FLEET流场测速时，荧光丝的形态和特征影响流场速度测量的精度和覆盖范围，而这些参数又取决于FLEET光学系统参数，有必要对FLEET光学系统参数对荧光丝的影响规律进行研究。本文研究了光学系统的主要参数飞秒激光脉冲能量和聚焦透镜焦距对荧光丝长度、光丝峰值强度、功率密度、信噪比的影响，并在最优实验参数条件下，对不同压强下空气飞秒荧光丝的寿命进行探测。实验表明，飞秒荧光丝的激发存在功率密度阈值，本实验中大致在2×10¹³ W/cm²，光学系统参数优化应以飞秒荧光丝信噪比高且光丝强度分布均匀为基准。飞秒荧光丝的寿命约为几微秒，因此，两次测速采样的时间间隔应小于微秒量级。本文研究结果可以为FLEET光学系统主要参数确定提供依据。

Overview: Femtosecond laser electronic excitation tagging (FLEET) is proposed in 2011 as Molecular tagging velocity measurement technique. FLEET uses one femtosecond laser and one optical detector (usually ICCD) and one signal generator, and works with nitrogen fluorescence as tagging tracer. Its experimental system is simple and no injection problem of tracer particle, hence has a wide application prospect. Fluorescent filament features such as size and intensity distribution are very vital to the velocity measurement accuracy, which are decided by the optical system parameters. Therefore, it is particularly important to study the influence of optical system parameters on it.Using FLEET to measure flow field velocity, the shape and feature of fluorescent filament impact on the accuracy and range of the velocity measurement, and are determined by the parameters of the FLEET optical system. Therefore, it is necessary to study the influence of FLEET optical system parameters on fluorescent filament. In this paper, the influences of main parameters for optical system, namely pulse energy of femtosecond laser and focal length of the focusing lens, on the length of filament, peak intensity, energy density of filament and signal to noise ratio are investigated by experiment. The lifetime of air fluorescent filament under different pressures are measured with the optimum experiment parameters. Experiments show that there is a power density threshold for femtosecond fluorescent filaments excitation, which is about 2×10¹³ W/cm² in this experiment. The optimization of optical system parameters should be based on the high signal-to-noise ratio and uniform intensity distribution of filament. The lifetime of femtosecond fluorescent filaments is about several microseconds. Therefore, the time interval between two velocity measurement samples should be less than microseconds. The results are useful for determining the main parameters of FLEET optical system.