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Integrated optical beamforming networks (OBFNs) are critical for photonics-assisted wideband microwave phased array antennas. Here, a 1x4 integrated OBFN based on low loss silicon nitride waveguide technology was demonstrated for millimeter wave (mmW) communications. Three cascaded optical ring resonators serve as tunable true time delays (TTDs) for each channel, which overcomes the beam squint issue associated with wideband communication. The tuning of rings was carefully calibrated using the lossless ring delay response theoretical model, and experiments were performed verifying the tuning accuracy. Theoretical simulations were performed to optimize the delay response of the OBFN using a genetic algorithm, which revealed tradeoffs among the delay response flatness, absolute delay value, and TTD bandwidth. Two topologies of the OBFN were theoretically and experimentally compared. A lookup table of the optimized ring parameters was generated, based on which single-delay channels with dynamic tuning ranges of 208.7 ps and 172.4 ps for TTD bandwidths of 6.3 GHz and 8.7 GHz were achieved, which correspond to phase shifts of 37.5π and 31π for a 90-GHz signal, respectively. Moreover, all four channels were tuned to a delay distribution with a differential delay around 4.2 ps, which is equivalent to a 49° beamsteering angle for a 90 GHz half-wavelength dipole antenna array. Using heterodyne upconversion and a single delay path, a 41 GHz mmW signal with 3-Gbps NRZ OOK data modulation was generated and delayed. Future work will focus on higher frequencies into the W-band and on beamsteering experiments.
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