Fig. 1: Optical intensity channels.

a A coherent source or Guidestar at position P1 on the input plane leads to a bright speckle at position P2 and a dark speckle at position P3 on the target plane. In the case of a bright speckle, the majority of the optical paths share a similar phase (denoted by the same color) and thus relatively constructively interfere. In contrast, the optical paths leading to the dark speckle are out of phase (denoted by different colors). b, c Based on the time-reversal symmetry of optical propagation, a bright speckle at position P2 will also lead to a bright speckle at position P1 (b). However, a bright speckle at position P3 will not result in a bright speckle at position P1 due to destructive interference (c). d This phenomenon is interpreted in a picture of optical channels. A ‘bright’ optical channel is established between position P1 and P2 for optical energy transmission while the optical channel between position P1 and P3 is ‘dark’.