Highly sensitive photoacoustic gas sensor using a hyperbolic nonlinear resonator integrated with a near-concentric multipass cavity

A hyperbolic nonlinear resonator (HNR) was proposed for the first time to greatly increase the sensitivity of a photoacoustic gas sensor owing to its better acoustic resonance performance than traditional cylindrical resonators. The HNR with a small resonator diameter down to 6 mm was closely integrated into a near-concentric multipass cavity (MPC) to further enhance the signal. The MPC were positioned outside the HNR so that the mirror surfaces would not be contaminated by the vapor and particles contained in the target gas due to the direct contact. The laser was collimated at a designed incident angle and reflected 42 times within the MPC, leading to a 19-fold increase in the PA signal. The sensor was also intended for use in power transformer fault diagnosis by measuring the concentration of dissolved acetylene (C₂H₂) in insulating oil. A minimum detection limit (MDL) of 10 ppb was achieved with an integration time of 100 s. The sensor demonstrated a record-low normalized noise equivalent absorption (NNEA) coefficient of 4.56 × 10^-11 cm^-1·W/Hz^1/2.