Fig. 1. Principle of space gating.

a Schematic of the imaging principle. Conventional confocal imaging method relies on the ballistic waves shown as green lines. When optical inhomogeneity is introduced, the intensity of the ballistic wave exponentially decreases with depth, and the multiply scattered wave (shown as solid blue and dotted blue lines) may obscure the ballistic wave. By implementing space gating at the object plane using an acousto-optic effect (indicated as a red spot), the multiply scattered wave that travels outside the acoustic focus (dotted blue lines) can be rejected, which in turn improves the ratio of the ballistic wave to the multiply scattered wave at the sensor element (marked as a blue pixel), whose position is conjugate to the illumination point r_d ~ r_i. b Intensity maps of illumination and detection transfer functions in a confocal gating scheme (where r_d ~ r_i), with respect to r_o on the object plane for a transparent medium. c Same as b, but in the presence of scattering. The optical thicknesses on the illumination and detection sides were L_i/l_s = 10.6 and L_d/l_s = 12.8, respectively. d Contribution map, |T_i(r_o; r_i)T_d(r_o; r_d)|², with respect to r_o calculated from the transfer functions in b. e Same as d, but in the presence of scattering, calculated from c. Scale bar: 100 μm.