Parameters optimization of FLEET optical system

Wang Jianxin; Chen Shuang; Chen Li; Yang Wenbin; Qiu Rong; Bai Fuzhong; Li Tonghong

doi:10.12086/oee.2022.210318

Article navigation > Opto-Electronic Engineering > 2022 Vol. 49 > No. 4 > 210318

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Wang J X, Chen S, Chen L, et al. Parameters optimization of FLEET optical system[J]. Opto-Electron Eng, 2022, 49(4): 210318. doi: 10.12086/oee.2022.210318

Citation:

Wang J X, Chen S, Chen L, et al. Parameters optimization of FLEET optical system[J]. Opto-Electron Eng, 2022, 49(4): 210318. doi: 10.12086/oee.2022.210318

Parameters optimization of FLEET optical system

1.
Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China
2.
Sichuan Civil-military Integration Institute, Mianyang, Sichuan 621010, China
3.
Facility Design and Instrumentation Institute, China Aerodynamic Research and Development Center, Mianyang, Sichuan 621000, China
4.
School of Mechanical Engineering, Inner Mongolia University of Technology, Huhhot, Inner Mongolia 010051, China
5.
National Center of Quality Inspection and Testing on Diamond Tools, Ezhou, Hubei 436000, China

Fund Project: National Natural Science Foundation of China (11902277, 62165011, 51765054)

More Information

Corresponding authors: wangjianxinkitty@163.com; chenli_03@163.com; fzbaiim@163.com

Received Date 30 September 2021

Revised Date 14 January 2022

Published Date 25 April 2022

Abstract

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.
- FLEET /
- velocity measurement /
- fluorescent lifetime /
- image processing

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References

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Overview

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.