Direct 3D printing of Vernier-enhanced Fabry-Pérot interferometers on fiber-tips for compact gas sensors

The detection of trace gases is crucial in environmental monitoring, industrial safety, and medical diagnostics. Optical sensing technologies, particularly those leveraging photothermal spectroscopy, offer high sensitivity and selectivity, enabling the identification of gases based on their unique absorption spectra. Among these, photothermal interferometry offers exceptional sensitivity due to its use of an interferometric signal transducer. In this work, we performed numerical simulations to systematically explore the influence of cavity geometry and mirror curvature on sensitivity. This guided the design of the most sensitive configurations. To validate the theoretical enhancement, we present a systematic comparison of 18 Fabry-Pérot interferometers (FPI) fabricated via two-photon polymerization (2PP) directly onto optical fiber-tips. These FPIs were rapidly prototyped using a commercial 2PP printer. They span three cavity lengths (110, 200, and 300 µm), each configured with flat or spherical mirrors. Single-cavity and Vernier-enhanced FPIs were implemented. The latter were also modified by gold coating of the terminal interface to enhance reflectivity. We evaluated the sensitivity optimization for collinear photothermal spectroscopy in a wavelength modulation setup. By exploiting the Vernier effect and tailored cavity geometries, we demonstrate a 12-fold improvement in the photothermal 2f-signal compared to a single-cavity FPI configuration. This highlights the versatility of 2PP-printed fiber-tip FPIs for next-generation trace gas sensors.