Traditional high-finesse fiber Fabry–Pérot (FP) cavities are often bulky, expensive, and rely on tunable laser sources. This work presents a low-cost, portable open-fiber cavity platform based on single-pulse CO₂-laser micromachining, which sculpts concave mirrors directly onto single-mode fiber end-faces. The resulting mirrors have a radius of curvature of approximately 160 µm and are coated for high reflectivity at 1550 nm, supporting both plane–concave and symmetric concave–concave cavity geometries.

Gaussian beam simulations covering cavity lengths of 0–160 µm (plane–concave) and 0–320 µm (concave–concave) identify design windows with mode-size mismatch below 5% and diffraction loss under 0.5 dB. A single 1 ms, 1 Hz CO₂-laser pulse produces concave surfaces with <10 nm RMS roughness, verified using a KEYENCE VK-X3000 optical profilometer. For a 100 µm cavity, the measured free spectral range is ~7.5 nm, and the resonance envelope closely follows an Airy profile. A finesse above 50 is achieved using a fixed-wavelength 1550 nm laser and a piezo-driven cavity length scan.

A proof-of-concept photoacoustic CO₂ gas sensing experiment demonstrates a two-fold signal-to-noise improvement compared to a commercial cavity of similar length. These results highlight the potential of CO₂-laser-fabricated fiber mirrors and fixed-wavelength laser operation for alignment-free, compact, and field-deployable Fabry–Pérot systems.

Keywords: fiber Fabry–Pérot cavity, CO₂-laser micromachining, concave micro-mirror, free spectral range, portable gas sensing