Long-lasting emission from femtosecond excitation of nitrogen-based flows shows promise as a useful mechanism for a molecular tagging velocimetry instrument. The technique, known as femtosecond laser electronic excitation tagging (FLEET), was invented at Princeton a decade ago and has quickly been adopted and used in a variety of high-speed ground test flow facilities. The short temporal scales offered by femtosecond amplifiers permit nonresonant multiphoton excitation, dissociation, and weak ionization of a gaseous medium near the beam's focus without the generation of a laser spark observed with nanosecond systems. Gated, intensified imaging of the resulting emission enables the tracking of tagged molecules, thereby measuring one to three components of velocity. Effects of local heating and acoustic disturbances can be mitigated with the selection of a shorter-wavelength excitation source. This review surveys the development of FLEET over the decade since its inception, as it has been implemented in several test facilities to make accurate, precise, and seedless velocimetry measurements for studying complex high-speed flows.

中文翻译：

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用于空气动力学的 FLEET 测速

氮基流的飞秒激发产生的持久发射有望成为分子标记测速仪器的有用机制。该技术被称为飞秒激光电子激励标签（FLEET），十年前由普林斯顿大学发明，并迅速被采用并用于各种高速地面测试流设施。飞秒放大器提供的短时间尺度允许光束焦点附近气体介质的非共振多光子激发、解离和弱电离，而不会产生纳秒系统观察到的激光火花。对所产生的发射进行门控强化成像，能够跟踪标记分子，从而测量速度的一到三个分量。通过选择较短波长的激励源可以减轻局部加热和声扰的影响。这篇综述调查了 FLEET 自成立以来十年来的发展，因为它已在多个测试设施中实施，以进行准确、精确和无种子测速测量，以研究复杂的高速流动。