Bernadette Johnson

Compact active hyperspectral imaging system for the detection of concealed targets

We have recently conducted a series of laboratory and field test to demonstrate the utility of combining active illumination with hyperspectral imaging for the detection of concealed targets in natural terrain. The active illuminator, developed at MIT Lincoln Laboratory, is a novel microlaser-pumped fiber Raman source that provides high- brightness, subnanosecond-pulse-length output spanning the visible through near-IR spectral range. The hyperspectral- imaging system is comprised of a compact, grating-based spectrometer that uses a gateable, intensified CCD array as the detector element. The illuminator and hyperspectral imaging system are mounted on a small platform that is itself mounted on a tripod and scanned in azimuth to build an image scene of up to several hundred spectral bands. The system has been deployed under a variety of environmental conditions, including night-time illumination, and on a variety of target scenes, including exposed and concealed plastic and metallic mine-like targets. Targets have been detected and identified on the basis of spectral reflectance, fluorescence signatures, degree of polarization, and range-to-target information. The combination of laser-like broadband illumination and hyperspectral imaging offers great promise in concealed or obscured target detection. On-going developments include the incorporation of broadband illuminators in the 1 to 2 micrometers and 3 to 5 micrometers spectral bands, with corresponding increases in spectral coverage of the imaging and detection systems.

Optical Techniques for Detecting and Identifying Biological-Warfare Agents

Rapid and accurate detection and identification of biological agents is an objective of various national security programs. Detection in general is difficult owing to natural clutter and anticipated low concentrations of subject material. Typical detection architectures comprise a nonspecific trigger, a rapid identifier, and a confirming step, often in a laboratory. High-confidence identification must be made prior to taking action, though this must be traded against regrets stemming from delay. Sensing requirements are best established by positing plausible scenarios, two of which are suggested herein. Modern technologies include the use of elastic scatter and ultraviolet laser-induced fluorescence for triggering and standoff detection. Optical and nonoptical techniques are used routinely in analyzing clinical samples used to confirm infection and illness resulting from a biological attack. Today, environmental sensing serves at best as an alert to medical authorities for possible action, which would include sample collection and detailed analysis. This paper surveys the state of the art of sensing at all levels.

Demonstration of spontaneous thermal-blooming phase-compensation instability

We have observed significant growth of phase modulation when thermal blooming is compensated by a 241-channel phase-conjugate adaptive-optics system. These modulations, which were not artificially seeded, occurred at a spatial frequency characteristic of the adaptive-optics system and resulted in a corresponding decay of the far-field irradiance. These experiments represent, to our knowledge, the first observations of a spontaneous phase-compensation instability.

Experimental observation of thermal-blooming phase-compensation instability

We have made the first experimental observations to our knowledge of thermal-blooming phase-compensation instability. We identified the instability by impressing a spatial intensity modulation on the laser beam and watching this modulation grow as thermal-blooming compensation was performed with a 69-channel phaseconjugate adaptive-optics system.

Pattern formation from thermal-blooming phase-compensation instability

We introduce a new, quantitative experimental signature–formation of a characteristic chain-link pattern in laser irradiance profiles–of thermal-blooming phase-compensation instability with uniform wind. We exhibit such chain links observed in computer simulation and in the laboratory, and we derive the main features of the observed chain link from first principles. This derivation represents what is to our knowledge the first practical application of linearized instability analysis to real systems with spontaneously generated unseeded phase-compensation instability.

Detection of manmade objects

Hyperspectral imagers have the unique capability of doing both material identification and anomaly detection. However, hyperspectral imagers with hundreds of co-registered contiguous bands are difficult to field particularly if real-time processing is required. With judicious choice of bands, the anomaly detection performance of a multispectral sensor can rival that of hyperspectral sensors. In order to achieve this performance, the choice of multispectral bands relies on the presence of exploitable target or background spectral features. The universality of these features will determine the overall utility of a multispectral system. We have discovered that water vapor features in the SWIR (Short Wave InfraRed) can be used to distinguish manmade objects from natural backgrounds. As an example, we will show that two broad bands chosen to exploit these features make most manmade objects detectable in the presence of natural clutter with few false alarms.

Wide-field-of-view heterodyne receiver at 1.06 μm with photorefractive InP:Fe

Photorefractive InP:Fe with an applied dc field was used to construct a wide-field-of view heterodyne receiver in a double phase-conjugate mirror configuration. Large double phase-conjugate mirror diffraction efficiencies (>20%) and stable (as a function of signal-beam angle) intermediate-frequency powers were measured over an approximately 8° field of view with a minimum rise time of approximately 65 ms. The influence of electric-field strength and sample temperature on rise time was measured.