Imaging devices are very attractive as sensors on modern airborne platforms and there is a continuing trend toward widespread employment of imaging either alone or in combination with complementary technologies. In the civil domain, modern silicon CCD and CMOS image sensors are becoming extremely small, so that the package size of commercial miniature cameras is becoming dominated by the image forming optics, even if the latter is only a structure supporting a pinhole. We have previously reported a biologically inspired, near IR, imaging system that offers a wide field-of-view, thanks to the use of a multi-aperture sensor based on micro-optics which can be used to observe simultaneously in different directions. In this paper we describe our recent work on the design, assembly and test of a novel, all solid state camera that exploits spherical lens technology to provide a robust, wide field-of-view camera. We have also considered active fluidic lenses that can operate in the mid infra-red in order to provide additional functionality.
There is a need for faster processing hardware to provide modern radar systems with advanced capabilities such as
multiple hypothesis tracking, real-time clutter removal and space-time adaptive beamforming (STAP) for jammer
nulling. One approach that may help to meet this need is to use analogue methods in parts of the signal processing chain
using optoelectronics. The vector-matrix multiplier is a powerful optical processing architecture that potentially offers
very large gains in computation speed, but has not so far become commercially successful. This paper reports
investigation of a novel arrangement for this type of processor that is aimed at improving the prospects for
commercialisation, using guided-wave and micro-optic components. This approach will assist miniaturisation of the
processor and improve ruggedness and scalability to large matrices.
Imaging devices are very attractive as sensors in modern airborne platforms and there is a continuing trend toward
widespread employment of imaging either alone or in combination with complementary technologies. In the civil
domain, modern silicon CCD and CMOS image sensors are becoming extremely small, so that the package size of
commercial miniature cameras is increasingly being dominated by the image forming optics, even if the latter is only a
structure supporting a pinhole. Recently, there have been demonstrations of ultra-flat, extremely light weight sensors
working in the visible region of the spectrum. Similar ideas for cameras developed to operate in the infra-red could help
to drastically reduce the size, weight and cooling requirements of imaging, also offering substantial cost reductions. In
addition, designs providing wide field-of-view can potentially eliminate the need for sightline steering hardware. This
paper describes work on a biologically inspired imaging system offering a wide field of view, thanks to the use of a
multi-aperture sensor based on micro-optics which can be used to observe simultaneously in different directions. Results
from a near-infrared, narrowband demonstrator are reported.
This paper reports the development of novel retroreflectors for use in free-space optical communication systems. It will be important for the retroreflectors to have a very wide field of view to make such systems practicable and affordable. Corner cube retroreflectors present a practical means of meeting the requirement for a wide field of view, but require use of materials with very high refractive index. Practical measurements on initial samples of high index corner cubes have shown encouraging optical performance. The measured results approximately confirm predictions of the variation of reflection efficiency with the angle of incidence. Retroreflectors based on graded-index, spherical (GRIN-sphere) lenses potentially offer an alternative with valuable technical advantages over the use of high-index corner cubes, if such lenses can be fabricated with a suitable combination of optical quality, size and relative aperture. The key property of GRIN-sphere lenses is that they can in principle suppress the most problematic feature of sphere lenses, that is, their strong spherical aberration. Predictions for practical graded-index sphere lens structures show valuable potential for improvement compared to uniform sphere lenses, including diffraction limited optical performance over significant fractions of the lens aperture.
This paper presents power budget predictions for retroreflective, free-space optical communication systems and examines options for producing retroreflectors with very wide field of view. Power budgets containing data representative of practical conditions show that operational ranges of many kilometres can be expected. Novel graded-index (GRIN) spherical retroreflectors have been examined in comparison with other types of retroreflector, and it is shown that they can offer technical advantages if they can be fabricated with suitable sizes and focal lengths. It is also shown that realisable high-index corner cube reflectors may be technically acceptable for this application.
In-shoe shear stress sensors are a required tool for the investigation of plantar ulcer development after the onset of diabetes. Recently, several transducers have been developed for measuring in-shoe shear stress using magneto- resistive technology, light intensity modulation, and copolymer piezoelectric materials. Common drawbacks in the previous methods are the relatively large size of the sensors and the difficulty in interrogating many sensors simultaneously in order to achieve distributed sensing. In this paper we demonstrate for the first time a shear stress sensor using Fiber Bragg gratings (FBGs). The small size and the multiplexing capability of FBGs enables quasi- distributed sensing of shear stress on the plantar surface by interrogating a large number of identical sensors. The sensor design is based on the theory of elastic bending of columns. The sensor consists of two FBGs fitted inside a metallic structure which is able to deform elastically under shear stress. This elastic deformation produces strain on the FBGs, which can be detected by measuring the Bragg wavelength shift of the reflected light of each FBG using a CCD spectrometer. Preliminary results on an enlarged version of the sensor have shown the applicability of FBGs for the implementation of the in-shoe sensor.
Two high resolution optical-fiber system for the distributed measurement of strain or temperature have been investigated. The first is a fully distributed system, based on a high spatial resolution POTDR which benefits from photon-counting detection, with a spatial resolution of 50mm. The second is a novel quasi-distributed system which uses coherent coupling of backscattered light in a high birefringence fiber, and also uses photon counting detection.
The ability to make distributed measurements on extended structures is of increasing importance. For example, the measurement of strain distribution on aircraft operating close to their performance limits, the distribution of temperature in boilers, pressure vessels, high voltage transformers etc.,
KEYWORDS: Signal to noise ratio, Spatial resolution, Sensors, Temperature metrology, Pulsed laser operation, Photodetectors, Photon counting, Time metrology, Interference (communication), Signal processing
A very high resolution distributed optical-fiber temperature sensor system has been demonstrated using a time-resolved photon counting technique. The spatial resolution of the system is 3.5 cm. A temperature sensitivity of 2 degrees Celsius has been achieved with 1 minute integration time when averaging the data points over 10 cm. The system offers a practical solution for life assessment and monitoring of hot spots along the steam pipes in power plants.
Distributed optical-fiber sensing promises to be a powerful technique for the monitoring and diagnosis of large structures. The technique provides the means by which the spatial and temporal distribution of, for example, strain and temperature can be measured, with good accuracy, throughout the structure. Backscatter methods in fibers are limited, in the performance which they can provide, by a low signal level. Better performance can be achieved by using forward-scatter methods which use polarimetric techniques and/or nonlinear optical effects. Such methods are reviewed in this paper and three of them are described in detail. Prospects for application are very promising.
Distributed optical-fiber sensing (DOFS) utilizes the unique advantages of the optical fiber as a passive, dielectric, flexible and one- dimensional measurement medium. It offers full spatial and temporal information concerning the behavior of a large range of measurand fields. Among the many potential application areas of DOFS are those in aerospace, petrochemicals, electricity supply, mining and civil engineering. Methods for realizing DOFS have hitherto concentrated on linear backscatter techniques in the fiber. New explorations to be described in this paper give the emphasis to nonlinear, forward-scatter techniques, and especially to two of these which rely on the optical Kerr effect. The primary advantage of this approach is a significantly improved spatial resolution, down to approximately equals 0.1. m. A description also will be given of a promising quasi-distributed (forward-scatter or backscatter) arrangement which used the (nonlinear) photosensitivity of fibers to devise a DOFS system for simultaneous quasi-distributed measurement of strain and temperature. Prospects for the future of DOFS technology will be reviewed.
Frequency-derived distributed optical-fiber sensing is a potentially-powerful technique for the distributed measurement of extended physical parameters, such as temperature and strain. The technique is capable, in principle, of high spatial resolution (better than 0.1 m) and accuracy (approximately 1%). This paper investigates the limitations on the performance of this system, and summarizes the experimental results achieved to date.
We have recently demonstrated1-3 that the optical Kerr effect can be used in pulsed-pump, cw-
probe configurations to detect changes in the linear birefringence of single mode optical fibres.
This phenomenon can be used in different ways to detect the presence of stress in the fibres, to
locate the stress and to measure the magnitude of the associated birefringence changes as a
function of position along the fibre.
Polarization mode coupling in hi-bi fibers may be achieved either statically, via the writing of photorefractive gratings, or dynamically, via the optical Kerr-effect of a propagating optical pulse. In each case the coupling can occur under the influence of an external measurand which can then be measured distributedly or quasi-distributedly. Examples are given of a static method and of a dynamic method for implementing good-performance distributed measurement systems.
A new arrangement is described for mapping transverse stress regions along a highly- birefringent fiber, using the optical Kerr effect in a pump-probe system. This arrangement provides a passively-biased detection system immune from thermal variations in sensitivity and, in principle, enables information to be extracted with only one laser pulse. It also enables real time location of transverse stress points which are moving along the fiber. As the laser sources employed are very compact, this system is potentially easily portable.
Photogenerated mode coupling filters for temperature and strain sensing are described. Mach- Zehnder fiber interferometers have been fabricated in elliptical-core, high-birefringence fibers where the directional couplers of a conventional fiber interferometer are replaced by two polarization rocking filters written on the same fiber. One interferometer having an operational bandwidth of 18 nm was used as a temperature sensor with a measured sensitivity of 9.1 mrad/($DEGC cm). Photoinduced polarization couplers written with 266 nm light can survive temperatures of 200$DEGC. This allows development of a temperature sensor based on the shift of the resonant wavelength of the coupler with temperature. A scheme for the simultaneous measurement of strain and temperature based on the use of a Bragg grating and a mode coupling filter is proposed.
Transverse stress applied to a highly birefringent fiber at an arbitrary angle (other than 0 or 90 degrees) to the fiber birefringence axes causes rotation of the birefringence axes and changes the beat length of the fiber in that section. If one of the polarization modes is excited at the input, coupling of light from one mode to the other will be observed at a stress point. The presentation describes a method for determining the locations of discrete mode coupling points spaced along a polarization maintaining fiber using a pump-prob architecture based on the optical Kerr effect. Probe light experiences coupling at different stress locations. Counterpropagating strong pump light also experiences coupling while inducing additional birefringence, and changing the polarization state of the probe at the output. This system may be made temperature independent by introducing a phase tracking/triggering system. The advantages and limitations of this technique are described.
Approaches to creating distributed optical fibre sensors arecompared. Pump-probe architectures have potential for realising fully- distributed systems having fast response. Quasi-distributed systems usingfibre components based on photosensitivity are also attractive.
Fully-distributed optical-fiber sensing is a potentially powerful tool for a range of industrial and research applications. Most methods which have been studied hitherto suffer from inadequate spatial resolution, inadequate sensitivity, or both. New methods are being explored in order to overcome these deficiencies. These are primarily polarization-optical methods which rely on birefringent fiber. Both linear and nonlinear optical effects are utilized in backscatter and forward-scatter systems. The methods are described and the future assessed.
We have observed several differing photorefractive effects on transmission in polarization maintaining optical fibers. Both the strain-induced and shape-induced birefringence fibers can exhibit strong photorefractive behavior, depending on fiber composition, structure and pre- treatment, while pre-treatment alone can influence a single fiber sample to exhibit differing behavior. Subsets of these transmission effects appear to originate from qualitatively differing photorefractive mechanisms which we have not identified. Contrasting behavior is most clearly seen in the characteristics of photo-generated components such as reflection gratings and polarization couplers. High-efficiency polarization couplers, including 100% and overcoupled versions, can be made in some fibers, but can apparently always be erased by heating to 240 degree(s)C. Reflection gratings written in a fresh fiber sample appear to be thermally stable. However, further gratings, written in the same fiber sample after optical erasure of the original grating, are thermally erasable.
Frequency-derived distributed optical-fiber sensing is a powerful and convenient method for measuring the spatial distribution of birefringence in a hi-bi fiber. The method relies on the special statistical characteristics of Rayleigh backscatter for its action, and these are analyzed in the context of the system. Some preliminary experimental results are also reported.
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