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arXiv 1809.01892physics.optics
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多级系统中的高效率太赫兹波产生

High Efficiency Terahertz Generation in a Multi-Stage System

Lu Wang, Arya Fallahi, Koustuban Ravi, Franz Kärtner

摘要:

我们描述了一个稳定的系统,用于激光驱动的窄带太赫兹产生与高转换效率的周期性极化铌酸锂(PPLN)。在多级太赫兹产生系统中,泵浦脉冲在每个PPLN级之后被回收用于进一步的太赫兹产生。通过将每个阶段中产生的太赫兹辐射耦合出去,避免了额外的吸收,并增加了有效的相互作用长度。在每个阶段的太赫兹和光脉冲的分离是通过适当设计的外耦合器来完成的。为了评估所提出的架构,在一个圆柱对称的几何形状的2-D耦合波方程数值求解使用有限差分法。与不能捕获自聚焦和衍射效应的1-D计算相比,我们的2-D数值方法捕获差频产生、自相位调制、自聚焦、光束衍射、色散和太赫兹吸收的效应。研究发现,通过对每一级泵浦脉冲的色散进行补偿,可以大大提高太赫兹波的产生效率。利用两级系统,我们预测了在$0.3$ THz下,使用1.1 J泵浦激光产生的总转换效率为$η_{\text{total}}=1.6\%$的$17.6$ mJ太赫兹脉冲,其中心光谱为1 $μ$ m.每一级的发电效率都在$0.8\%$以上,外耦合效率在$93.0\%$以上。

英文摘要:

We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion, and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a $17.6$ mJ terahertz pulse with total conversion efficiency $η_{\text{total}}=1.6\%$ at $0.3$ THz using a 1.1 J pump laser with a two-line spectrum centered at 1 $μ$m. The generation efficiency of each stage is above $0.8\%$ with the out-coupling efficiencies above $93.0\%$.

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