2Photon Flexible Lensless Endoscope

Recently, there has been a surge in reports of so-called `lensless endoscopes', a term which denotes the active control of the fiber injected wavefront from the proximal side only to enable imaging without any distal optics, thus allowing the miniaturization of the endoscope to the fundamental limit, the size of the optical fiber itself. Our approach sets itself apart in several ways: (i) the capability to perform nonlinear image contrast using two-photon fluorescence; and (ii) high image acquisition rates and (iii) the active control of the distal focused spot while bending the fiber. We have developed a multicore fiber (MCF) specifically for our approach. It consists of 169 homogeneous single-mode cores, each of which functions as one `pixel' of the wave front. These fiber cores are spatially arranged in an aperiodic pattern to eliminate the satellite grating induced repliqua and to extend the field of view. The composite waveform at the distal end is then generated by pre-shaping the beam before it is coupled into the fiber cores. Hence, the generation of a focal spot and its scanning can be performed without any opto-mechanical components on the fiber tip. The MCF is designed with a multi-mode inner cladding with a very high numerical aperture (NA) of 0.65. This permits the collection of the resulting fluorescence signal with a high efficiency. A dedicated twist fiber design makes the developed imaging endoscope insensitive to fiber bending.