Yuki Iyonaga

Spectrally interleaved, comb-mode-resolved spectroscopy using swept dual terahertz combs

Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10−7 in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.We demonstrated combination of gapless terahertz (THz) comb with dual-comb spectroscopy, namely gapless dual-THz-comb spectroscopy, to achieve the spectral resolution equal to width of the THz comb tooth. The gapless THz comb was realized by interpolating frequency gaps between the comb teeth with sweeping of a laser mode-locked frequency. The demonstration of low-pressure gas spectroscopy with gapless dual-THz-comb spectroscopy clearly indicated that the spectral resolution was decreased down to 2.5-MHz width of the comb tooth and the spectral accuracy was enhanced to 10-6 within the spectral range of 1THz. The proposed method will be a powerful tool to simultaneously achieve high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

Terahertz Comb Spectroscopy Traceable to Microwave Frequency Standard

The fine-structured spectrum of a terahertz (THz) frequency comb has been observed using asynchronous-optical-sampling THz time-domain spectroscopy with an extended time window covering multiple THz pulses. Fourier transformation of 10 consecutive THz pulses enables us to obtain the spectrum of THz comb mode having a linewidth of 25 MHz at intervals of 250 MHz. Further expansion of time window up to 100 consecutive THz pulses reduces the linewidth of THz comb mode down to 2.5 MHz while conserving interval of 250 MHz. The observed THz comb mode can be used as a precise and accurate frequency marker for broadband THz spectroscopy because it is phase-locked to a microwave frequency standard by laser control. The spectroscopy of pharmaceutical tablets and low-pressure molecular gas is used to demonstrate the utility of this approach, indicating a spectral resolution of 250 MHz. The proposed method enables frequency calibration of the THz spectrometer based on a microwave frequency standard.

Super-resolution discrete Fourier transform spectroscopy beyond time-window size limitation using precisely periodic pulsed radiation

Fourier transform spectroscopy (FTS) has been widely used in a variety of fields in research, industry, and medicine due to its high signal-to-noise ratio, simultaneous acquisition of signals in a broad spectrum, and versatility for different radiation sources. Further improvement of the spectroscopic performance will widen its scope of applications. Here, we demonstrate improved spectral resolution by overcoming the time window limitation using discrete Fourier transform spectroscopy (dFTS) with precisely periodic pulsed terahertz (THz) radiation. Since infinitesimal resolution can be achieved at periodically discrete frequencies when the time window size is exactly matched to the repetition period T, a combination of THz-dFTS with a spectral interleaving technique achieves a spectral resolution only limited by the spectral interleaving interval. Linewidths narrower than 1/(50T) are fully resolved allowing the attribution of rotational-transition absorption lines of low-pressure molecular gases within a 1.25 MHz band. The proposed method represents a powerful tool to improve spectrometer performance and accelerate the practical use of various types of FTS.

Frequency-swept asynchronous-optical-sampling terahertz time-domain spectroscopy

We proposed frequency-swept asynchronous-optical-sampling terahertz time-domain spectroscopy to further improve the spectral resolution. The spectral resolution achieved here was 2.2 MHz, which is two orders of magnitude smaller than the mode-locked frequency.

Optical frequency combs with gapless mode-resolved spectra

Optical frequency combs are innovative tools that link radio and optical frequencies to enable measurement of the latter. The ‘comb’ is a set of lines that read out when coherent light pulses create a broad spectrum. Applications include spectroscopy and optical frequency metrology, where a series of comb modes serve as frequency markers that are traceable to a microwave frequency standard.1 For broadband spectroscopy, combs can achieve high spectral resolution and accuracy across a wide spectrum. However, the resolution of many spectrometers, including Fourier transform, is insufficient to resolve each mode because these are distributed too densely. Recently, a dual-comb approach has unlocked further potential for broadband spectroscopy by providing a comb-moderesolved spectrum.2 However, the spectral sampling interval is limited to the comb mode spacing, rather than the comb mode spectral linewidth, because of the discrete mode distribution. To enhance the spectral sampling density, we filled the comb mode gaps by interleaving additional frequency marks.3 An optical comb in the terahertz (THz) region is a harmonic (or component) comb of the laser repetition frequency.2 Therefore, we can tune the absolute frequency of each comb mode by changing the repetition frequency. If we incrementally shift the mode-resolved spectrum frequency at an interval equal to the linewidth and all of the resulting comb spectra are overlaid in the spectral domain, we can completely remove the frequency gaps of the original comb. In this way, we have achieved a spectrally interleaved or gapless THz comb. To assess our system’s capacity to resolve fine spectral signatures, we measured the rotational transition 110 101 of lowpressure water-vapor molecules at 0.557THz. The gas sample Figure 1. Amplitude spectra of the original terahertz (THz) comb (a) and the gapless THz comb (b) around 0.557THz after passing through low-pressure water vapor contained in a gas cell. a.u.: Arbitrary units.

Sweeping of terahertz frequency comb for high-accuracy, high-resolution, and broadband terahertz spectroscopy

We fully interpolated frequency gaps between THz-comb modes by their incremental sweeping at intervals of their linewidth, showing the possibility of enhancing the spectral resolution in THz spectroscopy to the linewidth of THz comb mode.