Continuously tunable silicon optical true-time delay lines with a large delay tuning range and a low delay fluctuation

On-chip switchable optical true-time delay lines (OTTDLs) feature a large group delay tuning range but suffer from a discrete tuning step. OTTDLs with a large delay tuning range and a continuous tuning capability are highly desired. In this paper, we propose and experimentally demonstrate a silicon-based broadband continuously tunable OTTDL comprising a 7-bit delay line and a switch-based continuously tunable delay line. The group delay of the entire OTTDL can be continuously tuned from 0 to 1020.16 ps. A delay error within -1.27 ps to 1.75 ps, and a delay fluctuation of less than 2.69 ps in the frequency range of 2∼25 GHz are obtained. We analyze the causes of the delay fluctuation and its influence on beamforming. Moreover, we also propose a simplified non-invasive calibration method that can significantly reduce the complexity of the delay state calibration and can be easily extended to delay lines with more stages of optical switches. The high performance of our OTTDL chip and the calibration method drive practical applications of integrated OTTDLs.