Increasing the imaging capabilities of multimode fibers by exploiting the properties of highly scattering media

We present a novel design that exploits the focusing properties of scattering media to increase the resolution and the working distance of multimode fiber based imaging devices. Placing a highly scattering medium in front of the distal tip of the multimode fiber enables the formation of smaller sized foci at increased working distances away from the fiber tip. We perform a parametric study of the effect of the working distance and the separation between the fiber and the scattering medium on the focus size. We experimentally demonstrate submicron focused spots as far away as 800{\mu}m with 532nm light.Comment: 4 pages, 3 figure

Isotropic inverse-problem approach for two-dimensional phase unwrapping

In this paper, we propose a new technique for two-dimensional phase unwrapping. The unwrapped phase is found as the solution of an inverse problem that consists in the minimization of an energy functional. The latter includes a weighted data-fidelity term that favors sparsity in the error between the true and wrapped phase differences, as well as a regularizer based on higher-order total-variation. One desirable feature of our method is its rotation invariance, which allows it to unwrap a much larger class of images compared to the state of the art. We demonstrate the effectiveness of our method through several experiments on simulated and real data obtained through the tomographic phase microscope. The proposed method can enhance the applicability and outreach of techniques that rely on quantitative phase evaluation

A Learning Approach to Optical Tomography

We describe a method for imaging 3D objects in a tomographic configuration implemented by training an artificial neural network to reproduce the complex amplitude of the experimentally measured scattered light. The network is designed such that the voxel values of the refractive index of the 3D object are the variables that are adapted during the training process. We demonstrate the method experimentally by forming images of the 3D refractive index distribution of cells

Optimizing field-of-view of deep-tissue scanning microscopy

For centuries, the optical microscope has been a crucial instrument for new biological findings, as microscopes were the first devices allowing to observe the internal processes of the cell. Unfortunately, this observation requires the use of thin samples, as biological tissue scatters the incoming light, resulting in a blurred image. An ever increasing number of deep-tissue imaging technique have pushed the penetration depth of the optical microscope. Methods such as adaptive optics [1] allow focusing inside biological tissue by correcting for scattering introduced by the sample. However, adaptive optics methods can only correct for image distortions caused by scattering over a single small area (i.e., field-of-view) within tissue. Please click Additional Files below to see the full abstract

How inflation, market capitalization, industrial production and the economic sentiment indicator affect the EU-12 stock markets

In the present study we map the relationship between the EU-12 stock market price indices and four crucial macroeconomic factors, using panel data analysis. The examined variables are market capitalization, industrial production, the economic sentiment indicator, and inflation, while the twelve countries are those which have adopted the euro. The empirical results reveal a strong effect of the first three factors, while inflation has a negative but not statistically significant coefficient. Further, the variables that affect the stock markets positively are market capitalization and the economic sentiment indicator. Finally, an applied statistical model confirms the significant convergence of the EU-12 stock markets in the long run, indicating a low geographic diversification across European markets.peer-reviewe

Scattering compensation by focus scanning holographic aberration probing (F-SHARP)

A long-standing goal in biomedical imaging, the control of light inside turbid media, requires knowledge of how the phase and amplitude of an illuminating wavefront are transformed as the electric field propagates inside a scattering sample onto a target plane. So far, it has proved challenging to non-invasively characterize the scattered optical wavefront inside a disordered medium. Here, we present a non-invasive scattering compensation method, termed F-SHARP, which allows us to measure the scattered electric-field point spread function (E-field PSF) in three dimensions. Knowledge of the phase and amplitude of the E-field PSF makes it possible to optically cancel sample turbulence. We demonstrate the imaging capabilities of this technique on a variety of samples and notably through vertebrate brains and across thinned skull in vivo

Delivery of focused short pulses through a multimode fiber

Light propagation through multimode fibers suffers from spatial distortions that lead to a scrambled intensity profile. In previous work, the correction of such distortions using various wavefront control methods has been demonstrated in the continuous wave case. However, in the ultra-fast pulse regime, modal dispersion temporally broadens a pulse after propagation. Here, we present a method that compensates for spatial distortions and mitigates temporal broadening due to modal dispersion by a selective phase conjugation process in which only modes of similar group velocities are excited. The selectively excited modes are forced to follow certain paths through the multimode fiber and interfere constructively at the distal tip to form a focused spot with minimal temporal broadening. We demonstrate the delivery of focused 500 fs pulses through a 30 cm long step-index multimode fiber. The achieved pulse duration corresponds to approximately 1/30th of the duration obtained if modal dispersion was not controlled. Moreover, we measured a detailed two-dimensional map of the pulse duration at the output of the fiber and confirmed that the focused spot produces a two-photon absorption effect. This work opens new possibilities for ultra-thin multiphoton imaging through multimode fibers

Dynamic conjugate F-SHARP microscopy

Optical microscopy is an indispensable tool in biomedical sciences, but its reach in deep tissues is limited due to aberrations and scattering. This problem can be overcome by wavefront-shaping techniques, albeit at limited fields of view (FOVs). Inspired by astronomical imaging, conjugate wavefront shaping can lead to an increased field of view in microscopy, but this correction is limited to a set depth and cannot be dynamically adapted. Here, we present a conjugate wavefront-shaping scheme based on focus scanning holographic aberration probing (F-SHARP). We combine it with a compact implementation that can be readily adapted to a variety of commercial and home-built two-photon microscopes. We demonstrate the power of the method by imaging with high resolution over extended FOV (>80 µm) deeper than 400 μm inside a mouse brain through a thinned skull