Off-axis cavity-enhanced dual-comb absorption spectroscopy

High-sensitivity frequency comb spectroscopy can be realized using optical cavities, which typically requires precise alignment between cavity modes and comb modes. Here, we introduce an off-axis cavity-enhanced dual-comb absorption spectroscopy approach that delivers high spectral resolution and sensitivity, while enabling robust and efficient coupling of dual-frequency combs to an optical cavity. This method eliminates the need for complex optoelectronic feedback control. Utilizing an off-axis injection scheme, we demonstrated effective comb-cavity coupling across various dual-comb parameters, successfully allowing dual-combs to be passively coupled into the optical cavity simultaneously. As a proof of concept, we measured Doppler-broadened transitions of acetylene (C₂H₂) using an electro-optic dual-comb system in the near-infrared. For a concentration of 800 ppm at 0.1 atm, we measured C₂H₂ absorbance over a 350 GHz bandwidth with a spectral resolution of 200 MHz, yielding an enhancement factor of 380 and a signal-to-noise ratio of 335. This method offers significant advantages for high-resolution and high-sensitivity dual-comb spectroscopy for potential field deployment.