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We propose and demonstrate a continuous-wave vector THz imaging system utilizing a photonic generation of two-tone THz signals and self-mixing detection. The proposed system measures amplitude and phase information simultaneously without the local oscillator reference or phase rotation scheme that is required for heterodyne or homodyne detection. In addition, 2π phase ambiguity that occurs when the sample is thicker than the wavelength of THz radiation can be avoided. In this work, THz signal having two frequency components was generated with a uni-traveling-carrier photodiode and electro-optic modulator on the emitter side and detected with a Schottky barrier diode detector used as a self-mixer on the receiver side. The proposed THz vector imaging system exhibited a 50-dB signal to noise ratio and 0.012-rad phase fluctuation with 100-μs integration time at 325-GHz. With the system, we demonstrate two-dimensional THz phase contrast imaging. Considering the recent use of two-dimensional arrays of Schottky barrier diodes as a THz image sensor, the proposed system is greatly advantageous for realizing a real-time THz vector imaging system due to its simple receiver configuration.

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In the phase-shift measurement method, the distance light travels can be obtained via the phase difference between the reference and the measured signals. In this paper, a multiple-step phase demodulation method with time counting is proposed for measuring the phase difference, with resulting robustness against electric noise. When the phase differences are close to 2nπ (n = 0, 1, 2, ...]), 2π ambiguity problems commonly occur; here, a time-counting method using a 180°-shifted reference signal is proposed in order to resolve this problem.