42 research outputs found
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Experimental comparison of antenna clustering strategies in MIMO distributed antenna systems
In this paper the effect of partitioning arrays of transmitting antennas into spatially separated clusters on the condition number and capacity of MIMO wireless systems is examined using experimental channel measurements of an indoor MIMO-enabled distributed antenna system (DAS). It is first shown for a 3 _ 2 MIMO system that distributing the transmit antennas into spatially separated clusters provides an improvement in channel conditioning (_1dB) and hence capacity, in line with previous findings. Next, a configuration with 6 transmit antennas and 2 receive antennas is examined and it is found that when it is operated as a 6 _ 2 MIMO system, it makes negligible difference to the conditioning of the channel whether the transmit antennas are grouped into 3 clusters of 2 antennas or 2 clusters of 3 antennas. The capacity varies by only a small amount (_%1) between the two clustering schemes, which can be accounted for by environment-specific signal-tonoise ratio (SNR) effects. It is then concluded that for the two typical indoor DAS scenarios tested here, if sufficient transmit diversity is provided (i.e. there are a relatively large number of transmit antennas compared to receive antennas), greater spatial distribution through increased clustering provides diminishing performance improvements. Given the typically lower installation cost of less distributed arrangements, they may then be a preferred option in commercial DAS deployments.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/VTCFall.2014.696597
Holographic Predictive Search: Extending the scope
Holographic Predictive Search (HPS) is a novel approach to search-based hologram generation that uses a mathematical understanding of the optical transforms to make informed optimisation decisions. Existing search techniques such as Direct Search (DS) and Simulated Annealing (SA) rely on trialling modifications to a test hologram and observing the results. A formula is used to decide whether the change should be accepted. HPS operates presciently, using knowledge of the underlying mathematical relationship to make exact changes to the test hologram that guarantee the'best’ outcome for that change. In this work, we extend the scope of the original research to cover both phase and amplitude modulating Spatial Light Modulators (SLMs), both phase sensitive and phase insensitive systems and both Fresnel and Fraunhofer diffraction. In the cases discussed, improvements of up to 10x are observed in final error and the approach also offers significant performance benefits in generation time. This comes at the expense of increased complexity and loss of generality
Single-Step Fabrication of Multispectral Filter Arrays Using Grayscale Lithography and Metal-Insulator-Metal Geometry
© 2018 OSA. Metal-insulator-metal geometries can provide optical transmission filtering, with peak wavelength dependent on insulator thickness. Using grayscale electron beam lithography to control insulator thickness, we fabricate multispectral filter arrays, whereby dose determines wavelength
Coherent Imaging through Multicore Fibres with Applications in Endoscopy
Imaging through optical fibres has recently emerged
as a promising method of micro-scale optical imaging within
a hair-thin form factor. This has significant applications in
endoscopy and may enable minimally invasive imaging deep
within live tissue for improved diagnosis of disease. Multi-mode
fibres (MMF) are the most common choice because of their high
resolution but multicore fibres (MCF) offer a number of advantages
such as widespread clinical use, ability to form approximate
images without correction and an inherently sparse transmission
matrix (TM) enabling simple and fast characterisation. We
present a novel experimental investigation into properties of MCF
important for imaging, specifically: a new method to upsample
and downsample measured TMs with minimal information loss,
the first experimental measurement of MCF spatial eigenmodes,
a novel statistical treatment of behaviour under bending based
on a wireless fading model, and an experimental observation
of TM drift due to self-heating effects and discussion of how
to compensate this. We next present practical techniques for
imaging through MCFs, including alignment, how to parallelise
TM characterisation measurements to improve speed and how
to use non-interferometric phase and polarisation recovery for
improved stability. Finally, we present two recent applications
of MCF imaging: polarimetric imaging using a robust Bayesian
inference approach, and entropic imaging for imaging early-stage
tumours
Wide-field phase imaging for the endoscopic detection of dysplasia and early-stage esophageal cancer
© 2018 SPIE. Esophageal cancer has a 5-year survival rate below 20%, but can be curatively resected if it is detected early. At present, poor contrast for early lesions in white light imaging leads to a high miss rate in standard-of-care endoscopic surveillance. Early lesions in the esophagus, referred to as dysplasia, are characterized by an abundance of abnormal cells with enlarged nuclei. This tissue has a different refractive index profile to healthy tissue, which results in different light scattering properties and provides a source of endogenous contrast that can be exploited for advanced endoscopic imaging. For example, point measurements of such contrast can be made with scattering spectroscopy, while optical coherence tomography generates volumetric data. However, both require specialist interpretation for diagnostic decision making. We propose combining wide-field phase imaging with existing white light endoscopy in order to provide enhanced contrast for dysplasia and early-stage cancer in an image format that is familiar to endoscopists. Wide-field phase imaging in endoscopy can be achieved using coherent illumination combined with phase retrieval algorithms. Here, we present the design and simulation of a benchtop phase imaging system that is compatible with capsule endoscopy. We have undertaken preliminary optical modelling of the phase imaging setup, including aberration correction simulations and an investigation into distinguishing between different tissue phantom scattering coefficients. As our approach is based on phase retrieval rather than interferometry, it is feasible to realize a device with low-cost components for future clinical implementation
SLM-Based Mode Division Multiplexing System With 6×6 Sparse Equalization
We demonstrate a mode division multiplexing (MDM) system over an 8 km conventional graded index multimode fiber. Spatial light modulators (SLMs) are used to multiplex and demultiplex three linearly polarized (LP) modes (LP01, LP11a, and LP11b) in two polarizations. A 6 × 6 sparse frequency domain equalizer (FDE) is used as the channel impulse response of the SLM-based MDM system is found to be sparse due to the large crosstalk at the mode MUX/DEMUX and small coupling in the fiber. The signal transmitted on each mode is recovered with improved performance over conventional FDEs. The results indicate that this system can be used in short reach transmission applications to increase the system capacity
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Fabrication of nanostructured transmissive optical devices on ITO-glass with UV1116 photoresist using high-energy electron beam lithography
High-energy electron beam lithography for patterning nanostructures on insulating substrates can be challenging. For high resolution, conventional resists require large exposure doses and for reasonable throughput, using typical beam currents leads to charge dissipation problems. Here, we use UV1116 photoresist (), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40-50 μC cm, 1 nAs) to pattern over large areas on commercially coated ITO-glass cover slips. The minimum linewidth fabricated was ∼33 nm with 80 nm spacing; for isolated structures, ∼45 nm structural width with 50 nm separation. Due to the low beam dose, and nA current, throughput is high. This work highlights the use of UV1116 photoresist as an alternative to conventional e-beam resists on insulating substrates. To evaluate suitability, we fabricate a range of transmissive optical devices, that could find application for customized wire-grid polarisers and spectral filters for imaging, which operate based on the excitation of surface plasmon polaritons in nanosized geometries, with arrays encompassing areas ∼0.25 cm.EPSRC Integrated Photonics and Electronic Systems (Grant number: EP/L015455/1) Centre for Doctoral Training. Commonwealth, European and International trust
Adaptive, spatially-varying aberration correction for real-time holographic projectors
A method of generating an aberration- and distortion-free wide-angle holographically projected image in real time is presented. The target projector is first calibrated using an automated adaptive-optical mechanism. The calibration parameters are then fed into the hologram generation program, which applies a novel piece-wise aberration correction algorithm. The method is found to offer hologram generation times up to three orders of magnitude faster than the standard method. A projection of an aberration- and distortion-free image with a field of view of 90x45 degrees is demonstrated. The implementation on a mid-range GPU achieves high resolution at a frame rate up to 12fps. The presented methods are automated and can be performed on any holographic projector
Experimental observation of non-linear mode conversion in few-mode fiber
We show for the first time directly experimentally observed nonlinear spatial mode coupling in a 10 km long graded-index few-mode fiber
Degenerate mode-group division multiplexing using delayed adaptive frequency-domain equalization
A reduced complexity delayed adaptive frequency-domain equalizer is proposed for MIMO equalization in degenerate mode-group multiplexed systems. A factor of 2.8 reduction of the DSP complexity is obtained compared to the optimized delayed time-domain equalizer. © OSA 2014