Advances in highly coherent laser sources enable high-sensitivity, high-accuracy characterization of significant trace gases [1–4], facilitating the understanding of gas-related environmental and ecological issues, such as greenhouse gas emission evaluation [5], land vegetation photosynthetic monitoring [6], and atmospheric pollution source detection [7]. At present, multi-gas parallel sensing remains a significant challenge because of the limited bandwidth of coherent laser sources. Therefore, parallel gas spectral sensing typically employs several or tunable coherent light sources to cover the abundant absorption bands of multiple gas molecules [8–10], which hinders the rapid analysis of simultaneous variation processes. In the last decade, dual-comb spectroscopy (DCS) [11–15], consisting of two coherent optical frequency combs (OFCs), has been developed to achieve open-path sensing of greenhouse [16,17] and agricultural [18] gases in the near-infrared (NIR) region ($<2\mathrm{\mu m}$) by utilizing its high sensitivity, high resolution, and fast sampling efficiency. However, the common resonant absorption transitions of most atmospheric gas molecules are located in the mid-infrared (MIR) region of 3–5 μm, where laser gain media are extremely scarce [19,20], leading to a lack of directly produced MIR comb sources. To enable OFC spectroscopy in the MIR region, numerous methods have been proposed, including interband cascade lasers (ICLs) [21,22], quantum cascade lasers (QCLs) [23,24], micro resonators [25,26], optical parameter oscillators [27–29], optical parameter amplification [30–32], difference frequency generation (DFG) [33–36], and electro-optic modulation [37,38].

Although many multi-octave-spectra combs have been demonstrated [39–44], DCS-based gas sensing for the entire atmospheric window has not been achieved because of the significant challenges related to maintaining mutual coherence and promoting limited bandwidth, which critically restricts the capacity for parallel high-resolution measurement of multiple gas molecules. Fortunately, several advanced efforts have been made to improve the bandwidth and mutual coherence of MIR DCS systems. Using optical subharmonic generation in two gallium arsenide (GaAs) OPO cavities, high-coherence comb-mode-resolved DCS was realized over the atmospheric window to distinguish the isotopologues of gas mixtures [27]. However, the complex OPO cavity structure and precise phase-locking setups result in an unavoidably large volume and poor resistance to environmental noises [28,39], making it difficult to satisfy the molecule measurement requirements in complex outdoor atmospheres. A passively coherent dual-comb spectrometer without any phase-locking loop, which only uses a simple single-pass OPA in periodically poled lithium niobate (PPLN, ${\text{LiNbO}}_3$) and with a tunable spectral range of 3.3–3.9 μm, was developed, exhibiting good detection capacity for multiple greenhouse species [31]. In addition, tunable DFG was successfully incorporated in chirped PPLN, and this comb-tooth-resolution MIR DCS, to accomplish molecular spectral measurement of three greenhouse gases, exhibited a tunable span of mid-infrared atmospheric window [33,45]. PPLN crystals have a broad transparent range and strong nonlinearities in atmospheric spectral windows, providing great potential for integrated chip-level laser devices [41,46–48]. In addition, ${\text{LiNbO}}_3$ can be easily processed to form chirped periods and waveguide structures, enabling broadband phase-matching spectra and a high conversion efficiency. By leveraging the interpulse DFG (IPDFG) in chirped PPLN waveguides, we have established a portable, compact DCS system with comb-tooth spectral resolution at 3–4 μm [35,43]. We believe that these integrated PPLN-based MIR DCS systems will greatly contribute to parallel gas sensing in atmospheric open-path environments, with forthcoming breakthroughs in spectral width, mutual coherence, power consumption, volume, and weight.

In this study, we demonstrate parallel gas sensing for multiple greenhouse molecules by adapting a DFG-based MIR dual-comb spectrometer. In the DFG process, a pair of high-coherence NIR combs was employed as the driving laser source, which was separately divided into pump and signal branches for MIR generation. To support broadband DFG, the spectral ranges of the two branches were optimized using two length-designed highly nonlinear fibers (HNLFs). Meanwhile, PPLN waveguides were processed with optimized periods of 22–30 μm, directly realizing DCS with a total of 300,000 comb-tooth-resolved frequency lines. Moreover, the parallel measurement of four mixed gas molecules was performed at a high resolution of 100 MHz, in which the measured molecular absorption peaks were in good agreement with the results of the HITRAN database. The broadband MIR spectrometer provides an alternative high-efficiency choice for parallel gas sensing in open-path atmospheric environments.

The proposed broadband MIR DCS is illustrated in Fig. 1, mainly consisting of a high-coherent NIR dual-comb source, two pieces of SCG HNLFs (80414P5, OFS, typical nonlinear coefficient $10.7{\mathrm{W}}^{-1}·{\mathrm{km}}^{-1}$), two parallel MIR DFG sections, and a rapid broadband DCS module. The NIR OFCs were mode-locked at 1560 nm with repetition rates of 100 MHz, utilizing the mechanism of a nonlinear amplifying loop mirror (NALM) that directly emits two trains of femtosecond pulses with an average power of 35 mW [35]. Subsequently, two home-built PPLN-based f-2f units were constructed to obtain the repetition rate ($f_r$) and carrier-envelope offset frequency ($f_{\mathrm{ceo}}$) with a high signal-to-noise ratio (SNR) [43]. To realize high-precision coherent phase control, a piezoelectric transducer (PZT, slow actuator) and a phase-modulating electro-optical modulator (PM-EOM, fast actuator) were simultaneously used to stabilize $f_r$. Meanwhile, both the pump-driving current (slow actuator) and the amplitude-modulating electro-optical modulator (AM-EOM, fast actuator) were adopted to lock $f_{\mathrm{ceo}}$ [49]. Moreover, a four-channel function generator referenced to a hydrogen clock provided four radio-frequency (RF) standard signals (100, 100+0.000042, 20, and 20 MHz) for the phase stabilization of two $f_r$ and two $f_{\mathrm{ceo}}$.

The seed OFCs were scaled up to 450 mW by employing two single-mode (SM) Er-doped fiber amplifiers (EDFAs), which were separately pumped by two 1 W, 980 nm single-mode laser diodes. The amplified comb pulses were then divided into two pulse trains with durations of approximately 100 fs. A branch of 220 mW was delivered into an 8 cm HNLF to acquire the SCG spectrum, in which the spectral components at approximately 1064 nm were filtered out as a pump beam for the MIR DFG. Moreover, the pump beam was increased to 150 mW in a double-cladding Yb-doped fiber amplifier (YDFA). Another branch of 200 mW was delivered into a 3 cm HNLF to directly generate a 75 mW SCG spectrum at 1.3–1.7 μm, which served as a signal beam for broadband MIR DFG. Subsequently, two chirped PPLN waveguides (5% mole fraction, MgO:PPLN, Rayzer, Wuhan) were designed with well-designed periods of 22–30 μm by simultaneously considering the phase matching conditions and waveguide dispersions. Then we adopted dichroic mirrors (DMs) to combine the pump and signal beams in space, and a lens with matched numerical apertures was used to couple the combined beams into two chirped PPLNs for the parallel DFGs, in which a pair of broadband MIR combs was obtained with a corresponding average power of 2.6 mW. Thereafter, two MIR comb beams were recombined for spatial overlap using a beam splitter (BS) and were collectively directed into a designed multiple hybrid gas cell (pressure: 50 mbar). A commercially available balanced photodiode (BPD, HgCdTe PVI-2TE-5, VIGO System; 1 GHz response bandwidth, 3–5 μm response wavelength) was employed to acquire the MIR dual-comb interferogram, which was recorded using a fast data acquisition card synchronized with the same hydrogen clock. Finally, we performed phase correction, coherent averaging, and fast Fourier transform (FFT) on the recorded dual-comb interferogram [50], directly achieving broadband comb-line-resolved DCS for the parallel sensing of multiple greenhouse gases in the MIR region.

To accomplish broadband DFG, the spectral range of signal OFCs must be expanded to support phase matching across the entire atmospheric window (3–5 μm). The nonlinear coefficient of silicon dioxide fibers can be significantly improved by reducing the core diameter and determining the core-cladding refractive index. Therefore, the output spectra broadened under the combined actions of high-order nonlinearity and fiber dispersion when the injection peak power in the HNLF reached a certain threshold. Figures 2(a) and 2(b) show the simulation results of the temporal–spectral evolutions using non-linear Schrödinger equation (NLSE) when the femtosecond pulses, with actual initial dispersion, power, and duration, propagate in the HNLF. The pulse spectrum promptly expands with an almost unchanged temporal duration at a centimeter-level propagation length, which is beneficial to next-stage broadband MIR DFG. In fact, we directly adjusted HNLF length to optimize the output spectra. As shown in Fig. 2(c), we depict three expanding spectra at the propagation length of 2, 3, and 4 cm. The SCG spectrum covers a range of 1.2–2.0 μm when signal pulses evolve to the 4 cm position. However, the spectral components of $<1.3\mathrm{\mu m}$ and $>1.7\mathrm{\mu m}$ do not participate in this designed DFG process, which can result in pulse energy losses. Moreover, longer propagation lengths lead to spectral instability and power fluctuations. Therefore, the HNLF length was optimized to 3 cm, which broadened the SCG spectrum with a relatively high frequency intensity at 1.3–1.7 μm. Experimentally, 200 mW signal pulses were launched into a 3 cm HNLF. As shown in Fig. 2(d), we recorded the SCG spectrum using a combination of two spectrometers (Yokogawa AQ6370 and Bristol 771B), and the spectral profile and intensity distribution were in good agreement with the simulation results. In detail, the pulse energy is mainly distributed in the region of 1.3–1.7 μm, corresponding to the required signal spectral range for the designed atmospheric window DFG.

As mentioned above, the seed comb pulses were scaled up to 450 mW with a compressed pulse duration of approximately 100 fs. The pump and signal branches were separately delivered into 8 and 3 cm HNLFs for the SCG. In the pump branches, the spectral components centered at 1064 nm were extracted using a fiber filter, and the pulse power was amplified to 150 mW with a home-built YDFA. Meanwhile, the signal pulses were broadened to cover a range of 1.3–1.7 μm with an output power of 76 mW. To characterize the spectral properties of the NIR pump and signal pulses in both the master and slave paths, we recorded all the spectral profiles using two optical spectrometers (Yokogawa AQ6370 and Bristol 771B), as depicted in Fig. 3. Figures 3(a) and 3(b) show the spectral profiles of the two pump pulse trains, both centered at 1064 nm, with an available spectral range of 1040–1090 nm. Moreover, the inconformity on the spectral profile is mainly rooted in the parameter difference on the SCG process, which does not cause an obvious degradation on the MIR DCS. The broadband SCG spectra of the two signal pulse trains are shown in Figs. 3(c) and 3(d), where the pulse energy is mainly concentrated in the region of 1.3–1.7 μm. In view of the spectral distributions of the pump and signal lights, suitable PPLN periods must be finely designed for broadband phase-matching conditions, which is a key factor for successfully implementing broadband DFG in the MIR atmospheric window. Based on a 1064 nm pump wavelength, we calculated the phase-match wavelengths of the signal pulses and the corresponding PPLN polarization periods, as depicted by the blue lines in Figs. 4(a) and 4(b).

Considering the spectral distributions of signal lights, the suitable polarization periods of PPLN should be optimized to 23–32 μm. However, the geometric size of the PPLN waveguide is $15\mathrm{\mu m}\times 15\mathrm{\mu m}$, which is close to the MIR wavelength. Under these conditions, the introduced waveguide dispersion modulated the effective refractive index of the PPLN, resulting in a shift in the phase-matching period. For wideband DFG, the polarization period range of the PPLN should be finely designed to avoid wasting pump and signal frequency components. Considering the waveguide size, the dispersion characteristics of PPLN waveguides were calculated. Subsequently, we simulated the effective refractive index of the PPLN waveguide with a cross-section of $15\mathrm{\mu m}\times 15\mathrm{\mu m}$, obtaining the corrected phase-match conditions, as denoted by red lines in Figs. 4(a) and 4(b). The 1.7 μm phase-match polarization period exhibited an overall reduction to 30 μm from 32 μm. Therefore, the periodic structure of the PPLN waveguide was redesigned with a chirped polarization period of 22–30 μm and a length of 25 mm, supporting 3–5 μm MIR generation when signal pulses covering a range of 1.3–1.7 μm were injected. Thereafter, the pump and signal pulses were coupled to the PPLN waveguide for the MIR DFG. Two delayers were used to combine the pump and signal pulses in the temporal domain. After collimation and filtering, we obtained two 2.6 mW MIR combs with a bandwidth of 3–5 μm and an average comb line power of $\sim 8.67\mathrm{nW}$, which were recorded by infrared spectrometers (Bristol 771B), as described in Fig. 4(c). Owing to the spectral difference between the injected lights in the master and slave paths, the two MIR combs did not completely overlap in the spectral profile.

To verify the feasibility of broadband MIR DCS, a beam-splitting mirror was used to combine two comb pulse trains that were directly reflected onto a balanced detector using a gold mirror. Temporal dual-comb interferograms were recorded using a high-speed data-acquisition card synchronized with a hydrogen clock. Figures 5(a)–5(c) show the recorded temporal interferograms with time scales of 21 periods, single periods, and partial periods, respectively. We observed that the repetition frequency difference of two combs was set to 42 Hz by finely adjusting the cavity length of the slave laser, resulting in an interferogram period of 23.81 ms. Moreover, when scaling up the partial period, as shown in Fig. 5(c), we observed evident tailing arising from gas relaxation absorption in the transmission path, which contains molecular information on structure and motion. We then sampled a 200 s dual-comb temporal signal with 8400 interferogram packages, and coherent superposition and averaging were performed to improve the SNR. After applying an FFT to the optimized temporal interferograms, we obtained the RF dual-comb spectrum, which can be converted to the optical frequency domain by multiplying by a factor of $f_r/\mathrm{\Delta }{f}_r$. The converted dual-comb spectrum with a spectral range of 3.227–4.703 μm is exhibited in Fig. 5(d). Considering a spectral resolution of 100 MHz, we realized a broadband MIR dual-comb spectrometer with 300,000 comb-tooth-resolved frequency components. Moreover, four typical frequency windows at 3.2275, 3.6022, 4.0778, and 4.7033 μm are presented in Figs. 5(e)–5(h), respectively, exhibiting good mode-resolved capacity in MIR region. In detail, the MIR comb teeth linewidth was optimized to $\sim 1\mathrm{Hz}$, displaying an inherited consistency to the NIR combs [50]. This type of high-resolution spectrometer with a 30 THz spectral bandwidth will contribute to parallel gas spectral sensing in atmospheric environments.

Using a broadband MIR dual-comb spectrometer, we performed the parallel gas sensing of multiple mixed greenhouse molecules. As shown in Fig. 1, the MIR dual-comb lasers were combined to pass through an 8-cm-long gas cell filled with a gas mixture at a pressure of 50 mbar (component proportion: ${\mathrm{CH}}_4\text{:}{\mathrm{C}}_2{\mathrm{H}}_2\text{:}\mathrm{CO}\text{:}{\mathrm{N}}_2\mathrm{O}=1\text{:}5\text{:}3\text{:}1$). At room temperature (22°C), we sampled 200 s dual-comb temporal interferograms with 8400 periods, which were coherently averaged to implement FFT. Figure 6(a) shows the converted comb-tooth-resolved absorption spectra in the atmospheric window at a frequency resolution of 100 MHz. The spectral absorption peaks of ${\mathrm{CH}}_4$, ${\mathrm{C}}_2{\mathrm{H}}_2$, CO, and ${\mathrm{N}}_2\mathrm{O}$ (gas cell), together with those of ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{CO}}_2$ (open path), were clearly observed. Moreover, we scaled up eight typical frequency windows to present the measured molecular absorption lines (blue line), as depicted in Figs. 6(b)–6(i), which are in good agreement with the HITRAN database (red line). The average SNR of the dual-comb spectra as a function of the average time is shown in Fig. 6(j). The fitting slope is 0.503 in the exponential coordinate system, indicating that the average SNR is proportional to the square root of the average time. Therefore, the SNR can be effectively improved by increasing the measurement time, thereby helping solve weak-absorption situations. At an average time of 200 $\mathrm{s}$, the average SNR of the comb teeth was optimized to 32.59, corresponding to a quality factor of $7.34\times {10}^5$. Compared with NIR DCS, the gas sensing sensitivity can be significantly improved to $<10\mathrm{ppm}$ (${\mathrm{CH}}_4$; ppm, parts per million). However, the spectral range of this dual-comb spectrometer is still limited by the sensitivity of the BPD, which has an effective response wavelength only at 3–5 μm. Using Fourier transform infrared (FTIR) spectroscopy, we can perform spectral analysis at 2.7–5.2 μm. Therefore, a detector with the higher response and broader bandwidth will improve the dual-comb spectral measurement range. This PPLN-based scheme can be integrated using an all-fiber structure and fiber-waveguide edge coupling [42], and the spectral range can be converted to visible and UV by postpositive high-order harmonic generation in cascaded PPLN structure [51]. As expected, the MIR dual-comb spectrometer is capable of broadband high-resolution spectral analysis, providing a novel integrated spectral measurement device for non-damaging gas parallel sensing in MIR atmospheric windows.

In summary, we demonstrate the parallel sensing of multiple greenhouse gases using a broadband MIR dual-comb spectrometer. First, the NIR comb was divided into pump and signal branches for the MIR DFG, and the spectra were separately optimized using two HNLFs. Then, PPLN waveguides were processed, and we successfully achieved DCS in the range of 3.2–4.7 μm with a total of 300,000 comb-tooth-resolved frequency lines. In addition, a parallel analysis of four mixed gas molecules (${\mathrm{CH}}_4$, ${\mathrm{C}}_2{\mathrm{H}}_2$, CO, and ${\mathrm{N}}_2\mathrm{O}$) was performed with a 100 MHz spectral resolution, and the measured molecule absorption peaks were in good agreement with the results of the HITRAN database. We believe that this type of DFG-based broadband MIR spectrometer, which mainly consists of a small fiber oscillator, fiber amplifiers, and PPLN waveguide chips, has good application potential for parallel gas analysis in atmospheric open-path environments.