A Novel Interferometric Sub-THz Doppler Radar With a Continuously Oscillating Reference Arm

In this paper, we have built and tested a mixerless sub-terahertz (sub-THz) Doppler radar consisting of just a continuous wave (CW) source and a Schottky diode intensity detector based on optical interferometry technique. The reference arm features an oscillating mirror to modulate the low-frequency-band (LFB) Doppler signature to the high-frequency-band (HFB) centered at the reference arm frequency. The reference arm frequency needs to oscillate at a frequency that is higher than twice the Doppler frequency of the object to avoid overlapping of the LFB and HFB signals. Rigorous mathematical formulas have been derived to solve for both the amplitude and the unambiguous phase of the Doppler signal, by using both LFB and HFB signals. The unwrapped phase can be obtained in two ways: a simply phase unwrapping process and a universal fitting process. The Doppler frequency signature of a moving object can be obtained from the Fourier transform of the phase. Computer simulation was first used to show the validity of the derived mathematical formulas. Then a prototype at 0.15 THz was built and tested using a ball pendulum as target. Experimental scenarios for phase span of less than 2π and greater than 2π were studied. The measured amplitude and phase have been shown to agree well with the set up experimental parameters.