Nested Ring Resonant Sagnac-Enhanced Photothermal Gas Sensing Using a Nanofiber

Photothermal (PT) spectroscopy exhibits exceptional selectivity and high sensitivity for trace gas detection, yet developing PT gas sensors that combine fast response, high sensitivity, and practical usability remains a significant challenge. Here, we demonstrate a nested ring resonant Sagnac-enhanced PT gas sensor using a nanofiber. By inserting a nested ring resonator within the Sagnac loop, the proposed sensor reduces the full width at half-maximum of the resonance peak, thereby enhancing the cavity finesse and improving the sensitivity of PT phase modulation. With a 2 cm-long nanofiber and a resonator finesse of 73, the sensor achieves precise C₂H₂ detection featuring a noise equivalent concentration of 16 ppb, a 19 s response time, and <3% instability over 2 h. Compared to a nonresonant Sagnac interferometer using the same nanofiber, this configuration improves sensitivity by a factor of 10. By amplifying the PT effect within a short nanofiber, the sensor successfully integrates fast response with high sensitivity, overcoming the response delay typically associated with long sensing fibers. Furthermore, the sensor operates without complex wavelength-locking mechanisms, simplifying its operation and rendering it suitable for applications in remote monitoring and distributed sensing networks.