A coherent random-modulated continuous-wave (RMCW) LiDAR transmits a lightwave modulated by a pseudo-random binary sequence (PRBS). The lightwave backscattered from targets is received and used to reconstruct the PRBS. Then, the time-of-flight is extracted by correlating the reconstructed PRBS and the original PRBS. We propose a coherent RMCW LiDAR based on phase-coded subcarrier modulation, in which the impacts of internal reflection and optical Doppler frequency shift (DFS) are mitigated. A continuous lightwave is amplitude-modulated by an RF signal which is phase-coded with a PRBS. Coherent detection is used in the receiver. A beat signal that consisted of a low-frequency signal and a high-frequency signal is obtained by a single balanced photodetector (BPD). The optical DFS can be directly extracted from the low-frequency signal. It is used to compensate for the frequency offset of PRBS, which is extracted from the high-frequency signal. In addition, the background noise caused by internal reflection is suppressed by averaging over successive measurement spots. In this paper, the performance of a coherent RMCW LiDAR is firstly analyzed by numeric simulations and demonstration experiments. Then, line-scanning measurements for moving targets are implemented to demonstrate the 3D imaging capability of the proposed coherent RMCW LiDAR.

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Distributed RF systems are referred to as the fundamental structure for future wireless communication and sensors. To enable wideband coherent operation, synchronization across the distributed systems with massive parallel absolute time delay (ATD) measurements up to sub‐picosecond precision should be implemented. Traditional ways to achieve precise ATD measurements usually rely on probe signals with broad bandwidth, while that of RF systems is limited. Although an all‐optical solution has high precision, it is complex and not compatible with RF systems. Microwave photonics, which combines the merits of RF and photonics, potentially provides a low‐cost and high‐precision solution that can be deployed in existing RF systems. Here, ATD measurement with sub‐picosecond precision directly over an existing radio over free‐space optical (RoFSO) link, requiring no additional hardware beyond the link, is achieved. The bandwidth of the system is only 10 MHz, while the measurement results meet well with those obtained by optical combs. The measurement errors caused by atmospheric turbulence are well suppressed by a dynamic Kalman filter. The RoFSO links can be stabilized in a closed‐loop measurement. The standard deviation of the timing jitter is 0.28 ps, and the Allan variance is around 1.9 × 10−19 @ 1000 s, which is sufficient to synchronize millimeter‐wave signals.

In order to investigate the dynamics and vortex shedding of flexible supersonic canopies, a compressible permeability model combined with fabric structure parameters is proposed, and the periodic oscillation of the supersonic parachute which is referred to as breathing phenomenon is simulated based on the Arbitrary Langrangian Eulerian (ALE) method. The calculated results by new permeability model are consistent with the experimental results. The underlying mechanism of canopy breathing motion is then investigated. Moreover, the influence of canopy permeability on breathing phenomenon of supersonic parachute is analyzed. The results indicate that the periodic growth and shedding of the canopy vortex causes the variation of the pressure differential, which finally lead to the periodic oscillation of the canopy. With the increase of fabric permeability, the vortex rolled up from the canopy skirt move backward and become more slender. The influence of vortex shedding on canopy breathing motion weakened. Those lead to the decrease of the average value of canopy projected area and parachute dynamic load. So are the oscillation amplitude and frequency. The parachute deceleration performance decreases while the parachute swing angle decreases as the canopy permeability increases.

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