Abstract
Graded-index (GRIN) lenses serve as a key component for miniature endoscopes because of their small diameters and ease of assembly. However, the nonaplanatic nature of GRIN lenses causes inherent spatial aberrations that lower image resolution and sharpness. Here we present the diagnosis of the aberrations in GRIN probes and the use of adaptive optics to compensate for the wavefront errors in the endoscope. Two different operation schemes based on preset and in situ measurements are demonstrated, both resulting in a substantial reduction of the wavefront error from 0.42 to <0.1 μm.