Micron-Accurate Laser Fresnel-Diffraction Ranging System
- Tuesday, 01 January 2008
This system would exploit the variation of Fresnel diffraction with distance.
The figure schematically depicts two versions of an optoelectronic system, undergoing development at the time of reporting the information for this article, that is expected to be capable of measuring a distance between 2 and 10 m with an error of no more than 1 μm. The system would be designed to exploit Fresnel diffraction of a laser beam. In particular, it would be designed to take advantage of the fact that a Fresnel diffraction pattern is ultrasensitive to distance.
In either version, a Fresnel diffraction pattern would be generated by aiming a laser beam at a pinhole, the size of which could be varied. The diffracted laser light would illuminate the object, the distance to which was to be measured. The diffracted laser light reflected from that object would be collected by an optical receiver comprising a telescope equipped with an imaging photodetector array at its focal plane. The resulting Fresnel-diffraction-pattern readout from the array would be digitized and sent to a computer. In principle, the digitized Fresnel diffraction pattern could be compared computationally with a set of known Fresnel diffraction patterns for known distances. Once a match was found, the distance of the observed Fresnel pattern would be determined to within a micron. The range of the system would be limited only by the power of the laser, the maximum laser power tolerated by the optical train of the system, and the sensitivity of the photodetector array.
The two versions would differ in the following respects:
- In version 1, the focus of the telescope would be in the Fresnel region, and the telescope would have a small depth of focus. As a consequence, the Fresnel pattern would be imaged directly onto the photodetector array.
- In version 2, a multielement lens module would displace the Fresnel region from the vicinity of the pinhole to the vicinity of the optical receiver. As the distance to be measured varied, the location of the receiver relative to the displaced Fresnel-diffraction region would vary, thereby causing the Fresnel diffraction pattern on the focal plane to vary. The multielement lens module would also correct for aberrations.
The processing of the digitized Fresnel diffraction pattern in the computer might be accelerated by using only parts of the pattern or even only one small part — the central pixel. As the distance from the pinhole increased, the central pixel would rapidly cycle between maximum and minimum light intensity. This in itself would not be sufficient to uniquely determine the distance. However, by varying the size of the pinhole or the wavelength of the laser, one could obtain a second cycle of variation of intensity that, in conjunction with the first cycle, could enable a unique determination of distance. Alternatively, for a single wavelength and a single pinhole size, it should suffice to consider the data from only two different key pixels in the Fresnel pattern.
This work was done by David Lehner, Jonathan Campbell, and Kelly Smith of Marshall Space Flight Center; Duncan Earl and Alvin Sanders of the University of Tennessee; Stephen Allison of Oak Ridge National Laboratory; and Larry Smalley of the University of Alabama in Huntsville.