Broadband 1×8 Optical Beamforming Network Based on Anti-resonant Microring Delay Lines

We demonstrate a 1×8 binary tree optical beamforming network (OBFN) chip for a microwave photonic phased array antenna. The OBFN is formed by cascading multiple microring resonators (MRRs) working at the anti-resonant wavelengths. The binary tree topology reduces the MRR amount by reusing the MRRs in the preceding stage, and the anti-resonant MRRs realize continuous delay tuning with a reduced in-band delay fluctuation. The delay adjustment is achieved by digitally tuned MRRs and one continuously tuned MRR in each stage with an improved operating bandwidth. The random initial states of the MRRs are calibrated to produce a flat delay response. Experimental results show that the maximum delay is 560 ps. The beamforming performance of the OBFN is evaluated in 20 ps and 45 ps delay increments. In an 8-GHz operating bandwidth, the maximum delay fluctuation is less than 7.6 ps. The capability of our delay lines to suppress the delay ripple encountered in conventional on-resonance delay lines opens exciting opportunities for the development of compact integrated broadband OBFNs demanded in broadband microwave phased array antennas.