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Optimization of optical limiting devices based on excited-state absorption

机译：基于激发态吸收的光学限幅装置的优化

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Limiting devices protect sensitive optical elements from laser-induced damage (LID). Passive devices use focusing optics to concentrate the light through a nonlinear optical (NLO) element (or elements) to reduce the limiting threshold. Unfortunately, these NLO elements may themselves undergo LID for high inputs, restricting the useful dynamic range (DR). Recently, efforts at optimizing this DR have focused on distributing the NLO material along the propagation path z of a focused beam, resulting in different portions of the device (in z) exhibiting NLO response at different inputs. For example, nonlinear absorbers closer to the lens, i.e., upstream, protect device elements downstream near the focal plane. This results in an undesirable increase in the threshold, although the lowest threshold is always obtained with the final element at focus. Thus there is a compromise between DR and threshold. This compromise is determined by the material. We concentrate on reverse saturable absorber (RSA) materials (molecules exhibiting larger excited-state than ground-state absorption). We look at both tandem devices and devices in which the concentration of the NLO material is allowed to spatially vary in z. These latter devices require solid-state hosts. The damage threshold of currently available solid-state hosts is too low to allow known RSA materials to reach their maximum absorption, which occurs when all molecules are in their excited state. This is demonstrated by approximate analytical methods as well as by a full numerical solution of the nonlinear wave propagation equation over extremely large distances in z (up to 10(3) Z(0), where Z(0) is the Rayleigh range of the focused beam). The numerical calculations, based on a one-dimensional fast Fourier transform, indicate that proper inclusion of diffraction reduces the effectiveness of reverse saturable absorption for limiting, sometimes by more than a factor of 10. Liquid-based devices have higher damage thresholds (damage occurs to the cuvette wall) and, thus, larger nonlinear absorption. However, RSA material in liquid hosts may suffer from larger thermal lensing. (C) 1997 Optical Society of America.
机译：限位装置可保护敏感的光学元件免受激光引起的损坏（LID）。无源设备使用聚焦光学器件将光线集中通过非线性光学（NLO）元件，以减小极限阈值。不幸的是，这些NLO元素本身可能会接受高输入的LID，从而限制了有用的动态范围（DR）。最近，优化此DR的努力集中在沿着聚焦光束的传播路径z分布NLO材料，从而导致设备的不同部分（z中）在不同的输入端显示NLO响应。例如，更靠近透镜，即上游的非线性吸收器保护靠近焦平面的下游的器件元件。尽管总是在聚焦最终元素的同时获得最低阈值，但这会导致阈值出现不希望的增加。因此，DR和阈值之间存在折衷。这种折衷由材料决定。我们专注于反向饱和吸收器（RSA）材料（分子显示出比基态吸收更大的激发态）。我们研究了串联装置和允许NLO材料浓度在z空间上变化的装置。后面这些设备需要固态主机。当前可用的固态宿主的损伤阈值太低，无法使已知的RSA材料达到其最大吸收率，这在所有分子均处于其激发态时发生。这可以通过近似分析方法以及非线性波传播方程在z上的极大距离（最多10（3）Z（0），其中Z（0）是波的瑞利范围）的完整数值解得到证明。聚焦光束）。基于一维快速傅立叶变换的数值计算表明，适当包含衍射会降低反向饱和吸收限制的有效性，有时会降低10倍以上。液基设备的损坏阈值更高（发生损坏）到比色杯壁），从而产生更大的非线性吸收。但是，液体基质中的RSA材料可能会受到较大的热透镜作用。 （C）1997年美国眼镜学会。

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