  Rigid endoscopes like graded-index (GRIN) lenses are known tools in
biological imaging, but it is conceptually difficult to miniaturize them. In
this letter, we demonstrate an ultra-thin rigid endoscope with a diameter of
only 125 microns. In addition, we identify a domain where two-photon endoscopic
imaging with fs-pulse excitation is possible. We validate the ultra-thin rigid
endoscope consisting of a few cm of graded-index multi-mode fiber by using it
to acquire optically sectioned two-photon fluorescence endoscopic images of
three-dimensional samples.
