Two-photon lensless endoscopy through multicore fiber bundles (Conference Presentation)

Lensless endoscopes have generated a great deal of interest in the development of minimally invasive probes for imaging in sensitive and hitherto inaccessible regions as found in deep brain imaging. Lifting the requirement for the opto-mechanical elements at the distal end of the fiber reduces the footprint of the endoscope down to the fundamental limit, the fiber itself. Our approach, using specially designed multicore fibers allows us to i) generate two-photon fluorescence contrast with femtosecond pulses, ii) image at high speeds with resonant scanners, iii) simple and non-interferometric calibration schemes and iii) exhibits a high resilience to spatio-temporal distortion of the focus due to fiber bending. In this contribution, we will discuss how novel designs of the MCFs can lift several of the instrumental complexity typically associated wavefront shaping and high speed imaging. We show that the use of sparse arrays of fiber cores can provide pixelation-free imaging with no artifacts when employed in the wavefront domain as opposed to conventional fiber bundles. Furthermore, we examine the unique properties of the MCFs which allow for fast and non-interferometric calibration schemes and can tolerate severe bending with an intact focus. The inclusion of a secondary cladding on the MCF allows us high sensitivity detection though the fiber (NA = 0.6) whilst preserving the advantages of a sparse MCF. The combination of these new developments brings us towards the application of these ultrathin probes in realistic imaging conditions.