High-sensitivity dual-comb and cross-comb spectroscopy across the infrared using a widely-tunable and free-running optical parametric oscillator

Coherent dual-comb spectroscopy (DCS) enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning approaches. However, high system complexity and limited measurement sensitivity remain major challenges for DCS. Here, we address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with an up-conversion detection method. The OPO is tunable in the short-wave infrared (1300-1670 nm range) and mid-infrared (2700- 5000 nm range) where many molecules have strong absorption bands. Both OPO pump beams are generated in a single spatially-multiplexed laser cavity, while both signal and idler beams are generated in a single spatially-multiplexed OPO cavity. The near-common path of the combs in this new configuration enables comb-line-resolved and aliasing-free measurements in free-running operation. By limiting the instantaneous idler bandwidth to below 1 THz, we obtain a high power per comb line in the mid-infrared of up to 160 $\mu$W. With a novel intra-cavity nonlinear up-conversion scheme based on cross-comb spectroscopy, we leverage these power levels while overcoming the sensitivity limitations of direct mid-infrared detection, leading to a high signal-to-noise ratio (50.2 dB Hz$^{1/2}$) and record-level dual-comb figure of merit (3.5\times 10^8 Hz$^{1/2}$). As a proof of concept, we demonstrate the detection of methane with 2-ppm concentration over 3-m path length. Our results demonstrate a new paradigm for DCS compatible with high-sensitivity and high-resolution measurements over a wide spectral range.