In the field of modern optical communication, radar signal processing and optical sensors, true time delay technology, as a key means of signal processing, can achieve the accurate control of the time delay of optical signals. This study presents a novel design that integrates a 2 × 2 Multi-Mode Interference (MMI) structure with a Mach–Zehnder modulator on a silicon nitride–lithium niobate (SiN-LiNbO₃) heterogeneous integrated optical platform. This configuration enables the selective interruption of optical wave paths. The upper path passes through an ultralow-loss Archimedes’ spiral waveguide delay line made of silicon nitride, where the five spiral structures provide delays of 10 ps, 20 ps, 40 ps, 80 ps, and 160 ps, respectively. In contrast, the lower path is straight through, without introducing an additional delay. By applying an electrical voltage, the state of the SiN-LiNbO₃ switch can be altered, facilitating the switching and reconfiguration of optical paths and ultimately enabling the combination of various delay values. Simulation results demonstrate that the proposed optical true delay line achieves a discrete, adjustable delay ranging from 10 ps to 310 ps with a step size of 10 ps. The delay loss is less than 0.013 dB/ps, the response speed reaches the order of ns, and the 3 dB-EO bandwidth is broader than 67 GHz. In comparison to other optical switches optical true delay lines in terms of the parameters of delay range, minimum adjustable delay, and delay loss, the proposed optical waveguide digital adjustable true delay line, which is based on an optical switch and an Archimedes’ spiral structure, has outstanding advantages in response speed and delay loss.

Electro-optic phase modulation is a core technology for signal processing in optical communication and microwave photonics, yet the current multifunctional integrated devices for high-bandwidth phase modulation and efficient frequency doubling suffer from critical bottlenecks including large crosstalk, excessive chip area, bandwidth-voltage trade-off, and single material performance limitations. To address these issues, this study proposes a Si ₃ N ₄ -TFLN heterogeneous integrated design scheme by synergizing the low-loss property of silicon nitride and the strong electro-optic effect of thin-film lithium niobate. A Dual-Drive Mach-Zehnder Modulator (DD-MZM) and a racetrack Micro-Ring Resonator (MRR) with high quality-factor and large Free Spectral Range (FSR) are cascaded to realize the monolithic integration of high-precision phase modulation and microwave signal frequency doubling. Co-simulation results show that the designed device achieves an electro-optic modulation bandwidth of 72 GHz and a half-wave voltage-length product of 2.8 V · cm, which well reconciles the bandwidth-voltage trade-off. The device exhibits a frequency-doubling efficiency of 15%, 30.4% higher than that of the single TFLN-based MRR. The cascaded coupling structure suppresses the functional crosstalk to −38 dB, and the total chip layout area is only 3.648 mm ² , over 60% smaller than that of the existing similar devices. This work provides a high-performance on-chip solution for high-density integrated systems in optical communication, microwave photonics, and quantum information processing.

To address the contradiction between hardware non-idealities and broadband beam squint effects in microwave photonic beamforming systems, this paper proposes a beamforming algorithm framework based on true-time-delay and phase cooperative control. In this paper, we decouple and analyze the discrete tuning characteristics of optical true-time delay lines and the continuous phase tuning capability of electro-optic phase modulators, a mapping model from device constraints to system performance is established. A three-layer algorithm architecture is proposed, including discrete true-time delay solving, improved delay-and-sum algorithm, and robust MVDR algorithm. On this basis, a true-time-delay and phase cooperative control strategy is constructed, utilizing true-time delay to eliminate beam squint and electro-optic phase modulator to compensate for time-delay quantization residual errors. Simulation results show that the cooperative control reduces the beam pointing error from 0.86° to 0.06°, suppresses the sidelobe level to −31.5 dB, and achieves a squint coefficient lower than 0.01 ° /GHz. This study provides theoretical and algorithmic support for high-precision, low sidelobe, wide-angle scanning of broadband optically controlled phased array systems.

Traditional electronic phased array radars are constrained by electronic bottlenecks, resulting in inherent limitations including large form factor, fixed operational parameters, and narrow instantaneous bandwidth, which fail to meet the stringent requirements of next-generation high-performance radar systems. Optical true time delay (OTTD) technology based on integrated optical waveguides emerges as a core solution for realizing broadband, compact optically controlled beamforming systems. Traditional silicon-based waveguides suffer from severe mode competition (delay jitter > ±0.05 ps), energy leakage (transmission loss > 0.5 dB/cm) and large beamforming angle fluctuation (>0.3°) in OTTD systems, failing to meet the picosecond-level delay accuracy and broadband beam squint-free requirements of next-generation phased array radars. Thus, a customized mode-selective waveguide design for OTTD systems is urgently required. To address these critical challenges, this study proposes an OTTD-customized mode-selective integrated optical waveguide design tailored for OTTD systems, with three distinct innovations: (1) A systematic OTTD-oriented mode classification and selection methodology is established—instead of a conventional single-mode design, the fundamental TE₀ mode is identified as the optimal operating mode through Finite-Difference Time-Domain (FDTD) simulation, (95% TE polarization fraction and 2.0553 effective refractive index at 1548.39 nm, which cannot be achieved by other guided modes for OTTD applications). (2) The wavelength-dependent transmission characteristics of the TE₀ mode are quantitatively characterized, revealing a linear correlation between the effective refractive index (2.05–2.10) and wavelength (1500–1550 nm), alongside a controllable group delay range of 1.4315–1.4395 ps—this precise linear model fills the gap of lacking OTTD-specialized delay calibration theory in conventional waveguide research. (3) An OTTD-optimized practical mode selection criterion for OTTD applications is proposed by modifying the standard guided-vs-leaky condition for asymmetric waveguides: the effective refractive index of the operating mode must exceed the substrate refractive index with a fabrication tolerance margin (n_eff > 1.44 ± 0.02 for SiO₂ substrate) to mitigate leakage and adapt to OTTD picosecond-level delay precision. This criterion is validated through system-level beamforming experiments (rather than only device-level simulation), and the designed waveguide achieves a mode suppression ratio (MSR) of >30 dB for leakage modes and a transmission loss of <0.2 dB/cm, which is significantly superior to conventional single-mode waveguides in OTTD systems. Experimental results indicate that the angle fluctuation of the beamforming system is less than 0.08°, which is significantly superior to the 0.3° fluctuation observed in traditional silicon waveguide OTTD systems. This work provides a technical solution for improving the performance of optical phased array radar and laser radar and has broad engineering application prospects in microwave photonics and optical communication fields.