Wide-angle, open-faced retroreflectors of a proposed type would be constructed by use of traditional corner-cube reflectors as building blocks. Wide-angle retroreflectors are needed in optical stellar interferometry, and in other branches of highly precise laser metrology; in particular, two- and three-dimensional triangulation. All of these applications involve the use of reference structures (e.g., optical trusses) and the use of retroreflectors that establish fiducial points on the structures.
Ordinarily, single hollow corner-cube reflectors would be preferred as retroreflectors because they are not wavelength-dispersive and, in principle, return flat wavefronts with no distortion. Unfortunately, the range of useful acceptance angles of a corner-cube reflector in a given plane is only about 60°; this precludes the use of a corner-cube reflector at a fiducial point where two or more optical paths are required to intersect at an angle or angles greater than about 60°. Some non-corner-cube retroreflectors offer wider angular ranges; for example, a hemispherical lens offers a range as wide as 180°, but the reflected wavefront is subject to spherical and other aberrations, wavelength dispersion, and wavefront distortion associated with thermal expansion of the lens material along the optical path.
The proposed solution is to assemble multiple corner-cube reflectors for each fiducial point, subject to the following requirements: The corner-cube reflectors in the assembly must be mounted in various orientations such that, collectively, they provide acceptance angles to accommodate all optical paths required to intersect at the fiducial point (one or more beams can hit each corner). To establish the single desired fiducial point, the reflectors must be aligned so that their reflective faces intersect at that point.
Different geometries with two, three, even four corners have been considered. For the needs of JPL's space interferometry mission and its testbed, a triple corner cube design has been adopted, with a 30° wedge (see assembly sequence in Figure 1). A prototype has been successfully constructed in mid 1997, validating the concept of such a retroreflector and its manufacturability (see Figure 2).
The corner-cube reflectors would most likely be made from low-thermal-expansion glass prisms. The reflective corner faces would be polished flat to within 1/10 of a typical visible-light wavelength and coated for reflectivity. During assembly of the prisms to form the corners, special multiwavelength interferometric methods would be used to align the reflective faces to within arc seconds of the desired angles.
In practice, the fiducial point would have to be a virtual one, because it would not be possible to assemble multiple corner-cube reflectors with faces that continue, precisely, all the way into a common vertex. To enable assembly, it would be necessary to bevel at least some of the prisms, thereby leaving small gaps at the common vertex. If the bevels were made with custom polishing, then the gaps could be limited to < 50 ¼m. The quality of the reflected wavefronts would be limited only by the quality of the reflective faces and diffraction from the gaps.
This work was done by Edouard Schmidtlin of NASA's Jet Propulsion Laboratory.
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