5,110 research outputs found
Age Related Changes in Cerebrovascular Reactivity and Its Relationship to Global Brain Structure
ACKNOWLEDGMENTS This study was funded by Alzheimer’s Research UK (ARUK) and the Aberdeen Biomedical Imaging Centre, University of Aberdeen. GDW, ADM and CS are part of the SINASPE collaboration (Scottish Imaging Network - A Platform for Scientific Excellence www.SINAPSE.ac.uk). The authors thank Gordon Buchan, Baljit Jagpal, Nichola Crouch, Beverly Maclennan and Katrina Klaasen for their help with running the experiment and Dawn Younie and Teresa Morris for their help with recruitment and scheduling. We also thank the residents of Aberdeen and Aberdeenshire, and further afield, for their generous participation.Peer reviewedPublisher PD
Ultra-miniature dual-wavelength spatial frequency domain imaging for micro-endoscopy
There is a need for a cost-effective, quantitative imaging tool that can be
deployed endoscopically to better detect early stage gastrointestinal cancers.
Spatial frequency domain imaging (SFDI) is a low-cost imaging technique that
produces near-real time, quantitative maps of absorption and reduced scattering
coefficients, but most implementations are bulky and suitable only for use
outside the body. We present an ultra-miniature SFDI system comprised of an
optical fiber array (diameter 0.125 mm) and a micro camera (1 x 1 mm package)
displacing conventionally bulky components, in particular the projector. The
prototype has outer diameter 3 mm, but the individual components dimensions
could permit future packaging to < 1.5 mm diameter. We develop a phase-tracking
algorithm to rapidly extract images with fringe projections at 3 equispaced
phase shifts in order to perform SFDI demodulation. To validate performance, we
first demonstrate comparable recovery of quantitative optical properties
between our ultra-miniature system and a conventional bench-top SFDI system
with agreement of 15% and 6% for absorption and reduced scattering
respectively. Next, we demonstrate imaging of absorption and reduced scattering
of tissue-mimicking phantoms providing enhanced contrast between simulated
tissue types (healthy and tumour), done simultaneously at wavelengths of 515 nm
and 660 nm. This device shows promise as a cost-effective, quantitative imaging
tool to detect variations in optical absorption and scattering as indicators of
cancer.Comment: 26 Pages, 7 Figure
Single-ended Recovery of Optical fiber Transmission Matrices using Neural Networks
Ultra-thin multimode optical fiber imaging technology promises
next-generation medical endoscopes that provide high image resolution deep in
the body (e.g. blood vessels, brain). However, this technology suffers from
severe optical distortion. The fiber's transmission matrix (TM) calibrates for
this distortion but is sensitive to bending and temperature so must be measured
immediately prior to imaging, i.e. \emph{in vivo} and thus with access to a
single end only. We present a neural network (NN)-based approach that quickly
reconstructs transmission matrices based on multi-wavelength reflection-mode
measurements. We introduce a custom loss function insensitive to global
phase-degeneracy that enables effective NN training. We then train two
different NN architectures, a fully connected NN and convolutional U-Net, to
reconstruct complex-valued fiber TMs through a simulated
single-ended optical fiber with error. This enables image
reconstruction with error. This TM recovery approach shows
advantages compared to conventional TM recovery methods: 4500 times faster;
robustness to 6\% fiber perturbation during characterization; operation with
non-square TMs and no requirement for prior characterization of reflectors.Comment: 13 pages, 9 figure
Design of indoor communication infrastructure for ultra-high capacity next generation wireless services
The proliferation of data hungry wireless devices, such as smart phones and intelligent sensing networks, is pushing modern wireless networks to their limits. A significant shortfall in the ability of networks to meet demand for data is imminent. This thesis addresses this problem through examining the design of distributed antenna systems (DAS) to support next generation high speed wireless services that require high densities of access points and must support multiple-input multiple-output (MIMO) protocols.
First, it is shown that fibre links in DAS can be replaced with low-cost, broadband free-space optical links, termed radio over free-space optics (RoFSO) links. RoFSO links enable the implementation of very high density DAS without the need for prohibitively expensive cabling infrastructure. A 16m RoFSO link requiring only manual alignment is experimentally demonstrated to provide a spurious-free dynamic range (SFDR) of > 100dB/Hz^2/3 over a frequency range from 300MHz- 3.1GHz. The link is measured to have an 802.11g EVM dynamic range of 36dB. This is the first such demonstration of a low-cost broadband RoFSO system.
Following this, the linearity performance of RoFSO links is examined. Because of the high loss nature of RoFSO links, the directly-modulated semiconductor lasers they use are susceptible to high-order nonlinear behaviour, which abruptly limits performance at high powers. Existing measures of dynamic range, such as SFDR, assume only third-order nonlinearity and so become inaccurate in the presence of dominant high-order effects. An alternative measure of dynamic range called dynamic-distortion-free dynamic range (DDFDR) is then proposed. For two different wireless services it is observed experimentally that on average the DDFDR upper limit predicts the EVM knee point to within 1dB, while the third-order SFDR predicts it to within 6dB. This is the first detailed analysis of high-order distortion effects in lossy analogue optical links and DDFDR is the first metric able to usefully quantify such behaviour.
Next, the combination of emerging MIMO wireless protocols with existing DAS is examined. It is demonstrated for the first time that for small numbers of MIMO streams (up to ~4), the capacity benefits of MIMO can be attained in existing DAS installations simply by sending the different MIMO spatial streams to spatially separated remote antenna units (RAU). This is in contrast to the prevailing paradigm of replicating each MIMO spatial stream at each RAU. Experimental results for two representative DAS layouts show that replicating spatial streams provides an increase of only ~1% in the median channel capacity over merely distributing them. This compares to a 3-4% increase of both strategies over traditional non-DAS MIMO. This result is shown to hold in the multiple user case with 20 users accessing 3 base stations. It is concluded that existing DAS installations offer negligible capacity penalty for MIMO services for small numbers of spatial streams, including in multi-user MIMO scenarios.
Finally, the design of DAS to support emerging wireless protocols, such as 802.11ac, that have large numbers of MIMO streams (4-8) is considered. In such cases, capacity is best enhanced by sending multiple MIMO streams to single remote locations. This is achieved using a novel holographic mode division multiplexing (MDM) system, which sends each separate MIMO stream via a different propagation mode in a multimode fibre. Combined channel measurements over 2km of mode-multiplexed MMF and a typical indoor radio environment show in principle a 2x2 MIMO link providing capacities of 10bit/s/Hz over a bandwidth of 6GHz. Using a second experimental set-up it is shown that the system could feasibly support at least up to a 4x4 MIMO system over 2km of MMF with a condition number >15dB over a bandwidth of 3GHz, indicating a high degree of separability of the channels. Finally, it is shown experimentally that when a fibre contains sharp bends (radius between 20mm and 7.2mm) the first 6 mode-groups used for multiplexing exhibit no additional power loss or cross-coupling compared with unbent fibre, although mode-groups 7, 8 and 9 are more severely affected. This indicates that at least 6x6 multiplexing is possible in standard installations with tight fibre bends.For their financial support, I would like to thank the Rutherford Foundation of the Royal Society of New Zealand, the Cambridge Commonwealth Trust and the EPSRC
Global phase insensitive loss function for deep learning in holographic imaging and projection applications
Holographic imaging and projection are increasingly used for important applications such as augmented reality, 1 3D microscopy 2 and imaging through optical fibres. 3 However, there are emerging applications that require control or detection of phase, where deep learning techniques are used as faster alternatives to conventional hologram generation algorithms or phase-retrieval algorithms. 4 Although conventional mean absolute error (MAE) loss function or mean squared error (MSE) can directly compare complex values for absolute control of phase, there is a class of problems whose solutions are degenerate within a global phase factor, but whose relative phase between pixels must be preserved. In such cases, MAE is not suitable because it is sensitive to global phase differences. We therefore develop a ‘global phase insensitive’ loss function that estimates the global phase factor between predicted and target outputs and normalises the predicted output to remove this factor before calculating MAE. As a case study we demonstrate ≤ 0.1% error in the recovery of complex-valued optical fibre transmission matrices via a neural network. This global phase insensitive loss function will offer new opportunities for deep learning-based holographic image reconstruction, 3D holographic projection for augmented reality and coherent imaging through optical fibres
Grayscale-to-Color: Scalable Fabrication of Custom Multispectral Filter Arrays.
Snapshot multispectral image (MSI) sensors have been proposed as a key enabler for a plethora of multispectral imaging applications, from diagnostic medical imaging to remote sensing. With each application requiring a different set, and number, of spectral bands, the absence of a scalable, cost-effective manufacturing solution for custom multispectral filter arrays (MSFAs) has prevented widespread MSI adoption. Despite recent nanophotonic-based efforts, such as plasmonic or high-index metasurface arrays, large-area MSFA manufacturing still consists of many-layer dielectric (Fabry-Perot) stacks, requiring separate complex lithography steps for each spectral band and multiple material compositions for each. It is an expensive, cumbersome, and inflexible undertaking, but yields optimal optical performance. Here, we demonstrate a manufacturing process that enables cost-effective wafer-level fabrication of custom MSFAs in a single lithographic step, maintaining high efficiencies (∼75%) and narrow line widths (∼25 nm) across the visible to near-infrared. By merging grayscale (analog) lithography with metal-insulator-metal (MIM) Fabry-Perot cavities, whereby exposure dose controls cavity thickness, we demonstrate simplified fabrication of MSFAs up to N-wavelength bands. The concept is first proven using low-volume electron beam lithography, followed by the demonstration of large-volume UV mask-based photolithography with MSFAs produced at the wafer level. Our framework provides an attractive alternative to conventional MSFA manufacture and metasurface-based spectral filters by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability
Variable X-ray Absorption toward Gravitationally-Lensed Blazar PKS1830-211
We present X-ray spectral analysis of five Chandra and XMM-Newton
observations of the gravitationally-lensed blazar PKS1830-211 from 2000 to
2004. We show that the X-ray absorption toward PKS1830-211 is variable, and the
variable absorption is most likely to be intrinsic with amplitudes of about
2-30e22 cm^-2 depending on whether or not the absorber is partially covering
the X-ray source. Our results confirm the variable absorption observed
previously, although interpreted differently, in a sequence of ASCA
observations. This large variation in the absorption column density can be
interpreted as outflows from the central engine in the polar direction,
consistent with recent numerical models of inflow/outflows in AGNs. In
addition, it could possibly be caused by the interaction between the blazar jet
and its environment, or the variation from the geometric configuration of the
jet. While the spectra can also be fitted with a variable absorption at the
lens redshift, we show that this model is unlikely. We also rule out the simple
microlensing interpretation of variability which was previously suggested.Comment: 15 pages, 4 figures, Accepted to A
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