In this paper, we demonstrate a phase-sensitive photonic terahertz imaging system, based on two-tone square-law detection with a record-low phase noise. The system comprises a high-frequency photodiode (PD) for THz generation and a square-law detector (SLD) for THz detection. Two terahertz of approximately 300 GHz tones, separated by an intermediate frequency (IF) (7 GHz–15 GHz), are generated in the PD by optical heterodyning and radiated into free-space. After transmission through a device-under-test, the two-tones are self-mixed inside the SLD. The mixing results in an IF-signal, which still contains the phase information of the terahertz tones. To achieve ultra-low phase-noise, we developed a new mixing scheme using a reference PD and a low-frequency electrical local oscillator (LO) to get rid of additional phase-noise terms. In combination with a second reference PD, the output signal of the SLD can be down-converted to the kHz region to realize lock-in detection with ultra-low phase noise. The evaluation of the phase-noise shows the to-date lowest reported value of phase deviation in a frequency domain photonic terahertz imaging and spectroscopy system of 0.034°. Consequently, we also attain a low minimum detectable path difference of 2 µm for a terahertz difference frequency of 15 GHz. This is in the same range as in coherent single-tone THz systems. At the same time, it lacks their complexity and restrictions caused by the necessary optical LOs, photoconductive antennas, temperature control and delay lines.

中文翻译：

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基于双音平方律检测的超低相位噪声光子太赫兹成像系统

在本文中，我们演示了一种基于两音平方律检测且相位噪声低的相敏光子太赫兹成像系统。该系统包括用于产生THz的高频光电二极管（PD）和用于THz检测的平方律检测器（SLD）。在PD中，通过光学外差法产生了大约300 GHz音调的两个太赫兹，由中频（IF）（7 GHz-15 GHz）隔开，并辐射到自由空间中。通过被测设备传输后，这两个音会在SLD内部自混合。混合会产生IF信号，该信号仍包含太赫兹音调的相位信息。为了实现超低相位噪声，我们开发了一种使用参考PD和低频电本地振荡器（LO）的新混频方案，以消除其他相位噪声项。结合第二参考PD，SLD的输出信号可以下变频到kHz区域，以实现超低相位噪声的锁定检测。相位噪声的评估显示了迄今为止在0.034°频域光子太赫兹成像和光谱系统中最低的相位偏移报告值。因此，对于15 GHz的太赫兹差频率，我们还获得了2 µm的最小最小可检测路径差。该范围与相干单音THz系统的范围相同。同时，它缺乏必要的光学LO，光电导天线，温度控制和延迟线所造成的复杂性和限制。相位噪声的评估显示了迄今为止在0.034°频域光子太赫兹成像和光谱系统中最低的相位偏移报告值。因此，对于15 GHz的太赫兹差频率，我们还获得了2 µm的最小最小可检测路径差。该范围与相干单音THz系统的范围相同。同时，它缺乏必要的光学LO，光电导天线，温度控制和延迟线所造成的复杂性和限制。相位噪声的评估显示，迄今为止，在频域光子太赫兹成像和光谱系统中，相位偏移的最低报告值为0.034°。因此，对于15 GHz的太赫兹差频率，我们还获得了2 µm的最小最小可检测路径差。该范围与相干单音THz系统的范围相同。同时，它缺乏必要的光学LO，光电导天线，温度控制和延迟线所造成的复杂性和限制。该范围与相干单音THz系统的范围相同。同时，它缺乏必要的光学LO，光电导天线，温度控制和延迟线所造成的复杂性和限制。该范围与相干单音THz系统的范围相同。同时，它缺乏必要的光学LO，光电导天线，温度控制和延迟线所造成的复杂性和限制。