Study On Fabrication Of On-chip Silicon Nitride Microring Resonator And Generation Of Soliton Microcomb

Optical frequency combs,which is a series of equidistant coherent optical lines in the frequency domain,have received great attention and development in the past t-wo decades.The conventional optical frequency comb is produced by the mode-locked laser and has played an important role in the precise measurement of time and frequency.Based on the optical field enhancement characteristics of the microresonator,in 2007,the generation of optical frequency comb was realized in the microresonator by pumped with a continuous-wave laser,which opened a new field of optical frequency combs based on the microresonator.Due to the emergence of noise in the generation process,the early microresonator frequency combs showed low coherence,and its application value was not expected.In recent years,with the discovery of dissipated Kerr solitons,stable soliton pulses can be generated in microresonators and fully coherent frequency combs can be obtained by simultaneously balancing gain and loss,as well as dispersion and nonlinearity.Compared with the conventional optical frequency comb generated by the mode-locked laser,the soliton microcomb could also achieve parity in perfor-mance and has the advantages of low power consumption,small size,simple structure and integration.Therefore,in addition to providing an ideal platform for the study of nonlinear physics,the microresonator-based soliton microcomb has shown a promis-ing future in many applications,such as optical frequency synthesis,optical atomic clock,lidar,low-noise microwave source,coherent optical communication,dual-comb spectroscopy and optical coherence tomography.In the field of quantum information,quantum information can be encoded in the frequency and time degrees of freedom of a single photon.By using time-frequency entangled photons based on optical frequency comb,deterministic high-dimensional controlled quantum gate can be realized,which provides a feasible choice for scalable optical quantum computation.In this thesis,based on the silicon nitride microring resonator,the design and fab-rication of the silicon nitride microring chip,the generation of the soliton microcomb,and the frequency stability and tuning of the breathing soliton were studied.In addition,a kind of microresonator-based structure named self-interference structure is proposed in this thesis,and its application in optical sensing and optical frequency comb is pre-liminarily explored.Specific content has the following aspects:1.Fundamental theory of whispering gallery mode microresonator and microres-onator based optical frequency combFirstly,important parameters of the whispering gallery mode microresonator,like the quality factor,the free spectrum range,the effective mode volume and the disper-sion,are introduced in detail,and the linear shape of optical mode in microresonator is analyzed by using coupling mode equation.Then,starting from the third-order nonlin-earity,the role of the nonlinear effect in the generation of the optical frequency comb is introduced.The derivation process of coupling mode equation and Lugiato Lefever equation,which are commonly used in the study of optical frequency comb,is giv-en,and their equivalence is proved mathematically.Based on these two equations,the generation process of the optical frequency comb and the soliton is studied theoretical-ly,and the mathematical expressions of the generating conditions and some important parameters are given.2.Design and fabrication process improvement of the silicon nitride microring chipIn order to meet the requirement of soliton microcomb on microresonaotr disper-sion and improve the coupling efficiency of chip,we design and optimize the profile of the microring resonator and the shape of the edge coupler through simulation.As for the fabrication process of the microring chip,we introduce each step of the process one by one,and give the key points and improvement scheme for each step.optical modes with quality factors exceeding 3×106 have been observed in our samples using an improved fabrication process.3.Experimental generation of soliton microcomb in microring resonatorBased on our high quality factor silicon nitride microring resonator,the generation of soliton microcomb in microresonator is demonstrated experimentally,and the com-plete evolution process of the soliton microcomb from the primary comb state,mod-ulation instability state,breathing soliton state to the stable soliton state is recorded.Theoretical analysis and experimental characterization are carried out for each state of the evolutionary process.At the same time,in order to overcome the problem of low soliton production efficiency when single laser is directly scanned,we use the method of adding auxiliary laser to suppress the influence of thermal effect on generation process of the soliton,and realize the stable and efficient production of the soliton microcomb.4.Frequency stability and tuning of the breathing solitonThe breathing soliton state is a special state in the evolution process of the soliton microcomb.The theoretical analysis and experimental characterization of the breathing soliton state are carried out in detail,and several characteristic signals to prove the existence of the breathing soliton are given.Due to the periodic energy exchange,the stability of the breathing soliton state is worse than that of the stable soliton state.Based on the injection locking mechanism,we verified that the breathing frequency of the breathing soliton could be synchronized with the modulation frequency of the external signal,and observed that the stability of the breathing soliton in the locked state was significantly improved and the phase noise was effectively suppressed.By adjusting the power and frequency of the external modulation signal,the breathing frequency tuning of up to 50 MHz was realized in the experiment.The locked breathing soliton is expected to play a role in improving spectral resolution and wide-band microwave signal distribution.5.Principle and application of self-interference structureCompared with the traditional straight waveguide coupling structure,the self inter-ference structure has an extra U-shaped interference arm.By adjusting the phase on the interference arm,the phase difference between the interference arm and the microres-onator can be changed,so as to adjust the effective coupling loss of the microresonaor.The change of effective coupling loss will cause the change of the linewidth and extinc-tion ratio of the optical mode.Therefore,the self-interference structure can provide a new dissipative sensing mechanism,which can overcome the shortcomings of the tra-ditional sensing mechanism to some extent.We have successfully realized the electric power and solution concentration sensing based on the self-interference structure in the experiment,which shows the potential of its practical application.In addition,we al-so realize the optical frequency comb generation in the self-interference structure and observe the modulated spectral envelope.