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Light transport and imaging through complex media

Scientific meeting

Starts:

January
222018

10:00

Ends:

January
232018

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Theo Murphy international scientific meeting organised by Professor Daniele Faccio and Professor Stephen McLaughlin FREng.

Image by Ella Marushchenko

A multi-disciplinary meeting for the discussion a new challenge in the field of optical imaging: the control of light transport through complex and highly scattering media. The problems of seeing through fog, a multimode fibre or inside the human body were once thought to be intractable or simply impossible. Recent developments have shown that by combining novel computational and imaging approaches, not only is this possible but also within reach.

Enquiries: contact the Scientific Programmes team

Event organisers

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Schedule of talks

22 January

Session 1 09:00-12:30

4 talks Show detail Hide detail

Chairs

Professor Daniele Faccio

10:00-10:05 Welcome by the Royal Society

10:05-10:30 Double phase retrieval for imaging through scattering media

Professor Richard Baraniuk, Rice University, USA

Abstract

A transmission matrix describes the input-output relationship of a coherent complex wavefront as it passes through or reflects off of a multiple-scattering medium, such as frosted glass or a painted wall.  Knowledge of a medium's transmission matrix enables one to image through the medium, transmit signals through the medium, or even use the medium as a lens.  The double phase retrieval method is a recently proposed transmission matrix estimation technique that avoids complicated interferometric measurements.  Unfortunately, to perform high resolution imaging, existing double phase retrieval methods require an unreasonable amount of computation.  In this work, we reduce the computational complexity of double phase retrieval by developing a new computationally efficient and parallelizable phase retrieval algorithm.  Using modern GPU computing techniques, we achieve a 10,000× reduction in computation time compared to existing methods.  We illustrate with a range of high-resolution transmission matrix estimates.

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11:00-11:30 High energy photon detection of special nuclear materials

Professor Alfred Hero, University of Michigan, USA

Abstract

Special nuclear materials are fissile isotopes that include plutonium and enriched uranium. Safeguarding such materials against illegal diversions and non-peaceful use is of obvious importance to global nuclear security. Special nuclear materials emit high energy photons with distinct spectra that can identify the material and its properties but are often difficult to detect due to factors such as low detector efficiency, background interference, and spectral blurring. By coupling new radiation detection modalities to refined probabilistic models such materials can be more reliably detected and quantified.

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11:30-12:00 Open channels for transport and imaging in turbid media

Professor Allard Mosk, Utrecht University, Netherlands

Abstract

Random scattering of light, which takes place in paper, paint and biological tissue is an obstacle to imaging and focusing of light and thus hampers many applications. At the same time scattering is a phenomenon of basic physical interest as it allows the study of fascinating interference effects such as open transport channels which enable lossless transport of waves through strongly scattering materials.

The transmission of these open channels remains high even for a thick sample, while their statistical occurrence offers a new way to measure the scattering strength of a material. Single open channels can be elucidated by repeated phase conjugation, and this opens them up to detailed spectroscopy measurements, allowing space-time mapping of these remarkable transmission properties in three-dimensional optical systems.

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Session 2 13:30-19:00

4 talks Show detail Hide detail

Chairs

Professor Stephen McLaughlin FREng

13:30-14:00 Advances in refractive-index tomography: sparsity-based techniques for solving the inverse scattering problem

Professor Michael Unser, EPFL, Lausanne, Switzerland

Abstract

Optical diffraction tomography (ODT) is a microscopy method that allows to do quantitative imaging of the distribution of refractive indices in biological samples. It proceeds by solving an inverse scattering problem from holographic measurements of the scattered field produced when the sample is illuminated by an incident wave. The nonlinear nature of the scattering phenomenon, which is governed by the wave equation, makes this reconstruction problem a challenging task. While classical reconstruction algorithms were relying on linear approximations of the forward model (Born or Rytov), more recent works have shown the benefit of combining advanced physics (nonlinear multiple scattering models) and sparsity constraints. In this talk, we present a reconstruction algorithm that deploys the nonlinear Lippmann-Schwinger model together with total-variation regularization. In particular, we show the ability of the method to obtain high quality reconstructions for difficult configurations (high contrasts and few illuminations).
Joint work with Emmanuel Soubies and Thanh-An Pham

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14:00-14:30 Quantum optics and information science in multi-dimensional photonics

Professor Christine Silberhorn, University of Paderborn, Germany

Abstract

Classical optical networks have been widely used to explore a broad range of transfer phenomena based on coherent interference of waves, which relate to different disciplines in physics, information science, and even biological systems. At the quantum level, the quantized nature of light, this means the existence of photons and entangled states, gives rise to genuine quantum effects that can appear completely counter-intuitive. Yet, to date, quantum network experiments typically remain very limited in terms of the number of photons, reconfigurability and, maybe most importantly, network size and dimensionality. Here we present three differing approaches to overcome current limitations for the experimental implementation of multi-dimensional quantum networks for photonic systems.

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15:30-16:00 Photon-Efficient Imaging Through Scattering Media

Professor Vivek Goyal, Boston University

Abstract

The resolution achieved in photon-efficient active optical range imaging systems can be low due to non-idealities such as propagation through a diffuse scattering medium.  We propose a constrained optimization-based framework to address extremes in scarcity of photons and blurring by a forward imaging kernel.  We provide two algorithms for the resulting inverse problem: a greedy algorithm, inspired by sparse pursuit algorithms; and a convex optimization heuristic that incorporates image total variation regularization.  We demonstrate that our framework outperforms existing deconvolution imaging techniques in terms of peak signal-to-noise ratio.  Since our proposed method is able to super-resolve depth features using small numbers of photon counts, it could be useful for observing fine-scale phenomena in remote sensing through a scattering medium and through-the-skin biomedical imaging applications.

Joint work with Dongeek Shin and Jeffrey H. Shapiro.

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16:00-16:30 Resolving fast neuronal impulses in scattering brain tissue

Dr Amanda Foust, Imperial College London

Abstract

Voltage and calcium fluorescence imaging have become mainstays of neuronal network research. Mammalian brain tissue, however, is highly scattering and precludes simple widefield imaging configurations.  Two-photon laser scanning microscopy (2PLSM) mitigates scattering through squared dependence of fluorescence on laser pulse energy. A near-infrared laser excites fluorescence in a femto-liter volume that is rastered in two or three dimensions, and a single area detector collects non-descanned fluorescence. Restriction of fluorescence to a diffraction-limited spot confers optical sectioning, even up to ~1 mm in scattering brain tissue. The scanning requirement severely limits temporal resolution, which decreases as field-of-view and signal-to-noise ratio increase. Typical frame and volume rates range from 1-10 Hz, too slow to capture the one-millisecond impulses with which neurons communicate at >100 Hz repeat rates. I present two strategies to increase the temporal resolution of neuronal calcium and voltage imaging in scattering brain tissue. The first uses computer-generated holography to sculpt light over structures of interest. We image fluorescence with a scientific complementary metal-oxide semiconductor (sCMOS) camera to resolve neuronal action potentials with high spatial specificity at >2000 Hz.  Secondly, we have developed a multifocal two-photon system that rasters a spatially sparse line of foci and collects non-descanned fluorescence with an sCMOS camera. Our novel source localization strategy increases image contrast at depth and reduces functional crosstalk between pixels, capturing calcium transients at frame rates up to 200 Hz.  I discuss the performance, advantages and limitations of these two systems compared to traditional widefield imaging and 2PLSM. 

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23 January

Session 3 09:00-12:30

4 talks Show detail Hide detail

Chairs

Professor Miles Padgett FRS, University of Glasgow, UK

09:00-09:30 Imaging Reconstruction for photon-limited atmospheric lidar

Professor Rebecca Willett, University of Wisconsin-Madison, USA

Abstract

Atmospheric lidar observations provide a unique capability to directly observe the vertical column of cloud and aerosol scattering properties. Detector and solar-background noise, however, hinder the ability of lidar systems to provide reliable backscatter and extinction cross-section estimates. Standard methods for solving this inverse problem are most effective with high signal-to-noise ratio observations that are only available at low resolution in uniform scenes. In this talk, I will describe novel methods for solving the inverse problem with high-resolution, lower signal-to-noise ratio observations that are effective in non-uniform scenes. In particular, we will examine a regularized maximum likelihood formulation of the reconstruction problem, where the regularizer is based on state-of-the-art patch-based imaging denoising methods like BM3D. This regularizer is nonconvex, so care must be taken when initializing any iterative optimization method. We develop a novel coarse-to-fine proximal gradient optimization algorithm in which we step across different levels of image resolution to compute successively better initial points for the proposed optimization procedure. Two case studies of real experimental high spectral resolution lidar data illustrate the advantages associated with the proposed method over the standard approach.

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09:30-10:00 Phase sensitive amplification for sub-shot-noise phase measurement and enhanced quantum imaging

Maria Chekhova, Max-Planck Institute for the Science of Light, Germany

Abstract

The use of quantum states brings information technologies to a principally new level. At the same time, quantum information is difficult to deal with due to its fragility to detection losses and noise. A remedy is phase sensitive amplification, proposed theoretically more than two decades ago but applied to experiments only recently. 

In particular, phase measurement below the shot-noise level, possible with squeezed light, is strongly affected by detection losses. At the same time, amplification of the quadrature containing the phase information enables overcoming any level of loss. This tolerance to loss has been demonstrated in a recent experiment with two coherently pumped high-gain parametric amplifiers, the first of them producing squeezed light testing the phase and the second one amplifying and protecting the phase information. 

The same principle of phase sensitive amplification can be used to protect from loss the protocol of sub-shot-noise imaging, in which an object is placed into one of twin beams and the image is restored in the difference intensity distribution. By amplifying the image before detection, the limitations imposed by losses can be lifted, and the protocol can be extended to ‘difficult’ spectral ranges such as infrared. 

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11:00-11:30 Time-of-flight computational imaging

Professor Andreas Velten, University of Wisconsin-Madison, USA

Abstract

Time of Flight Non-Line-of-Sight (NLOS) imaging has been used to reconstruct scenes that are blocked from direct view via non-specular reflections in the scene. Current methods image scenes of few meter diameter at centimetre resolution There are currently several challenges in this area. I this talk I will review the current state of our research and the field and point out some of the challenges and opportunities of this method.

Including these challenges are speed and efficiency of the reconstruction; scene features like occlusions and multiple reflections, that cannot be handled by current reconstruction methods; and the development of hardware systems that can capture relevant information at reasonable speed and resolution while maintaining acceptable requirements to size, weight, power, and cost. 

We will highlight several recent developments that seek to address these problems, talk about fundamental limitations and requirements of NLOS imaging , and speculate on application areas and scenarios where this method addresses problems that cannot be better addressed by other competing options, such as non-line-of-sight sonar and RADAR imaging through walls.

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11:30-12:00 Optical manipulation of neuronal circuits by optical wave front shaping

Dr Valentina Emiliani, Univeristy Paris Descartes, France

Abstract

Optogenetics has revolutionized neuroscience by enabling remote activation or inhibition of specific populations of neurons in intact brain preparations through genetically targeted light sensitive channels and pump. Nevertheless, studying the role of individual neurons within neuronal circuits is still a challenge and requires joint progresses in opsin engineering and light sculpting methods. 

Here we show that computer generated holography using an amplified pulse laser combined with high light sensitive and somatic opsins enable precise in vitro and in vivo control of neuronal firing in mouse brain with millisecond temporal precision, single cell resolution and unprecedented low illumination level. 

We also show a new optical system generates multiple extended excitation spots in a large volume with micrometric lateral and axial resolution. Two-dimensional temporally focused shapes are multiplexed several times over selected positions, thanks to the precise spatial phase modulation of the pulsed beam. This permits, under multiple configurations, the generation of tens of axially confined spots in an extended volume, spanning a range in depth of up to 500 m. We demonstrate the potential of the approach by performing multi-cell volumetric excitation of photoactivatable GCaMP in the central nervous system of Drosophila larvae, a challenging structure with densely arrayed and small diameter neurons, and by photoconverting the fluorescent protein Kaede in zebrafish larve. Our technique paves the way for the optogenetic manipulation of a large number of neurons in intact circuits.

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Session 4 13:30-17:00

3 talks Show detail Hide detail

Chairs

Professor Alfred Hero, University of Michigan, USA

13:30-14:00 Single Pixel Camera and data inversion

Professor Miles Padgett, University of Glasgow, UK

Abstract

Cameras are often marketed in terms of the number of pixels they have – the more pixels the “better” the camera.  Rather than increasing the number of pixels we ask the question “how can a camera work when it only has a single pixel?” This talk will link the field of computational ghost imaging to that of single-pixel cameras explaining how components found within a standard data projector, more commonly used for projecting films and the like, can be used to create both still and video cameras using a single photodiode.
These single pixel approaches are particularly useful for imaging at wavelengths where detector arrays are either very expensive or even unobtainable. The ability to image at unusual wavelengths means that one can make cameras that can see through fog or smoke or even image invisible gases as they leak from pipes.
Beyond imaging at these unusual wavelengths, by using pulsed illumination and adding time resolution to the camera it is possible to use a single-pixel camera to see in 3-dimensions, perhaps useful for autonomous vehicles and other robotic applications. 


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14:00-14:30 Correlations between the transmitted and reflected speckle patterns in scattering media

Dr Jacopo Bertolotti, University of Exeter, UK

Abstract

When monochromatic light propagates through a scattering medium, it is scrambled and produces a seemingly random speckle pattern. This randomization process prevents information to pass through a turbid material, which behave like a screen and prevents us to see through it. Yet, multiple scattering is not enough to completely decouple the two sides of a turbid medium, as interference between the scattered waves is known to produce correlations in the speckle patterns. In most cases these correlations can be used to extract information about the turbid medium itself. An exception is the optical memory effect, that connects the incident with the transmitted wavefront, and thus can be exploited to retrieve information through an otherwise opaque screen. Here we present the experimental characterization of a novel form of correlation that connects the reflected and the transmitted speckle measuring only the reflected light. We characterize the rich phenomenology of this correlation, and show that it can be used to image non-invasively through a strongly scattering medium.

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15:30-16:00 Transmission matrix approach to light control through complex media : imaging and beyond

Professor Sylvain Gigan, Laboratoire Kastler-Brossel, UPMC Paris, France

Abstract

Coherent light transport through disordered media gives rise to the well known speckle pattern. This pattern can be understood as the result of of a complex mixing of the incident coherent light through linear elastic multiple scattering, i.e. the propagation can be described by a transmission matrix, linking input to output modes. I will show how the knowledge of the transmission matrix allows imaging through complex media, and how we can extend these concepts in the realms of computational imaging and even optical computing.

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Light transport and imaging through complex media

Theo Murphy international scientific meeting organised by Professor Daniele Faccio and Professor Stephen McLaughlin FREng

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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