The figure is a simplified diagram of an apparatus, now undergoing development, for range imaging based on measurement of the round-trip phase delay of a pulsed laser beam. Variants of this apparatus could be used to provide range information needed for navigation of autonomous robotic ground vehicles and robotic aircraft, and for navigation and aiming in numerous military applications.
The apparatus would operate in a staring mode. A pulsed laser would illuminate a target. Laser light reflected from the target would be imaged on a verylarge- scale integrated (VLSI)-circuit image detector, each pixel of which would contain a photodetector and a phase-measuring circuit. The round-trip travel time for the reflected laser light incident on each pixel, and thus the distance to the portion of the target imaged in that pixel, would be measured in terms of the phase difference between (1) the photodetector output pulse and (2) a local-oscillator signal that would have a frequency between 10 and 20 MHz and that would be synchronized with the laser-pulse-triggering signal.
This apparatus offers several advantages over prior laser range imagers (essentially, scanning lidar systems based on explicit measurement of round-trip pulse travel times). A typical scanning lidar system consumes tens of watts of power, must be large because of the need for complex optics and mechanical scanning, and must include a clock running at a frequency of the order of a gigahertz. Moreover, because of the need for mechanical scanning to build up a range image, it is not possible to achieve an update rate (frame rate) sufficient for most applications.
In contrast, because of its staring mode of operation, the developmental apparatus could utilize simpler optics and would contain no moving parts. Because of the elimination of mechanical scanning and the use of VLSI circuitry, the power demand of this apparatus would be only about 100 mW. Moreover, because a complete range image could be constructed for each successive laser pulse, it would be possible to achieve an update rate, greater than the standard video frame rate of 30 Hz, that would be sufficient for most robotic applications. It has been estimated that the apparatus could provide a range resolution of 1 cm. The maximum range of the apparatus would depend on the details of the design and the specific application: for example, on the basis of the minimum detectable photocurrent density, the maximum range would be about 1 km for a 15°-wide field of view or about 100 m for a 60°-wide field of view.
A prototype of the phase-measuring VLSI image detector has been demonstrated. In each pixel, the output of the photodiode and the local-oscillator signal are fed as inputs to a current-mirror circuit to obtain output currents proportional to the value of the local-oscillator sinusoid at the time of return of the laser pulse. In each of several phasedetector circuits, one of the currentmirror output currents is integrated in a capacitor to obtain a low-noise voltage indicative of the phase difference. This design enables accurate measurement of the phase difference because it is possible to measure voltage very accurately (to within microvolts) in VLSI circuits. The use of several phase detectors, each excited with a differently delayed replica of the local oscillator signal, makes it possible to measure the target distance accurately in the presence of unknown background illumination, unknown target albedo, and fullcycle phase ambiguities.
This work was done by Bedabrata Pain and Bruce Hancock of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
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