System-Theoretical Modeling and Analysis of Phase Control in a Photonically Steered Terahertz Phased Array Transmitter

The use of photonic phase shift beamforming is highly attractive in terahertz transmitters due to its seamless integration with terahertz generation by photomixing. By feeding several photodiodes with coherent infrared signals with well-defined phase relationships, highly directed and steerable terahertz radiation can be generated. At the same time, the total radiated power is increased by free-space power combining. Since optical phase modulators are readily available in planar lightwave circuit technology and do not need to scale with RF frequency, planar optic integration is simplified compared to photonic true time delay beamforming. However, phase fluctuations due to thermal and mechanical influences become a major issue if fiber-optic components are used and necessitate the implementation of a phase control circuit. Since direct detection of the terahertz phases is not feasible, an alternative approach is needed. In this paper, a concept for controlling the terahertz phases using a lower-frequency reference tone generated by a second set of infrared lasers is proposed and a simplified system-theoretical model is developed and analyzed.