The mid-infrared (MIR) region, with wavelengths spanning from 2 to 20 µm, is of paramount importance in spectroscopy due to its rich molecular absorption features^[1–3]. This region encompasses the fundamental vibrational and rotational transitions of many molecules, providing a unique “molecular fingerprint”^[4] that enables precise identification and analysis of chemical compounds. As a result, MIR spectroscopy has become an indispensable tool in fields such as environmental monitoring^[5], biomedical diagnostics^[6,7], and material science. However, the complexity of molecular spectra in this region demands high-resolution and broadband measurement capabilities to fully exploit its potential^[8,9].

Optical frequency combs (OFCs) have emerged as a transformative technology in spectroscopy, offering unparalleled precision and broad spectral coverage^[10–12]. By generating a series of equally spaced frequency lines, OFCs enable high-resolution measurements across wide bandwidths, making them ideal for studying complex molecular structures. In the MIR region, OFCs have been successfully applied to trace gas detection, combustion analysis, and the study of ultrafast molecular dynamics^[13–16]. Despite these advancements, a significant challenge remains: conventional OFCs often struggle to simultaneously achieve narrow line spacings and large frequency spans. This limitation restricts their ability to resolve closely spaced spectral features, which is critical for detailed molecular analysis.

The resolution of OFC-based spectroscopic systems is fundamentally limited by the linewidth of the comb lines rather than their spacing. While the linewidth of OFC can be as narrow as a few kHz, the typical spacing between comb lines is on the order of hundreds of MHz or even GHz^[17]. This gap between linewidth and line spacing presents a barrier to achieving the high resolution required for advanced spectroscopic applications. To address this challenge, spectral interleaving has been proposed as a promising technique^[18–21]. By sweeping the center frequency of the OFCs and combining interleaved measurements, it is possible to achieve spectral resolution beyond the native comb line spacing, effectively bridging the gap between high resolution and broad bandwidth. OFCs generated based on electro-optic modulation offer high flexibility in setting the spectral bandwidth and line spacing^[22,23]. Based on the generation mechanism of electro-optic frequency combs (EOFCs), a broadband and high-resolution dual-comb interferometer (DCI) can be produced through methods such as cascaded modulation and spectral interleaving^[24,25].

In this paper, we present an experimental scheme for generating MIR DCI with high resolution. At 1550 nm, we utilize a dual-drive Mach-Zehnder modulator (DD-MZM) to generate EOFCs with a repetition rate of 18 GHz and 27 comb lines. Utilizing the injection locking^[26], we generate a linearly frequency-swept lightwave spanning a bandwidth of 18 GHz. This swept lightwave serves as the center wavelength, enabling the entire EOFC to shift in the frequency spectrum. Theoretically, gapless spectral measurement across the full bandwidth of the electro-optic comb can be achieved. This method for generating high-resolution OFC spectroscopy is termed spectral interleaving. The interleaved spectrum generated at 1550 nm is converted to the MIR region through a difference frequency generator (DFG). Consequently, we obtain a dual-comb spectrum with a bandwidth of 486 GHz and a frequency resolution of 100 MHz at 3.3 µm. Our demonstration indicates that higher-frequency accuracy can be achieved under conditions of longer swept lightwaves and higher nonlinear efficiency.

The specific experimental setup of the high-resolution MIR dual electro-optic comb interferometer is illustrated in Fig. 1(a). A coherent continuous-wave (CW) narrow linewidth fiber laser (NKT Adjustik) centered at 1550 nm serves as the seed laser for the system. The CW laser is modulated by a Mach-Zehnder modulator (MZM). The MZM is driven by the arbitrary waveform generator (AWG, Keysight M8195A). The driven signal is a sinusoidal radio frequency signal with a linear frequency sweep from 6 to 15 GHz in 3.6 ms. The linearly swept optical wave generated by modulation passes through a circulator and is injected into a distributed feedback (DFB) laser without an isolator. The AWG generates a current amplitude-modulated signal to drive the DFB laser, aligning the center wavelength of the DFB laser with the positive second-order sideband of the swept lightwave. Based on the injection locking effect, a linearly swept optical wave with a bandwidth of 18 GHz is generated. The swept lightwave is modulated by two dual-drive Mach-Zehnder modulators (DD-MZMs) to generate dual EOFCs with line spacings of 18 and 17.999 GHz, featuring a repetition frequency difference of 1 MHz. The electrical signals generated by signal generators (SGs) are amplified to drive the DD-MZMs. An acousto-optic modulator introduces a frequency shift of 40 MHz to avoid overlapping between the positive and negative sidebands in the probe and local branches. The dual comb at 1550 nm serving as a signal laser is collimated and input to the DFG system. Then, it results in MIR DCI, which is detected by an MIR photodetector (MPD, PVI-4TE) and digitized by a digital oscilloscope (OSC, Keysight DSOS204A).

The experimental setup of the DFG system is illustrated in Fig. 1(b). The 1550 nm lightwave from the dual-comb interferometer, amplified by an erbium-doped fiber amplifier (EDFA), is collimated and output as the signal lightwave. Meanwhile, a 1064 nm CW laser, amplified by an ytterbium-doped fiber amplifier (YDFA), is collimated and output as the pump lightwave. The two half-wave plates serve to align the polarization of the signal lightwave and the pump lightwave. After spatial alignment, the two beams are focused and input into a magnesium oxide-doped periodically poled lithium niobate (MgO-PPLN) crystal. The temperature of the crystal is set according to the quasi-phase matching condition. Ultimately, an MIR lightwave is generated after passing through a long-pass filter (LPF).

Due to the limitation of a 32 GS/s sampling rate, the AWG is unable to directly generate an electrical frequency-swept signal with a bandwidth of 18 GHz. Therefore, we utilize injection locking technology to lock the DFB laser onto the second-order sideband generated by the MZM^[27]. This allows us to drive the MZM with an electrical frequency-swept signal of 9 GHz bandwidth generated by the AWG. As a result, the second-order sideband produced by the MZM modulation becomes a swept lightwave with a bandwidth of 18 GHz. The prerequisite for this scheme is that the MZM operates under a bias voltage condition that amplifies even-order sidebands. Figure 2(a) illustrates the output spectrum of the MZM operating at a bias voltage setting that enhances even-order sidebands. The second-order sidebands generated by modulation can achieve a doubling of the modulation frequency. However, when the modulator operates in a state where even-order sidebands are amplified, the carrier is 10 dB higher than the second-order sidebands. To produce an ideal linear frequency-swept signal, only one of the symmetrical second-order sidebands should be retained, while the carrier and other sideband frequency components are filtered out. Figure 2(b) illustrates the spectrum after lock-amplifying the second-order sideband utilizing the injection locking effect. Due to the injection locking effect, the second-order sideband is selected and amplified to 5 dBm and the carrier suppression ratio is more than 40 dB.

The lightwave output from the MZM exhibits swept frequency bands in both the positive and negative sidebands. In the experimental system, spectral interleaving only requires the swept frequency band of one sideband. Typically, in the field of optical communications, optical filters are used to remove unwanted frequency bands. As shown in Fig. 3(a), we employ an optical tunable filter (OTF) to remove signals outside the swept frequency band of the positive second-order sideband. Figure 3(c) presents the optical spectrum after filtering with the OTF. It can be observed that the OTF removes most of the unwanted frequency bands. However, due to the inherent non-flatness in the amplitude-frequency responses of both the AWG and the OTF, this results in a non-flat amplitude spectrum of the swept frequency signal. Furthermore, the OTF cannot completely remove the carrier and the positive first-order sideband, which can degrade the signal-to-noise ratio (SNR) of the swept lightwave.

As shown in Fig. 3(b), the modulated lightwave acting as the master laser is injected into an isolator-free DFB laser acting as the slave laser via a circulator. The amplitude-modulated digital current signal output from the AWG serves as the driving signal for the DFB laser, causing the central wavelength of the DFB laser to follow the frequency-swept wavelength of the master laser. Due to the injection locking effect, the second-order sideband is locked and amplified to 5 dBm. Figure 3(d) presents the spectrum of the locked and amplified positive second-order sideband. It can be observed that the carrier and other sidebands are suppressed, resulting in a very flat frequency-swept amplitude spectrum.

The linearly frequency-swept lightwave serves as the seed laser for the dual electro-optic comb interferometer, entering two DD-MZMs to generate EOFCs. A broadband flat electro-optic comb spectrum is generated by controlling the direct current (DC) bias voltage of the DD-MZM and the initial phase of the radio frequency (RF) signal^[28,29]. Figure 4(a) presents the spectrum measured by a spectrometer for the probe comb when the seed laser is a CW laser. Figure 4(b) presents the optical comb spectrum measured by the spectrometer for the probe comb when the seed laser is a frequency-swept lightwave. Due to the utilization of injection locking technology, the swept lightwave exhibits stable power, resulting in a flat spectrum to cover the gaps between the comb lines.

The interleaved dual-electro-optic comb interferometer signal is converted to the MIR region by the DFG system and directly detected by an MPD. According to the principle of spectral interleaving, the upper limit of spectral resolution for the MIR interleaved optical comb is determined by both the Nyquist sampling rate upper limit and the frequency sweep linearity of the swept lightwave^[30]. The measured standard deviation of nonlinear frequency errors in the linearly frequency-swept lightwave is approximately 20 kHz. Therefore, the theoretical resolution limit of this scheme is sub-MHz. However, due to the limited nonlinear efficiency of the DFG system, an averaging time of 20 µs is required during the demodulation to obtain a complete MIR spectrum, and the SNR of each comb line is above 10 dB. An averaging time of 20 µs corresponds to a frequency resolution of 100 MHz for an 18 GHz bandwidth swept signal over 3.6 ms. Figure 5(a) presents the demodulated spectrum of MIR DCI with a line spacing of 100 MHz and a bandwidth of 486 GHz at 3.3 µm. Figure 5(b) presents the zoomed-in figure of Fig. 5(a) around 90.8 THz.

In conclusion, we proposed a novel high-resolution MIR DCI system based on spectral interleaving. We utilized electro-optic modulation and injection locking to generate a swept lightwave with a bandwidth of 18 GHz and a duration of 3.6 ms. The swept lightwave serves as the seed laser to generate interleaved dual electro-optic combs with a line spacing of 18 GHz. Through DFG, we generated an interleaved MIR dual-comb interferometer at 3.3 µm. Ultimately, we demodulated an MIR comb spectrum with a bandwidth of 486 GHz and a resolution of 100 MHz. This scheme is not yet fully optimized and it is believed that it can be further improved. It holds potential for generating gapless interleaved optical comb spectroscopy with kHz-level resolution in the MIR region.