Jain Ayush, Parajuli Pradeep, Wang Yejun, Kulatilaka Waruna D
Opt Lett. 2020 Sep 1;45(17):4690-4693. doi: 10.1364/OL.400930.
Ultra-short optical pulses in the ultraviolet (UV) region are of significant interest for combustion and reacting flow diagnostics, as most important chemical species have electronic resonance transitions in the UV region. Optical parametric amplifiers are typically used for frequency conversion of femtosecond (fs) pulses from near-IR to UV; however, their implementation for practical imaging applications is limited because of the low conversion efficiency and extreme sensitivity to ambient conditions. In this work, we report the implementation of direct-frequency-tripled, fs laser pulses from a tunable amplified laser system for high-resolution imaging of hydroxyl (OH) radical in flames. The fundamental laser output near 850 nm is frequency tripled to obtain approximately 283.3-nm UV radiation. OH planar laser-induced fluorescence (PLIF) imaging at 1 kHz is demonstrated in turbulent flames with image sheet heights in excess of 45 mm and a signal-to-noise ratio better than 25. These results represent over 3× increase in the imaging dimensionality compared to traditional OPA-based systems. Additionally, the third-harmonic generation apparatus is compact, robust, and easy to operate while providing near-Gaussian beam profiles. Simple power scaling suggests another factor of 3 or more increase in sheet height can be achieved for kilohertz-rate practical combustion diagnostics applications.
紫外（UV）区域的超短光脉冲对于燃烧和反应流诊断具有重要意义，因为大多数重要的化学物质在紫外区域都有电子共振跃迁。光学参量放大器通常用于将飞秒（fs）脉冲从近红外频率转换到紫外；然而，由于其低转换效率和对环境条件的极端敏感性，它们在实际成像应用中的实现受到限制。在这项工作中，我们报告了一种来自可调谐放大激光系统的直接三倍频飞秒激光脉冲用于火焰中羟基（OH）自由基高分辨率成像的实现情况。接近850 nm的基频激光输出被三倍频以获得约283.3 nm的紫外辐射。在湍流火焰中演示了1 kHz频率下的OH平面激光诱导荧光（PLIF）成像，图像片高度超过45 mm，信噪比优于25。与传统的基于光学参量放大器的系统相比，这些结果表明成像维度提高了3倍以上。此外，三次谐波产生装置紧凑、坚固且易于操作，同时提供近高斯光束轮廓。简单的功率缩放表明，对于千赫兹速率的实际燃烧诊断应用，片高度可以再提高3倍或更多。
