针对太赫兹量子级联激光器光频梳射频信号提取与传输存在的阻抗不匹配问题，设计并提出了一种输入阻抗为20 Ω、输出阻抗为50 Ω的渐变微带线结构，研究了该结构的准TEM模电场分布，并根据S参数建立了相应的LC等效电路模型分析其物理机理。实验中，将渐变微带线结构应用于太赫兹量子级联激光器光频梳射频信号提取与测试，以验证渐变微带线传输射频信号效果，成功测量得到太赫兹量子级联激光器重复频率，其线宽为3.7 kHz，信噪比达到60 dB，在注入电流为700~900 mA范围内重复频率表现为稳定的单模信号，测量得到的最大保持和幅度艾伦方差结果同样表现出重复频率的稳定性。实验结果表明，所设计的渐变微带线具有良好的阻抗匹配效果，能够实现对太赫兹量子级联激光器光频梳射频信号稳定的提取和传输。

Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the single-mode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (f_s) and the carrier envelope offset frequency (f_CEO) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

A real-time imaging system based on a compact terahertz laser is constructed by employing one off-axis parabolic mirror and one silicon lens. Terahertz imaging of water, water stains, leaf veins, human hairs, and metal wire is demonstrated. An imaging resolution of 68 µm is achieved. The experiments show that this compact and simplified imaging system is suitable for penetration demonstration of terahertz light, water distribution measurement, and imaging analysis of thin samples.