HC-ARF gas sensors based on PTS. (a) Experimental setup of a heterodyne interferometric MZI-PTS gas sensor based on a HC-ARF [94]. (b, c) FPI-PTS-based ¹³CO₂ gas sensor [96]: (b) schematic of the F-P cavity; (c) schematic of the experimental setup. (d) Comparison of two FPI sensing processes: top, at the quadrature point; bottom, at the resonance point with wavelength dither [98]. (e) Schematic of the MPD-PTS principle [51]. (f, g) Comparative experiment on the sensing performance of MZI-PTS and MIPTS [100]: (f) experimental setups of (top) MZI-PTS and (bottom) MI-PTS; (g) Allan-Werle deviation as a function of integration time for both MZI-PTS and MI-PTS.