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摘要:
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。
Abstract: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.
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Key words:
- FLEET /
- velocity measurement /
- fluorescent lifetime /
- image processing
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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.
In this paper, the effects of laser pulse energy and focal length are researched on the length, peak intensity, power density and signal-to-noise ratio of fluorescent filament by the experiment, and the threshold laser power of excited air fluorescent filament and the optimal system parameters are given. With optimal optical system parameters, the lifetime of still air fluorescence under 4 different pressures are shown.
The results show that there is a power density threshold for excitation of femtosecond fluorescent filaments. Only when femtosecond laser power is greater than threshold, the fluorescent filaments will be generated near the rear focal plane of the focusing lens. The minimum power density required for femtosecond laser to excite air fluorescent filaments is about 2×1013 W/cm2. When the femtosecond laser energy is constant, the longer the focal length is, the longer the filament and the lower the signal-to-noise ratio are; When the focal length is constant, the length of the fluorescent filament becomes longer with the increase of laser energy, and the central position of the fluorescent filament in the x-axis direction moves toward the focusing lens. Within the parameter range of the experimental study, when the focal length of the focusing lens is 500 mm and the laser energy is 3 mJ, the quality of the fluorescent filament is relatively excellent. The filament has relative uniform intensity distribution and high signal-to-noise ratio. Under these parameters, the lifetime of air fluorescent under four pressures are about few microseconds. Therefore, when the flow field velocity is measured by FLEET, the time interval between two measurement samples should be less than a few microseconds. This research provides the basis for the optimization of optical system parameters in the flow field velocity measurement by FLEET.
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图 3 不同焦距和激光能量下的空气飞秒荧光丝对比图。
Figure 3. Air fluorescent filament images excited by femtosecond laser with different focal length and laser energy.
图 4 飞秒荧光丝的有效区域界定流程。
Figure 4. Process for definition of effective area of fluorescent filament.
图 6 激光功率和聚焦透镜焦距，对飞秒荧光丝的信噪比和光丝长度影响。
Figure 6. Fluorescent filament SNR and length vs laser power and focal length.
图 7 不同压强下，空气飞秒荧光丝归一化峰值强度随延迟时间变化情况
Figure 7. The normalized peak density of air fluorescent filament for different time delay under different pressures
表 1 激光功率和聚焦透镜焦距长度对光丝最强峰值的影响
Table 1. The maximum gray affected by laser power and focal length
Laser energy
/mJFocal length of focus lens/mm 175 300 500 1000 1500 0.5 23759 1216 764 750 无信号 1.0 205599 3840 2375 1950 无信号 1.5 345641 5377 3614 2882 1071 2.0 541298 9239 6504 3711 1265 2.5 575889 13876 7844 4508 1497 3.0 610190 17881 10900 5118 1795 
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表 2 透镜后焦平面处的激光功率密度随激光能量和聚焦透镜焦距变化情况
Table 2. The laser power density at back focal plane with different laser power and focal length/(W/cm2)
Laser energy
/mJFocal length of focus lens/mm 175 300 500 1000 1500 0.5 7.22×1014 2.46×1014 8.84×1014 2.21×1013 9.82×1012 1.0 1.44×1015 4.91×1014 1.77×1014 4.42×1013 1.96×1013 1.5 2.17×1015 7.37×1014 2.65×1014 6.63×1013 2.95×1013 2.0 2.89×1015 9.82×1014 3.54×1014 8.84×1013 3.93×1013 2.5 3.61×1015 1.23×1015 4.42×1014 1.11×1014 4.91×1013 3.0 4.33×1015 1.47×1015 5.30×1014 1.33×1014 5.89×1013
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