Fig. 1. Schematic of the experimental configuration of continuous-wave terahertz diffraction tomography (CW THz DT). PM1 and PM2, off-axis parabolic mirrors; BS, THz beam splitter; M, gold-coated mirror; RS, rotational stage.

Fig. 2. Reconstructed results of digital holography for single polystyrene (PS) foam sphere: (a1), (b1) holograms with and without the object at 0°; (c1), (d1) reconstructed amplitude and wrapped phase images of (a1); (a2)–(d2) reconstructed phase images at 0°, 45°, 90°, and 180°; (e1) phase profiles of the black dotted line in (a2)–(d2); (e2) result of (e1) after alignment.

Fig. 3. Reconstructed refractive index (RI) profiles of DT for a single foam sphere: (a1)–(c1), (a2)–(c2) 3D RI profiles at cross sections x–y, y–z, and x–z by FDI and FBPP algorithm, respectively; (d) RI profiles of the red dotted line in (c1) and (c2), and ideal values obtained by the THz-TDS system.

Fig. 4. Amplitude of the spectrum distribution of (a) FDI and (b) FBPP on the logarithmic scale along the f_x–f_y, f_x–f_z, and f_y–f_z cross sections, respectively.

Fig. 5. RI distributions of DT for two foam spheres: (a)–(c) 3D RI profiles at cross sections x–y, y–z, and x–z; (d) volume rendering of 3D RI profiles (see Visualization 1).