The pulse detection method obtains distance and speed information by measuring the round-trip time delay of pulsed electromagnetic waves. A balance must be reached between range and velocity resolution for pulse detection. Otherwise, a longer bandwidth pulse is required to overcome the aforementioned limitations of a single pulse. The double-pulse waveform markedly increases the time?bandwidth product (TBP) and has a higher energy than a single pulse with the same ranging ability, which is beneficial for detecting weak signals. The double-pulse system does not require time-frequency domain conversion, which reduces computation time to some extent. Therefore, the double-pulse waveform can also be applied to vibration measurement instruments, three-dimensional coherent imaging, and detection of hard and aerosol targets using Doppler radar. We propose a double-pulse coherent detection system to prevent interference from background light. By establishing theoretical models and conducting experiments, the ranging and velocity measurements in a double-pulse coherent system were optimized. This study is expected to provide a new direction for LIDAR detection and lays a foundation for improving detection accuracy of double-pulse systems.

Theoretical models for ranging and velocity measurements were established to study the effects of pulse interval, width, and period on the performance of double-pulse LIDAR. The low- and high-speed targets can be measured by controlling the time interval between the front and rear pulses of a pulse pair, in theory and simulations. A mathematical model of the relationship between the parameters of the double-pulse system and characteristics of the detected target was constructed through noise analysis. Based on the above parameters, the experiment on the ranging and velocity measurements of the targets in a double-pulse coherent system was optimized by demodulating the phase information of the detected double-pulse signal using the Hilbert transform and Python software.

The influence of micromotion velocity and pulse interval on the double-pulse phase was considered (Fig. 4). The detection of the targets at different speeds corresponds to different pulse intervals. Simulation results show that the pulse interval of double pulse system used for detecting low- and high-speed moving targets is on the order of μs. The positive and negative polarities of the obtained phase differences were identical (Fig. 5). Assuming an object with an angular frequency greater than 500 Hz, the pulse interval of the dual-pulse system for detecting such moving targets would be on the order of ms (Figs. 6, 7). The faster the micromotion speed of the target, the higher the pulse interval of the double-pulse system (Table 1). Theoretically, it has been proven that low- and high-speed targets can be measured by controlling the time interval between the front and back pulses in a pulse pair, which demonstrates the velocity measurement capability of a double-pulse coherent system over a wide speed range. Noise analysis indicates that the variance hardly changes with CNR (carrier-to-noise ratio) at a small value of N (average number of independent waveforms). In general, a continuous-wave seed source with a CNR within the range of 0?5 dB, can provide good measurement accuracy (Fig. 8). When ΔT is in the range of 0.2T_c?1.2T_c, the system has good detection accuracy (Fig. 9). Because of the special principle of double-pulse coherent detection, the signal processing method differs from that of single-pulse and continuous light detection. The time required for signal acquisition varies depending on the loop-length and target speed. The optimized experimental system exhibits high-precision ranging and velocity measurement capabilities. The inversion of the distance and velocity is achieved through the Hilbert transform (Table 2, Table 3). Compared to the double-pulse ranging system at the same distance, the error in the velocity measurement is larger. Overall, the errors in the above experiments are all less than 0.25%, which demonstrates double-pulse coherent detection system owning the good capacity in high-precision ranging and velocity measurements.

In this study, a theoretical model was constructed for ranging and velocity measurements, and noise analysis was conducted for a double-pulse detection system. The effects of the interval between front and rear pulses, pulse period, and width on detection distance and micromotion speed were theoretically determined. The control of detection parameters can be realized by simulations. The experiment focused on the high-precision ranging and velocity measurements of targets in a double-pulse coherent system. The Hilbert transform was used to accurately extract phase information. The experimental and simulation results were in good agreement. The above research not only proves the capability of long-range and high-speed target measurement for a double-pulse system but also demonstrates its feasibility in wide-range high-precision detection.