Architecture for microcomb-based GHz-mid-infrared dual-comb spectroscopy

ARTICLE https://doi.org/10.1038/s41467-021-26958-6 OPEN Architecture for microcomb-based GHz-midinfrared dual-comb spectroscopy 1234567890():,; Chengying Bao1,3,5, Zhiquan Yuan1,5, Lue Wu Kerry J. Vahala 1 ✉ 1, Myoung-Gyun Suh1,4, Heming Wang 1, Qiang Lin2 & Dual-comb spectroscopy (DCS) offers high sensitivity and wide spectral coverage without the need for bulky spectrometers or mechanical moving parts. And DCS in the mid-infrared (mid-IR) is of keen interest because of inherently strong molecular spectroscopic signatures in these bands. We report GHz-resolution mid-IR DCS of methane and ethane that is derived from counter-propagating (CP) soliton microcombs in combination with interleaved difference frequency generation. Because all four combs required to generate the two mid-IR combs rely upon stability derived from a single high-Q microcavity, the system architecture is both simplified and does not require external frequency locking. Methane and ethane spectra are measured over intervals as short as 0.5 ms, a time scale that can be further reduced using a different CP soliton arrangement. Also, tuning of spectral resolution on demand is demonstrated. Although at an early phase of development, the results are a step towards mid-IR gas sensors with chip-based architectures for chemical threat detection, breath analysis, combustion studies, and outdoor observation of trace gases. 1 T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA. 2 Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627, USA. 3Present address: State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments, Tsinghua University, Beijing 100084, China. 4Present address: Physics & Informatics Laboratories, NTT Research, Inc. 940 Stewart Dr, Sunnyvale, CA 94085, USA. 5These authors contributed equally: Chengying Bao and Zhiquan Yuan. ✉email: NATURE COMMUNICATIONS | (2021)12:6573 | https://doi.org/10.1038/s41467-021-26958-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-26958-6 D ual-comb spectroscopy (DCS) works by mapping an optical comb of frequencies into radio-frequencies by multi-heterodyne beat with a second comb having a slightly different repetition rate. Because the two combs sample absorption spectra with a resolution set by their line spacing (or repetition rate), analysis of the corresponding comb of radio frequencies reveal these spectra in a multiplexed fashion without the use of scanning gratings or interferometers1–3. Comb generation in the mid-infrared (mid-IR) has traditionally used methods that rely upon mode-locked pulse generation, including difference-frequency-generation (DFG), optical parametric oscillation, and supercontinuum generation3,4; and there is considerable progress using such systems for mid-IR DCS5–14. More recently, mid-IR comb generation by DFG using electro-optic frequency combs (EO-comb) has also been demonstrated15,16. In contrast to conventional mode-locking, this approach offers rate tunability to the X-band range (8−12 GHz) and higher16; and because DCS systems can trade-off spectral resolution for higher acquisition rates, such higher rates can be useful for the study of dynamics17. With the advent of thin-film lithium niobate technology, EO-combs have potential for chip-integration18. Indeed, on-chip lithium niobate microcavity-based EO-combs have been used for DCS in the near-IR19. Also offering high repetition rates and chip integration are soliton microcombs20,21. On account of their compact size, these devices operate readily in the X to millimetre-wave bands. Microcomb-based DCS has been reported at rates of 22 and 450 GHz in the near-IR22,23 and 127 GHz in the mid-IR5. And while offering extremely short acquisition times, these rates are too high for spectroscopy of many species, leading to spectral undersampling of gas samples. For balance between acquisition rate and spectral resolution, DCS at GHz rates is considered to be relatively optimal for sensing of ambient gases (with linewidths narrower than 10s of GHz)24–26. Special efforts have been directed to reduce near-IR microcomb rates to the single-digit GHz range27,28, but these require very high Q resonators to reduce increased threshold pumping power associated with larger mode volumes. Thermal tuning of large spacing microcombs has also been used to improve the resolution for near-IR DCS at the expense of measurement speed29. Aside from microcombs, an onchip III−V laser frequency comb with a line spacing of 1 GHz (together with an EO-comb) has also been used for near-IR DCS30. Nonetheless, mid-IR DCS with GHz resolution remains quite challenging for chip-based devices, including quantum cascaded laser frequency combs31,32. Here, we report microcomb-based DCS with GHz resolution in the mid-IR band. The two GHz-rate mid-IR combs are generated by interleaved difference-frequency-generation (iDFG)33 applied to four near-IR combs. These four combs are linked to counterpropagating (CP) solitons34 formed within a single microcavity. The frequency stability of the resulting mid-IR DCS spectra is high on account of this simplified architecture in combination with the high mutual coherence of the CP solitons. DCS measurements of methane and ethane near 3.3 p μm ffiffi are performed. Normalized precision as high as 1.0 ppm ⋅ m
s is demonstrated. Results Architecture of the DCS system. The experimental setup is illustrated in Fig. 1a. It shows two 3.3 μm frequency combs generated in upper and lower branches of the optical train, followed by combining (far right in the figure) for input to the test gas cell. In accordance with the DCS procedure as described elsewhere2 the two combs are photodetected after passage through the gas cell, and this multi-heterodyne process creates a radio-frequency spectrum that contains the mid-IR absorption 2 spectrum of the gas. The spectrum is obtained by fast Fourier transform (FFT) of the time-domain interferogram signal of the dual combs. The gas cell (Wavelength Reference) has a length of 5 cm and contains ~2% methane (CH4) and ~0.5% ethane (C2H6) buffered by nitrogen to a total pressure of 760 Torr (parameters can have ±5% uncertainty). Such a methane concentration is equivalent to about 1 ppm in an ambient environment when passing the comb light through a 1 km open path for field measurements. Each mid-IR comb is generated by iDFG in a PPLN crystal (4 cm long, NTT Electronics) of two near-IR combs: a soliton microcomb at 1.55 μm (Fig. 1b) and an EO-comb (Fig. 1c) at 1.06 μm. Counter-pumped clockwise (cw) and counter-clockwise (ccw) solitons formed in a single silica resonator35 are input to upper and lower branches of the optical train. On account of the ccw silica Raman response, the soliton repetition rates (f cw r or f r ) can be independently fine-controlled b (...truncated)