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

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 ($\textit{Dow Chemical Company}$), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40-50 μC cm$^{-2}$, 1 nAs$^{-1}$) 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$^{2}$.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