  Multimode fibers (MMFs) are attractive ultra-thin replacements for
state-of-the-art endoscopes, but the phase randomization in propagation through
MMFs poses a major hurdle for imaging and focusing of light. Recently, this
challenge has been addressed by pre-measuring the compensation for the fiber's
complex input-output modes relations. Unfortunately, the sensitivity of this
approach to fiber bending and temperature variations renders it inappropriate
for many applications. Here, we demonstrate a truly endoscopic robust method
for controlled in-situ focusing and scanning through a flexible uncharacterized
MMF, whereby all the instrumentation is situated at the proximal end. We show
that in graded-index (GRIN) fibers, light patterns at the proximal end allow
retrieving information about the distal light distribution. We utilize these
properties and two-photon fluorescence for robust controlled focusing through
bended GRIN fibers. Our results carry potential for lensless two-photon
micro-endoscopy.
