The antenna array technology overcomes the aperture limitation of single antenna detection and improves the microwave detection capability by several or even dozens of times, which is regarded as a revolutionary change in microwave detection technology. Stable transmission of optically carried microwave signals via fiber links enables the long-distance distribution of microwave signals with delay (or phase) stability required in antenna arrays and plays an irreplaceable role in the new generation of microwave measurement technology with multiple antenna coordination. The core issue of this stable transmission technology is the unstable transmission delay due to environmental factors such as temperature variation and physical vibration. The key to this technology lies in how to accurately measure and compensate for the fiber transmission delay variation, thereby ensuring the delay or phase stability of the signal transmitted to the remote end.

Increasing the signal frequency not only improves the measurement accuracy of transmission delay but also weakens the effect of other sources of noise. Therefore, transmission stability can be improved by increasing the signal frequency (Fig. 2). To achieve stable transmission, it is necessary to first detect the phase variation of the high-frequency signal with high sensitivity. Therefore, we propose a phase detection method by dual optical and electrical heterodyne mixing (Fig. 3). The RF signal to be transmitted at the local end after electrical frequency shifting is modulated onto an optical carrier to generate a reference. It is then optically mixed with the signal returned to the local end after optical frequency shifting at the remote end. The resulting intermediate frequency signals are then obtained through a low-speed photodiode. Finally, the two intermediate frequency signals are mixed electrically to obtain the phase variation of the returned signal representing the transmission delay variation. This method significantly improves receiving sensitivity and phase detection precision. High-precision control of the phase of high-frequency microwave signals or the optical delay is another key technique to be addressed for stable transmission. To improve the phase control precision of microwave signals in stable-phase transmission, we put forward a high-frequency signal phase control method based on single-sideband modulation. By adding the phase of the MHz-level intermediate frequency signal to the high-frequency signal via single-sideband modulation, accurate phase control of the high-frequency signal can be achieved by controlling the intermediate frequency signal. To achieve high-precision control of optical delay in stable-time transmission, we adopt a cascaded optical delay control method. This method employs a high-precision (fs-level), small-range (ps-level) piezoelectric fiber stretcher and a medium-precision (ps-level), large-range (ns-level) motorized adjustable delay line in series, achieving remarkable performance in delay compensation accuracy, speed, and range.

Based on the above-mentioned transmission scheme and key technical solutions, we realize phase-stable and time-stable optically carried microwave signal transmission systems. For the phase-stable transmission system, the frequency of the transmitted signal is 100 GHz. Phase detection via dual optical and electrical mixing and phase control via single-sideband modulation are adopted (Figs. 6 and 7). Under 10000 s of averaging, the frequency stability after 100, 120, 140, and 160 km transmission are 1×10^-17, 1.2×10^-17, 4×10^-17, and 6×10^-17 respectively (Fig. 8). It should be noted that the Allan deviation maintains linear decrease with the increasing averaging time, indicating excellent long-term stability of the system. The corresponding root mean square (RMS) value of time jitter is 33, 37, 52.5, and 62 fs respectively. For the time-stable transmission system, the frequency of the transmitted signal is 25.00 GHz. Phase detection via dual optical and electrical mixing and delay control via cascaded optical delay lines are utilized to stabilize fiber transmission delay (Figs. 9 and 10). The phase variation of the 25.00 GHz signal transmitted over 21 km measured by a vector network analyzer is less than 0.09° (RMS) within 3800 s, corresponding to a time error of 10 fs (RMS). Due to the high-precision optical delay control capability, the stability of the transmission system is greatly improved.

Stable transmission of optically carried microwave signals is currently the most effective means for signal synchronization in antenna array systems. After analyzing the relationship between the transmission stability of the optically carried microwave and signal frequency, we propose the idea of increasing the signal frequency for achieving high transmission stability. High-sensitivity phase detection and high-precision phase and delay control have been realized by dual optical and electrical mixing, single-sideband modulation, and cascaded optical delay control. Based on these techniques, we demonstrate a phase-stable and time-stable transmission system. For the phase-stable transmission, the frequency stability of the 100 GHz signal after 160 km transmission reaches 6×10^-17, and the time jitter is 62 fs. For the time-stable transmission, the RMS value of the time jitter is 10 fs within 3800 s at a transmission distance of 21 km. Our study provides an effective technical approach to signal synchronization in multi-antenna cooperative microwave measurements, such as antenna array systems.