KEYWORDS: Sensors, Interferometry, Signal processing, Data modeling, Bayesian inference, Chemical species, Electroluminescent displays, Monte Carlo methods, Data processing, Interference (communication)
Recent advances in the field of quantum technology offer the exciting possibility of gravimeters and gravity gradiometers capable of performing rapid surveys with unprecedented precision and accuracy. Measurements with sub nano-g (a billionth of the acceleration due to gravity) precision should enable the resolution of underground structures on metre length scales. However, deducing the exact dimensions of the structure producing the measured gravity anomaly is known to be an ill-posed inversion problem. Furthermore, the measurement process will be affected by multiple sources of uncertainty that increase the range of plausible solutions that fit the measured data. Bayesian inference is the natural framework for accommodating these uncertainties and providing a fully probabilistic assessment of possible structures producing inhomogeneities in the gravitational field. Previous work introduced the probability of excavation map as a means to convert the high-dimensional space belonging to the posterior distribution to an easily interpretable map. We now report on the development of the inference model to account for spatial correlations in the gravitational field induced by variations in soil density.
There is an increasingly important requirement for day and night, wide field of view imaging and tracking for both
imaging and sensing applications. Applications include military, security and remote sensing. We describe the
development of a proof of concept demonstrator of an adaptive coded-aperture imager operating in the mid-wave infrared
to address these requirements. This consists of a coded-aperture mask, a set of optics and a 4k x 4k focal plane array
(FPA). This system can produce images with a resolution better than that achieved by the detector pixel itself (i.e. superresolution)
by combining multiple frames of data recorded with different coded-aperture mask patterns. This superresolution
capability has been demonstrated both in the laboratory and in imaging of real-world scenes, the highest
resolution achieved being ½ the FPA pixel pitch. The resolution for this configuration is currently limited by vibration
and theoretically ¼ pixel pitch should be possible. Comparisons have been made between conventional and ACAI
solutions to these requirements and show significant advantages in size, weight and cost for the ACAI approach.
Adaptive coded aperture imaging systems can resolve objects that are smaller than the pixel-limited resolution of the
detector focal plane array. This is done by combining multiple frames of data, where different frames are taken with
different coding patterns on the coded-aperture mask. In the mid-wave infrared the required signal to noise ratio
necessitates some form of light concentration. Optical design software has been used to model candidate optical systems
with the aim of achieving up to four times resolution enhancement along each linear dimension. As in some other
computational imaging systems, the requirements on the optical system are found to be different to those that are
normally used in more classical optical designs. The basic needs are a point-spread function of suitable extent that
changes gradually with angle and does not vary significantly with the expected changes in input spectra or system
temperature. Novel metrics have been derived and used to inform the optical design. The modeling and design trade-offs
and resulting performance are discussed.
Adaptive coded aperture imaging (ACAI) has the potential to enhance greatly the performance of sensing systems by
allowing sub detector pixel image and tracking resolution. A small experimental system has been set up to allow the
practical demonstration of these benefits in the mid infrared, as well as investigating the calibration and stability of the
system. The system can also be used to test modeling of similar ACAI systems in the infrared. The demonstrator can use
either a set of fixed masks or a novel MOEMS adaptive transmissive spatial light modulator. This paper discusses the
design and testing of the system including the development of novel decoding algorithms and some initial imaging
results are presented.
Coded Aperture Imaging (CAI) is a new approach to system design whereby the optics are simplified in a controlled way
so that system performance can be recovered using appropriate computer based algorithms. Adopting Coded Aperture
approaches to sensor designs opens up possibilities of increasing the system design trade-space thereby giving the system
designer greater degrees of freedom to optimise the system. A comparison has been made between a system adopting CA
in its optical train with systems based on conventional optics approaches. These comparisons show that CA based
systems can provide significant benefits to the user in some applications.
Coded aperture imagery poses a challenge for traditional image tracking algorithms because of the highly distributed
nature of the coded imagery. Traditional algorithms would require this imagery to undergo a computationally expensive
decoding operation before subsequent processing and tracking. In this paper, a novel tracking algorithm is described that
can track point-source targets at sub-pixel accuracy without requiring the coded aperture imagery to be decoded.
Furthermore, it is shown that the algorithm is robust to changes in the coded aperture mask pattern, and so is suitable for
use in adaptive coded aperture imaging systems. Some results of the algorithm on synthetic and initial MWIR
experimental data will be given.
We report on a laser communications experiment over a kilometre optical range where we have used a retro-reflective
transponder incorporating an optical modulator based on silicon micro-electro-mechanical systems (MEMS) device. This
employs interference to provide modulation and relies on performing as a coherent array to modulate incident light in the
near-IR band (1550nm) over a wide angular range (120 degrees). Modulation is achieved by tuning a large array of
Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the
underlying silicon substrate.
The micro-mirrors have a strong mechanical resonance, and modulate light by adjusting the spacing between the micromirrors
and the substrate. We use a 'release and catch' technique to exploit the mechanical resonance, and we time the
motion of the micro-mirrors to be synchronised with the arrival of an interrogator pulse to ensure that the etalon spacing
provides the required modulation, whatever the angle of incidence.
We describe experiments over a one kilometre path where simple strings were sent at 200kbit per second. We also
discuss approaches to adapting the link to a given angle of incidence.
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.
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.
The development of a micro-opto-electro-mechanical system (MOEMS) technology employing interference effects to
modulate incident light in the near-IR band (1550nm) over a wide angular range (120 degrees) is reported. Modulation is
achieved by tuning a large array of Fabry-Perot cavities via the application of an electrostatic force to adjust the gap
between a moveable mirror and the underlying silicon substrate.
The optical design determines the layer thicknesses; however, the speed and power are determined by the geometry of
the individual moveable elements. Electro-mechanical trade-offs will be presented as well as a key innovation of
utilising overshoot in the device response in reduced pressure environment to reduce the drive voltage.
Devices have been manufactured in a modified polysilicon surface micromachining process with anti-reflection coatings
on the back of the silicon substrate. Measurements of individual mirror elements and arrays of mirrors at 1550nm show
excellent uniformity across the array. This enables good response to an incident signal over a wide field of view when
integrated with a silicon retroreflector in a passive optical tag. In conjunction with appropriate anti-stiction coatings,
lifetimes of over 100 million cycles have been demonstrated.
Key advantages of the modulator are that it is low cost being based on standard polysilicon micromachining; high speed
(>100kHz) and robust due to utilising a massively parallel array of identical compact devices; low power for portable
applications; and operates in transmission - allowing simple integration with a retroreflector in a passive tag for halfduplex
free-space optical communications to a remote interrogator.
The advent of non-linear photonic crystal fibres with engineered optical properties has enabled the production of
compact high-brightness super-continuum sources with a spectral power density in excess of 1 mW/nm throughout
the visible and near-infrared spectral regions. Such sources have intrinsically good beam quality and, when properly
collimated, the various spectral components propagate in a co-linear fashion, thus retaining spectral fidelity along the
beam path. These properties are ideal for an active hyper-spectral remote sensing system.
We report the construction and testing of a white light transceiver for measurement of the spectral reflectivity of
remote targets. The transmission section of the transceiver comprises a commercial white light source with
apochromatic optics to ensure simultaneous collimation at all wavelengths. The receiver section comprises a
telescope coupled to a fibre-optic visible band spectrometer. A portion of the received light is directed onto a camera
to facilitate accurate pointing of the system.
The transceiver has been used to measure the spectral reflection from both diffuse and retro-reflecting targets at an
outdoor range. The spectral return from retro-reflective targets was successfully measured at ranges up to 1.2 km.
For diffuse targets, the useful range was limited to a few hundred metres, beyond which the signal was dominated by
ambient daylight. The propagation of the white light beam along the 1.2 km has been studied. The fidelity of
measured spectra was affected by atmospheric turbulence which caused the beam to break up into a time-varying
pattern of coloured regions. This effect imposed a lower limit on the integration time required to measure individual
spectra, independent of the signal to noise ratio.
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.
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.
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.
Recent progress on the development of a long-range, high-resolution 3D active imaging sensor is described. Diffraction limited angular resolution of 20μrad and sub-metre down range resolution are demonstrated at stand-off ranges of 8km. A scanned single pixel arrangement was employed using an all-fiber coherent lidar operating in a chirp-pulse-compression mode. The monostatic antenna had an aperture of 150mm and the image was built up using a piezoelectric tip/tilt stage positioned prior to the final expansion of the beam. Transmit/receive multiplexing was achieved with a fiber optic circulator. Examples of recently acquired images consisting of 150x150 pixels with 1000, 30cm range cells per pixel at a stand-off range 8km are presented.
A retro-reflective communications system comprises a laser transmit/receive station and a remote retroreflector that can be switched between "on" and "off" states. The laser illuminates the remote station and a collection telescope directs the reflected light to an associated detector the output of which is interpreted as logic 1 or 0. Atmospheric turbulence affects the outgoing illumination beam, resulting in beam spreading and in fluctuations in the intensity (scintillation)1. The reflected beam undergoes further turbulence induced spreading and there is an enhancement in the intensity fluctuations. These fluctuations mean that the logic level of signals may be wrongly identified, leading to bit errors. The signal may fade below detectable levels for periods of time, leading to sections of the bit stream being lost. We develop a description of the intensity fluctuations in terms of the Gamma-Gamma distribution2, and incorporate the effect of "aperture averaging" associated with the retro-reflector and the collection aperture. We characterise signal-to-noise ratios and calculate bit error rates as a function of retro-reflector cross-section and contrast for a variety of ranges, turbulence levels and system configurations. We also identify the characteristic timescale over which the atmosphere causes changes of intensity and discuss the implications.
A summary is presented of some of the design criteria relevant to the realisation of silicon micromachined modulator arrays for use in free-space optical communication systems. Theoretical performance levels achievable are compared with values measured on experimental devices produced using a modified Multi-User MEMS Process (MUMPS). Devices capable of realising modulation rates in excess of 300 kHz are described and their optical characteristics compared with published data on devices based on multiple quantum well technology.
We have previously shown that amplitude weighting can improve the accuracy of measurements of the frequency offset of a signal contaminated by multiplicative Gaussian noise. We have investigated the more general non-Gaussian case through study of the statistics of a simple phase-screen scattering model and derived formulae for the low-order moments of the intensity-weighted phase-derivative. In this paper we extend numerical simulation of the problem to the case of a phase screen with Kolmogorov spectrum. We also report the results of some preliminary experimental measurements.
The process of Brillouin amplification when the input pump and signal beams have Gaussian spatial profiles is investigated. Two distinct regimes are considered. In the small signal regime, it is shown that the signal beam undergoes spatial beam narrowing as it propagates through the amplifier owing to the exponential dependence of signal beam gain on pump beam intensity. The relative change in spot size is found to depend on both the peak signal intensity gain and on the input signal to pump spot size ratio. In the large signal (depleted pump) regime, it is found that the signal beam becomes broadened if the pump depletion is significant and the output spot size depends on the input signal beam intensity. The theoretical predictions are verified experimentally using a Q-switched Nd-YAG laser to provide pulses of 30 ns FWHM duration and wavelength 1.06 micrometers , and using CCl2FCCIF2 as the nonlinear medium.
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