Design of an on-chip optical beamforming network based on dense wavelength division multiplexing

The optical beamforming network (OBFN) is a promising alternative of traditional electronic phase shifter due to the advantage of eliminating the so-called beam squint behavior. Herein, a series of optical true time delay lines (OTTDLs) based on silicon on insulator (SOI) platform are proposed to produce OBFNs with multi-beam steering characteristics. A series of OBFN chips is designed to realize 34 specific delay steps between -21~+21 ps in order to achieve a nominal equal difference beam steering angle between ±45°. To pursue good beam quality, 32 delay waveguide channels are proposed according to the number of array elements. The optical carriers in each channel are densely multiplexed off-chip with a frequency difference of 100 GHz in C-band, and coupled on-chip by a grating coupler. Variable optical attenuators (VOAs) based on Mach-Zehnder interferometer (MZI) structure are introduced to tune the optical outputs of the 32 channels with a discrepancy less than ±1 dB. The OTTDLs are compatible with CMOS process, and appeared in a compact footprint of only 0.163 mm² for a maximum true time delay of 651 ps. Such a compact waveguide layout leads to the footprint of a single chip within 2.5 mm×7.5 mm. Meanwhile, the operating temperature is under closed-loop control via a pair of thermoelectric cooler (TEC) and negative temperature coefficient thermistor (NTC). Together with hermetic package, the reliability of the OBFN chips can also be promoted.