Overcoming the resolution limit in retinal imaging using the scattering properties of the sclera

In-vivo imaging of the eye's fundus is widely used to study eye's health. State of the art Adaptive Optics devices can resolve features up to a lateral resolution of 1.5 um. This resolution is still above what is needed to observe subcellular structures such as cone cells (1-1.25 um diameter). This limit in resolution is due to the small numerical aperture of the eye when the pupil is fully dilated (max 0.24). In our work, we overcome this limit using a non-standard illumination scheme. A laser beam is shined on the lateral choroid layer, whose scattered light is illuminating the eye's fundus. Thanks to a Spatial Light Modulator the scattered light from the choroid layer can be manipulated to produce a scanning focus spot on the fundus. The intensity of the reflected light from the fundus is collected from the pupil and used for reconstructing the image.

Control of pulsed light propagation through multimode optical fibers

Edgar Emilio Morales Delgado

Visualization of organs and cells in the interior of living beings is a challenging task due to the light absorption and multiple light scattering occurring in biological tissue that prevents the di-rect transmission of images. A standard visualization approach is based on the use of endo-scopes which is accomplished through apertures in the body. Since their invention, the light transmission capability of optical fibers has enabled fiber optic based endoscopy with devices based on bundles of optical fibers that can directly relay images. Due to their small diameter, fiber optic endoscopes are standard in minimally invasive applications either with a white light illumination or as a fluorescent imaging device. The principle behind fluorescent endoscopy consists on the scanning of light intensity over a fluorescent sample, which is achieved by se-quential illumination of each one of the fiber bundles or by mechanical scanning of a single-mode double-cladding fiber. Collecting the fluorescent signal through the light guiding medium, an image can be reconstructed. The main drawback of fiber bundle endoscopes is the pixelated limited resolution given by the presence of thousands of cores that conform the bundle. In the double cladding fiber endoscopes, there is a limit to the miniaturization of the endoscope due to the requirement of mechanical elements at the tip of the fiber to scan the illumination over the sample. In this thesis, methods for focusing and digital scanning of optical light pulses through multimode optical fibers without any distal mechanical element are developed. Such techniques are based on spatial light modulation. Spatial phase modulation allows the control of light propagation in scrambling media, as in a multimode fiber, enabling the generation of intensity light foci or arbitrary intensity profiles. The high intensity of the transmitted pulses permits multi-photon imaging through multimode optical fibers, resulting in imaging probes of higher resolution than in the case of fiber bundles. We demonstrate in a working prototype that this approach provides all the advantages of multi-photon microscopy at the tip of an ultra-thin probe, such as a reduced photo bleaching of the sample, optical sectioning and enhanced image contrast. The developed methods can also be employed for material processing that requires high in-tensity light pulses. In specific, we demonstrate additive manufacturing, also known as 3D printing, at the tip of an ultra-thin needle. The working principle is based on two-photon polymerization, which is accomplished by scanning intense light pulses on a photosensitive material that hardens when exposed to light. All other additive manufacturing methods require very large nozzles or components in close proximity to the structure that they build, up to now. With an ultra-thin 3D printing probe we enable micro-fabrication through very small apertures, in places of difficult access or in the interior of living beings. We call it, endofabrication.

EPFL2017

Application of Adaptive Optics to Focusing and Imaging

Daniel Iwaniuk

Adaptive optics is used to abate aberrations with a wavefront correction. It is widely used in astronomy and is starting to expand into applications of laser focusing and imaging with high numerical apertures, e.g. microscopy. The physical background of focusing laser light and imaging is the same, so we can use the same methods. In both cases, light is propagating through a lens; either it becomes focused into a small region or light from a small region becomes imaged on a camera. Modulations applied on the lens to implement wavefront corrections behave similarly in both applications. A powerful simulation tool was created to characterize the impact of those modulations. As an example, we validated a design for a Fresnel lens produced on a glass fibre tip to focus its emitting light. We have developed solutions mainly for three different problems. First, a high depth of focus enables keeping a laser beam focused within a larger length or imaging objects from different positions simultaneously. In photography, this can be attained by stopping down the aperture, which introduces a huge loss of light. State-of-the-art for focusing into a line segment also shows an inefficient performance. We present an elegant lens design, which enables highly efficient elongation of the depth of focus. Preliminary studies have shown that it might be a feasible alternative for current intra-ocular lens implants in ophthalmology or for 3D visualization in imaging and microscopy. A high depth of focus also has a large potential in optical lithography and data storage, because the focal position is enlarged and does not have to be adjusted precisely to obtain a useful spot. Second, specimen-induced aberrations can affect even a perfectly adjusted, diffraction limited lens system. A planar refraction index mismatch introduces spherical aberrations, degrading the optical resolution. As a first step to correct them, we were able to characterize simultaneously the refractive index and the thickness of an unknown medium that is placed between the lens and its focal region. This was done by clever manipulation of the beam angles with the same adaptive optics element in the focusing and in the imaging system. Finally, the medium-induced spherical aberrations were corrected based on the characterization results. The point spread function degradation of a focused laser beam was completely removed, which might be useful in optical tweezers or in laser processing of biological samples. While imaging through the planar refractive index mismatch, the bending of the object field was corrected and the diffraction limited performance restored.

EPFL2012

Digital confocal microscopy through a multimode fiber

Salma Farahi, Alexandre Goy, Damien Claude-Marie Loterie, Christophe Moser, Ioannis Papadopoulos, Demetri Psaltis

Acquiring high-contrast optical images deep inside biological tissues is still a challenging problem. Confocal microscopy is an important tool for biomedical imaging since it improves image quality by rejecting background signals. However, it suffers from low sensitivity in deep tissues due to light scattering. Recently, multimode fibers have provided a new paradigm for minimally invasive endoscopic imaging by controlling light propagation through them. Here we introduce a combined imaging technique where confocal images are acquired through a multimode fiber. We achieve this by digitally engineering the excitation wavefront and then applying a virtual digital pinhole on the collected signal. In this way, we are able to acquire images through the fiber with significantly increased contrast. With a fiber of numerical aperture 0.22, we achieve a lateral resolution of 1.5 mu m, and an axial resolution of 12.7 mu m. The point-scanning rate is currently limited by our spatial light modulator (20Hz). (C)2015 Optical Society of America