Photonic True-Time Delay Beamforming Using a Switch-Controlled Wavelength-Dependent Recirculating Loop

A photonic true-time delay beamforming network to provide tunable progressive time delays for phased array beamforming is proposed and demonstrated. In the proposed system, a microwave signal is modulated on multiwavelength optical carriers and sent to an optical dispersive recirculating loop that contains a linearly chirped fiber Bragg grating (LCFBG) to provide a wavelength-dependent round trip time. As the microwave-modulated optical signals recirculate in the loop, the microwave signals carried by different wavelengths will experience different time delays at the output of the loop. By controlling the number of round trips, tunable progressive true-time delays are achieved, which are required for true-time delay phased array beamforming. The proposed true time delay beamforming network is experimentally evaluated. Two dispersive loops that can introduce a single-round-trip true-time delay difference of 2.5 ns and 160 ps are designed, which are implemented by using two LCFBGs with two different dispersion coefficients of 2500 and 200 ps/nm. The progressive time delays for beam steering achieved by the two dispersive loops can cover a beam scanning range from -60 degrees to 60 degrees.