Ultrasensitive photoacoustic detection in a high-finesse cavity with Pound–Drever–Hall locking

We demonstrate an ultrasensitive photoacoustic sensor using a low laser power (4 mW) and high-finesse (>9000) optical cavity. The Pound–Drever–Hall (PDH) method is adopted to lock the external cavity diode laser at 1531.58 nm to the Fabry–Pérot cavity. By placing a photoacoustic cell inside the 130-mm-long optical cavity, we obtain an enhancement of more than 630 times in laser power for acetylene (C2H2) detection. The present photoacoustic spectroscopy (PAS) sensor achieves a normalized noise equivalent absorption coefficient of 1.1×10−11  cm−1 WHz−1/2, which is unprecedented sensitivity among all the current PAS sensors. Our results demonstrate the feasibility of merging PAS with a high-finesse cavity using PDH locking for ultrasensitive trace gas detection.