Improved depth range and resolution of inline holographic microscopy through fiber imaging bundles

Inline holographic microscopy has recently been demonstrated through fiber imaging bundles, opening up the possibility of ultra-miniaturized microscopy probes. In order to minimize artefacts arising due to the multimode behaviour of the fiber bundle cores, a partially coherent light source was used: an LED coupled into a multimode fiber. However, partial coherence limits the maximum working distance between the bundle and the sample before the resolution begins to degrade. The resolution is also limited by the finite core spacing in the fiber bundle, leading to under-sampling of the finer details of the hologram. Here, we demonstrate and evaluate several techniques for improving the resolution and working range, including tailoring the source coherence and using multiple sources, demonstrating that we can achieve at least a two-fold improvement in performance.