David I. Klick

Laser radar reflective tomography utilizing a streak camera for precise range resolution

Tomography is used to reconstruct 2-D images from 1-D range-resolved laser radar data. A doubled mode-locked Nd:YAG pulsed laser illuminates a conical object, and a receiver utilizing a streak camera resolves the reflected light in time.

Three-Dimensional Imaging Using A Single Laser Pulse

We describe results from an optical detector capable of producing three-dimensional images using single laser pulses. The method consists of detecting reflected light from an object illuminated by a short pulse from a laser with a detector that resolves many pixels in the objects image into fine time bins equivalent to range resolution of 4 cm. The detector utilizes a fiber optic image converter to transform a square focal plane into a line array that is input to a streak camera to obtain high time resolution in all the pixels. We show data from simple objects, like posts and cones, as well as more complicated objects. This work builds upon our results reported in September 1988 at the Laser Radar III SPIE conference. In that work we imaged the entire object to a point that was a single input to the streak camera. In addition we viewed the object from many aspect angles and used the range measurements to produce a two-dimensional projection image of the object. That method of reflective tomography requires gathering data from many aspect angles while the method reported here, which we call angle-angle-range, provides a three-dimensional image with data from a single aspect angle using a single laser pulse. Here we will compare the methods in detail.

Visible laser radar: range tomography and angle-angle-range detection

This paper compares two detectors for visible laser radar: a 1-D detector that resolves a target in range and a 3-D detector that resolves a target in angle and range. For both, a short pulse laser illuminates the target. For both, the receiver is based on a streak camera, which detects reflected light from the illuminated target and resolves the light in time. The time resolution is 250 ps, so the target is resolved into 4 cm range cells. The 1-D detector focuses the reflected light to a point. The output is the 1 -D, range-resolved projection of the target. The 3-D detector images the target on a focal plane, which is dissected by a fiber optic image converter attached to the streak camera. The output is a 3-D image of the target. For both detectors, we show data from two simple targets. The paper also compares two methods of remote sensing using these detectors: 2-D range tomography using data from the 1-D detector and angle-angle-range imagery using the 3-D data.

Two-Dimensional Tomographs Using Range Measurements

We demonstrate a method of constructing two-dimensional projection images of objects using reflective tomography. The method consists of detecting reflected light from an object illuminated by short pulses (FWHM ≈ 100 ps) from a laser with high time resolution (FWHM ≈ 250 ps) to produce one-dimensional range-resolved data. Repetition at many aspect angles provides input to a filtered back projection algorithm that produces a 2D projection image of the object. The receiver consists of 1) a lens to image the object to a point, 2) a streak camera to provide time resolution, 3) a tv detector to record the streaked light, and 4) electronics to control the system and to store data. This paper describes the concept on which the receiver is based, the details of the prototype receiver, and the characteristics of images of many objects.

Detection of targets in infrared clutter

Measurements of IR background variation or clutter are important for determining target detectability. Image sequences of widely varied ground-clutter types were recorded with the Airborne Infrared Imager (AIRI), housed in a wing pod of the Airborne Seeker Test Bed (ASTB) aircraft. Target detection statistics were derived for various backgrounds (ocean, ocean glints, desert, forest, shoreline, and urban). MWIR and LWIR images were processed to determine the minimum point-target contrast temperature detectable in various clutter types. This clutter metric was found to be relatively insensitive to changes in wave- length, season, or spatial scale, but to vary strongly with clutter type. These statistics are used to predict clutter- limited detection ranges for generalized targets in appropriate scenarios. Reduction in detection range from most benign clutter (ocean) to most severe clutter (urban) was found to be 7-9 dB, depending on waveband.

Fourier Transform Infrared (FTIR) Kinetics Diagnostics Of Thin Film Polymerization Photoinitiated By Excimer Laser Pulses

Rapid-scan Fourier transform infrared (FTIR) spectroscopy is becoming a useful method for monitoring reaction kinetics. The excimer laser has recently appeared as a source of intense UV radiation for laser-assisted photochemistry. These two tools are combined in this study of the polymerization rates of commercially useful transparent thin films. Liquid mixtures made up of 3 parts epoxy oligomer and 2 parts di- or tri-acrylate monomer, with 3-5% of various photoinitiators and 3% amine, were spread on NaCl plates in 7 μm layers. Transmission FTIR spectra were taken with the sample in an N2 atmosphere. Excimer laser pulses at either 308 or 351 nm and spanning a range of fluences from 10 -4 to 10 -2 J/cm 2 (below the damage level) illuminated the sample. The photoinitiator absorbed the UV light and initiated a chain reaction that led to full polymerization of the coating. FTIR scans of 4 cm -1 resolution were taken every 80 msec during the reaction. Extent of reaction was monitored by the disappearance of the acrylate C=C bond at 810 cm -1, with reference to the unchanging peak at 830 cm -1. Typical experiments at high fluence involved a pulse or fast series of pulses (sufficient to lead to complete polymerization) followed by rapid FTIR scanning as the reaction proceeded. At low fluence, many series of pulses were required to complete the reaction, and each series was followed by an FTIR scan. Kinetics data were taken with several fluences, wavelengths, and sample compositions. These and other simpler kinetics diagnostics measurements are discussed in the context of a search for an industrially useful laser-cured polymer coating process.

Applications of a streak-camera-based imager with simultaneous high space and time resolution

A high-speed imaging device has been built that is capable of recording several hundred images over a time span of 25 to 400 ns. The imager is based on a streak camera, which provides both spatial and temporal resolution. The systems current angular resolution is 16 X 16 pixels, with a time resolution of 250 ps. It was initially employed to provide 3-D images of objects, in conjunction with a short-pulse (approximately 100 ps) laser. For the 3-D (angle-angle-range) laser radar, the 250 ps time resolution corresponds to a range resolution of 4 cm. In the 3-D system, light from a short-pulse laser (a frequency-doubled, Q-switched, mode-locked Nd:YAG laser operating at a wavelength of 532 nm) flood-illuminates a target of linear dimension approximately 1 m. The returning light from the target is imaged, and the image is dissected by a 16 X 16 array of optical fibers. At the other end of the fiber optic image converter, the 256 fibers form a vertical line array, which is input to the slit of a streak camera. The streak camera sweeps the input line across the output phosphor screen so that horizontal position is directly proportional to time. The resulting 2-D image (fiber location vs. time) at the phosphor is read by an intensified (SIT) vidicon TV tube, and the image is digitized and stored. A computer subsequently decodes the image, unscrambling the linear pixels into an angle-angle image at each time or range bin. We are left with a series of snapshots, each one depicting the portion of target surface in a given range bin. The pictures can be combined to form a 3-D realization of the target. Continuous recording of many images over a short time span is of use in imaging other transient phenomena. These applications share a need for multiple images from a nonrepeatable transient event of time duration on the order of nanoseconds. Applications discussed for the imager include (1) pulsed laser beam diagnostics -- measuring laser beam spatial and temporal structure, (2) reflectivity monitoring during pulsed laser annealing of microelectronics, and (3) detonics or shock wave research, especially microscopic studies of shocks produced by laser pulses.

Time-resolved beam profiler for pulsed lasers

A high-speed imaging device based on a streak camera has been demonstrated, which provides multiple images from non-repeatable transient events of time scale >= 1 ns. It can be employed for pulsed laser beam diagnostics, measuring laser beam spatial and temporal structure on a single-pulse basis. The system currently has angular resolution of 16 X 16 pixels, with a time resolution of 250 ps. The laser beam width is sized to fill the input optic, and the image is dissected by a square array of optical fibers. At the other end of the fiber optic image converter, the 256 fibers form a line array, which is input to the slit of a streak camera. The streak camera sweeps the input line across the output phosphor screen so that position is directly proportional to time. The resulting 2-D image (fiber position vs. time) at the phosphor is read by an intensified (SIT) vidicon TV tube, and the image is digitized and stored. A computer subsequently decodes the image, unscrambling the linear pixels into an angle-angle image at each time. We are left with a series of snapshots, each one depicting the laser beam spatial profile (intensity cross-section) at succeeding moments in time. The system can currently record several hundred images over a span of 25 to 400 ns. This detector can study lasers of pulse width >= 1 ns and with a visible wavelength (200 - 900 nm). Candidate lasers include doubled Nd:YAG, excimer, ruby, nitrogen, metal vapor, and Ti:Sapphire. The system could also be simply configured as an 8 X 8 element wavefront sensor to record the cross-sectional distribution of phase, as well as amplitude. Finally, suggestions for system improvement are detailed, and the ultimate limitations of the method in terms of spatial and temporal resolution are discussed.