Optical: systems and elements – Signal reflector – 3-corner retroreflective
Reexamination Certificate
2003-03-14
2004-05-18
Phan, James (Department: 2872)
Optical: systems and elements
Signal reflector
3-corner retroreflective
C359S834000, C359S850000
Reexamination Certificate
active
06736518
ABSTRACT:
BACKGROUND
Cube corner reflectors are well-known optical elements that are used in a variety of optical systems. A cube corner reflector
100
as illustrated in
FIG. 1
has three planar reflective surfaces
110
,
120
, and
130
that intersect at right angles in the same manner as the intersection of faces at the corner of a cube. Reflective surfaces
110
,
120
, and
130
can be formed on three sides of a tetrahedral glass block that also has a transparent face
140
for input of an incident beam and output of a reflected beam. The tetrahedral glass block in cube corner reflector
100
is symmetric so that the perimeter of transparent face
140
forms an equilateral triangle and the perimeters of reflective surfaces
110
,
120
, and
130
are congruent isosceles right triangles.
Cube-corner reflector
100
is a retroreflector, and therefore a reflected beam from cube-corner reflector
100
is parallel to but offset from an incident beam regardless of the direction of the incident beam.
FIG. 1
illustrates an example of an incident beam
180
that enters cube corner reflector
100
through transparent face
140
and reflects from one or more of reflective faces
110
,
120
, and
130
before exiting as a reflected beam
190
. Reflected beam
190
is parallel to incident beam
180
and offset from incident beam
180
by twice the perpendicular separation between incident beam
180
and a vertex
150
of cube corner reflector
100
.
The tetrahedral shape of cube corner reflector
100
includes more glass than is generally required for the optical function of cube corner reflector
100
, particularly in optical systems where the location and direction of the incident beam is well controlled. Cube corner reflector
100
can thus be trimmed to remove glass that is not required for the optical function of cube corner reflector
100
. One conventional way to trim cube corner reflector
100
is to take a cylindrical core of cube corner reflector
100
, which results in transparent face
140
having a circular perimeter. Another known trimming scheme gives transparent face
140
a rectangular boundary
145
.
FIG. 2
shows a cube corner reflector
200
resulting from trimming cube corner
100
at boundary
145
. Cube corner reflector
200
is small for a retroreflector capable of reflecting an incident beam
280
to provide an offset reflected beam
290
. The minimum required size of cube corner reflector
200
to perform this optical function depends on the desired offset between incident and reflected beams
280
and
290
, the diameters or areas of beams
180
and
290
, and the path of the beams inside cube corner reflector
200
. To minimize the area of the face of cube corner reflector
200
, incident beam
280
(or alternatively reflected beam
290
) is centered at a point on an edge
235
of cube corner reflector
200
.
Analysis of the beam paths in cube corner reflector
200
shows the if incident beam
280
is parallel to a central axis of cube corner reflector
200
then the beam paths will remain within a band having boundaries at the upper and lower edges of beams
280
and
290
in FIG.
2
. For example, a ray
282
at a top edge of incident beam
280
reflects from a reflective face
210
toward a reflective face
230
and then reflects from a point on reflective face
230
that is at the same height as the bottom edge of incident beam
280
. From there, the ray travels horizontally to reflective surface
220
and exits as a reflected ray
292
at the bottom of reflected beam
290
. Similarly, a ray
284
at the bottom of incident beam
280
reflects from reflective surface
230
to a point on reflective surface
210
at the same height as the top of incident beam
280
, travels horizontally to the top of reflected beam
290
, and exits as reflected ray
294
. The height of cube corner reflector
200
can thus be as small as the diameter of beams
280
and
290
plus an added margin for beam variations or misalignments.
FIG. 3
illustrates a known multi-axis plane mirror interferometer
300
employing four cube corner reflectors
200
. U.S. Pat. No. 09/876,531, entitled “Multi-Axis Interferometer With Integrated Optical Structure And Method For Manufacturing Rhomboid Assemblies” further describes some examples of multi-axis interferometers containing retroreflectors that can be implemented using cube corner reflectors.
Interferometer
300
has four input beams IN
1
to IN
4
that are direction into a polarizing beam splitter
310
. Polarizing beam splitter
310
splits input beams IN
1
to IN
4
into components according to polarization. Components of one polarization from input beams IN
1
to IN
4
become respective measurement beams M
1
to M
4
, and components of an orthogonal polarization in input beams IN
1
to IN
4
become reference beams (not shown). Measurement beams M
1
to M
4
travel from polarizing beam splitter
310
to a planar measurement reflector (not shown) that is mounted on an object being measured. The measurement reflector returns measurement beams M
1
to M
4
along the same paths.
Polarization changing elements (e.g., quarter-wave plates)
320
are in the paths of outgoing and returning measurement beams M
1
to M
4
and change the polarization of measurement beams M
1
to M
4
so that polarization beam splitter
310
directs the returning measurement beams M
1
to M
4
to respective cube corner reflectors
200
.
Cube corner reflectors
200
reflect returning measurement beams M
1
to M
4
so that offset measurement beam M
1
′ to M
4
′ can traverse polarizing beam splitter
310
and elements
320
, reflect from the measurement reflector, and return through elements
320
and polarizing beam splitter
310
to form parts of respective output beams OUT
1
to OUT
4
. Each measurement axis of interferometer
300
corresponds to a pair of beams M
1
to M
1
′, M
2
and M
2
′, M
3
and M
3
′, or M
4
and M
4
′ and to a measured point that is halfway between the centers of the incident areas of the corresponding pair on the measurement mirror. Accordingly, cube corner reflectors
200
must be small enough to fit within the spacing of measurement beams M
1
to M
4
and M
1
′ to M
4
′ that is required for the desired measurement axes.
The reference beams have paths that include first reflections from a reference reflector (not shown), reflections from respective cube corner reflectors
200
, and second reflections from the reference reflector before the reference beams rejoin respective measurement beams M
1
′ to M
4
′ in output beams OUT
1
to OUT
4
. The two reflections of each measurement beam from the measurement reflector, the two reflections of each reference beam from the reference reflector, and the intervening reflections from the associated cube corner reflector
200
are well known to eliminate an angular separation that misalignment of the measurement or reference mirror might otherwise cause between the reference and measurement beams in the combined output beam.
A measurement along a measurement axis of interferometer
300
requires measuring and analyzing the phases of the measurement and reference beams that are within the output beam associated with the measurement axis. These measurements are most accurate if the wavefronts of measurement and reference beams are uniform because the measured phase information is generally an integral or average of the phase information over a cross-section of the output beam. Further, the integrated/analyzed portion of the measurement beam typically changes because of beam “walk-off”. Beam walk-off occurs when the object being measured changes angular orientation. The walk-off changes the matched portions of the measurement and reference beams, causing an erroneous phase shift when the beam wavefront is nonuniform. Wavefront distortion can thus cause errors and lower signal-to-noise ratios in phase information measurements and correspondingly in the measurements along the measurement axes of interferometer
300
.
Returning to
FIG. 2
, edge
23
Belt R. Todd
Bockman John J.
Agilent Technologie,s Inc.
Phan James
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