Fig. 1. Absorption coefficient of water vapor around 1392 nm (P = 1 atm, T = 296 K) based on HITRAN^[19]. The non-absorbing region is indicated by the dotted rectangle.

Fig. 2. Second harmonic R_2f demodulated from the absorption signal.

Fig. 3. Schematic of the experiment setup.

Fig. 4. Detected signal from the photodetector. The non-absorbing region is indicated by the dotted rectangle.

Fig. 5. Response of ln-WMS-2f (black dots) and WMS-2f (red inverted triangles). The black line represents the linear fit of ln-WMS-2f versus volume fraction.

Fig. 6. (a) η retrieved with the η-seeking algorithm during 90 s of continuous measurement (0.1 s per measurement); (b) frequency distribution histogram of η; (c) mean of R1fNA for each measurement. The integral time of each point is 0.1 s.