A frequency-modulated continuous-wave laser ranging method using low-duty-cycle linear-frequency-modulated (LFM) signals is proposed. A spectrum consisting of a dense Kronecker comb is obtained so that the frequency of the beat signal can be measured with finer resolution. Since the dense comb is provided, super-resolved laser ranging can be achieved using a single-parametric frequency estimation method. Therefore, the run times of the estimation are reduced which promises real-time applications. A proof-of-concept experiment is carried out, in which an LFM signal with a bandwidth of 5 GHz and a duration of 1 µs is used. The duty-cycle of the LFM signal is 10%. The time delay of a scanning variable optical delay line is obtained in real time from the frequency of the highest comb tooth, of which the measurement resolution is 20 ps. Moreover, a single-parametric nonlinear least squares method is used to fit the envelope so that the time delay can be estimated with super-resolution. The standard deviation of the estimation displacements is 2.3 ps, which is 87 times finer than the bandwidth-limited resolution (200 ps). Therefore, the variation of the time delay can be precisely monitored. The proposed method may be used to achieve real-time high-resolution laser ranging with low-speed electronic devices.

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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.

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