Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented.

中文翻译：

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使用双10 GHz飞秒频率梳的实时实时太赫兹频率计量

使用飞秒光学采样通过射频下变频进行的太赫兹频率计量依赖于太赫兹频率和光学采样率之间的谐波因子检索。在典型的飞秒激光重复频率下，这给频率计量带来了歧义。我们报告了一个双梳状采样系统，用于以赫兹级精度对太赫兹量子级联激光器进行明确的频率测量。两个具有10 GHz重复频率的Ti：蓝宝石振荡器用于电光采样由9.7 GHz和19.6 GHz重复频率的有源锁模太赫兹量子级联激光器发出的2.5 THz的太赫兹辐射。通过相干下变频，在射频域中测量发射的太赫兹波形。太赫兹频率梳通过在射频拍音信号上采用锁相环来稳定。第二个红外采样梳用于测量太赫兹辐射的绝对频率。这种基于采样重复率失谐的方法，允许实时直接直接检索量子级联激光器的绝对频率，而无需使用额外的光学频率基准进行校准。为了证明稳定和读出技术的可行性，提出了对气态甲醇的高分辨率光谱测量。允许实时直接检索量子级联激光器的绝对频率，而无需使用其他光学频率基准进行校准。为了证明稳定和读出技术的可行性，提出了对气态甲醇的高分辨率光谱测量。允许实时直接检索量子级联激光器的绝对频率，而无需使用其他光学频率基准进行校准。为了证明稳定和读出技术的可行性，提出了对气态甲醇的高分辨率光谱测量。