Rebecca Anne Wilson

Coded aperture systems as non-conventional lensless imagers for the visible and infrared

Coded aperture imaging (CAI) has been used extensively at gamma- and X-ray wavelengths, where conventional refractive and reflective techniques are impractical. CAI works by coding optical wavefronts from a scene using a patterned aperture, detecting the resulting intensity distribution, then using inverse digital signal processing to reconstruct an image. This paper will consider application of CAI to the visible and IR bands. Doing so has a number of potential advantages over existing imaging approaches at these longer wavelengths, including low mass, low volume, zero aberrations and distortions and graceful failure modes. Adaptive coded aperture (ACAI), facilitated by the use of a reconfigurable mask in a CAI configuration, adds further merits, an example being the ability to implement agile imaging modes with no macroscopic moving parts. However, diffraction effects must be considered and photon flux reductions can have adverse consequences on the image quality achievable. An analysis of these benefits and limitations is described, along with a description of a novel micro optical electro mechanical (MOEMS) microshutter technology for use in thermal band infrared ACAI systems. Preliminary experimental results are also presented.

An investigation of the potential for the use of a high resolution adaptive coded aperture system in the mid-wave infrared

Previous applications of coded aperture imaging (CAI) have been mainly in the energetic parts of the electro-magnetic spectrum, such as gamma ray astronomy, where few viable imaging alternatives exist. In addition, resolution requirements have typically been low (~ mrad). This paper investigates the prospects for and advantages of using CAI at longer wavelengths (visible, infrared) and at higher resolutions, and also considers the benefits of adaptive CAI techniques. The latter enable CAI to achieve reconfigurable modes of imaging, as well as improving system performance in other ways, such as enhanced image quality. It is shown that adaptive CAI has several potential advantages over more traditional optical systems for some applications in these wavebands. The merits include low mass, volume and moments of inertia, potentially lower costs, graceful failure modes, steerable fields of regard with no macroscopic moving parts and inherently encrypted data streams. Among the challenges associated with this new imaging approach are the effects of diffraction, interference, photon absorption at the mask and the low scene contrasts in the infrared wavebands. The paper analyzes some of these and presents the results of some of the tradeoffs in optical performance, using radiometric calculations to illustrate the consequences in a mid-infrared application. A CAI system requires a decoding algorithm in order to form an image and the paper discusses novel approaches, tailored to longer wavelength operation. The paper concludes by presenting initial experimental results.

Sub-pixel super-resolution by decoding frames from a reconfigurable coded-aperture camera: theory and experimental verification

In a previous paper we presented initial results for sub-detector-pixel imaging in the mid-wave infra-red (MWIR) using an imager equipped with a coded-aperture based on a re-configurable MOEMS micro-shutter. It was shown in laboratory experiments that sub-pixel resolution is achievable via this route. The purpose of the current paper is to provide detail on the reconstruction method and to discuss some challenges which arise when imaging real-world scenes. The number of different mask patterns required to achieve a certain degree of super-resolution is also discussed. New results are presented to support the theory.

Phased array antenna beamforming using a micromachined silicon spatial light modulator

We describe an RF/microwave/MM-wave free-space optical phased array antenna beamformer operating at 10 GHz in transmission or reception with reconfiguration speed of 30 kHz through the use of a micromachined silicon phase Spatial Light Modulator (SLM).

Visible band lens-free imaging using coded aperture techniques

Traditionally, coded aperture techniques have been applied to short-wavelength imaging: X-rays and γ-rays. For these wavelengths, it is valid to neglect diffraction and describe the operation of the imager in purely geometric-optics terms. We have investigated coded aperture imaging in the visible band. The much longer wavelengths in this region of the spectrum mean that diffraction effects cannot be neglected. We describe the effects of diffraction and the implications for image resolution. We present experimental results from a lens-free coded-aperture imager operating in the visible band and describe the techniques used to obtain good quality images of complex greyscale scenes.

Adaptive coded aperture imaging in the infrared: towards a practical implementation

An earlier paper [1] discussed the merits of adaptive coded apertures for use as lensless imaging systems in the thermal infrared and visible. It was shown how diffractive (rather than the more conventional geometric) coding could be used, and that 2D intensity measurements from multiple mask patterns could be combined and decoded to yield enhanced imagery. Initial experimental results in the visible band were presented. Unfortunately, radiosity calculations, also presented in that paper, indicated that the signal to noise performance of systems using this approach was likely to be compromised, especially in the infrared. This paper will discuss how such limitations can be overcome, and some of the tradeoffs involved. Experimental results showing tracking and imaging performance of these modified, diffractive, adaptive coded aperture systems in the visible and infrared will be presented. The subpixel imaging and tracking performance is compared to that of conventional imaging systems and shown to be superior. System size, weight and cost calculations indicate that the coded aperture approach, employing novel photonic MOEMS micro-shutter architectures, has significant merits for a given level of performance in the MWIR when compared to more conventional imaging approaches.

PSK communication enhancements to an optically controlled phased array antenna

We report the addition of low error rate phase shift keyed (PSK) communications to a previously reported compact optical beamformer, demonstrating BPSK, QPSK, MSK modulation schemes and also single channel operation at 35 GHz.

A high-speed readout scheme for fast optical correlation-based pattern recognition

We describe recent developments to a novel form of hybrid electronic/photonic correlator, which exploits component innovations in both electronics and photonics to provide fast, compact and rugged target recognition, applicable to a wide range of security applications. The system benefits from a low power, low volume, optical processing core which has the potential to realise man portable pattern recognition for a wide range of security based imagery and target databases. In the seminal Vander Lugt correlator the input image is Fourier transformed optically and multiplied optically with the conjugate Fourier transform of a reference pattern; the required correlation function is completed by taking the inverse Fourier transform of the product optically. The correlator described here is similar in principle, but performs the initial Fourier transforms and multiplication electronically, with only the final most computationally demanding output Fourier transform being performed optically. In this scheme the Fourier transforms of both the input scene and reference pattern are reduced to a binary phase-only format, where the multiplication process simplifies to a simple Boolean logic XOR function. The output of this XOR gate is displayed on a state-of-the-art Fast Bit Plane Spatial Light Modulator (FBPSLM). A novel readout scheme has been developed which overcomes the previous system output bottleneck and for the first time allows correlation frame readout rates capable of matching the inherently fast nature of the SLM. Readout rates of up to ~1 MHz are now possible, exceeding current SLM capabilities and meeting potential medium term SLM developments promised by SLMs based on novel materials and architectures.

Low-phase-noise optoelectronic microwave source

A low phase noise optoelectronic source has been developed operating at 10 GHz. Custom surface acoustic wave (SAW) devices operating at high frequency and high acoustic power have been developed and are used as the critical element in an optical phase locked loop. Improved performance could be attained via lasers with larger control loop bandwidths. The source is antenna remoteable via the use of optical fibre and is compact and lightweight, suggesting applications in phased array radar.

Laser stabilisation for use in microwave photonics

Demonstrates a 50 dB improvement to the close-in phase noise spectra of diode-pumped YAG lasers and DFB laser diodes by locking the outputs to a fibre-optic delay line frequency discriminator.