Dual-input Dual-output (DIDO)/Multi-input Multi-output (MIMO)-based Off-axis Integrated Cavity Output Spectroscopy for Cavity Mode Noise Suppression2

Off-axis integrated cavity output spectroscopy (OA-ICOS)-based gas sensor is widely used in industry, environment and other fields due to its advantages of high sensitivity, good selectivity, and in-situ detection capability. In OA-ICOS, once the scanned laser is coupled into the cavity, the mode amplitude will fluctuate due to the jitter of the wavelength between different modes, resulting in the generation of cavity mode noise. Mode noise is the main noise type in OA-ICOS, in which a small gas absorption signal needs to be resolved from the cavity mode baseline. Therefore, a smooth absorption spectrum with low cavity mode noise is particularly important for trace gas sensing. However, how to eliminate the mode noise is still a key issue to be solved. Herein, we demonstrated a novel dual-input dual-output (DIDO)/multiinput multi-output (MIMO) laser-to-cavity coupling scheme of OA-ICOS for cavity mode noise suppression. By dividing the laser beam into multiple parts and coupling into the cavity at the same time, multiple beams were transmitted back and forth in the cavity without interference. In this way, the cavity mode density can be effectively increased, also the light intensity fluctuation caused by the coherent oscillation in the cavity can be effectively reduced. Therefore, the cavity mode noise can be decreased and the detection sensitivity can be improved. In this work, theoretical and experimental investigation was performed through methane (CH
4)
detection for DIDO pattern. The noise suppression factor was improved by a factor of 2.2 both in simulation and experiment compared to the regular single-input single-output (R-SISO) approach. For MIMO pattern, Theoretical analysis was carried out to explore the relation between the noise suppression factor and the number of splitting beam. Compared to R-SISO, a maximum noise suppression factor of 7 was obtained with the maximum 14 splitting beams. This work reveals a novel method to eliminate the cavity mode noise and provides a new idea for improving the OA-ICOS sensor performances.