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Towards Multimode-fiber-based Two-photon Endoscopy

Multimode fibers (MMFs) show great promise in imaging applications where space is limited, due to their small diameter, yet high NA [1] , [2] . One such application is endoscopy, where a multimode fiber can be used as a thin and flexible probe. Unfortunately, the perturbation sensitive mode mixing in a multimode fiber makes it difficult to reconstruct an image that is transmitted through the fiber [3] . Methods to overcome this difficulty, such as spatial wavefront shaping, still need re-optimalization after significant fiber perturbations. Such methods are also usually based on linear imaging. However, nonlinear imaging can provide increased resolution, reduced background and the ability for 3D imaging [4] . Combining ultrashort pulses with MMFs is challenging because of the complex spatiotemporal response of an MMF. So far, only methods based on spatial domain wavefront shaping have been used to selectively focus an ultrashort pulse through an MMF [5] , which are still perturbation-sensitive.