A reconfigurable and compact circular multi-pass sensor for ultra-sensitive methane detection

Abstract

Atmospheric methane (CH4) monitoring requires an optimal balance between ultra-high sensitivity and mechanical field-readiness. Conventional multi-pass architectures, such as Herriott cells, are fundamentally constrained by rigid geometries and alignment sensitivities that limit real-time optimization capability. To overcome this limitation, we introduce an angularly tunable circular multi-pass absorption cell (CMPC) integrated with digital wavelength modulation spectroscopy (WMS). By leveraging a 20 cm radius “optical cage” architecture, continuous optical path-length tunability is achieved through precise beam injection angle control, resulting in a large effective optical path length of 134.8 m via 337 reflections. Targeting the R4 transition near 1.651 μm ensures strong spectral isolation from ambient H2O and CO2 interference. The system incorporates a balanced dual-beam detection scheme that suppresses common-mode noise and laser intensity fluctuations, yielding a signal-to-noise ratio (SNR) of 180 and a short-term precision of 50.1 ppb. Allan deviation analysis confirms a detection limit of 55 ppb at 1 s integration time, reaching an ultimate precision of 6.25 ppb at an optimum integration time of 841 s. These results demonstrate that the proposed architecture provides a compact, mechanically robust, and highly sensitive alternative to conventional fixed-geometry methane sensing systems.