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

Abstract

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.

Gapless Dual THz Comb Spectroscopy

“…The individual mode-locked frequencies of the fiber lasers (f 1 = 250,000,000 Hz and f 2 = 250,000,050 Hz) and the frequency difference between them (∆f = f 2 -f 1 = 50 Hz) were stabilized by two independent laser control systems, and are respectively used for generation and detection of the pulsed THz radiation. Details of this system are given elsewhere [3]. We acquired the temporal waveform of the THz pulse train when the measurement time window was set to 40 ns.…”

“…Therefore, if incremental sweeping of the comb mode in an interval equal to its linewidth is repeated and then all of the comb mode spectra are overlaid in the spectral domain as shown in Fig. 2, the gaps between comb modes can be fully interpolated [3]. This situation is equivalent to continuous sweeping of a singlemode, narrow-linewidth CW-THz wave across a broadband THz spectral region.…”

“…If this narrow linewidth can be used for THz spectroscopy, the spectral resolution will be greatly enhanced. In this paper, we demonstrated gapless dual THz comb spectroscopy by frequency sweeping of THz comb mode [3].…”

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

“…The first demonstration of broadband THz-FCS employed an Er:Fiber oscillator system with a comb tooth spacing of 25 MHz and a tooth width of 2.5 MHz was interleaved at 2.5 MHz steps to partially resolve the rotational structure of a single K-stack of acetonitrile at 150 mTorr. 23 The use of an Er:Fiber oscillator limits the THz bandwidth of this system, but its general principles of operation are similar to those of the system described here. Very narrow comb spacing and resolution via coherent multi-bunch THz emission from synchrotrons has recently been used to report similar measurements on acetonitrile 24 up to a few hundred GHz, but the broadband generation of THz radiation requiring a synchrotron source limits the potential for widespread spectroscopic applications and is not discussed further here.…”

Applications using high-Tc superconducting terahertz emitters

“…Among the many imaging and sensing systems using THz EMWs that have been reported to date are a three-dimensional computed tomography (3D CT) imaging system10, a time-of-flight imaging system that can obtain depth information11, an imaging system below the diffraction limit using near-field lighting12, a THz chemical microscope13, and a high-resolution spectrometer using a THz-comb14. Systems based on THz time domain spectroscopy (THz-TDS) using pulsed-wave sources have made especially significant progress, and some systems have already been commercialized by Advanced Photonix Inc, Tera View, ADVANTEST Corp., etc .…”