Double-cycle circular cavity-enhanced Raman spectroscopy for trace gas detection

To address the challenges of weak signals and poor system stability in Raman spectroscopy for trace gas detection, an enhanced spectroscopy technique based on a circular confocal cavity is proposed. This technique is centered on a circular multi-pass cell comprising multiple independent spherical mirrors, whose unique confocal configuration affords the system exceptional stability and alignment tolerance. A double-cycle optical path, realized through an integrated retro-reflector, doubles the effective optical path length while simultaneously collecting both forward and backward scattered signals, thereby maximizing collection efficiency. Under ambient conditions, a limit of detection (LOD) of 19 ppm for carbon dioxide was achieved within a 20-s integration time. This apparatus paves the way for the development of portable, high-sensitivity Raman gas analyzers.