| Terahertz parametric sources based on stimulated polariton scattering(SPS)are laser driven terahertz radiation sources with many advantages such as high temporal and spatial coherence,wide frequency tunability and room temperature operation,etc.In the last two decades terahertz parametric sources have become one of popular topics in related area.Although there has been a lot of achievements about terahertz parametric sources in the last few decades,there are still some problems unsolved.1.There are a lot of achievements on theoretical models for terahertz parametric sources,but these models are only for terahertz parametric generators or intracavity-pumped terahertz parametric oscillators,none of them are specifically for extracavity-pumped terahertz parametric oscillators.2.By now the output average power of terahertz parametric source is relatively low.The highest average power reported is 367 μW.The experiment device and parameters are still to be optimized and improved.3.Tunable Stokes lasers based on combinations of stimulated polariton scattering and other nonlinear effects such as stimulated Raman scattering(SRS)are important methods to expand tuning range of near-infrared lasers.Relevant theoretical and experimental investigations on such lasers are valuable.Be directed against the problems above,a series of research are done in this dissertation.1.A theoretical model for extracavity-pumped terahertz parametric oscillators was established.The modelling process contains three steps.(1)Expressing the amplitude of THz wave as a function of amplitudes of the fundamental and Stokes waves.(2)Establishing and numerically solving the rate equation of the Stokes wave.(3)Numerically solving the coupled wave equations of the fundamental and THz waves.The model was used to analyzed an extracavity terahertz parametric oscillator pumped by a Q-switched Nd:YAG laser,and the simulation results basically matched the experiment data,demonstrating the effectiveness of the proposed model.This model can play an important role in the design and optimization of extracavity-pumped Q-switched TPOs.2.Methods to furtherly improve the output power of terahertz parametric sources were investigated,and a terahertz parametric oscillator with milli watt level output power was achieved.Using the model for intracavity TPO,the influences of the fundamental beam size and pulse repetition frequency on the THz output power were calculated,and the results indicate that there are optimal values for the fundamental beam size and pulse repetition frequency.Based on the calculation results,the experiment device was optimized as possible by changing the reflectivity of the Stokes cavity mirror,the curvature of the fundamental cavity mirror,and the pulse repetition frequency.Eventually 0.91 mW average power at 3.92 THz was obtained,and it is the first time for terahertz parametric sources to reach milli watt level of average power.3.The SPS characteristics of KTP crystal was studied,and its parameters in THz region was calculated,including the absorption coefficient and refractive index in THz region,and the parametrical gain coefficient and Raman gain coefficient in SPS process.These results can provide data for relevant studies.4.A theoretical model for SPS-SRS cascaded Stokes lasers was established.This theoretical model is based on the combination of the model for intracavity-pumped TPO and coupled wave equations of SRS.This model can provide theoretical guidance for studies on the corresponding type of cascaded lasers.A tunable SPS-SRS cascaded Stokes laser based on KTP crystal was experimentally investigated.The laser used the SPS-Stokes wave to arouse SRS effect.The tuning range of the laser was 1112.08-1113.64 nm,1114.94-1115.77 nm,1117.371119.92 nm,and the maximum output power was 734 mW(1186.86 nm)when the pump power was 6.0 W and the pulse repetition frequency was 7 kHz.The tuning characteristic,pulse evolutions,and the input-output characteristic of the laser basically matched with the simulation results. |
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