Study On The Thermal Effect And Stabilization For The Soliton Microcomb

High-quality optical microcavities can bind light in the micrometer scale for a long time,forming a very high optical density,which greatly enhances the light-matter interaction and provides a good experimental platform for frequency conversion of nonlinear optics,especially microcavity frequency comb.The third-order Kerr effect,a nonlinear effect in which the refractive index of a material changes with the light intensity,is one of the important mechanisms in optical fiber communication.The optical frequency comb based on Kerr effect in optical microcavities and its phase-locked soliton state were realized in SiO2 microtoroid and MgF2 crystalline cavity in 2007 and 2014,respectively,and have been verified in various material and structural systems since then,where on-chip microring represented by Si3N4 and LiNbO3 materials have received more attention and show very high potential for heterogeneous integration.Compared with conventional mode-locked lasers,optical soliton in microcavity have the advantages of small size,high repetition rate,tunable dispersion,and low power consumption,which have great prospects for applications in optical clock,communication,ranging,coherent imaging,and absorption spectrum measurement.Due to the high power optical field in the microcavity,the material thermal absorption will cause the change of microcavity temperature,i.e.,the thermal effect in the cavity,resulting in the change of its size and effective refractive index with the change of power in the cavity,and the resulting drift of the resonant mode can even far exceed the linewidth of the optical mode,which cannot be ignored in the experiment.Especially,the thermal effect can shorten the soliton step in the soliton generation process and make the soliton unstable.The research in this thesis aims to solve the problem of the generation and stability of the soliton state under the thermal effect.The main results of this thesis include:The design and experimental preparation of a hybrid athermal microcavity structure and the study on the thermal oscillation phenomenon in it.Investigation about the optimization effect of thermal elimination on the soliton generation and thermal noise.The experimental realization of a thermally stable region based on a stretchable and easily deformable microbubble cavity.The theoretical and computational work on the breathing and repetition rate of the soliton state based on injection locking,which corroborates the experimental results.The major research work I have completed during my graduate studies is as follows:1.Theoretical simulation of the athermal effect of the hybrid microcavity and investigation of its optimization effect on soliton generation and thermorefractive noise.Using the negative thermorefractive property of TiO2 combined with the on-chip Si3N4 microring cavity,three hybrid microcavity designs are proposed to eliminate the thermal effect inside the microcavity,and the finite element method is used to simulate the equivalent thermorefractive when the Si3N4 waveguide is covered with different TiO2 thickness.The hybrid microcavity can effectively suppress the thermal effect in the cavity and prolong the soliton steps.The thermal noise in the hybrid microcavity is also studied based on the fluctuation-dissipation theorem,and the thermal noise in the cavity can be reduced by an order of magnitude when the equivalent thermorefractive is eliminated.2.Experimental observation of thermal oscillations in the hybrid microcavity,and demonstration of the simulation and theoretical analysis based on the thermal oscillation model,in agreement with the experimental results,providing a theoretical basis for avoiding thermal oscillations.The hybrid microcavity may introduce the phenomenon of thermal oscillation while eliminating the thermal effect,which makes the microcavity mode in an unstable state within a certain power and detuning range,leading to an important problem when eliminating the thermal effect.We explain the mechanism of thermal oscillation occurrence based on the evolution of the coupled mode equation of the hybrid microcavity under thermal effect;then the thermal oscillation model is expanded perturbatively near the steady-state temperature to derive the theoretical conditions for the occurrence of thermal oscillation,which provides a basis for effectively avoiding the thermal oscillation region in applications.3.In the microbubble cavity fixed at both ends,the optical mode with temperature insensitive region was achieved experimentally by using the opposite effect of the thermal expansion of the metal and the thermal effect of the microcavity.By fixing the stretchable and easily deformable microbubble cavity on an aluminum frame with high thermal expansion rate to form a hybrid structure,another scheme to eliminate the thermal drift of the optical mode inside the cavity was realized in the experiment.When the operating temperature of the system gradually changes,the optical mode will exist a thermally stable region where the mode is insensitive to temperature change under the combined effect of thermorefractive,thermal expansion,and stretching force,providing a new scheme to eliminate the mode thermal drift in the cavity.4.A soliton microcomb is implemented in the microcavity to achieve stabilization of the breathing frequency or repetition rate using injection locking,and a theoretical analysis is performed,which is consistent with the experimental results.To address the problem of thermal effects arising during soliton generation,a fast sweeping technique for pump light is implemented to generate soliton stably by using high-speed modulation of electro-optical modulators.The injection locking of the soliton breathing frequency is performed based on the coupled mode equation,and the narrowing of the linewidth after locking and time-domain evolution are simulated and demonstrated to provide tools for studying the fast kinetic evolution of the breathing state locking.In addition,the injection locking on the repetition rate is investigated by introducing the soliton pulse dynamics based on the Lugiato-Lefever equation,deriving the equation of motion of the pulse in the potential field and the analytical conditions satisfied by the pulse as it moves with the potential field,while the evolutionary solution of the coupled mode equation is used to verify the analytical conclusion and the existing experimental results,providing a theoretical basis for the locking capability of the repetition rate of the soliton pulse.