Uniform line fillings

Deterministic fabrication of random metamaterials requires filling of a space with randomly oriented and randomly positioned chords with an on-average homogenous density and orientation, which is a nontrivial task. We describe a method to generate fillings with such chords, lines that run from edge to edge of the space, in any dimension. We prove that the method leads to random but on-average homogeneous and rotationally invariant fillings of circles, balls and arbitrary-dimensional hyperballs from which other shapes such as rectangles and cuboids can be cut. We briefly sketch the historic context of Bertrand's paradox and Jaynes' solution by the principle of maximum ignorance. We analyse the statistical properties of the produced fillings, mapping out the density profile and the line-length distribution and comparing them to analytic expressions. We study the characteristic dimensions of the space in between the chords by determining the largest enclosed circles and balls in this pore space, finding a lognormal distribution of the pore sizes. We apply the algorithm to the direct-laser-writing fabrication design of optical multiple-scattering samples as three-dimensional cubes of random but homogeneously positioned and oriented chords.Comment: 10 pages, 12 figures; v3: restructured paper, more references, more graph

Spatiotemporal focusing through a multimode fiber via time-domain wavefront shaping

We shape fs optical pulses and deliver them in a single spatial mode to the input of a multimode fiber. The pulse is shaped in time such that at the output of the multimode fiber an ultrashort pulse appears at a predefined focus. Our result shows how to raster scan an ultrashort pulse at the output of a stiff piece of square-core step-index multimode fiber and in this way the potential for making a nonlinear fluorescent image of the scene behind the fiber, while the connection to the multimode fiber can be established via a thin and flexible single-mode fiber. The experimental results match our numerical simulation well.Comment: V2:29 pages including appendices, 9 figures (1 new), several updated, many improvements throughou

Comparison of round- and square-core fibers for sensing, imaging and spectroscopy

Multimode fibers (MMFs) show great promise as miniature probes for sensing, imaging and spectroscopy applications. Different parameters of the fibers, such as numerical aperture, refractive index profile and length, have been already optimized for better performance. Here we investigate the role of the core shape, in particular for wavefront shaping applications where a focus is formed at the output of the MMF. We demonstrate that in contrast to a conventional round-core MMF, a square-core design doesn't suffer from focus aberrations. Moreover, we find that how the interference pattern behind a square-core fiber decorrelates with the input frequency is largely independent of the input light coupling. Finally, we demonstrate that a square core shape provides an on-average uniform distribution of the output intensity, free from the input-output correlations seen in round fibers, showing great promise for imaging and spectroscopy applications.Comment: 9 pages, 5 figure

Towards Multimode-fiber-based Two-photon Endoscopy

Multimode fibers (MMFs) show great promise in imaging applications where space is limited, due to their small diameter, yet high NA [1] , [2]. One such application is endoscopy, where a multimode fiber can be used as a thin and flexible probe. Unfortunately, the perturbation sensitive mode mixing in a multimode fiber makes it difficult to reconstruct an image that is transmitted through the fiber [3]. Methods to overcome this difficulty, such as spatial wavefront shaping, still need re-optimalization after significant fiber perturbations. Such methods are also usually based on linear imaging. However, nonlinear imaging can provide increased resolution, reduced background and the ability for 3D imaging [4]. Combining ultrashort pulses with MMFs is challenging because of the complex spatiotemporal response of an MMF. So far, only methods based on spatial domain wavefront shaping have been used to selectively focus an ultrashort pulse through an MMF [5] , which are still perturbation-sensitive

Towards multimode-fiber-based two-photon endoscopy

We demonstrate a method towards two-photon endoscopy based on time-domain wavefront shaping through a multimode fiber. This allows grid scanning of an ultrashort pulse over the output facet of the fiber with a perturbation-insensitive input

Superiority of a square-core multimode fiber for imaging and spectroscopy

For fiber based imaging and spectroscopy, a round-core multimode fiber MMF is commonly used. We experimentally and theoretically demonstrate that because of the homogeneous mode distribution, a square-core MMF is superior to a round-core MMF

Superiority of a Square-core Multimode Fiber for Imaging and Spectroscopy

An optical fiber is a promising tool for remote imaging [1]. A multimode fiber (MMF) offers multiple advantages: compactness, flexibility as well as the ability to transmit a large amount of information via multiple spatial modes. There are several methods to do optical imaging through a MMF. One can use wavefront shaping (WFS) to compensate for the modal dispersion and mode mixing so that the incident light can converge to a desired pattern at the fiber output [2]. By using WFS, we can sequentially generate focal spots at the distal fiber facet to scan the sample. Another imaging method through a MMF is compressive imaging (CI). The interfering fiber modes create a speckle pattern, which varies with wavelength and input light position, which. can be used as a basis for CI [3]. CI allows to reconstruct the image with a resolution beating the Abbe limit with fewer measurements than the number of pixels [4]. Finally, the decorrelation of the speckle patterns with wavelength encodes the spectral information of input light, which enables MMF spectroscopy applications [5]