1.  Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to an improved approach for testing small light beams by lateral-shear interferometry. 
2.  Description of the Related Art
Lateral-shear interferometry is known in the art as a method of testing optical components, including the testing of collimation of light beams.  The method consists of displacing a wavefront laterally by a small amount and obtaining an interference pattern between the original and the displaced wavefronts.  A simple and common physical arrangement to obtain lateral shear consists of two plane glass surfaces used as beam dividers and by the introduction of a small tilt between the surfaces. 
Such devices are commercially known as shear-plate interferometers.  As shown in FIG.  1, a thin glass plate 10 is positioned at approximately 45-degree angle to an incident light beam W of predetermined width or diameter D produced by a source (not shown), such as a laser.  Because of the reflectivity of the front and back surfaces 12, 14 of the plate 10, two wavefronts W1, W2 result from a single incident beam W. As illustrated in the figure, the wavefront W1 reflected from the front surface 12 is displaced from the wavefront W2 reflected from the back surface 14 by a lateral shear S. As a result, the two reflected beams overlap over a common region of overlap O. The size of the overlap O depends on the width D of the beam W relative to the thickness T of the plate 10.  As one skilled in the art would readily understand, the shear plate 10 must be progressively thinner as the beam W is reduced in size in order to ensure that a zone of overlap exists. 
By introducing a small angle between the front and back surfaces 12,14 (illustrated in exaggerated proportion by the angle α in FIG.  1), interference is produced between the two reflected wavefronts W1 and W2 in the region of overlap O. Thus, this simple optical device can be used to analyze the quality of the beam W, or the quality of optical elements in the path of the collimated wavefront.  For example, a perfectly collimated, aberration-free, wavefront incident to the plate 10 produces a pattern of parallel fringes, as illustrated in FIG.  2, oriented in the direction normal to the direction of shear.  Predictably, converging and diverging wavefronts produce clockwise and counterclockwise rotation of the parallel fringes, respectively, as shown in FIGS.  3 and 4.  Other fringe patterns identify other optical aberrations in the tested beam of light (whether the result of light-source or lens imperfections), such as spherical aberration, coma, astigmatism, curvature of field, and chromatic aberration.  Thus, the fringe pattern produced by the shear interferometer 10 is advantageously utilized to produce a detailed analysis of the quality of the incoming wavefront W. See Chapter 4 of Optical Shop Testing, John Wiley & Sons, Inc., Second Edition (1992), for details about the theory underlying lateral-shearing interferometers. 
A disadvantage of this type of lateral-shear interferometer resides in the fact that the reflected images will not overlap if the beam W is too small relative to the thickness T of the plate 10.  Therefore, the thickness of the glass plate limits how small the beam W can be.  For example, lateral-shear interference on beam sizes less than 5 mm in diameter requires a thickness T of about 1 mm or less.  In practice, it is very difficult to manufacture a glass plate less than a few millimeters thick with very precise flat surfaces and a shear wedge therebetween.  Thus, conventional lateral-shear interferometers are not suitable for testing beams less than about 5-8 mm in diameter. 
As illustrated in FIG.  5, one solution to this problem has been to use two glass plates 20,22 separated by an air gap 24, which can easily be manufactured as small as needed in the form of a wedge, instead of a single thin plate.  The inside surfaces 26 and 28 of the plates 20 and 22, respectively, are uncoated, so that about 4% of the incoming light is reflected by them (simply based on the reflectivity of untreated glass).  The outer surfaces 30 and 32 are coated with a high-quality antireflection coating, so that the intensity of light reflected from these surfaces is reduced as much as possible in order to decrease interference with the reflected wavefronts W1,W2. 
Although this design of lateral-shear interferometer is advantageously suitable for small-beam applications, it suffers from several drawbacks that disadvantage its commercial utilization.  Because antireflection coatings are not perfect and cannot work over a broad range of wavelengths, the resulting fringe image retains unwanted interference patterns due to reflections from the treated surfaces (30,32).  Moreover, antireflection coatings are quite expensive, particularly the highly efficient coatings required for lateral-shear interferometry.  Therefore, the cost of these devices is relatively high.  Because the two glass plates are mounted on a connecting frame, they are subject to relative movement due to mechanical stresses and environmental effects on the frame that render the reflective surfaces unstable over time.  In addition, conventional lateral-shear interferometers have not been coupled to image magnification optics, thereby making analysis of small beams impractical.  Finally, conventional shear-interferometer designs produce diffraction rings from the edges of the wavefront under test, which causes serious problems of image degradation when testing small beams.  Therefore, there is still a need for an improved lateral-shear interferometer that addresses these problems.