Compact Trace Gas Sensing by Balanced-Detection Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS)

The miniaturization of sensitive as well as selective laser-based gas detectors is of big request among different fields of activity due to specific characteristics such as a fast sensor response or simply a small footprint. However, it still remains challenging. While methods based on direct absorption spectroscopy show a limited potential for miniaturization due to their dependence of sensitivity on the optical path length according to the Lambert-Beer law, indirect spectroscopic techniques of photothermal nature inherently exhibit high miniaturization potential, even down to integration onto a chip.
The Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of a Fabry-Perot interferometer (FPI) as transducer for photothermal spectroscopy. The implementation of a balanced detection scheme to our developed system is a key improvement, which enhances the sensor’s performance by efficient cancellation of noise.
Within the presentation, recent results of a setup employing individual interferometers will be shown. Here, balanced-detection was realized by using two identical cavities having a path length of 1 mm and a total sample gas volume of a few mm³. The system uses an all fiber-coupled probe laser configuration, which detected the reflectance of the interferometers, enabling sensor operation close to the fundamental limit of shot noise. The metrological figures of merit were investigated by detection of different trace gases such as SO2, CO and NO using QCLs as powerful mid-infrared excitation sources. The induced refractive index changes were monitored by a near-infrared probe laser. For the targeted molecules a minimum detection limit down to the sub-ppbv level was achieved with a 1s integration time, corresponding to a normalized noise equivalent absorption of the order of 10⁻⁹ cm⁻¹ W Hz^−1/2.
Additionally, latest progress regarding sensor miniaturization will be discussed. FPIs made solely from single-crystalline silicon with Bragg mirrors consisting of silicon-air dielectric multilayers [2] were designed and fabricated. The FPIs allow easy coupling of the near-infrared beam by optical fibers positioned in the chip along aligned grooves. First demonstration of ICAPS gas sensing employing a silicon FPI will be shown.

References
[1] J. P. Waclawek, H. Moser, and B. Lendl, “Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe laser configuration,” Opt. Express 29, 7794-7808 (2021).
[2] M. Malak Karam, et al., “Micromachined Fabry–Perot resonator combining submillimeter cavity length and high quality factor”, Appl. Phys. Lett., 21 (98), id. 211113 (2011).