Multichannel-GaAsP-photomultiplier-based fiber bundle ISM-STED microscope

The usage of a GaAsP (gallium arsenide phosphide) photomultiplier for microscopical imaging allows the evaluation of low-light luminescent objects. We designed a setup for collecting a confocal microscopic image signal, which is divided into 14 equal-sized input channels. The division is achieved with a beamsplitter and two fiber bundles consisting of seven fibers each. Re-imaging the confocal pinhole by such a densely packed fiber bundle permits the utilization of a photon re-localization approach to overcome the optical resolution limit. The center fiber creates a real-time image, while the outer fibers enable a higher-resolution image via an image scanning microscope (ISM) signal calculation. The fiber bundles are enclosed in a fused silica capillary and are drawn out to create one solid fiber bundle. During the drawing process, the fiber bundles are tapered down to an outer diameter size of 400 μm, with each fiber having a less than 0.3 Airy unit diameter. For the photomultiplier interface, all fibers of both fiber bundles are integrated into a v-groove array, with each fiber representing a detection input, which is followed by projection optics for imaging onto the multichannel detector. The resulting confocal super-resolution microscope is suitable for the application of time-correlated single photon counting (TCSPC) techniques such as fluorescence lifetime imaging (FLIM), time-resolved anisotropy, or Förster resonance energy transfer (FRET) imaging