| Laser has a wide range of applications in industry,national defense,scientific research,biomedicine and other fields,but different applications have different requirements for laser wavelength.Nonlinear frequency conversion is an important technique to expand laser wavelength,and third harmonic conversion is one of the commonly used methods.In the existing optical third harmonic conversion,the crystal-based scheme has been relatively mature,but it needs to control the incident direction of light to achieve phase matching,the spatial optical path is complex,high integration difficulty,difficult to make the laser source portable;Photonic crystal fiber(PCF)can flexibly adjust the dispersion to meet the phase matching condition of harmonic conversion process,but it is difficult to design and prepare,and it is not easy to connect with conventional fiber devices.Third harmonic conversion can also be achieved by air-clad step-index microfiber,but the strength and stability of the fiber need to be improved,and the conversion efficiency needs to be further improved.A step-index fiber with solid cladding has good mechanical strength,simple structure,easy design and preparation,and is not easily disturbed by external interference.It can be easily connected with conventional optical fiber devices.Through proper design,it is expected to produce stable and efficient third-harmonic conversion devices,and promote the all-fiber frequency conversion technology from laboratory to practical application.In this thesis,the generation law of third harmonic in step-index fiber is studied in detail.The sulfide glass GGSS(15Ge-3Ga-12Sb-70S)with high optical nonlinearity is selected as the core material and telluride glass TZLB(78Te O2-5Zn O-12Li2O-5Bi2O3)as the cladding material,and a special step-index fiber with solid cladding for third harmonic conversion is designed.Optical fiber cascade scheme is used to improve the conversion efficiency and output characteristics of the third harmonic.The main work completed in this thesis is as follows:1)Design of step-index fiber with solid cladding and analysis of its third harmonic conversion law.GGSS was selected as the core material and TZLB was selected as the cladding material.The phase matching conditions of the third harmonic generation in the fiber were analyzed by using the eigenequation,and the diameter of the fiber core was preliminarily determined.The detailed process of third harmonic conversion is studied by coupling mode equations,and the diameter of optical fiber core is optimized.The results show that GGSS?TZLB fiber has large core diameter and tolerance of manufacturing error,which is difficult to fabricate.Moreover,a single long fiber can achieve conversion efficiency of more than 10%by optimizing incident pump power,which can be used to fabricate stable and efficient all-fiber third-harmonic conversion devices.2)Construction and optimization of cascade system composed of step-index fibers with solid cladding.GGSS?TZLB optical fiber was used to construct a cascade system,according to the mathematical model of the optical field by analytical and numerical methods of three harmonic conversion features are analyzed,and found that conversion efficiency mainly depends on the frequency conversion level and duplicate yuan for the length of the fiber,conduction fiber diameter and length will change the conversion efficiency of the main great the pump power of the corresponding interval.GGSS?TZLB fiber has high optical nonlinearity,its cascade system uses lower pump power and shorter fiber to achieve efficient third-harmonic conversion,and can reduce the influence of random roughness of fiber,inhibit other competitive nonlinear effects,so as to improve the regularity and stability of third-harmonic output. |
PDF Full Text Request