Polarization-mode-phase-difference photothermal gas sensing with an optical microfiber coupler

We report an evanescent-wave photothermal interferometry gas sensor by measuring the phase difference between two polarization modes in a single-mode bi-conical tapered microfiber coupler. With a 2-cm-long, wavelength-scale-diameter microfiber coupler, the noise-reduction capability to external perturbations is significantly improved, resulting in a detection limit of 28 ppb for acetylene with a 1-s lock-in time constant.