Gas detection with quantum cascade lasers: An adapted photoacoustic sensor based on Helmholtz resonance

A photoacoustic gas sensor exploiting a quantum cascade laser as a radiation source is demonstrated. A detection limit of ∼1 ppm with 1 ms response time is found using a Peltier-cooled Fabry–Pérot InGaAs-based quantum cascade laser emitting at 9.4 μm, and a commercial microphone as a detector. The photoacoustic cell consists of a Helmholtz resonator preceeded by a low-pass acoustic filter. This geometry is well adapted to the shape of the laser beam and allows for an effective filtering of ambient acoustical noise. The relative simplicity of the system is particularly attractive for applications where sensitivity, robustness, and ease of fabrication are all fundamental requirements.