The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of ${\lt}1\;{\rm Hz}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range $\pm{0.3}\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.

中文翻译：

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太赫兹激光器的精确频率和相位控制

高分辨率光谱的准确性主要取决于激光源的稳定性，频率控制和可追溯性。在这项工作中，我们报告了太赫兹激光器的精确可调频率合成和相位控制。太赫兹激光器首次被太赫兹注入锁相环锁定，太赫兹信号由超快光检测器中的全光纤红外频率梳状发生器的选定线杂散产生。梳状线的频率间隔由锁定全球定位系统的微波频率合成器精确确定，可将太赫兹激光频率追溯到主要标准。锁定技术将太赫兹激光的外差线宽减小到测量仪器限制的线宽$ {\ lt} 1 \; {\ rm Hz} $，可抵抗短期和长期的环境波动。太赫兹激光器的频率可以仅由微波合成器的分辨率确定的增量进行调谐，并且相对于参考，激光器的相位被独立且精确地控制在$ \ pm {0.3} \ pi $的范围内。这些发现有望在相位分辨高精度太赫兹气相光谱和辐射测量中得到应用。