Photoacoustic Brillouin spectroscopy of gas-filled anti-resonant hollow-core optical fibers

Photoacoustic spectroscopy, a powerful tool for gas analysis, typically uses bulky gas cells and discrete microphones. Here we exploit light-gas-acoustic interaction in a gas-filled anti-resonant hollow-core-fiber (AR-HCF) to demonstrate photoacoustic Brillouin spectroscopy (PABS). Pump absorption of gas molecules excites the acoustic resonances of the fiber, which modulates the phase of a probe beam propagating in the fiber. Detection of the phase modulation enables spectroscopic characterization of gas species and concentration as well as the fiber microstructure. Studying the acoustic resonances allows us to characterize the longitudinal inhomogeneity of the fiber microstructure. By tuning the pump modulation frequency to a wine-glass-like capillary mode of a 30-cm-long AR-HCF and the pump wavelength across a gas absorption line, we demonstrate detection of acetylene at the parts-per-billion level. PABS has great potential for high sensitivity gas sensing and non-invasive fiber characterization.