Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
1999-04-26
2002-10-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Spectroscopy
C356S455000
Reexamination Certificate
active
06469790
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The objects of the present invention are to generate interferometric signals more accurately and more precisely, and in some cases, more rapidly than is possible with present art. Accordingly a new class of tilt-compensated interferometer designs for generating interferometric signals is disclosed.
The subject area of the invention is tilt-compensation of multiple reflecting surfaces. A recently-approved application, Ser. No. 08/959,030 which disclosed optics for tilt-compensation of the moving mirror of an interferometric spectrometer, is included by reference for the entirety of its disclosure. The tilt-compensation was effected by the use the use of two complementary reflections at a flat moving mirror. The present disclosure expands the use of this tilt-compensation mechanism to a larger class of interferometers in which the compensated complementary reflections occur at one or more planar surfaces which may include the beamsplitter. A variety of motions may be applied to the moving planar surfaces to introduce path difference scanning. In conventional Michelson interferometers, tilt errors of the planar mirrors compromise photometric accuracy and interferometric efficiency. Baseline errors will also be introduced into spectra measured with instruments having tilt errors. Considerable effort has been expended in constructing interferometers which have electronic servomechanisms to adjust the tilt of planar interferometer mirrors. Considerable effort has also been expended in constructing interferometers having intrinsic optical tilt-compensation. The present invention expands the area of intrinsic optical tilt-compensation by applying a novel approach to tilt-compensate moving planar mirrors and applying a known approach for tilt-compensating beamsplitter. Thus, the present invention allows construction of interferometers that may be permanently aligned and more stable than known ways. A variety of applications will benefit from these improvements.
The tilt-compensation approach for the beamsplitter is known in, for example, Schindler, U.S. Pat. Nos. 3,809,481, 4,181,440, 4,193,693, Frosch, U.S. Pat. No. 4,278,351 and Woodruff, U.S. Pat. No. 4,391,525. The primary moving mirrors are retroreflectors and the planar mirrors were generally fixed. In the cases where a planar mirror did move, it was for correcting path difference errors introduced by imperfections in the motion of the retroreflectors. The planar reflectors make these interferometers more compact by folding the beams. Reference is also made to Solomon, U.S. Pat. No. 5,675,412 and Turner and Mould, U.S. Pat. No. 5,808,739 as well as a commercial product from Bomem (450, avenue St-Jean-Baptiste, Quebec, Quebec, G2E 5S5, Canada), the MB-100 Fourier spectrometer. The Bomem instrument uses beamsplitter, as is also shown in, for example Learner, U.S. Pat. No. 4,779,983 and Izumi, U.S. Pat. No. 4,932,780.
Tilt compensation by complementary reflections is shown in
FIG. 1. A
primary beam of radiation from a collimated source
10
propagates to a beamsplitter
30
. The beamsplitter
30
may have a coating
32
intended to be partially reflective and partially transmitting. The primary radiation beam divided at the beamsplitter coating
32
propagates in two directions. The first energy beam is reflected by coating
32
and enters a first optical path. The second energy beam is transmitted by coating
32
and enters a second optical path. The term arm may be used interchangably with first or second optical path.
The reflected first energy beam, in the case of
FIG. 1
, propagates to a retroreflector
70
which returns the beam with an offset, but with a propagation angle exactly antiparallel to the incident beam. The returned first energy beam propagates to the beamsplitter
30
where it may impinge on a reflective coating
34
. The beam then makes a second reflection from the beamsplitter at
34
and propagates to a fixed reflector
80
which may be flat. The first energy beam propagating towards
80
is necessarily parallel to the primary energy beam to the extent that the beamsplitter coatings
32
and
34
are exactly parallel and to the extent that the retroreflector
70
is optically perfect. In practice, these conditions can be met with sufficient accuracy for useful interferometric measurements. If the reflector
80
is oriented perpendicularly to the primary energy beam, the reflection which occurs for the first energy beam will be at exactly normal incidence causing this beam to exactly reverse its course through the first optical path where it will reach the coating
32
and recombine with a portion of the second energy beam which has traversed the second optical path.
FIG. 1
only indicates such a moving mirror in the second optical path. It will be shown that one or more moving planar reflectors may be included in either or both of the first and second optical paths.
The second energy beam initially transmitted through the coating
32
impinges on a movable flat mirror
50
then propagates to a retroreflector
60
. The retroreflector
60
returns the second energy beam exactly parallel and inverted. The inverted beam may then impinge a second time on the planar surface of
50
and then propagate to return reflector
80
. The beam may pass through an uncoated portion of the substrate
30
, or a compensator plate according to Woodruff, in transit from mirror
50
to reflector
80
and vice versa. The second energy beam as it propagates to the return reflector
80
is necessarily perpendicular to the primary energy beam to the extent of optical perfection of the components. As before, to the extent that reflector
80
is aligned perpendicular to the primary energy beam from the source
10
, then the impingement of the second energy beam on
80
will be at exactly normal incidence. This completes one half of the traversal of the second optical path. Because of the perpendicular incidence, reflector
80
returns the second energy beam exactly on the inverse of the first half of its traversal of the second optical path, thus returning it to the beamsplitter
30
with optical precision. The four reflections at the mirror
50
are pairwise complementary such that the beam returning to the beamsplitter
30
via the second optical path is exactly antiparallel to the second energy beam initially entering the second optical path from the beamsplitter
30
.
The two reflections of the first energy beam from the beamsplitter at coatings
32
and
34
are complementary. Hence, the beam propagating from the reflective coating
34
to reflector
80
is exactly parallel to the primary beam propagating from the source
10
to the beamsplitter
30
and its coating
32
. Likewise, the beam propagating from retroreflector
60
to reflector
80
, which may pass through a compensator plate, or an uncoated portion of the substrate
30
or pass around substrate
30
, will be exactly parallel to the beam propagating from beamsplitter
30
and its coating
34
to reflector
80
. At reflector
80
, the first and second energy beams reverse their direction of propagation and return to the beamsplitter
30
by the exact inverse of their paths from it. At the beamsplitter
30
, particularly coating
32
, the returning first and second energy beams are both split again and form two recombined beams at coating
34
. One of the recombined radiation beams returns to the source
10
and is effectively lost. The other recombined energy beam propagates to a detector
20
by a path which may include other optics and/or material to be measured as is commonly practiced in the use of interferometers.
Tilt of reflector
80
will produce only second order misalignment (motion of the source
10
image on the detector
20
) because the optical alignment of the wavefronts will be preserved by the equal effect of the tilt of reflector
80
on both the first and second energy beams. It will be appreciated that the misalignment of any component in
FIG. 1
will have at most second order effect. The path difference betw
Font Frank G.
Lee Andrew H.
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