Compact trace gas sensing via single-cavity balanced-detection interferometric cavity-assisted photothermal spectroscopy

• New balanced detection architecture for photothermal gas sensing, featuring: a reduced system complexity by utilizing a single 1-mm air-spaced Fabry-Perot interferometer, an improved signal-to-noise ratio by a factor of 18, a gas cell having a sample probe value of only 1.8 mL. • Robust sensing scheme: Refractive index changes which are induced by a mid-infrared excitation laser are monitored by a near-infrared probe laser intersecting the excitation beam in transverse configuration. • CO gas sensing achieving a minimum detection limit of 2 ppbv, which corresponds to a NNEA of 7.4 x 10 −9 cm −1 W Hz −1/2 . • Investigation of the influence of a varying water vapor concentration on the molecular relaxation process of CO at a modulation frequency of ∼ 300 Hz. This work reports on the implementation of an alternative balanced-detection scheme to the ICAPS sensing method employing solely a single cavity. The new concept was realized by simultaneous detection of the interferometer’s reflectance and transmission. The use of only one cavity significantly reduces the system’s complexity in a balanced configuration by simplifying the detection architecture. Additionally, it increases the sensor’s sensitivity by noise cancellation and an improvement of the detectable signal up to a factor of 2. The set-up employed an optical cavity with a mirror spacing of 1 mm. A mid-infrared laser served as excitation source to induce refractive index changes in the sample, and a near-infrared laser served as probe source to monitor the photo-induced variations. The sensor’s metrological figures of merit were investigated by detection of CO. Moreover, the influence of varying water vapor concentration on the molecular relaxation of CO and thus the monitored photothermal signal employing a modulation frequency of ∼ 300 Hz was investigated. For the targeted absorption band centered at 2179.77 cm −1 a 1σ minimum detection limit of 2 ppbv was achieved using an integration time of 1 s. This result corresponds to a normalized noise equivalent absorption of 7.4 × 10 −9 cm −1 W Hz −1/2 .