Near-infrared methane sensor system using off-axis integrated cavity output spectroscopy with novel dual-input dual-output coupling scheme for mode noise suppression

Mode noise suppression in off-axis integrated cavity output spectroscopy (OA-ICOS) is a key for improving the signal-to-noise ratio (SNR) of a sensor system, which basically depends on the ability to smooth out the cavity mode structure and to reduce the cavity mode linewidth. We demonstrated a novel dual-input dual-output (DIDO) coupling scheme of OA-ICOS for mode noise suppression. The influences of beam splitting ratio and light reflection number to the output intensity and cavity mode linewidth were theoretically studied. Experimental investigation on the suppression of cavity mode noise was carried out in terms of detection sensitivity and SNR through methane (CH₄) measurements. A SNR of 221 and a detection sensitivity of 1.73 × 10⁻⁸ cm^-1 Hz^-1/2 was obtained for the DIDO-based OA-ICOS sensor system. The SNR was improved by a factor of ∼ 2.5 and the sensitivity was improved by a factor of ∼ 2.2 compared to the regular single-input single-output (R-SISO) approach. Multi-input multi-output (MIMO) configuration was further numerically studied to verify the noise suppression ability of this detection concept. This work reveals a new laser-to-cavity coupling method and provides a novel way to exploit OA-ICOS sensors with improved SNR and sensitivity.