Optical frequency switching scheme for a high-speed broadband THz measurement system based on the photomixing technique.

This study presents an optical frequency switching scheme for a high-speed broadband terahertz (THz) measurement system based on the photomixing technique. The proposed system can achieve high-speed broadband THz measurements using narrow optical frequency scanning of a tunable laser source combined with a wavelength-switchable laser source. In addition, this scheme can provide a larger output power of an individual THz signal compared with that of a multi-mode THz signal generated by multiple CW laser sources. A swept-source THz tomography system implemented with a two-channel wavelength-switchable laser source achieves a reduced time for acquisition of a point spread function and a higher depth resolution in the same amount of measurement time compared with a system with a single optical source.

Continuous-wave THz vector imaging system utilizing two-tone signal generation and self-mixing detection.

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.

Robust terahertz self-heterodyne system using a phase noise compensation technique.

We propose and demonstrate a robust terahertz self-heterodyne system using a phase noise compensation technique. Conventional terahertz self-heterodyne systems suffer from degraded phase noise performance due to phase noise of the laser sources. The proposed phase noise compensation technique uses an additional photodiode and a simple electric circuit to produce phase noise identical to that observed in the terahertz signal produced by the self-heterodyne system. The phase noise is subsequently subtracted from the terahertz signal produced by the self-heterodyne system using a lock-in amplifier. While the terahertz self-heterodyne system using a phase noise compensation technique offers improved phase noise performance, it also provides a reduced phase drift against ambient temperature variations. The terahertz self-heterodyne system using a phase noise compensation technique shows a phase noise of 0.67 degree in terms of a standard deviation value even without using overall delay balance control. It also shows a phase drift of as small as approximately 10 degrees in an open-to-air measurement condition without any strict temperature control.

Implementation of a pulse-type laser detection and ranging system based on heterodyne detection for long-range measurement with high repetition rate.

This paper presents the implementation of a pulse-type LAser Detection And Ranging (LADAR) system based on heterodyne detection for long-range measurement. A pulse-type LADAR based on an intensity direct-detection is certainly simple and mature, but it requires a high peak-power laser and a low-noise avalanche photodiode for long-range measurement, which restricts the scope of the application due to the weight, power consumption, and cost of the laser and the photodetector. In this work, heterodyne detection using a PIN photodiode is implemented to increase receiver sensitivity instead of using a low-noise avalanche photodiode. An optical phase-locked loop is adopted to generate an optical local oscillator signal for heterodyne detection. The proposed heterodyne detection scheme achieves a minimum detectable signal level of -52.6 dBm at a bandwidth of 1.2 GHz, and it is adopted in a pulse-type LADAR system for long-range measurement. The pulse-type LADAR system can measure a distance of 2.77 km at a repetition rate of 40 kHz, and it demonstrates great advantages for realizing real-time 3D imaging for long-range measurement with a high frame rate.

A phase-shift laser scanner based on a time-counting method for high linearity performance.

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.