Study Of Optical Frequency Combs Based On Tapered Optical Fiber Coupled Microsphere Structure

Whispering gallery mode(WGM)microcavity is a new type of optical microcavity with ultra-high quality factor and low mode volume that significantly enhances the photon-matter interaction.Optical frequency combs(OFC)based on WGM microcavity has the advantages of high coherence,wide spectral range,integration and low power consumption,which makes it have broad application prospects in intergrated microwave photonics,radio frequency signal generation,miniature optical clock and so on.In this paper,WGM microcavity coupled biconical fiber structure is used as a carrier,and two kinds of experimental systems for generating microcavity OFC are mainly introduced.The effects of different physical quantities on OFC microcavity characteristics are also studied.In chapter 2,Maxwell's equations are firstly used to solve the electromagnetic field distribution and eigenvalue equation in microsphere cavity.The basic concepts for characterizing WGM microcavity are introduced,and the preparation of tapered optical fibers and microspheres is demonstrated.In chapter 3,the mode-coupled theory combined the experimental results is used to describe the basic principle of third-order nonlinear optical effect in microcavity.The influence of thermal effect on the microcavity resonance is theoretically analyzed and experimentally proved.The different dispersion parameters are described in detail.The total dispersion simulation and zero-dispersion wavelength of microcavity are calculated according to the refractive indices of different materials.Two complementary classical theoretical models for describing the formation of OFCs are introduced.In chapter 4,the OFC generation of single-frequency laser directly pumped silica microspheres using an evanescently coupled tapered optical fiber have been realized.The competition of stimulated Raman scattering(SRS)with four-wave mixing(FWM),the transformation between different mode families and the method of obtaining pure microcavity OFC are analyzed.Comparing experimental results with theoretical models,the influence of mode coupling,pump power,pump wavelength,dispersion,frequency detuning and other factors on the formation of OFC is qualitatively analyzed by using control variable method.The experimental results well verify the relationship between these factors and the frequency interval of pump—first sideband.A new system for generating OFC based on tapered optical fiber microsphere/erbium-doped fiber dual-cavity structure have been introduced in chapter 5.The system embeds the microsphere coupling tapered optical fiber structure into the ring fiber gain cavity composed of an erbium-doped fiber.This system uses the functions of resonant frequency selection,narrow-band reflection and nonlinear parametric oscillation of the microcavity to get rid of the dependence of microcombs on tunable single-frequency laser,and combines with the periodic gain amplification of the gain fiber to achieve tunable high repetition rate wideband OFCs.First,the working optical path of the system is described in detail.The characteristics of single frequency laser generated from this system are analyzed from four aspects:signal-to-noise ratio,thermo-optic effect,power conversion efficiency and coupling condition.Then,OFCs with different pump power,different microsphere size and different coupling conditions are obtained;and the reasons for the formation of OFC with multi-mode family and variable repetition rate are analyzed.According to the characteristics of this dual-cavity system,an extension scheme for generating OFC in 2 ?m waveband is proposed.Finally,some methods for optimizing the conversion efficiency,stability and adjustability of the system are described.With the growing demand for communication capacity and speed,microcavity-based OFCs can meet the requirements of multi-wavelength laser source or ultrashort pulse laser source well.