Research On Efficient Third Harmonic Conversion In Microfibers

Nonlinear frequency conversion can obtain the optical signal of new frequency through nonlinear effect,while third harmonic generation(THG)can convert the existing light into output signal of triple frequency.The scheme of crystal is relatively mature,providing high conversion efficiency but strict incident conditions.The scheme of integrated optical waveguide has a simple waveguide structure but lower conversion efficiency.Microfiber(MF)has attracted the attention of researchers because of its large evanescent field area,high nonlinear coefficient,low optical loss,and easy access to fiber systems.The third-order nonlinear effect can be used to third harmonic conversion in mcrofiber.To realize the practical application of this scheme,it is necessary to improve the efficiency.In this thesis,third harmonic generation in microfiber is studied in detail.Based on the coupled mode equations,the main factors influencing THG output efficiency are analyzed.On this basis,a THG enhancement scheme with cascaded microfiber is proposed.The enhancement characteristics and main factors of this structure are analyzed by analytical model and numerical simulation.The research results in this thesis provide a novel idea for efficient third harmonic conversion.The specific research content has two parts below:1.The basic scheme of third harmonic generation in microfiber is studied,such as the establishment of coupled mode equations,analysis of several main factors that affect the THG efficiency,systematical illustration of the design method related to phase matching diameter.In this chapter,a microfiber random roughness model of Gaussian distribution is established.The effect of random roughness and adaptive adjustment of pump power are also analyzed,finding that there is a optimal pump power that makes third harmonic efficiency peak.2.A THG enhancement scheme using cascaded microfibers is proposed.The cascaded structure of microfibers are designed,with the guiding fiber selected.The analytical model describing the THG enhancement of cascaded structure is derived.Comparing the numerical simulation results with analytical results,the main factors affecting THG output efficiency are analyzed.The efficiency tolerance of preparation errors is researched.The influence of other factors in cascaded structure is also studied.At last,the specific procedure and advantages of cascaded microfiber structure are explained.The simulation results show that interference enhancement effect of cascaded structure can improve the THG efficiency,with low sensitivity to random roughness and good tolerance for preparation errors.