Applications of HC-ARFs in PAS for gas sensing. (a) Experimental setup of the HC-ARF-based PABS system [102]. (b, c) Flexible longitudinal photoacoustic resonator based on HC-ARF [103]: (b) schematic structure of the resonator; (c) normalized acoustic pressure of the second-order eigenmode inside the resonator under different bending radii. (d, e) H₂-filled HC-ARF used as a PAS excitation light source [104]: (d) absorption spectrum of CH₄ and laser spectra around 1650 nm at different laser temperatures; (e) central wavelength of the sixth-order Stokes line as a function of laser temperature.