Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs

www.nature.com/scientificreports OPEN Broadband high‑resolution molecular spectroscopy with interleaved mid‑infrared frequency combs A. V. Muraviev, D. Konnov & K. L. Vodopyanov* Traditionally, there has been a trade-off in spectroscopic measurements between high resolution, broadband coverage, and acquisition time. Originally envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet, optical frequency combs are now commonly used for probing molecular ro-vibrational transitions throughout broad spectral bands in the mid-infrared providing superior resolution, speed, and the capability of referencing to the primary frequency standards. Here we demonstrate the acquisition of 2.5 million spectral data points over the continuous wavelength range of 3.17–5.13 µm (frequency span 1200 cm−1, sampling point spacing 13–21 MHz), via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequencycomb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying eight spectra with gradually shifted comb lines we fully resolve the amplitude and phase spectra of molecules with narrow Doppler lines, such as carbon disulfide (CS2) and its three isotopologues. Coherent laser beams in the 3 to 20 μm mid-infrared (mid-IR) region provide a unique prospect for sensing molecules through addressing their strongest absorption bands. Thanks to their coherent and broadband nature, optical frequency combs can probe molecular signatures over an extensive (e.g. more than an octave) spectral span simultaneously1,2. When one (or several) of the comb teeth is phase locked to a narrow-linewidth reference laser(s), frequency comb spectroscopy can provide the spectral resolution, which is on par with tunable laser spectroscopy—limited only by the absolute comb-tooth linewidth. However, comb spectroscopy has a strong advantage of massive parallelism of data collection and, most importantly, the absolute optical frequency referencing to an accurate external standard (e.g. atomic clock) over the whole spectral span of the comb. Combs with high degree of phase coherence can be used in the dual comb spectroscopy (DCS)—one of the most advanced spectroscopic techniques. In a dual-comb spectrometer, a sensing comb is transmitted through a sample and then multi-heterodyned against a local oscillator (LO) comb which has a repetition rate fr that differs by a small fraction fr from that of the sensing comb. Compared to classical Fourier transform infrared spectroscopy, DCS demonstrates remarkable improvement of spectral resolution, data acquisition speed, and sensitivity, all at the same t ime3. With a high degree of mutual coherence between the two combs in a DCS system, it is possible to obtain comb-tooth resolved s pectra3–5. Using DCS in the near-IR (comb span 1.36–1.69 µm), Zolot et al. resolved the phase and amplitude of over 400,000 individual comb modes at a mode spacing of 100 MHz5. In the mid-IR, by utilizing frequency combs near 3.3 µm (spectral span 12 cm−1) achieved by applying a nonlinear mixing step to near-IR combs produced by electro-optic modulation (EOM) technique, Yan et al. resolved 1,200 comb lines with the line spacing of 300 MHz6. Subsequently, two groups demonstrated comb-tooth resolved spectra over a broad span of frequencies: Ycas et al. resolved 270,000 comb lines between 2.6 and 5.2 µm in four overlapping spectral sequences (with mode spacing 200 MHz and a combined frequency span 1800 cm−1)7, and Muraviev et al. resolved 350,000 comb lines with a finesse of 4,000 within a single frequency comb spanning 3.1–5.5-µm (mode spacing 115 MHz, frequency span 1400 cm−1) and also demonstrated simultaneous detection of more than 20 molecular species in a mixture of g ases8. Once comb teeth are resolved, the spectral resolution is defined by the comb tooth linewidth, which can be orders of magnitude narrower than the comb-line spacing. Then, high-resolution measurements can be CREOL, College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA. * email: Scientific Reports | (2020) 10:18700 | https://doi.org/10.1038/s41598-020-75704-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ implemented by interleaving spectra taken with discretely stepped—either comb repetition rate fr or carrierenvelope offset (CEO) frequency fceo2,9–13. For example, using a near-IR comb (span of 1.5–1.64 µm) and a mechanical Fourier transform spectrometer with mode-resolving capability, Rutkowski et al. performed measurements, which yielded 2.4 million sampling points with a step of 20 kHz—performed by interleaving spectra with frequency-shifted comb l ines14. In the mid-IR, Baumann et al. used difference frequency combs near 3.4 µm (spectral span 30 cm−1), to attain a high-resolution spectrum of methane, by interleaving spectra acquired by shifting the combs by 25 MHz—one-quarter of the 100-MHz comb-tooth s pacing10. Using quantum cascade laser (QCL) combs, Villares et al. performed DCS measurements near 7 µm (span 16 cm−1) with the sampling point spacing that was improved from the original comb-tooth spacing of 7.5 GHz to 80 MHz by frequency sweeping the combs via QCL current modulation15. Similarly, by utilizing dual QCL combs near 8.3 µm (span 55 cm−1), Gianella et al. achieved an improvement in the sampling point spacing from 9.8 GHz to 30 MHz by sweeping the frequencies of both the sensing and LO combs via synchronized current m odulation16. In the two above QCL scenarios the combs were free running with no absolute optical frequency referencing; rather, the frequency scale was calibrated by comparing the spectra with the HITRAN d atabase17. Overall, the spectral −1 width of mid-IR measurements with interleaved combs does not exceed 60 cm with one exception of a silicon microresonator comb with a span of 3–3.5 µm that was scanned with a step of 80 MHz via tuning both the pump laser frequency and the cavity resonance. However, the comb lines were scanned over 16 GHz—a small portion of the 127-GHz mode s pacing18. One of the challenges of DCS is to get high spectral resolution over a broad bandwidth—required, for example, in applications related to multi-species detection in gas mixtures. To achieve high resolution, one needs long mutual coherence time between the two combs3. If the combs are not fully locked, one can track the relative phase drifts and correct for these in real time, at the expense of having two stable continuous-wave reference lasers, as has been demonstrated in the near-IR19–21, or perform phase correction by a posteriori data processing, as has been shown in the THz and mid-IR ranges22,23. However, both of these methods lack the absolute frequency referencing—a setup needs to be calibrated using, for example, a well-known gas absorption feature. Degenerate (subharmonic) optical parametric oscillators (OPOs) pumped by mode-locked lasers are noteworthy source (...truncated)