Zhiwei Dong

Visualization study on vortex structure over an airfoil using planar laser induced fluorescence

Abstract The research discussed the application of planar laser-induced fluorescence technique for visualizing the flow over an airfoil at different Reynolds numbers and angles of attack. The proposed arrangement allows visualizing the development process of the flow field structure around the airfoil. The focus of the study is the influence of the angles of attack and the Reynolds number on the separation boundary layer, which causes the wake vortex or separation bubble. Compared with the traditional visualization technology, the technology of planar laser-induced fluorescence has better time resolution (50 ns) and followability (Reynolds number = 219,500). In order to achieve high-speed flow field visualization, planar laser-induced fluorescence provides a reliable means of measurement to solve the related engineering problems.

Research of elevation error based on points cloud data of stripe principle LiDAR

Waveform sampling LiDAR is a hot topic in LiDAR technique due to its high precision geodesy and multi-layer target detection ability. Especially, the LiDAR systems applying streak tube have encouraging application due to their special properties about high-sensitivity and full waveform sampling ability. This paper describes a kind of LiDAR system applying the full waveform sampling stripe principle array. Basing on the planar fitting of square object, the elevation error of points cloud got from flight experiment is analyzed. The statistical properties of elevation error are got.

100 kHz, 3.1 ns, 1.89 J cavity-dumped burst-mode Nd:YAG MOPA laser

We demonstrated a cavity-dumped burst-mode 1.06 μm side-pumped Nd:YAG laser and its dual-stage dual-pass amplified laser performance. The cavity dumping process has been theoretically studied and the output performance has been experimentally investigated. At the pumping duration of 2 ms and pumping frequency of 10 Hz, burst energy, peak power and pulse width of the amplified laser reached 1.89 J, 2.87 MW and 3.1 ± 0.3 ns, respectively, at the Q-switch repetition rate of 100 kHz. The maximum energy extraction efficiency reaches to 30%.

2.4 THz/s continuously linearly frequency-modulated Nd:YVO 4 laser

We demonstrate a linearly frequency-modulated laser from a laser diode (LD) pumped Nd:YVO4 laser. A fast frequency tuning of 2.40 THz/s with a tuning range of 6 GHz is achieved in LD pumped Nd:YVO4 1064 nm laser by using RbTiOPO4 (RTP) crystals as the frequency modulator. The continuous tuning range is more than 3 times the longitudinal mode spacing of the resonator. The maximum output power of frequency-modulated laser reaches 160 mW at 1064 nm. Linewidth of the single frequency laser is measured to be 190 kHz by a delay self-heterodyne interferometer. A deviation lower than 60 MHz is obtained during linear modulation.

Depth resolution improvement of streak tube imaging lidar using optimal signal width

Abstract. Streak tube imaging lidar (STIL) is an active imaging system that has a high depth resolution with the use of a pulsed laser transmitter and streak tube receiver to produce three-dimensional (3-D) range images. This work investigates the optimal signal width of the lidar system, which is helpful to improve the depth resolution based on the centroid algorithm. Theoretical analysis indicates that the signal width has a significant effect on the depth resolution and the optimal signal width can be determined for a given STIL system, which is verified by both the simulation and experimental results. An indoor experiment with a planar target was carried out to validate the relation that the range error decreases first and then increases with the signal width, resulting in an optimal signal width of 8.6 pixels. Finer 3-D range images of a cartoon model were acquired by using the optimal signal width and a minimum range error of 5.5 mm was achieved in a daylight environment.

High-repetition-rate, high-peak-power passively Q-switched ceramic Nd:YAG 946 nm laser

We demonstrate a high-repetition-rate, high-peak-power diode-pumped passively Q-switched ceramic Nd:YAG quasi-three-level laser at 946 nm. The maximum peak power reaches 12.4 kW with a pulse width of 13.0 ns at 3.3 kHz.

Effect factors of temperature measurements by femtosecond time-resolved CARS

Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy is utilized to measure the premixed methane/oxygen/nitrogen flame temperature at atmospheric-pressure. The procedure for fitting theoretical spectra to experimental spectra is explained. The experimental results show good agreements with theoretical ones and present a good repeatability. Laser parameters are very important for accurate temperature measurements. The effects of laser parameters on temperature measurements are discussed. Laser parameters in our discussion are shown as follows. Laser pulse shape is hyperbolic secant and Lorentz, respectively. The delay time between the pump and Stokes is from -40 fs to +40 fs. The central wavelength of the pump/probe pulses is from 650 nm to 700 nm. Pulse duration is from 40 fs to 120 fs. In 2000 K, variations of delay time between the pump and Stokes pulses lead to less than 5% error and while variations of the other three parameters lead to less than 1.5% error. Timing jitter is added to the pump/probe pulses and Stokes pulses. In 2000 K, the results indicate that timing jitter of 10% lead to less than 2% error for temperature measurements. In the higher temperature measurement, the impact of laser parameters’ error is greater.

Measurements of flame temperature by femtosecond CARS

Time-resolved femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS) is utilized to measure the methane/oxygen/nitrogen flame temperature at atmospheric-pressure. The measurements are performed using the CARS signal of N2 with 40-fs laser pulses in the first few picoseconds after the initial in-phase excitation. Flame temperatures at 300 and 1 325 K are measured, the experimental results show good agreements with theoretical ones and present a good repeatability.