Leaky Hollow-Core Fiber-Based All-Optical Photoacoustic Gas Sensor for Highly Sensitive Detection of Carbon Dioxide.

Photoacoustic (PA) gas sensing technology is well known for its high sensitivity. However, the sensitivity decays when detecting carbon dioxide because of the long vibrational-translational (V-T) relaxation time. In this work, we systematically exploited and optimized a leaky hollow-core fiber (LHCF)-based all-optical low-frequency resonant photoacoustic gas sensor via theoretical modeling, finite element analysis, and experimental validation. An optimized LHCF-PA gas sensor with an operation frequency of 1020 Hz was constructed to limit the influences from the long V-T relaxation time. The total gas volume of the gas sensor was only ∼6 mL. Experimental results showed that the minimum detectable limit (MDL) could be as low as 238.4 ppb when the lock-in integration time was 227 s. The normalized noise equivalent absorption (NNEA) coefficient was calculated to be 1.4 × 10-9 W·cm-1·Hz-1/2. This work develops a model for considering the gas viscous damping, light transmission loss, and the V-T relaxation effect to help optimize the geometrical parameters of the LHCF, which paves the way to design and optimize a low-frequency resonant LHCF-PA cell for highly sensitive detection of other gas molecules with a long V-T relaxation time.