Viktor Tsvirkun

Extended field-of-view in a lensless endoscope using an aperiodic multicore fiber

We investigate lensless endoscopy using coherent beam combining and aperiodic multicore fibers (MCF). We show that diffracted orders, inherent to MCF with periodically arranged cores, dramatically reduce the field-of-view (FoV), and that randomness in MCF core positions can increase the FoV up to the diffraction limit set by a single fiber core, while maintaining a MCF experimental feasibility. We demonstrate experimentally pixelation-free lensless endoscopy imaging over a 120 μm FoV with an aperiodic MCF designed with widely spaced cores. We show that this system is suitable to perform beam scanning imaging by simply applying a tilt to the proximal wavefront.

Widefield lensless endoscopy with a multicore fiber

We demonstrate pixelation-free real-time widefield endoscopic imaging through an aperiodic multicore fiber (MCF) without any distal opto-mechanical elements or proximal scanners. Exploiting the memory effect in MCFs, the images in our system are directly obtained without any post-processing using a static wavefront correction obtained from a single calibration procedure. Our approach allows for video-rate 3D widefield imaging of incoherently illuminated objects with imaging speed not limited by the wavefront-shaping device refresh rate.

Improving endoscopic imaging with disordered multi-core fiber bundles (Conference Presentation)

The periodic arrangement of core positions in multi-core fiber bundles introduces ‘ghost’ artifacts to endoscopic images obtained through them, whether in wide-field imaging (based on either direct imaging or speckle correlations) or in confocal scanning microscopy using wavefront shaping. Here we introduce partially disordered multi-core bundles as a means to overcome these artifacts. The benefits of their use will be discussed in the context of multiphoton scanning microscopy utilizing a spatial light modulator in the proximal end, and in the more general case of widefield imaging. We also show that both numerically and experimentally that the presence of disorder also enables to apply phase retrieval methods to characterize the phase distortion introduced due to propagation in the bundle without the need of an interferometrically stabilized reference. Thus, in addition to overcoming the challenge of ghost artifacts, disordered multi-core fibers have the potential to overcome another challenge, movement-induced phase distortions, by enabling real-time characterization of this phase distortion in reflection mode only via the proximal end.

Phase retrieval in multicore fiber bundles

Multicore fiber bundles are widely used in endoscopy due to their miniature size and their direct imaging capabilities. They have recently been used, in combination with spatial light modulators, in various realizations of endoscopy with little or no optics at the distal end. These schemes require characterization of the relative phase offsets between the different cores, typically done using off-axis holography, thus requiring both an interferometric setup and, typically, access to the distal tip. Here we explore the possibility of employing phase retrieval to extract the necessary phase information. We show that in the noise-free case, disordered fiber bundles are superior for phase retrieval over their periodic counterparts, and demonstrate experimentally accurate retrieval of phase information for up to 10 simultaneously illuminated cores. Thus, phase retrieval is presented as a viable alternative for real-time monitoring of phase distortions in multicore fiber bundles.

Bending-induced inter-core group delays in multicore fibers

We examine the impact of fiber bends on ultrashort pulse propagation in a 169-core multicore fiber (MCF) by numerical simulations and experimental measurements. We show that an L-shaped bend (where only one end of the MCF is fixed) induces significant changes in group delays that are a function of core position but linear along the bending axis with a slope directly proportional to the bending angle. For U- and S-shaped bends (where both ends of the MCF are fixed) the induced refractive index and group delay changes are much smaller than the residual, intrinsic inter-core group delay differences of the unbent MCF. We further show that when used for point-scanning lensless endoscopy with ultrashort pulse excitation, bend-induced group delays in the MCF degrade the point-spread function due to spatiotemporal coupling. Our results show that bend-induced effects in MCFs can be parametrized with only two parameters: the angle of the bend axis and the amplitude of the bend. This remains valid for bend amplitudes up to at least 200 degrees.