Coupling optimized bending-insensitive multi-core fibers for lensless endoscopy.

We report a bending-insensitive multi-core fiber (MCF) for lensless endoscopy imaging with modified fiber geometry that enables optimal light coupling in and out of the individual cores. In a previously reported bending insensitive MCF (twisted MCF), the cores are twisted along the length of the MCF allowing for the development of flexible thin imaging endoscopes with potential applications in dynamic and freely moving experiments. However, for such twisted MCFs the cores are seen to have an optimum coupling angle which is proportional to their radial distance from the center of the MCF.

Miniature 120-beam coherent combiner with 3D-printed optics for multicore fiber-based endoscopy.

In this Letter, we report a high-efficiency, miniaturized, ultra-fast coherent beam, combined with 3D-printed micro-optics directly on the tip of a multicore fiber bundle. The highly compact device footprint (180 µm in diameter) facilitates its incorporation into a minimally invasive ultra-thin nonlinear endoscope to perform two-photon imaging.

Line-scan compressive Raman imaging with spatiospectral encoding.

We report a line-scanning imaging modality of compressive Raman technology with a single-pixel detector. The spatial information along the illumination line is encoded onto one axis of a digital micromirror device, while spectral coding masks are applied along the orthogonal direction. We demonstrate imaging and classification of three different chemical species.

Spatial frequency modulated imaging in coherent anti-Stokes Raman microscopy.

For sparse samples or in the presence of ambient light, the signal-to-noise ratio (SNR) performance of single-point-scanning coherent anti-Stokes Raman scattering (CARS) images is not optimized. As an improvement, we propose replacing the conventional CARS focus-point illumination with a periodically structured focus line while continuing to collect the transmitted CARS intensity on a single detector. The object information along the illuminated line is obtained by numerically processing the CARS signal recorded for various periods of the structured focus line.

Compressive Raman imaging with spatial frequency modulated illumination.

We report a line scanning imaging modality of compressive Raman technology with spatial frequency modulated illumination using a single pixel detector. We demonstrate the imaging and classification of three different chemical species at line scan rates of 40 Hz.

Supercritical angle fluorescence for enhanced axial sectioning in STED microscopy.

We demonstrate subwavelength axial sectioning on biological samples with a stimulated emission depletion (STED) microscope combined with supercritical angle fluorescence (SAF) detection. SAF imaging is a powerful technique for imaging the membrane of the cell based on the direct exploitation of the fluorophore emission properties. Indeed, only when fluorophores are close to the interface can their evanescent near-field emission become propagative and be detected beyond the critical angle.

High-resolution multimodal flexible coherent Raman endoscope.

Coherent Raman scattering microscopy is a fast, label-free, and chemically specific imaging technique that shows high potential for future in vivo optical histology. However, the imaging depth in tissues is limited to the sub-millimeter range because of absorption and scattering. Realization of coherent Raman imaging using a fiber endoscope system is a crucial step towards imaging deep inside living tissues and providing information that is inaccessible with current microscopy tools.

Single-shot noninterferometric measurement of the phase transmission matrix in multicore fibers.

A simple technique for far-field single-shot noninterferometric determination of the phase transmission matrix of a multicore fiber with over 100 cores is presented. This phase retrieval technique relies on the aperiodic arrangement of the cores.

Nonlinear imaging through a Fermat's golden spiral multicore fiber.

We report two-photon lensless imaging through a novel Fermat's golden spiral multicore fiber. This unique layout optimizes the sidelobe levels, field of view, crosstalk, group delay, and mode density to achieve a sidelobe contrast of at least 10.9 dB.

A straightforward STED-background corrected fitting model for unbiased STED-FCS analyses.

Combining stimulated emission depletion and fluorescence correlation spectroscopy (STED-FCS) provides a powerful and sensitive tool for studying the molecular dynamics in live cells with high spatio-temporal resolution. STED-FCS gives access to molecular diffusion characteristic at the nanoscale occurring within short period of times. However due to the incomplete suppression of fluorescence in the STED process, the STED-FCS point spread function (PSF) deviates from a Gaussian shape and challenges the analysis of the auto-correlation curves obtained by FCS.

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.

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.

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.

Ultrathin endoscopes based on multicore fibers and adaptive optics: a status review and perspectives.

We take stock of the progress that has been made into developing ultrathin endoscopes assisted by wave front shaping. We focus our review on multicore fiber-based lensless endoscopes intended for multiphoton imaging applications. We put the work into perspective by comparing with alternative approaches and by outlining the challenges that lie ahead.

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.

Single-shot polarimetry imaging of multicore fiber.

We report an experimental test of single-shot polarimetry applied to the problem of real-time monitoring of the output polarization states in each core within a multicore fiber bundle. The technique uses a stress-engineered optical element, together with an analyzer, and provides a point spread function whose shape unambiguously reveals the polarization state of a point source. We implement this technique to monitor, simultaneously and in real time, the output polarization states of up to 180 single-mode fiber cores in both conventional and polarization-maintaining fiber bundles.

Ultra-thin rigid endoscope: two-photon imaging through a graded-index multi-mode fiber.

Rigid endoscopes like graded-index (GRIN) lenses are known tools in biological imaging, but it is conceptually difficult to miniaturize them. In this letter, we demonstrate an ultra-thin rigid endoscope with a diameter of only 125 μm. In addition, we identify a domain where two-photon endoscopic imaging with fs-pulse excitation is possible.

Confocal supercritical angle microscopy for cell membrane imaging.

We demonstrate subwavelength sectioning on biological samples with a conventional confocal microscope. This optical sectioning is achieved by the phenomenon of supercritical angle fluorescence, wherein only a fluorophore next to the interface of a refractive index discontinuity can emit propagating components of radiation into the so-called forbidden angles. The simplicity of this technique allows it to be integrated with a high numerical aperture confocal scanning microscope by only a simple modification on the detection channel.