Terahertz-Wave Homodyne Detection With IQ Demodulation Based on Circular Polarization

Abstract

We propose a homodyne detection method in the terahertz (THz)-wave range to significantly improve the signal-to-noise ratio in imaging and other applications. The THz wave is generated by a resonant-tunneling diode (RTD) oscillator array. We split this wave into RF and local oscillator (LO) beams, and allow the RF beam to propagate to the target. When it returns, it mixes with the LO beam in the detector, achieving homodyne detection. However, since this is equivalent to an interferometric setup, the detected signal strongly depends on the distance to the target. To remove this dependence, we implement an IQ demodulation technique by using a second detector that provides a quadrature signal with respect to the first. This allows measuring both the intensity of the wave returning from the target and the propagation distance, modulo the wavelength. To obtain the pair of quadrature signals, the RF beam is circularly polarized using a quarter-wave plate. We present the preliminary results of this homodyne detection technique, including SNR measurements to demonstrate its noise-reduction effectiveness and an imaging illustration.