High-precision formic acid sensing using a whispering-gallery-mode microbubble resonator with integrated and reference-assisted temperature compensation

High-sensitivity detection of formic acid gas is crucial for industrial safety and environmental monitoring, yet temperature fluctuations often distort sensor responses and compromise measurement accuracy. Here, we present a chitosan-coated whispering-gallery-mode (WGM) microbubble resonator for formic acid sensing with effective temperature compensation. Two compensation strategies, namely fiber Bragg grating (FBG)-assisted correction and modal decoupling between the fundamental and higher-order WGMs, are implemented on the same platform and comparatively evaluated. The FBG-assisted strategy achieves superior accuracy, with a root mean square error (RMSE) of 0.99 ± 0.01 ppm, whereas the modal decoupling strategy yields an RMSE of 2.48 ± 0.67 ppm while offering improved compactness by avoiding additional optical components. The gas sensor exhibits a sensitivity of 3.34 pm/ppm over a concentration range of 0–120 ppm and an experimental limit of detection (LoD) of 1 ppm, together with excellent long-term stability, repeatability, and selectivity. These results demonstrate a practical and compact route for high-precision formic acid sensing under temperature-varying conditions, with promising prospects for environmental monitoring and industrial process control.