A class of miniaturized imaged systems based on multicore fibers (MCFs) and wavefront shaping, also known as lensless endoscopes, have emerged as promising candidates for non-invasive imaging deep inside the tissue. At the current stage of their development, these systems already provide features like pixelation-free wide-field and point-scanning imaging and are compatible with the majority of nonlinear imaging techniques, offering diffraction-limited resolution over a field of view only limited by the single fiber numerical aperture.
Widely spaced single-mode cores in such an MCF are designed for very weak inter-core coupling, which ensures an infinite memory effect and provides good ultrashort pulse delivery framework for nonlinear imaging. In a true endoscopic setting, however, where the fiber geometry is subject to continuous deformation (at a rate from several Hz to several tens of Hz), results in inter-core phase and group delay dispersion (GDD) that changes as the fiber bends. This consequently degrades the PSF in terms of size and power in the focal spot, eventually rendering impossible to produce non-linear imaging.
We addressed this issue by implementing an active measurement and compensation loop at the proximal end of an MCF, allowing to correct in real time the GDD changes and to minimize the temporal dispersion of the delivered laser pulses. We evaluate this approach against a passive scheme where a static pre-compensation is used in a compliment with a specifically designed MCF, allowing to drastically increase the MCF bending robustness.