Enhanced spectral resolution in mid-infrared dual-comb spectroscopy via synchronous offset frequency tuning

Mid-infrared dual-comb spectroscopy offers significant advantages by combining the high sensitivity of mid-infrared spectroscopy with the high spectral resolution and rapid acquisition of the dual-comb method. However, its effective resolution, constrained by the inherent comb line spacing, hinders its ability to resolve narrow absorption features, common in critical applications such as sub-Doppler spectroscopy, low-pressure gas analysis, and construction of the atmospheric profile. To address this challenge, we present a synchronous offset frequency tuning method for the mid-infrared dual-comb system to improve effective resolution far beyond comb line spacing. In our system, the mid-infrared dual-comb source is generated from a near-infrared dual-comb source and a continuous-wave pump laser via difference frequency generation in a single periodically poled lithium niobate bulk. By adjusting the phase-lock frequency of the pump laser to one of the near-infrared combs, we synchronously tune the offset frequencies of both mid-infrared combs without changing the near-infrared dual-comb source. We demonstrated that this method enabled the high resolution of overlapped spectral lines of ethane around 3000 cm^-1, achieving a uniform spectral sampling interval of 10 MHz in the interleaved spectrum and a 25-fold enhancement in effective resolution. This approach allows for sub-MHz spectral resolution in mid-infrared dual-comb spectroscopy without any modifications to the data acquisition system, offering possibilities for high-precision spectral analysis.