Pulsed laser scanning design technology under high-speed target detection

  Objective  The tail flame radiation intensity (W/sr) of large high-speed moving targets is about tens of thousands of watts, while the radiation intensity of the target body is hundreds of watts, a difference of two orders of magnitude, and it is difficult for a single infrared imaging to achieve tail flame anti-interference and body identification, which in turn causes deviation in tracking position. In order to solve the accurate detection of the strong tail flame and weak body of high-speed targets, the laser scanning composite mode is introduced to accurately locate the target position according to the tracking deviation. Based on this, in order to make up for the shortcomings caused by the system detection requirements, the design of high-speed and high-precision scanning trajectory schemes has become a crucial part. Since the existing research has not proposed a design method for lidar rose scanning to match high-speed target detection, it is particularly important to design scanning algorithms and study trajectory uniformity and density to achieve high detection probability of targets under these high-speed conditions, which can effectively guide the application of high-speed laser scanning.

  Methods  A single-pulse laser rose scanning system is constructed (Fig.1). The single-pulse laser detection system reduces the beam scattering angle (not more than 360 μrad) and increases the peak power of the transmission to achieve a detection distance of more than 4 km at a high altitude (APD detector sensitivity is better than 3×10⁻⁸ W), and achieve a detection response time of more than 2 s under high-speed movement of the target (Fig.2). At the same time, in order to make up for the lack of laser detection field of view caused by the smaller beam scattering angle, the scanning field is expanded by scanning, the piezoelectric method is used to achieve sufficient resonant frequency, the effective detection area of 2°×2° is completed, the pulse frequency and scanning algorithm are designed, and the density and uniformity of the pulse point and target opening angle are evaluated (Fig.3) to achieve target search with a large field of view and no less than 90% of the target detection efficiency (Fig.4). Blind-spot-to-detectable-area distance (BDD) is introduced to calculate its maximum value and rms to visually measure the size, uniformity of the continuous blind zone and proportion of the detection area (Fig.5), and realize the optimal design of the rose line scanning mode.

  Results and Discussions   Probing design simulation is used to calculate the optimal sweep parameters through experiments. The scanning trajectory f₁=500 Hz, f₂=450 Hz, the peak power of laser single pulse emission is 40 kW, the repetition rate is 40 kHz, and the single field of view covers 20 rose leaves and 800 pulse points. The scanning closed-loop bandwidth is 1000 Hz, the field center repetition rate can reach 1000 times per second, and there are at least 50 field refresh cycles per second (Fig.6). In the optimal scanning mode, the BDD statistic is the smallest, the continuous dead zone is the smallest, and most of the blind spots are in the peripheral area of the detection field of view (Fig.7-8). Under a single field of view update, 100% detection probability feedback can be achieved, the scanning point distribution is uniform, the maximum proportion of detectable area is 55.34%, and the minimum BDD is 1.26 (Tab.1), which has the ability to reliably detect high-speed targets.

  Conclusions  In this study, a laser pulse long-range, high-frequency, high-probability scanning application system is designed, and the target detection system model and the optimal scanning algorithm scheme of laser pulse matching high-speed target detection are optimized. High peak power and small laser beam scattering angle are designed to achieve active laser long-distance detection at a high altitude of more than 4 km. The highly dynamic piezoelectric scanning mechanism stroke is designed to expand the laser detection field of view by more than 2°. Research on high-speed pulse frequency and sweep trajectory algorithms is conducted. The density, uniformity and coverage of the laser pulse point opening angle and the target opening angle in the scanning projection field of view are evaluate. A scanning closed-loop bandwidth of not less than 1000 Hz and a field of view refresh rate of 50 Hz (matching 20 ms image cycle) are achieved. The 100% detection probability feedback under a single field of view refresh of high-speed targets is realized. The detectable area under the condition of breaking through the limit detection (head-on or tail chase) accounts for more than 55%, the quantitative evaluation of BDD index was carried out, which achieved the purpose of accurately aiming at the target body, completed the real position and angle feedback of the target, and verified the achievability and good detection performance of high-speed target detection.