Fig. 1. The model of the dynamic speckle

Fig. 2. TIL wave propagation configurations. Transmitted wave with complex amplitude A(r') propagates in an optically homogeneous medium toward a target, and after scattering off the target surface at the plane (r', z=L), the return wave propagates back to the multi-channel detectors. (a) Spot with a diameter of 1.5 mm focused on the target. (b) The speckle pattern corresponds to (a). (c) Spot with a diameter of 4 mm focused on the target, and (d) shows the speckle pattern corresponding to (c).

Fig. 3. The simulation results of the dynamic speckles normalized cross-correlation function. (a) ρ=3 m; (b) ρ=6 m

Fig. 4. The dynamic speckle signals according to the model of the movement of Scatters. (a) Speckle signal fluctuations. (b) The frequency bandwidth of the power spectra decreases as the target spot size changes from 0.2 mm to 1.2 mm

Fig. 5. Simulation results of the speckle-metric varying with the target spot size from 0.2 mm to 1.2 mm, with an interval of 0.01 mm

Fig. 6. (a) Temporal spectrum obtained by a single-channel, and the spatio-temporal spectrum obtained by a four-channel. (b) The normalized speckle-metric obtained by the four-channel varying with the target spot size

Fig. 7. Experimental platform for collecting laser echo speckle with two single-point detectors

Fig. 8. Experimental results of the speckle spectroscopy method. The horizontal coordinates in (a) and (b) indicate the frequency of intensity fluctuations, and the vertical coordinates are the normalized temporal spectra obtained from the two detectors. (c) Results of the speckle-metric J_one obtained by two point detectors, and (d) shows the speckle-metrics obtained by averaging the two-channel results (J_ave) and the spatial-temporal spectrum (J_spt).