An optical cavity can be utilized as an excellent transducer for highly sensitive gas
detection with the application of photothermal spectroscopy, featuring the beneficial property
of an ultra-low absorption volume within a rugged sensing element. We report the novel
implementation of balanced detection in Fabry-Perot photothermal interferometry via two
identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous
cavity-based photothermal sensors was effectively rejected close to the fundamental limit of
shot noise. A quantum cascade laser served as mid-infrared excitation source to induce
refractive index changes in the sample, and a near-infrared fiber laser served as probe source
to monitor the photo-induced variations. The metrological qualities of the sensor were
investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm−1, a
5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a
normalized noise equivalent absorption of 7.5 × 10−9 cm−1 W Hz−1/2. Additionally, the sensor
showed excellent long-term stability, enabling integration times of a few thousand seconds.
