Schmidt Jacob B, Schaefer Zane D, Meyer Terrence R, Roy Sukesh, Danczyk Stephen A, Gord James R
Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, USA.
Appl Opt. 2009 Feb 1;48(4):B137-44. doi: 10.1364/ao.48.00b137.
Time-gated ballistic-photon imaging is a form of shadowgraphy in which an ultrashort, optical-Kerr-effect (order 2 ps) time gate is used to enhance the relative intensity of ballistic versus multiply scattered photons. In the current work, this technique is adapted for what is believed to be the first time for use in the moderately dense environment (optical density approximately 1.5 to 2) of a high-speed 5 to 15 mm diameter rocket spray to improve image contrast and observe liquid-breakup phenomena. Unlike coherence gating, which is another form of ballistic imaging, the time-gating approach allows sufficient signal levels from ballistic and near-ballistic photons to enable time-resolved single-shot imaging. Direct comparisons with non-time-gated shadowgraphy indicate that the two techniques are sensitive to different features of the flowfield, with regions composed of a dense field of droplets being highly attenuated in conventional shadowgrams but appearing transparent to ballistic photons. This enables significant image contrast enhancement (approximately 6.6:1) of liquid-core structures and facilitates improved understanding of the primary and secondary breakup processes in sprays of moderate optical density.
时间选通弹道光子成像技术是一种阴影成像形式，其中使用超短的光学克尔效应（约2皮秒）时间选通门来增强弹道光子与多次散射光子的相对强度。在当前工作中，该技术首次被应用于直径5至15毫米的高速火箭喷雾的中等密度环境（光密度约为1.5至2），以提高图像对比度并观察液体破碎现象。与弹道成像的另一种形式——相干选通不同，时间选通方法允许来自弹道光子和近弹道光子的足够信号水平，从而实现时间分辨单次成像。与非时间选通阴影成像的直接比较表明，这两种技术对流场的不同特征敏感，由密集液滴场组成的区域在传统阴影图中高度衰减，但对弹道光子而言却显得透明。这使得液核结构的图像对比度显著增强（约6.6:1），并有助于更好地理解中等光密度喷雾中的一次和二次破碎过程。
