Fig. 4.

Experimental demonstration of the multilayer acoustic backpropagation algorithm. (a) Two 300 μm diameter pencil leads were placed orthogonal to each other in a petri dish at a depth difference of about 1 mm. The dish was filled with DI water and covered with a 3.44-mm-thick PDMS block. (b) The hologram taken at the top surface of the block (left) and the extracted surface displacement map (right). (c) Same as (b), but taken at the bottom surface of the PDMS block. The white and black scale bars in (b)-(c): 200 μm and 2 mm, respectively. (d) Sharpness metric calculated with various acoustic velocities $v_c$ and $v_m$. (e) The 3D image reconstructed without accounting for the propagation angle-dependent attenuation due to acoustic impedance mismatch. (f) The same image but reconstructed after compensating the propagation angle-dependent attenuation. Scale bars in (e)-(f): 2 mm.