Lateral-transfer optical retroreflectors can now be made extremely stable to both external and internal fluctuations and gradients of temperature. As explained in more detail in the fourth paragraph, this high stability is achieved through improvements in design and fabrication.
A lateral-transfer retroreflector is similar to the more familiar corner-cube reflector in that (1) it includes three optically flat mirrors co-aligned and bonded in place so that each optical face is perpendicular to the other two faces and (2) by virtue of this mutual
perpendicularity, regardless of its orientation, it always reflects a beam of light back along a line parallel to the direction of incidence. However, unlike in a corner-cube reflector, only two of the mirrors are adjacent. The third mirror is mounted out on an arm, away from the other two mirrors, so that the retroreflected light beam is displaced laterally from the incident light beam by a distance that depends on the length of the arm. Thus, a lateral-transfer retroreflector is useful for picking off a portion of a light beam and sending it back to a location laterally displaced from the source.
Heretofore, in the fabrication of a retroreflector, the usual practice has been to bond the mirrors together by use of epoxy along back seams as shown on the left side of the figure. When the retroreflector is then subjected to a temperature below the fabrication temperature, the epoxy shrinks, giving rise to tensile stresses on the backs of the mirrors. These stresses cause the mirrors to undergo angular misalignments that are small but nevertheless unacceptable because they give rise to degradation or loss of the retroreflective optical function.
In the improved design, the seams (where the faces are bonded) have a tongue-in-groove configuration, which makes it possible to put epoxy on two different surfaces in each seam. This configuration (depicted on the right side of the figure) causes the stresses engendered by cooling the retroreflector to below the fabrication temperature to become distributed across two planes perpendicular to each other. The result is a bond that does not allow change in either direction because the epoxy is essentially working against itself and unable to pull the mirrors out of alignment.
As part of a demonstration of this concept, a lateral-transfer retroreflector with a beam deviation of 11 arc seconds and a peak-to-valley wavefront error of 0.3 wave at a wavelength of 633 nm was constructed. The beam deviation was shown to change by less than 1 arc second when the retroreflector was subjected to an internal temperature gradient characterized by a temperature difference of 30 °C between the seam of the dihedral mirror subassembly and the mirror at the other end of the arm.
This work was done by James J. Lyons III of Goddard Space Flight Center. No further documentation is available. GSC-14056