Research On Mechanism,Characteristics And Devices Of Soliton Frequency Comb In Fiber Microsphere Cavity

Optical frequency comb refers to a spectrum composed of a series of discrete,equally spaced frequency components with a stable phase relationship.As the awardwinning technology of the Nobel Prize in Physics in 2005,its unprecedented measurement capabilities have brought revolutionary technological innovations to the field of precision measurement,and has shown great application potential in astronomy,microwave photonics,precision spectroscopy,atomic clocks,molecular fingerprint measurements,communications,sensing and so on.Optical frequency combs mainly include fiber laser frequency combs,electro-optic frequency combs and microcavitybased Kerr frequency combs.Unlike laser frequency combs and electro-optical frequency combs,which require additional saturable absorbers or electro-optical modulators,microcavity-based Kerr frequency comb are generated by continuous lasers through a parametric nonlinear frequency conversion process in an optical microresonator,with larger comb line interval and higher pulse repetition rate.At the same time,the optical field in the cavity is confined in a very small volume,thereby enhancing the optical energy density and nonlinear interaction intensity in the cavity,which makes the microcavity optical frequency comb device with the characteristics of miniaturization and low power consumption,and has great research value and application potential.Microcavity-based Kerr frequency combs(Microcombs),as one of the advanced high-performance light sources,have received extensive attention at home and abroad.With the expansion of the application range and the increase of the demand,realizing the functionalization of microcomb is an important development direction for the application of microcomb devices in the future.However,the stable working conditions of microcombs are very harsh,and functionalization(such as: fast generation,dynamic control,and sensing)for specific applications is difficult to achieve on the existing research platforms,which limits the further expansion of microcomb applications.Fiber microsphere cavity is one of the ideal platforms for the research of nonlinear optics and functionalized devices because of its characteristics including high quality factor,simple structure preparation,rich mode characteristics and easy surface functionalization.With supports of the National Natural Science Foundation of China(G0561975025,U213010043),the National Ministry of Science and Technology Key R&D Project(2021YFB2800602)and the Zhijiang Laboratory Major Project(202012KFY00562),this thesis focuses on the mechanism,characteristics and device research of soliton frequency comb in fiber microsphere cavity,and conducts an in-depth exploration on the functionalization of soliton frequency comb.The research contents include: firstly proposed three new structures of optical fiber microsphere cavity(supermode,active,graphene-fiber microsphere cavity),carried out systematic research on the new mechanisms and new characteristics of soliton optical frequency comb,formed functionalized Prototype of new device for soliton optical frequency combs.The research content of this thesis has platform scalability,which has a guiding role in the research of soliton frequency combs based on on-chip and other microcavity platforms,and has important scientific significance and application value for promoting the functional development of microcombs.The main research works and innovations are briefly introduced as follows:1.Exploring the new mechanism of Kerr-Raman/Brillouin multi-nonlinear hybrid gain and avoided mode crossing multi-mode interaction based on over-modal fiber microsphere cavity.Demonstrating the new features of the multi-comb co-evolution based on nonlinear cross-gain and the soliton slingshot based on mode cross-coupling.Realizing new devices of multi-comb co-generation and soliton burst.Specific research contents include:Taking the research on the energy conversion relationship and phase matching process of the optical frequency comb in the Raman/Brillouin gain interval,under the multi-nonlinear hybrid gain of Raman/Brillouin gain and Kerr parametric gain.Realizing the Raman/Brillouin soliton exaction and demonstrating optical frequency comb band extension and new features of multi-comb co-evolution.In addition,with the multi-mode interactions from avoided mode crossing,the additional energy accumulation caused by multi-mode coupling is investigated,and the rapid and direct burst of soliton is realized,revealing new properties of the soliton slingshot.Furthermore,through the integrated packaging of over-modal fiber microsphere cavity and the tapered fiber core element,a new device for the co-generation of multiple soliton combs and the rapid and direct burst of soliton in a single microresonator is developed.This new mechanism of nonlinear hybrid gains and multi-mode interaction provides a new technical solution for the realization of highly integrated multi-comb source devices and user-friendly turnkey soliton devices,filling the gap of microcombs from laboratory to practical applications.2.Proposing a new mechanism of rare-earth and parametric crossover gain based on active fiber microsphere cavity.Demonstrating the characteristics of soliton performance enhancement and all-optical control with active gain assistance.Realizing a new highperformance soliton control device without amplification integration.Specific research contents include:By sintering erbium ions on the surface of quartz microspheres,the active optical assistance of parametric microcavities is achieved by auxiliary lasers.The combination of coherent parametric oscillation and active gain assistance greatly compensates the inherent loss in the cavity and get a soliton performance boost.And through the dynamic control of the auxiliary laser,the all-optical switch control of the soliton output is realized.With a flexible switching rate of 7 MHz,the soliton performance enhancement and alloptical modulation with active gain assistance are demonstrated.Furthermore,benefit from the compensation of microcavity loss by active gain assistance,reducing the requirements of soliton optical frequency comb for microcavity performance and pump power,and a new high-performance controllable soliton device without amplification is realized.This non-trivial optical assistance not only provides a route to integrate erbiumdoped fiber amplifiers into microresonators,improving their practicality,but also inspires a new paradigm of microscopic optical frequency combs,enabling actively controllable soliton devices.3.Taking the research on the new mechanism of graphene-microcavity refractive index modulation based on graphene-fiber microsphere cavity.Demonstrating new properties in the sensitivity and selectivity of gas molecules of multi-soliton combs enhanced by graphene.Realizing a new sensing device for single-molecule and multispecies.Specific research contents include:Through the asymmetric deposition of graphene in over-modal microsphere cavity,breaking the insensitivity limit of conventional microcavities to environmental changes.Combining the gas sensitivity of graphene and the advantages of soliton frequency comb,like high stability and low noise,and the natural characteristics of the optical frequency comb as an "optical-electrical" bridge,leveraging the the advanced heterodyne lock-in amplification technology,finally achieving high-sensitivity detection of weak and small signals and demonstrating the new properties of graphene-enhanced multi-soliton comb sensitivity and selectivity to gas molecules.Furthermore,by encapsulating graphene-fiber microsphere cavity,a new soliton sensing device for single-molecule and multispecies is developed.This combination of atomic-layer thin-film materials and microcombs shows the potential for integrated spectroscopy with unprecedented performance,introduces a new technological scheme based on soliton optical frequency comb light sources for fiber-optic sensing systems,and provide new insights into the design of functionalized microcavity photonic devices.