Turning optical loss into interferometric gain: hybrid-mode Vernier photothermal spectroscopy for gas sensing

A hybrid-mode Vernier photothermal spectroscopy (HM-VPTS) sensing architecture is proposed, employing a Bragg hollow-core fiber (BHCF) to overcome the inherent sensitivity bottlenecks of conventional interferometric sensors. By exploiting coherent interference between a pair of dominant hollow-core modes and a corresponding pair of dominant cladding modes, the system establishes a mode Vernier effect (MVE) that optically amplifies weak photothermal phase shifts. Notably, this mechanism inherently enables common-mode noise suppression (CMS) without the need for sophisticated nanocoatings or active mechanical stabilization. Experimentally, despite an ultra-small gas-light interaction volume of merely 1.1 nL within the BHCF microcavity, the system achieves a minimum detection limit of 13 ppb at an integration time of 430 s, corresponding to a noise-equivalent absorption coefficient of approximately 1.4 × 10⁻⁸ cm⁻¹, with a response time of around 2.2 s. This compact configuration shows significant potential for in situ monitoring of dissolved acetylene in oil-immersed power transformers. Moreover, further miniaturization enhances its integration potential, laying the foundation for advanced applications, including robotic olfaction for embodied intelligence and environmental monitoring in space stations.