Absolute Time Delay Measurement Over an Existing Radio Over Free‐Space Optical Link with Sub‐Picosecond Precision

Abstract

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.