Active systems, using a near-infrared pulse laser and a fast, gated detector, are now adopted for most long range imaging
applications. This concept is often called laser-gated imaging (LGI) or burst-illumination LIDAR (BIL). The SELEX
solid state detector is based on an array of HgCdTe avalanche photodiodes, and a custom-designed CMOS multiplexer to
perform the fast gating and photon signal capture. This paper describes two recent developments. The first is aimed at
reducing the size, weight, power and cost of steerable platforms which often have to contain a large number of electrooptic
tools such as lasers, range finders, BIL, thermal imaging and visible cameras. A dual-mode infrared detector has
been developed with the aim of shrinking the system to one camera. The detector can be switched to operate as a passive
thermal imager, a laser-gated imager or a solar flux imager. The detector produces a sensitivity in the MW thermal band
of 16-18mK and a sensitivity in the BIL mode as low as 10 photons rms, in other words close to the performance of
dedicated imagers. A second development was to extend the current BIL capability to 3D. In complex scenes, with camouflage and concealment, the ability to generate 3D images provides a signal-to-clutter advantage. Also in airborne applications, especially, it is useful to have 3D information to provide agile, feedback control of the range gating in a dynamic environment. This report describes the development of the 3D detector and camera, and the results of field trials
using a prototype system.
As part of a project to optimize hybrid laminates for resistance to fatigue failure, arrays of fiber Bragg gratings are being used to monitor small-scale strain perturbations in composite materials. A remote multiplexed sensing system with 40 remote sensing sties using fiber optic technology, has been developed to monitor the strain field developed across the composite lamina of a hybrid laminate in the vicinity of a fatigue crack. Developed primarily for fatigue-critical aerospace applications, i.e. fuselage and lower wing skins, the hybrid laminates are orthotropic materials having lower density and higher strength than a simple alloy monolith without reinforcement. Fatigue crack growth in hybrid laminates is a complex process that involves a combination of delamination and fiber bridging. The fiber optic system has been applied to the problem of characterizing delamination zone development about a fatigue crack, initiated at a through-thickness fastener hole.
Three different fiber sensor systems using gratings are presented. Firstly, using an acousto-optic tunable filter and closed loop feedback, arrays of fiber Bragg gratings are being used to monitor small-scale perturbations in composite materials by mapping the strain field around a defect. Gratings are also used as distributed sensors by measuring the wavelength as a function of distance. Low-coherence interferometry selects the location under interrogation and a tunable filter measures the local wavelength. An open loop interrogation technique using a commercially available optical coherence domain reflectometer is demonstrated. The reflectivity of the sensor grating is measured as a function of both distance and wavelength. Gratings 40 cm long are interrogated and several distributed grating sensors are multiplexed in an array. Thirdly, a new sensing concept using subcarrier fiber gratings (SFGs) has been proposed and modelled. The SFG is a periodic reflective array resonant at RF frequencies. The resonance may be measured using subcarrier interferometry. Modelling has demonstrated the SFG to have superior performance over other subcarrier sensors.
The need for new instruments to measure the optical properties of natural waters, at higher spectral resolutions than existing commercial instruments, has become apparent in recent years. Such instruments will be required to support the calibration of the next generation of remote sensing platforms by providing in-situ measurements of the optical light field. In addition, high resolution spectral measurements will also support the development of optical closure models which are used to relate the observed optical properties of the water to its biological and geological content. This paper describes the design and construction of a flexible, general purpose, high resolution underwater spectrometer which can be re- configured to perform a variety of optical measurements using interchangeable, optical fiber based sensor heads. Typical results obtained during the first deployment of the instrument at sea are also presented.
An instrument has been developed for the simultaneous in situ measurement of inherent and apparent optical properties, enabling various radiative transfer models to be tested. The Southampton Underwater Multi-parameter Optical Spectrometer System measures up- and downwelling irradiance, beam transmission and forward, side and backward scatter. These inherent optical properties can be measured both at 670 nm using a laser diode, or spectrally using a white light. The instrument resolution ranges form 0.47nm to 6.7nm. This extended abstract describes the new instrument, and presents preliminary data from an experiment to monitor the optical properties at the mouth of the Hamble river over a spring tidal cycle. Optical data are compared with measurements of suspended particulate matter and chlorophyll concentrations.
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