Simultaneous Multi-Species Detection Based on an Opposite Two-Way Off-Axis Cavity-Enhanced Absorption Spectroscopy

To address the limitations of conventional off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) operated in a one-way transmission configuration-such as the low utilization efficiency of the optical integrated cavity and the difficulty in achieving simultaneous multi-species detection-an OA-CEAS system with an opposite two-way coupled detection configuration is proposed in this work. Ray-tracing simulations using TracePro and optical field analysis based on MATLAB were employed to optimize and determine key system parameters, including the cavity mirror spacing, incident aperture position, incident angle, as well as the position, radius of curvature, and relative orientation of the re-injection mirror. Based on these optimizations, a high-precision OA-CEAS optical integrated cavity with an opposite two-way configuration was constructed. Furthermore, by integrating frequency-division multiplexing (FDM)–assisted wavelength modulation spectroscopy (WMS), the output beams of four tunable diode lasers with center wavelengths of 1.567 µm, 1.571 µm, 1.620 µm, and 1.653 µm were coupled into a single optical integrated cavity. Simultaneous detection of CO, CO₂, C₂H₄, and CH₄ was achieved by extracting the second-harmonic (2f) signals of the corresponding absorption transitions.

In addition, comparative analysis with the conventional one-way OA-CEAS configuration demonstrates that the concave mirror used for reflecting and focusing the detected beam can also act as a re-injection mirror, effectively promoting light re-entry into the cavity and significantly enhancing the intracavity optical power. As a result, both the signal amplitude and the signal-to-noise ratio are improved by approximately a factor of 1.5, leading to enhanced detection sensitivity. This work highlights a new strategy for simultaneous multi-species detection using multiple lasers in a single optical integrated cavity, which improves cavity utilization efficiency, reduces system cost, and broadens the application prospects of OA-CEAS for complex gas mixture sensing.