Berton C. Willard

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.

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.

Spectral calibration of the EO-1 Advanced Land Imager

Spatial calibrations have been performed on the Advanced Land Imager (ALI) of the EO-1 satellite. Topics discussed in this paper include end-to-end imaging test, measurements of system modulation transfer function (MTF), and pixel lines of sight. The MTF measurements were made by recording scans of a knife-edge past the pixels. The techniques used to place the focal plane at the correct focal position are described, since they make use of MTF measurements. Line-of- sight measurements combine theodolite measurements of the telescope distortions and the photolithographic patterns of the detector arrays with images of a stationary Ronchi ruling recorded with the instrument at its normal operating conditions in a thermal vacuum chamber.

Wide-field-of-view Schmidt-sphere imaging collimator

A collimator was required to qualify the Advanced Land Imager (ALI) instrument of the EO-1 new millennium satellite for focus, MTF measurements, and distortion. It was used during assembly of the instrument and during thermal cycling while the instrument was in a vacuum tank. It had to be diffraction-limited over a 3 X 3 degree field of view and over a waveband of 400 to 2500 nm, have no obstruction, and have a virtual exit pupil that could be imaged onto the entrance pupil of the telescope. To satisfy these requirements the collimator, external to the vacuum tank, was built comprising a spherical mirror with exit pupil at its center of curvature, a full-aperture beamsplitter in collimated space, and a single lens to flatten the field. The optical layout and method of verifying collimation will be presented as well as optical performance, of interest since no corrector plate could be used as in the usual Schmidt camera configuration.

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.

Rotational shearing interferometer.

An interferometer with 1800 rotational shear does not show rotationally symmetrical aberrations. The instrument, however, is suitable for aligning zooming laser optical systems, monitoring wavefront irregularities, and verifying beamsteering tilt directions.