Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-09-30
2001-06-19
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S520000
Reexamination Certificate
active
06249352
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to a lateral shearing interferometer system, and more particularly to a lateral shearing interferometer system that utilizes selected non-adjacent portions of the interference pattern to provide an accurate measurement of the phase difference between two optical wavefronts.
DESCRIPTION OF THE PRIOR ART
Lateral shearing interferometers (LSIs) are employed as wavefront sensors and are extremely well known in the art. LSIs are disclosed, for example, in “Optical Shop Testing”, Second Edition, edited by Daniel Malacara, John Wiley and Sons, Inc., 1992, Chapter 4, entitled: “Lateral Shearing Interferometers” by M. V. Mantravadi, which chapter is hereby incorporated by reference.
In one use of unit shear LSI wavefront sensors, a copy of the wavefront is normally made and shifted in the X-direction by a distance equal to the spacing between actuators on a deformable mirror. The original and shifted beams are interfered in order to find the phase difference therebetween. The interference pattern is applied to an array of detectors. The intensity of the light in the entire interference pattern provides a measure of the wavefront X-tilt. This is also done with a shear in the Y-direction to get the Y-tilt. The output signals from the detectors are processed to provide tilt signals which, in turn, are applied to a plurality of actuators of a deformable mirror to correct for any distortions sensed in the wavefront.
As described in the related U.S. patent application Ser. No. 09/257,162 output signals from the LSI and from a Hartman wavefront sensor are known to be processed by a so-called real reconstructor which provides relatively accurate results between conditions in which the wavefront phase change between adjacent subapertures is &pgr; radians or less. As is well known, wavefront sensors processed by real reconstructors, are blind to branch point discontinuities in the wavefront. Consequently, the wavefront sensors are known to provide rather poor results when large wavefront phase changes rapidly occur. Hence the system described in the related patent application utilizes a complex reconstructor that uses the entire light in the interference pattern to process the optical output signals from the unit shear LSI wavefront sensor to provide relatively accurate estimates of the tilt signals at the discontinuities.
An example of a prior art lateral shearing interferometer is illustrated in FIG.
1
and designated as
10
. The LSI employs a Mach-Zehnder interferometer including an input beam splitter
12
, and output beam combiner
14
, and two plane reflectors
16
and
18
arranged in two beam paths. Two identical glass plane parallel plates
20
and
22
of the same thickness and material are employed in each beam path of the LSI. The two plates are arranged and adjusted in such a way that they rotate in equal amounts as shown in FIG.
1
. When a wavefront
23
from a convergent beam of light is incident on the beam splitter
12
, the beam splitter
12
separates the beam into two paths and through the two identical rotating glass plates
20
and
22
to obtain lateral shear at the output of the LSI as indicated at
24
. To determine tilt or displacement between two successive incident beams, or wavefronts, and with reference to
FIG. 2
, in operation, the LSI
10
takes a portion of a first beam
26
and a portion of a successive beam
28
and interferes them over the entire area. This results in the interfered pattern
30
in which the portions
26
and
28
are in the same spatial location. It should be recognized that the output
24
of the LSI is the integral of the interference of the entire area from the two incident wavefronts. If the interfered patterns are in-phase the pixel
30
is very bright. If the interfered patterns are out of phase then no light is present at the pixel
30
. As shown, since the entire light in the interfered pattern is used, the pixel
30
fills its entire area.
What is wanted for the reconstruction of the wavefront is the phase difference between the points where the actuators are located. When interfering an extended region, as is usually done, the LSI produces an output that depends on the integral of the sine of the phase difference, not the phase difference itself. When the phase variations are small this does not produce a large error, however when the phase is varying rapidly the output of the LSI sensor is innacurate.
What is needed, therefore, is an improved LSI system that provides substantially enhanced performance, and one which produces more accurate phase difference measurements between incident wavefronts, even when the phase difference varies rapidly.
SUMMARY OF THE INVENTION
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides, in a first aspect an improved lateral shearing interferometer system for use with an original optical wavefront and a displaced optical wavefront having a phase difference from the original optical wavefront that utilizes only selected portions of the interfered light to achieve more accurate results. The LSI system comprises a lateral shearing interferometer for developing an optical interference pattern representative of the optical phase difference between the original and displaced wavefronts. An optical detector positioned at the focal plane develops an array of electrical signals representative of the interference pattern. A data processor is programmed to utilize only spatially separated electrical signals from the array, which thus represents separated portions of the interference pattern. This effectively uses approximately one-quarter of the area of the interfered light and essentially can be considered to function as an array of separated small light sources.
In another implementation, the present invention provides an improved lateral shearing interferometer that employs a mask over the focal plane optical detector with openings therethrough at preselected locations and creates an interfered pattern that comprises an array of non-adjacent regions. The optical detector senses these regions and provides an array of electrical signals representative of the phase differences in the incident wavefront. A data processor responds to the electrical signals and provides an indication of the resulting interfered pattern. This also uses approximately one-quarter of the area of the interfered light.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention. Like numerals refer to like elements throughout both the drawing figures and the written description.
REFERENCES:
patent: 4692027 (1987-09-01), MacGovern et al.
“Optical Shop Testing”, Chapter 4, Second Edition, Edited by Daniel Malacara, 1992.
Davies Donald W.
Hutchin Richard A.
Slater Mark
Keller Robert W.
TRW Inc.
Turner Samuel A.
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