  Thick biological tissues give rise to not only the scattering of incoming
light waves, but also aberrations of the remaining unscattered waves. Due to
the inability of existing optical imaging methodologies to overcome both of
these problems simultaneously, imaging depth at the sub- micron spatial
resolution has remained extremely shallow. Here we present an experimental
approach for identifying and eliminating aberrations even in the presence of
strong multiple light scattering. For time-gated complex-field maps of
reflected waves taken over various illumination channels, we identify two sets
of aberration correction maps, one for the illumination path and one for the
reflection path, that can preferentially accumulate the unscattered signal
waves over the multiple-scattered waves. By performing closed-loop optimization
for forward and phase- conjugation processes, we demonstrated a spatial
resolution of 600 nm up to the unprecedented imaging depth of 7 scattering mean
free paths.
