Side-polished fiber evanescent wave quartz-enhanced photoacoustic spectroscopy employing dielectric coatings for evanescent field enhancement

We report an all-fiber laser gas analyzer (LGA) based on quartz-enhanced photoacoustic spectroscopy (QEPAS) that exploits strong evanescent wave (EW) enhancement using a dielectric coating on side-polished fiber. The dielectric coating increases the fraction of the evanescent field in air, significantly amplifying light-gas interaction within the polished region. A single-mode fiber with a 17 mm polished section passes through two millimeter-scale resonator tubes and a custom quartz tuning fork (QTF) with 0.8 mm prong spacing. The optimized EW coupling efficiently generates photoacoustic waves that excite the QTF's fundamental flexural mode. Methane-nitrogen mixtures at 800 mbar were used to evaluate performance, achieving a detection limit of 2.5 ppmv for CH₄ with 300 ms integration time. By enhancing the evanescent interaction within a compact, robust fiber geometry, this EW-QEPAS sensor eliminates free-space optics and offers a miniaturized, field-deployable solution for gas detection in industrial and agricultural environments.