequency comb（>360 GHz）.Theoretical analysis shows that the attenuation of the Q can be compensated by the phase modulation indexβ,therefore a Fabry-Perot type resonantly enhanced modulator is designed to enhance the phase modulation.By optimizing the design and fabrication,the microwave propagation loss near 20 GHz is 70 m^-1,and the optical Q_intof the microring can be reach to 1.2×10⁵.According to this,the calculated phase modulation of the standing wave electrode can be increased by more than 2 times compared with the traveling wave type electrode.The measured spectral bandwidth under the traveling wave electrode can be up to 1.64 nm,while the standing wave electrode can be up to 2.96 nm.These results demonstrate the advantage of the dual-resonance enhanced electro-optic modulation structure in broadening the electro-optical frequency comb.