Abstract
The spectral resolution and accuracy of asynchronous-optical-sampling terahertz time-domain spectroscopy (ASOPS-THz-TDS) were evaluated by examining low-pressure gas-phase samples. Use of dual 56-MHz, erbium (Er)-doped, mode-locked femtosecond fiber lasers enhanced the spectral resolution to as low as 50.5 MHz and the spectral accuracy to as low as 6.2 × 10(-6). The results indicate that ASOPS-THz-TDS has the potential to achieve high spectral resolution, high spectral accuracy, and wide spectral coverage at the same time. ASOPS-THz-TDS will open a new door to gas-phase spectroscopy of multiple chemical species in the field of atmospheric gas analysis.
Highlights
Terahertz (THz) spectroscopy has emerged as a new mode for sensing and material characterization
The results indicate that ASOPS-THz time-domain spectroscopy (THz-TDS) has the potential to achieve high spectral resolution, high spectral accuracy, and wide spectral coverage at the same time
An assured traceability of the frequency scale to the frequency standard is a big advantage of this method over conventional THz spectroscopic methods without any traceability system, and opens the door for establishment of frequency metrology in THz spectroscopy
Summary
Terahertz (THz) spectroscopy has emerged as a new mode for sensing and material characterization. One interesting application of THz spectroscopy is in the analysis of atmospheric molecular gases, because their rotational transitions give them rich spectral fingerprints in the THz region Measuring these densely distributed absorption lines correctly in THz spectroscopy requires high spectral resolution, high spectral accuracy, and broad spectral coverage. THz time-domain spectroscopy (THz-TDS) with pulsed THz radiation is a popular spectroscopic technique for obtaining a broadband THz spectrum [1, 2] In this method, the frequency scale of the THz spectrum is established by mechanical timedelay scanning with a motor-driven translation stage. Limited stroke length and poor positioning precision of usual stepping-motor-driven translation stages are barriers to achieving high spectral resolution and accuracy in this method. The spectral accuracy can be improved by phase-locking the two CW lasers to a single optical frequency comb [5], the spectral range of continuous tuning covered in this method is usually narrower than that in THz-TDS
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