High Stability Lithium Niobate-based Cavity Optomechanical Oscillator

The rapid development of modern navigation and communication systems has put forward higher requirements for the accuracy of reference clocks.Atomic clocks have emerged in this era,and their long-term stability has reached 10^-18 orders of magnitude.Among them,the rubidium/cesium atomic clocks have been widely used.Combining MEMS technology with atomic clock technology can provide excellent frequency stability,but the system is relatively complex and requires the use of active circuits such as frequency synthesizers and frequency dividers with high noise and high power consumption.By interlocking the cavity optomechanical system with high short-term stability and the rubidium atomic clock with high long-term stability,a chip-scale atomic clock with low power consumption and ultra-high stability can be realized.In order to realize chip-scale atomic clocks based on cavity optomechanical systems in the future,this thesis uses lithium niobate materials with a wide transparent window spanning the visible light to mid-infrared bands to design cavity optomechanical systems based on high-quality factor photonic crystal microcavities,and then fabricate A high-stability optomechanical oscillator.The main research work and contributions of this thesis are as follows:1.Utilizing the high-efficiency optical-mechanical energy coupling characteristics in the cavity optomechanical system,a high-stability cavity optomechanical oscillator based on lithium niobate material was designed.The optical microcavity is simulated and optimized using Lumerical software.For the triangular lattice photonic crystal array,the influence of lattice constant,air hole radius and plate thickness on the energy band is studied,and the finally optimized photonic crystal band gap range is 1.45-1.65μm;For the line-defect photonic crystal waveguide,the coupling loss between the optical waveguides in the on-chip integrated waveguide was studied,and finally the tapered waveguide was used to optimize the transition,and the obtained photonic crystal waveguide transmittance was better than 0.9;The slit significantly enhances the light confinement ability of the cavity,and the influence of line defect width,different perturbation and slit width on the resonance frequency and quality factor of the cavity mode is studied,and the finally optimized operating wavelength of the cavity fundamental mode is 1491nm,the mode volume is as low as 0.17（λ/n）³,and the quality factor is as high as 1.29×10⁶.The optomechanical oscillator is simulated and optimized by Comsol software.For the double cantilever mechanical oscillator structure,the influence of the size of the corrosion block on both sides of the oscillator and the length of the slit on the intrinsic mechanical oscillation frequency is studied.The final optimized operating frequency is67MHz;for the oscillator The optomechanical interaction in the cavity verified the high-efficiency optomechanical coupling in the optomechanical system of the cavity,and studied the influence of the size of the corrosion block and the length and width of the slit on the optomechanical coupling rate,and the final optimized optomechanical coupling rate was as high as 1.63×10⁵Hz.2.After the simulation design is completed,the lithium niobate optomechanical oscillator is fabricated.Lithium niobate etching uses the ICP-RIE process,using Cr metal as the etching hard mask,which solves the product deposition problem caused by the low selectivity mask;then optimizes the design of the air groove width,breaking through the slit area due to Etching difficulties caused by the micro-loading effect;finally,using photoresist negative resist HSQ as a mask,a lithium niobate-based optomechanical oscillator with a slit width of 300nm was prepared,and the sidewall inclination angle reached 71.6 degrees.3.After the chip preparation is completed,a test platform is built under atmospheric conditions.The test and analysis of the silicon-based optomechanical oscillator with the same air-gap-loaded waveguide-modulated microcavity structure shows that the optical quality factor of the photonic crystal microcavity can reach 2.61×10⁴,and an obvious thermal increase can be observed by increasing the input power of the pump laser.Nonlinear dragging phenomenon;under different laser detuning conditions,phenomena such as chaos in the optomechanical cavity,parametric optomechanical oscillation,and free carrier oscillation have been observed.The mechanical quality factor of the optomechanical oscillation signal at 65.5MHz can reach 1.64×10⁵,the phase noise performance is better than-102.71d Bc/Hz@10k Hz,and the Allan standard deviation of10ms integration time under 1000s working time is about 4.5×10^-7.