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A decade-spanning high-resolution asynchronous optical sampling terahertz time-domain and frequency comb spectrometer

机译：十年跨度的高分辨率异步光学采样太赫兹时域和频梳光谱仪

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We present the design and capabilities of a high-resolution, decade-spanning ASynchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ∼80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region.
机译：我们介绍了基于高分辨率，跨越十年的异步光学采样（ASOPS）的TeraHertz时域光谱（THz-TDS）仪器的设计和功能。我们的系统采用双锁模飞秒Ti：Sapphire振荡器，其重复频率通过锁相环（PLL）以〜80 MHz振荡器重复频率的第60次谐波偏置，锁定在100 Hz。在12.5 ns的窗口中以10 ms的实验室扫描时间扫描各个激光脉冲的各自时间延迟，从而支持高达15.6 fs的时间延迟分辨率。泵浦振荡器的重复频率通过PLL在振荡器重复频率的12次谐波下同步至Rb频率标准，从而实现了毫赫兹（mHz）的稳定性。我们使用空的标准具，光学互相关器和PLL的相位噪声频谱来表征系统的时序抖动。通过在短光程气室中测量0.55至3.35 THz的水蒸气吸收线和0.13至1.39 THz的乙腈吸收线，证明了ASOPS-THz-TDS的光谱应用。通过70分钟的数据采集，可获得50 dB的信噪比。与JPL分子光谱数据库相比，实测水线中心频率的均方根偏差为14.6 MHz，平均偏差为11.6 MHz。此外，在相同的仪器和数据采集硬件的情况下，我们使用控制泵浦振荡器的重复频率的功能来启用THz频率梳谱（THz-FCS）。在此，产生齿宽为5 MHz的频率梳，并用于以多普勒极限精度完全解析乙腈的纯旋转谱。如果需要，通过我们的PLL锁定电路实现的振荡器重复频率稳定性可实现亚MHz齿宽的产生。该仪器提供从80 MHz到亚MHz的前所未有的跨度，可调范围的分辨率，并预示了太赫兹地区的新一代气相光谱仪。

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