Opt. Express 23, 32158–32170 (2015)

An endoscope typically uses multicore fibres (MCFs) to visualize the interior of the body. Unfortunately, MCFs deliver pixelated images due to the gap between the cores (3–5 μm). Now, Edgar Morales-Delgado and colleagues from École Polytechnique Fédéral de Lausanne in Switzerland demonstrate high-resolution two-photon excitation imaging through a 20-cm-long graded-index multimode fibre (MMF) of 350 μm diameter. In their approach, a focused 120 fs pulse is delivered and scanned over the sample, and digital phase conjugation is performed using mode selection by time-gating with an ultrafast reference pulse. The excited two-photon emission is collected through the same fibre. Scanning the focused pulse in a 3D volume over a sample consisting of fluorescent beads suspended in a polymer achieves lateral and axial resolutions of 1 μm and 15 μm, respectively. The team says that the high resolution, small probe diameter, large collection efficiency and sectioning capability opens the possibility for in vivo minimally invasive multiphoton endoscopy in the brain, the ear or the eye, for example.