  Quantitative phase imaging (QPI) is a label-free technique providing both
morphology and quantitative biophysical information in biomedicine. However,
applying such a powerful technique to in vivo pathological diagnosis remains
challenging. Multi-core fiber bundles (MCFs) enable ultra-thin probes for in
vivo imaging, but current MCF imaging techniques are limited to amplitude
imaging modalities. We demonstrate a computational lensless microendoscope that
uses an ultra-thin bare MCF to perform quantitative phase imaging of biomedical
samples with up to 1 {\mu}m lateral resolution and nanoscale axial resolution.
The incident complex light field at the measurement side is precisely
reconstructed from a single-shot far-field speckle pattern at the detection
side, enabling digital focusing and 3D volumetric reconstruction without any
mechanical movement. The accuracy of the quantitative phase reconstruction is
validated by imaging the phase target and hydrogel beads through the MCF. With
the proposed imaging modality, 3D imaging of human cancer cells is achieved
through the ultra-thin fiber endoscope, promising widespread clinical
applications.
