The broadband phased-array system based on traditional phaser shifters is troubled by a phenomenon called “beam squint” [1]. Hence, true time delay (TTD) techniques are widely used to solve such a problem, and thus achieving a larger operational bandwidth in microwave phased-array antenna systems. In general, TTD technology can be categorized into electrical true time delay (ETTD) technology [2] and optical true time delay (OTTD) technology [3]. OTTD typically has a larger operating bandwidth, which enables faster communication rates. Therefore, OTTD is an ideal choice in many broadband systems. In addition, OTTD finds wide application in the development of microwave photonics technology, such as tunable high-resolution microwave filters, and beamforming in phased-array antennas [4,5].

The OTTD realized by a cascaded MZI structure based on the optical switches has been widely researched recently [[6], [7], [8], [9], [10], [11], [12], [13], [14]]. The reasons lie in two aspects. On one hand, reconfigurable true time delay lines (RTTDL) can be implemented in chip form, typically with total delays in the nanosecond range and very short waveguide lengths. Due to the very short waveguide lengths inside the chip, the effect of dispersion can be negligible within the common C-band. Hence this kind of OTTD has a far wider bandwidth compared with that based on stimulated Brillouin scattering or ring resonators [[15], [16], [17], [18]]. On the other hand, the utilization of optical switches can provide a more remarkable reconfigurable ability compared with that relying on the Bragg grating or multicore photonic crystal fiber. However, this kind of RTTDL can only provide discrete delay increments, which depend on the states of the optical switches. Therefore, researchers have thought of using a ring resonator to provide a small and continuous delay to compensate for the delay gaps in RBTTDL [9]. This method involves cascading RBTTDL with micro-ring resonators, thereby integrating two distinct delay structures on the same chip. However, although using a ring resonator can provide a small but continuously adjustable delay, it will impose its detrimental impact on the broadband performance of RBTTDL. While the greatest advantage of RBTTDL lies in its broadband characteristics, this combined approach needs to be improved. One improvement is to cascade two micro-ring resonators behind the RBTTDL [9]. By adjusting the operating wavelengths and wavelength spacing of the two micro-ring resonators, this structure can achieve continuous delay tuning over a relative wider frequency range. The collaboration of two micro-ring resonators can extend the operational bandwidth, but it comes at the cost of higher power consumption and more complex control algorithms. Moreover, ensuring these two micro-ring resonators provide precise delay poses a significant challenge in practical applications.

In 2022, we proposed a continuously adjustable delay scheme based on interference principles at the backend of the traditional RBTTDL structure [10]. Similarly, there have also been some delay structures based on interference using MZI switches [[11], [12], [13], [14]]. The active interference with MZIs used in all these schemes is to achieve the continuous delay functionality. In our attempt, we use a 3 dB coupler instead of an MZI at the rear end of the former interference structure, to avoid the complexity of control and the delay error introduced by inaccurate allocation ratio of the MZI. With the help of the pair of PDs behind the last optical switch, arbitrary interference instead of optical path switching can be easily realized by adjusting the allocation ratio. The front traditional RBTTDL structure enables the delay to exponentially increase with the number of MZI switches. Theoretical analysis and simulation calculations have been carried out to discuss the capability of continuous delay.

In this paper, a self-designed TTDL chip is used to experimentally demonstrate the feasibility of continuous delay using the proposed scheme. The broadband performance and the methods for achieving continuous delay are also discussed. The rest of the paper is organized as follows: In Section 2, the continuously adjustable delay scheme for the binary RTTDL is analyzed with the performance calculated. In Section 3, the experimental performance of the chip is tested based on this scheme, and comparison is carried out with the theoretical one. Finally, the conclusions are drawn in Section 4.