Two-photon excitation endoscopy through a multimode optical fiber

The vast number of propagating solutions to the wave equation in multimode optical fibers represents a larger information capacity than provided by fiber bundles of the same diameter. Therefore, in the field of imaging, multimode fibers potentially allow the transmission of images with higher resolution. However, image transmission through multimode fibers is not direct as in the fiber bundle case, in which each of the fiber cores can relay a portion of the distal image. In multimode fiber transmission, a distribution of intensity is scrambled in time and space by the propagating modes, leading to a speckle-like pattern that does not resemble the initial distribution. Here, we demonstrate two-photon excitation imaging of fluorescent beads through a multimode optical fiber. We show that our method maintains the advantages of two-photon excitation microscopy compared to single-photon excitation such as reduced photo-bleaching, deeper penetration depth and sectioning capability. Our method is based on time-gated digital phase conjugation, which allows the generation of focused pulses on the other side of a multimode fiber. To acquire an image, the focused femtosecond pulse is scanned in a three-dimensional mesh, producing two-photon excitation on each spatial location of the sample. By collecting the fluorescence through the fiber, a 3D two-photon image is reconstructed.