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.