A Four-Mirror Incoherent Broadband Cavity-Enhanced Absorption Spectrometer for Trace Gas Detection.

A four-mirror resonator-based incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) is demonstrated for the first time for sensitive trace gas detection. The proposed geometry addresses limitations of conventional linear cavities by enabling enhanced alignment flexibility and multiplexed detection through multiple output ports, allowing selective sensitivity optimization for simultaneous multispecies measurements. The resonator, constructed using four high-reflectivity dielectric mirrors (∼99.9% and ∼99.98% over 400-550 nm), provides a long effective optical path length within a compact footprint, while the transmitted light is detected using a CCD array for high-resolution broadband analysis. The optical stability of the cavity is evaluated theoretically based on cavity length and mirror radius of curvature. Laboratory experiments demonstrate a detection limit of 0.3 Mm1- at 60 s averaging in the 480-550 nm region. Instrument performance is validated using calibrated concentrations of nitrogen dioxide and iodine. Owing to its flexibility, stability, and capability for selective sensitivity enhancement, the proposed system is well suited for field deployment and identifying wide range of gas detection in space-constrained environments.