A two-photon lensless endoscope with a double-clad tapered multi-core fiber

We present a novel, to the best of our knowledge, multi-core fiber-based lensless micro-endoscope incorporating several key innovations: a Fermat's golden spiral core layout, a low-index polymer cladding, and a longitudinally varying fiber diameter. This fiber design enables efficient focusing of ultrashort pulses and collection of the fluorescence signal with both diffraction-limited resolution and field of view. These advancements address several long-standing challenges in the development of ultra miniaturized non-linear endoscopes. We characterize the device's performance by evaluating the Strehl ratio, field of view, and memory effect. Finally, we demonstrate its application to biological cellular samples by performing two-photon excited fluorescence imaging in an endoscopic configuration.