We propose and experimentally demonstrate a continuous-wave terahertz (THz) vector imaging system based on an optical frequency comb, which can avoid interference of phase noise induced by both laser and RF sources. As a benefit, the proposed THz system is capable of extracting stable amplitude and phase information and supporting robust THz imaging measurements. The system uses a phase modulator to create a wide optical frequency comb, and an intensity modulator to generate first-order harmonics of each comb line. The THz signal is photonically generated by an uni-traveling carrier photodiode (UTC-PD), and then detected in a Schottky barrier diode (SBD) detector. With respects to the reception, the THz signal is firstly down-converted into an electrical signal at the SBD based on the square-law detection and self-mixing, which is then mixed with a local oscillator (LO) and fed into a lock-in amplifier (LIA) to extract amplitude and phase information for imaging. The experimental results show the fluctuations of amplitude and phase in this system are about 0.14 mV and 3.3°, respectively. The system has a great potential in real-time THz security inspection and biomedical science.
We propose an opto-electronic THz vector imaging system based on a self-mixing detection receiver, which can be used to measure thickness of samples. In the proposed system architecture, a THz signal is generated by microwave photonics technology and received by a Schottky barrier diode detector. In the THz free space transmission link, four parabolic mirrors are used to collimate the THz signals, and the sample under test is placed at the focal point of a parabolic mirror. At the receiver side, the transmission signal is down-converted by Schottky barrier diode detector, and the phase information is acquired by a lock-in amplifier, which is used for sample imaging and thickness detection. In the experiment demonstration, a Mach-Zehnder modulator with carrier-suppression is used to generate a two-tone optical signal, and coupled with an external cavity laser for photomixing generation of a two-tone THz signal in the 300GHz frequency band at an uni-traveling carrier photodiode. Several samples with different thicknesses are imaged and compared in the experiment, and the measured thickness error is estimated to be about 3.84%.
We propose and experimentally demonstrate a continuous-wave (CW) terahertz (THz) imaging system based on microwave photonic signal generation scheme. Compared to the traditional opto-electronic THz imaging by using unmodulated lasers, our proposed system obtains the amplitude information by using microwave photonic optical double sideband modulation (DSB). As a result, the phase noise and frequency drift of the free-running laser sources are negligible. We experimentally demonstrate a THz imaging system for measuring various 3D-printed samples by using a 300 GHz DSB signal. In the experiment, the optical DSB modulation is realized with an in-phase and quadrature Mach-Zehnder modulator, and THz signals are generated by photo-mixing at a uni-traveling-carrier photodiode. The THz signals passing through the sample is received by a self-mixing-based Schottky barrier diode. The experimental results show signal-tonoise ratio (SNR) a ~25 dB . In addition, we verify that the optical DSB system operating in the linear regime is superior to the carrier-suppressed scheme, which is attributed to higher THz power and higher SNR. Therefore, the proposed scheme based on microwave photonic double sideband modulation is potentially promising in high-resolution object recognition.
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