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Abstract
Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.
Highlights
The advantages of THz time-domain spectroscopy (THz-TDS) and THz frequency-domain spectroscopy (THz-FDS), namely, broadband coverage and high resolution, because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line continuous-wave THz (CW-THz) radiation characteristics
If the temporal waveform of the distorted RF pulse train is acquired in synchronization with an adaptive sampling clock, reflecting the rapid fluctuations of the temporal magnification factor (TMF) caused by the timing jitter, the fluctuation of the time scale in the RF pulse train can be cancelled, as shown in the right part of Fig. 1(b)
The adaptive sampling method seems to be more powerful than the constant sampling method with stabilized lasers, which is limited by the residual timing jitter
Summary
The advantages of THz-TDS and THz-FDS, namely, broadband coverage and high resolution, because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. The temporal waveform of a THz pulse train is acquired over many repetition periods using asynchronous optical sampling (ASOPS) with two mode-locked femtosecond lasers (repetition frequencies = frep[1] and frep2) with a frequency offset between them (= foffset = frep2 - frep1)[22,23,24,25]. Such fluctuation of the time scale propagates to the frequency scales of the RF comb and the THz comb via the FT and frequency calibration, leading to severe degradation of the spectral resolution and accuracy. The simplicity of generating the adaptive sampling clock is advantageous compared with adaptive sampling DCS in the visible and near-infrared regions, where two kinds of adaptive sampling clocks are required to correct variations of both frep[1] and fceo[15,16]
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