A double-walled functionalized single-ended top-opened ultrathin microbubble resonator for highly sensitive CO 2 sensing

This paper presents a carbon dioxide (CO 2 ) optical sensor based on a double-walled functionalized single-ended top-opened ultrathin microbubble resonator (DW-SO-UTMBR). The microbubble resonator is fabricated from silicon dioxide (SiO 2 ) capillary glass tubing using an improved arc discharge process, which facilitates the creation of ultra-thin walls measuring approximately 500 nm. An open structure is introduced at the apex of the microbubble. Functionalization layers of polyhexamethylene biguanide (PHMB) deposited on both the inner and outer walls enhance the interaction between the optical field and CO 2 molecules. This structural design enables the fundamental radial whispering gallery mode (WGM) excited in the sensor to achieve a high sensitivity of −1.09 pm/ppm and a low detection limit of 16.5 ppm, with a response time of approximately 130 seconds. Furthermore, the sensor demonstrates excellent low cross-sensitivity to variations in environmental temperature and humidity. The DW-SO-UTMBR is compact and straightforward to fabricate, offering an effective solution for high-performance CO 2 fiber sensing.