| Supercontinuum(SC)is a kind of spectrum that narrow-band ultrashort laser pulse spreads in the nonlinear optical medium,through the combination of dispersion and nonlinear optical effects of the medium,making the spectrum of the incident pulse broadened.SC is widely used in optical coherence tomography,frequency combing,frequency metering,air pollution monitoring,disease diagnosis and fluorescence imaging because of its good spatial coherence,high brightness and wide bandwidth.At present,the working wavelength range of SC has expanded from visible light to mid-far infrared,and future supercontinuum sources will be widely used in national defense and people's livelihood.Currently candidates for producing MIR SC light sources include tellurite,fluoride,and chalcogenide glasses(Ch Gs),among which Ch Gs have an and excellent transmission performance from MIR to far-infrared(up to more than 20 ?m)and high nonlinearities(about three orders of magnitude of silica fiber),very low phonon energy coefficient,low two-photon absorption coefficient ?2,high refractive indices(about 2.0-3.5),thus it have become one of the excellent candidate for MIR SC generation.In order to obtain a wide SC broadening,the researchers usually choose to pump at near zero-dispersion wavelength(ZDW)in the anomalous dispersion region,but because the ZDW of the general Ch Gs fiber is >4.5 ?m,the OPA or OPO lasers pumping source must be carried out as pumping source,which will not be conducive to the miniaturization of the supercontinuum source.In addition,since the main mechanism of spectral broadening in the anomalous dispersion region is the soliton effect and the influence of modulation instability(MI),the coherence of the general output supercontinuum is not high.The tapered fiber has the advantages of improving coupling efficiency,increasing nonlinearity and harnessing dispersion.The high coherence SC is mainly achieved by designing the structure of fibers to achieve all-normal dispersion(ANDi),and tapered fibers can achieve this goal more simply.Currently,high coherence broadband SC is generated in Ch Gs tapered fibers and the effects of its structure on SC coherence are rarely reported.In this paper,As2S3 single-mode Ch Gs fiber is prepared,and the ANDi fiber structure is designed by simulation calculation of dispersion,and this structure is realized by taper,then the effects of pump wavelength and pump power on the SC of the ANDi fiber were studied,and the high coherence broadband SC is obtained.The coherence is theoretically demonstrated.And the influence of waist core diameter on the coherence of SC is investigated.The influence of the SC coherence results in the fiber structure that achieves the best balance point of coherence and spectral broadening.In this paper,we first introduce the application of SC in various aspects,and introduces the research progress of SC in recent years,the research content,purpose and significance of this paper.Then,based on the nonlinear Schr?dinger equation,the SC is derived.The main broadening mechanism of nonlinear effects and dispersion involved in the broadening process;then the factors that have great influence on the SC broadening are elaborated,such as fiber dispersion and its measurement method and self-phase modulation(SPM),solitons effect,and Stimulated Raman scattering(SRS);next,the preparation process of Ch Gs glass,fiber preform and tapered fiber is introduced,related performance test methods,self-made taper platform and fiber taper process,the infrared SC generated measurement platform are introduced.Then the optical properties of single-mode As2S3 fiber were measured.Then the fundamental-mode(FM)dispersion of single-mode fiber under different core waist diameters was calculated.The propagation modes of untapered fiber and tapered fiber were analyzed,and the influence of wavelength and different pump power on the SC broadening of the tapered fiber were investigated,the coherence of the output SC of the tapered fiber is simulated,and the coherence of SC under different core diameters is calculated under ANDi;finally,the main research results obtained in this thesis,and explains the shortcomings in the research process and the issues that need to be discussed in depth in the future work. |
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