Several terahertz-wave radar techniques based on a resonant-tunneling-diode (RTD) oscillator are under investigation.First, an amplitude-modulated continuous-wave (AMCW) radar able to measure the absolute distance was achieved bydemodulating the wave returning from the target and measuring its phase, either by determining the time delay with anoscilloscope, or by using an IQ demodulator to determine the sine and cosine of the phase. Our current record with thistype of radar is a ranging error of 0.063 mm; however, it can only measure the distance up to a single reflecting surface.Second, a subcarrier frequency-modulated continuous-wave (FMCW) radar has been demonstrated to work and is expectedto allow the simultaneous ranging of several targets. The ranging error for this radar was found to be 0.73 mm.Additionally, a radar technique based on the swept source optical coherence tomography (SS-OCT) principle is proposed.Terahertz-wave 2D imaging can be combined with these radar techniques to obtain a 3D imaging system.
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