  Flexible fiber-optic endoscopes provide a minimally-invasive solution for
imaging at depths beyond the reach of conventional microscopes. Current
endoscopes require focusing and/or scanning mechanisms at the distal end, which
limit miniaturization and frame-rate, and induce aberrations. Alternative
lensless solutions are based on adaptive wavefront-correction, but are
extremely sensitive to fiber bending. Here, we demonstrate a novel endoscopic
approach, which enables single-shot imaging at a variable working distance
through a conventional fiber bundle, without the use of any distal optics. Our
approach computationally retrieves the object image from a single speckle
pattern transmitted through the bundle, exploiting phase information preserved
through inherent angular speckle correlations. Unlike conventional fiber-bundle
endoscopes, the resulting image is unpixelated, the resolution is
diffraction-limited, objects can be imaged at variable working distance, and
the technique is completely insensitive to fiber bending. Since no optical
elements are required, miniaturization is limited only by the bundle diameter.
