Feedback interferometry with integrated 260 GHz BiCMOS emitter

Compact terahertz emitters are highly regarded devices for a wide range of applications. In this work, we present a study on the effect of feedback interferometry which manifests itself in devices implemented in a 130 nm SiGe BiCMOS integrated-circuit technology. The emitters are based on a voltage-controlled oscillator (VCO) that employs a differential Colpitts configuration with optimized emission frequency at fundamental harmonic. The radiation is outcoupled through the substrate side using a hyper-hemispheric silicon lens. The source emits up to 0.3 mW of propagating power, which is tuneable in the frequency range from 258 to 262 GHz. When the part of radiation gets reflected from an arbitrary object located in the beam path and couples back into the oscillator, it interferes and produces a self-mixing current which in magnitude can exceed several percent compared with the bias current. Produced current change can be measured either in the dc regime or by employing signal modulation techniques. Furthermore, the magnitude of self-mixing strongly depends on the selected bias conditions. Finally, we demonstrate the applicability of the self-mixing effect to perform coherent reflection-type imaging.