U.S. patent application number 09/858076 was filed with the patent office on 2001-10-11 for apparatus to transform two nonparallel propagating optical beam components into two orthogonally polarized beam.
This patent application is currently assigned to Zygo Corporation, a Delaware corporation. Invention is credited to Groot, Peter de, Hill, Henry A..
Application Number | 20010028461 09/858076 |
Document ID | / |
Family ID | 22039060 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028461 |
Kind Code |
A1 |
Hill, Henry A. ; et
al. |
October 11, 2001 |
Apparatus to transform two nonparallel propagating optical beam
components into two orthogonally polarized beam
Abstract
The invention features systems and methods for generating
optical beams having substantially orthogonal polarizations for use
in distance measuring interferometry. In one embodiment, the
invention features a system including a source which during
operation generates two nonparallel propagating source beams; and a
retarder element positioned to receive the two nonparallel
propagating source beams and convert them into two nonparallel
propagating output beams that are polarized substantially
orthogonal to one another. The system can further include a
birefringent prism positioned to receive the two nonparallel
propagating output beams and produce two parallel output beams.
Inventors: |
Hill, Henry A.; (Tucson,
AZ) ; Groot, Peter de; (Middletown, CT) |
Correspondence
Address: |
ERIC L. PRAHL
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Assignee: |
Zygo Corporation, a Delaware
corporation
|
Family ID: |
22039060 |
Appl. No.: |
09/858076 |
Filed: |
May 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09858076 |
May 15, 2001 |
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09061928 |
Apr 17, 1998 |
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6236507 |
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Current U.S.
Class: |
356/493 ;
359/487.06; 359/489.04; 359/489.07; 359/489.09; 359/489.19 |
Current CPC
Class: |
G01B 9/02003 20130101;
G01B 2290/70 20130101; G02B 27/283 20130101 |
Class at
Publication: |
356/493 ;
359/496 |
International
Class: |
G01B 009/02 |
Claims
What is claimed is:
1. A system comprising: a source which during operation generates
two nonparallel propagating source beams; and a retarder element
positioned to receive the two nonparallel propagating source beams
and convert them into two nonparallel propagating output beams that
are polarized substantially orthogonal to one another.
2. The system of claim 1, wherein the retarder element is a
retardation plate having substantially parallel entry and exit
faces.
3. The system of claim 1 further comprising an additional retarder
element positioned along a path defined by the source and output
beams.
4. The system of claim 3, wherein the additional retarder element
is positioned to receive the output beams and change their
polarizations.
5. The system of claim 4, wherein the additional retarder is a half
waveplate.
6. The system of claim 4, wherein the additional retarder is a
quarter waveplate.
7. The system of claim 3, wherein the additional retarder element
compensates for temperature dependent changes in the birefringence
of the first mentioned retarder element.
8. The system of claim 3, wherein the additional retarder element
is separate from the first mentioned retarder element.
9. The system of claim 3, wherein the additional retarder element
is positioned to receive the nonparallel propagating output beams
and generate nonparallel propagating output beams that exit from
the additional retarder.
10. The system of claim 9, further comprising a third retarder
element positioned to receive the nonparallel propagating output
beams and generate substantially coextensive and collinear output
beams that exit from the third retarder.
11. The system of claim 10, wherein the third retarder element is a
birefringent prism.
12. The system of claim 1, wherein an optical axis of the retarder
element lies substantially in a plane defined by the source
beams.
13. The system of claim 1, wherein the retarder element is
uniaxial.
14. The system of claim 1, wherein the optical frequencies of the
two nonparallel propagating source beams differ from one
another.
15. The system of claim 14, wherein the source comprises: a laser
generating a single-frequency, polarized beam; and a Bragg cell
positioned to receive a beam derived from the polarized beam and
generate the two nonparallel propagating source beams having
optical frequencies that differ from one another.
16. The system of claim 15, wherein the source further comprises: a
source retarder element positioned to receive the beam derived from
the polarized beam and transform it into ordinarily-polarized and
extraordinarily-polarized beams, wherein immediately before exiting
the source retarder element, the ordinarily-polarized and
extraordinarily polarized beams generate a composite beam formed by
a pair of overlapping beams, and wherein the Bragg cell is
positioned to receive the composite beam and generate the two
nonparallel propagating source beams having frequencies that differ
from one another.
17. The system of claim 15, wherein the source further comprises: a
beam expander positioned to receive the beam derived from the
polarized beam and expand the size of the polarized beam, and
wherein the Bragg cell is positioned to receive the expanded beam
and generate the two nonparallel propagating source beams having
frequencies that differ from one another.
18. The system of claim 15, wherein the beam derived from the
polarized beam is the polarized beam.
19. The system of claim 1, further comprising a beam contractor
positioned to receive the nonparallel propagating output beams and
contract the size of the nonparallel propagating output beams.
20. A system comprising: a source which during operation generates
first and second source beams propagating along nonparallel
directions; and a retarder element positioned to receive the first
and second source beams and to transform each of the first and
second source beams into an ordinarily-polarized beam and an
extraordinarily-polarized beam, wherein immediately before exiting
the retarder element, the ordinarily-polarized and
extraordinarily-polarized beams generated from the first source
beam differ in optical phase by a first amount and the
ordinarily-polarized and extraordinarily-polarized beams generated
from the second source beam differ in optical phase by a second
amount and wherein the first and second amounts differ by a value
that is substantially equal to .pi. radians (modulo 2.pi.).
21. The system o: claim 20, wherein the first amount is
substantially equal to .pi. radians (module .pi.).
22. The system of claim 20, wherein the first amount is
substantially equal to .pi./2 radians (modulo .pi.).
23. A system comprising: a source which during operation generates
first and second source beams propagating along nonparallel
directions; and a retarder element positioned to receive the first
and second source beams and transform each of the first and second
source beams into overlapping ordinarily-polarized and
extraordinarily-polarized beams, wherein upon exiting the retarder
element the overlapping portions of the ordinarily-polarized and
extraordinarily-polarized beams produced from the first source beam
form a first output beam and the overlapping portions of the
ordinarily-polarized and polarized beams produced from the second
source beam form a second output beam and wherein the first and
second output beams are polarized substantially orthogonal to one
another.
24. The system of claim 23, wherein an optical axis of the retarder
element lies substantially in a plane defined by the first and
second source beams.
25. The system of claim 24, wherein the optical axis makes an angle
of about 45.degree. with an axis collinear with the first source
beam.
26. A system comprising: a retarder element positioned to receive
two nonparallel propagating input beams and convert them into two
nonparallel propagating output beams that are polarized
substantially orthogonal to one another; and a birefringent prism
positioned to receive the two nonparallel propagating output beams
from the retarder element and convert them into two substantially
parallel optical beams that are polarized substantially orthogonal
to one another.
27. The system of claim 26, wherein the retarder element and the
birefringent prism are integral with one another.
28. The system of claim 26, wherein one of the two nonparallel
propagating output beams propagates within the birefringent prism
as an ordinarily polarized beam and the other of the two
nonparallel propagating output beams propagates within the
birefringent prism as an extraordinarily polarized beam.
29. The system of claim 26, wherein the birefringent prism is made
from a material in the group consisting of LiNbO.sub.3, KDP,
quartz, and TeO.sub.2.
30. The system of claim 26, wherein the retarder element is made
from a material in the group consisting of LiNbO.sub.3, KDP,
quartz, and TeO.sub.2.
31. The system of claim 26, wherein the birefringent prism is a
Wollaston prism.
32. The system of claim 26 further comprising a waveplate
positioned between the retarder element and the birefringent
prism.
33. A system comprising: a source which during operation generates
first and second source beams propagating along nonparallel
directions; a retarder element positioned to receive the first and
second source beams and produce first and second intermediate
beams; and a birefringent prism positioned to receive the first and
second intermediate beams and transform each of the first and
second intermediate beams into ordinarily-polarized and
extraordinarily-polarized beams, wherein the prism has a shape and
a birefringence that causes the ordinarily-polarized beam produced
from the first intermediate beam to produce a first output beam and
the extraordinarily-polarized beam produced from the second
intermediate beam to produce a second output beam, wherein the
first and second output beams exit the prism substantially parallel
to one another and wherein the combined energy of the first and
second output beams is greater than half of the combined energy of
the two source beams.
34. The system of claim 33, wherein the polarizations of the first
and second source beams are substantially the same.
35. A system comprising: a source which during operation generates
two nonparallel propagating source beams that are polarized
substantially parallel to one another; a retarder plate positioned
to receive the two nonparallel propagating source beams and produce
two nonnparallel propagating intermediate beams, wherein the
retarder plate has a thickness, birefringence, and orientation that
causes the two nonparallel propagating intermediate beams to be
polarized substantially orthogonal to one another upon exiting the
retarder plate; and a birefringent prism positioned to receive the
two nonparallel propagating intermediate beams and produce two
output beams that are polarized substantially orthogonal to one
another, wherein the prism has a shape and a birefringence that
causes the two output beams to be substantially parallel to one
another.
36. The system of claim 35 further comprising a half waveplace
positioned between the retarder plate and the birefringent prism to
change the polarizations of the two nonparallel propagating
intermediate beams.
37. The system of claim 36, wherein an optical axis of the retarder
plate is substantially orthogonal to an optical axis of the
birefringent prism.
38. A method comprising: generating first and second beams which
propagate along nonparallel directions; separating each of the
first and second beams into overlapping ordinarily-polarized and
extraordinarily-polarized beams; retarding the
extraordinarily-polarized and ordinarily-polarized beams produced
from the first beam relative to one another, wherein the
overlapping portions of the extraordinarily-polarized and
ordinarily-polarized beams produced from the first beam form a
first output beam; and retarding the extraordinarily-polarized and
ordinarily-polarized beams produced from the second beam relative
to one another, wherein the overlapping portions of the
extraordinarily-polarize- d and ordinarily-polarized beams produced
from the second beam form a second output beam, and wherein the
first and second output beams are polarized substantially
orthogonal to one another.
39. The method of claim 38, further comprising the step of making
the first and second output beams propagate parallel to one
another.
40. The method of claim 38, further comprising the step of making
the first and second output beams substantially coextensive with
one another.
41. The method of claim 38, wherein the first and second output
beams have optical frequencies that differ from one another.
42. A distance measuring interferometry system comprising: the
system of claim 1; an interferometer that directs at least a
portion of one of the output beams along a reference optical path
and at least a portion of the other of the output beams along a
variable optical path and thereafter combines the portions of the
output beams into a signal beam; and a detector for measuring an
intensity of the signal beam.
43. The interferometry system of claim 42, wherein the detector
comprises a polarizer for producing a polarized signal beam having
a polarization different from the polarizations of the output
beams, and wherein the intensity of the signal beam measured by the
detector is an intensity of the polarized signal beam.
44. The interferometry system of claim 42 further comprising
measurement electronics for determining changes in the variable
optical path from the measured intensity.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to electro-optical systems used to
perform extremely accurate measurement of changes in either length
or optical path length, e.g., interferometry systems. More
particularly, the invention relates to an apparatus for use with an
interferometry system in which the apparatus transforms a single
frequency, linearly polarized laser beam into a beam with two
frequency components that are orthogonally polarized.
[0002] The use of optical interferometry to measure changes in
either length, distance, or optical path length has grown
significantly due not only to technological advances in lasers,
photosensors, and microelectronics but also to an ever increasing
demand for high precision, high accuracy measurements [cf. N.
Bobroff, "Recent advances in displacement measuring
interferometry," Meas. Sci. Technol., 4(9), 907-926 (1993)]. The
prior art interferometers can be generally categorized into two
types based on the signal processing technique used, i.e., either
homodyne or heterodyne. The interferometers based on the heterodyne
technique are generally preferred because (1) they are insensitive
to low frequency drift and noise and (2) 7 hey can more readily
have their resolution extended. Within the heterodyne type of
interferometers, of particular interest are the ones based on the
use of two optical frequencies.
[0003] In the prior art two-optical frequency heterodyne
interferometers, the two optical frequencies are produced by one of
the following techniques: (1) use of a Zeeman split laser, see for
example, Bagley et al., U.S. Pat. No. 3,458,259, issued Jul. 29,
1969; G. Bouwhuis, "Interferometrie Mit Gaslasers," Ned. T.
Natuurk, 34, 225-232 (August 1968); Bagley et al., U.S. Pat. No.
3,656,853, issued Apr. 18, 1972; and H. Matsumoto, "Recent
interferometric measurements using stabilized lasers," Precision
Engineering, 6(2), 87-94 (1984); (2) use of a pair of
acousto-optical Bragg cells, see for example, Y. Ohtsuka and K.
Itoh, "Two-frequency Laser Interferometer for Small Displacement
Measurements in a Low Frequency Range," Applied Optics, 18(2),
219-224 (1979); N. Massie et al., "Measuring Laser Flow Fields With
a 64-Channel Heterodyne Interferometer," Applied Optics, 22(14),
2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic
Two-frequency Interferometry for Small Displacement Measurements,"
Optics and Laser Technology, 16, 25-29 (1984); H. Matsumoto, ibid.;
P. Dirksen, et al., U.S. Pat. No. 5,485,272, issued Jan. 16, 1996;
N. A. Riza and M. M. K. Howlader, "Acousto-optic system for the
generation and control of tunable low-frequency signals," Opt.
Eng., 35(4), 920-925 (1996); (3) use of a single acousto-optic
Bragg cell, see for example, G. E. Sommargren, commonly owned U.S.
Pat. No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly
owned U.S. Pat. No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et
al., ibid.; or (4) use of two longitudinal modes of a randomlv
polarized HeNe laser, see for example, J. B. Ferguson and R. H.
Morris, "Single Mode Collapse in 6328 .ANG. HeNe Lasers," Applied
Optics, 17 (18), 2924-2929 (1978).
[0004] As for the prior art use of a Zeeman split laser to produce
the two optical frequencies, this approach is only applicable to
certain lasers (e.g., HeNe) and limits the frequency difference
between the two optical frequencies to about 2 MHz. This imposes a
limit on the maximum rate of change of the length or optical length
being measured. In addition, the available power from a Zeeman
split laser is less than 500 microwatts, which can be a serious
limitation when one laser source must be used for the measurement
of multiple axes, such as three to six axes.
[0005] The acousto-optical modulator with a single acousto-optical
Bragg cell of Sommargren, commonly owned U.S. Pat. No. 4,684,828
and of Dirksen, et al., ibid., and the acousto-optical modulator
with two acousto-optical Bragg cells of Dirksen, et al., ibid., are
based on normal Bragg diffraction in both non birefringent and
birefringent Bragg cells. The normal Bragg diffraction generates a
diffracted beam wherein the state of linear polarization of the
diffracted beam is the same state of linear polarization as the
incident, undiffracted beam. However, the objectives of the
heterodyne interferometry are usually best served when the two
optical beam components from an acousto-optical modulator are
frequency shifted one with respect to the other, orthogonally
polarized, and collinear. The process of converting the output beam
components generated by a normal Bragg diffraction acousto-optical
modulator, i.e., two non collinear beams in the same linear
polarization state into two collinear beams in orthogonally
polarized beams, has had an efficiency significantly less than
100%.
[0006] Accompanying the increasing demand for improved high
precision, high accuracy distance measurements is a demand to
increase the number of axes being measured with distance measuring
interferometry. The demand to increase the number of axes being
measured translates to either increasing the number of laser
source-acousto-optical modulator units, increasing the power of the
laser source, and/or increasing the conversion efficiency with
respect to power of the two frequency heterodyne source. An
increase in the conversion efficiency is clearly an attractive
option from a commercial point of view.
SUMMARY OF THE INVENTION
[0007] The present invention relates to an apparatus for providing
light beams of orthogonal states of polarization and of different
frequency for use in precision metrology applications such as in
the measurement of length or length changes using interferometric
techniques. The light beams of orthogonal states of polarization
are typically parallel but may beneficially have a predetermined
angle of divergence or convergence between them. Different
embodiments of the invention are disclosed in the form of optical
devices for efficiently converting an input optical beam comprising
two components having differing frequency profiles, the same states
of linear polarization, and directions of propagation differing by
a small predetermined angle from a light source, typically
comprising a single frequency laser and acousto-optical modulator,
to an output beam having two principal, typically parallel, output
beams of differing orthogonal states of polarization, one principal
output beam comprising substantially the same frequency components
as one of the input beam components and another principal output
beam comprising substantially the same frequency components as
another of the input beam components. The frequency profiles of the
input beam components are typically different but may beneficially
have the same frequency profiles for some applications. The energy
flux profiles of the principal output beams may be spatially
separated, partially coextensive, or substantially coextensive in
accordance with the details of particular device embodiments. The
input beam is introduced to a series of at least one phase
retardation plate where it experiences phase retardations via
optical birefringence of the at least one phase retardation plate
to form two sets of orthogonally polarized internal beam components
diverging by a small angle. The two sets of orthogonally polarized
internal beam components subsequently become four external beams
two of which, the principal ones, are available outside of the at
least one phase retardation plate for use in anticipated downstream
applications. The remaining two of the four output beams are
typically reduced to nominally zero intensities compared to the
intensity of the input beam so as to achieve a high efficiency
conversion of the input beam into the principal output beams, thus
rendering the two output beams with reduced intensities spurious.
Spatial filtering may be used to further control any negative
impact of the spurious beams.
[0008] Depending on the specific embodiment, progenitor beam
components of selected ones of the external beams are either
intercepted within or outside the series of at least one phase
retardation plate so that the selected ones of the external beams
are rendered typically parallel by a collimating means. The
collimating means can be in the form a of internal reflecting
and/or integral refracting surfaces and/or external elements.
However, if desired, the selected ones of the external beams can be
non-parallel such that they have a predetermined angle of
divergence or convergence between them.
[0009] The degree of overlap or spatial separation between the
energy flux profiles of the principal, linearly-orthogonally
polarized, external beams is controlled by various internal
reflecting and refracting properties of the series of at least one
phase retardation plate including the birefringence and optical
properties of the material of the series of at least one phase
retardation plate, the length of the physical path of travel
experienced by the internal beam components, and/or the use of
external control elements.
[0010] Thermal compensation can be provided via the use of thermal
compensating birefringent elements or the arrangement of external
components with respect to the series of at least one phase
retardation place or some combination of both. The surfaces of the
series of at least one phase retardation plate, thermal
compensating birefringent elements, the external elements, and the
external control elements may be anti-reflection coated where
appropriate to improve efficiency.
[0011] In general, in one aspect, the invention features an optical
system including: a source which during operation generates two
nonparallel propagating source beams; and a retarder element
positioned to receive the two nonparallel propagating source beams
and convert them into two nonparallel propagating output beams that
are polarized substantially orthogonal to one another.
[0012] The optical system may include any of the following
features. The nonparallel propagating source beams are diverging.
The nonparallel propagating source beams are converging. The
nonparallel propagating output beams are diverging. The nonparallel
propagating output beams are converging. The retarder element is a
retardation plate having substantially parallel entry and exit
faces. The system further includes an additional retarder element
positioned along a path defined by the source and output beams. The
additional retarder element is positioned to receive the output
beams and change their polarizations. The additional retarder is a
half waveplate. The additional retarder is a quarter waveplate. The
additional retarder element compensates for temperature dependent
changes in the birefringence of the first mentioned retarder
element. The additional retarder element is separate from the first
mentioned retarder element. The additional retarder element is
positioned to receive the nonparallel propagating output beams and
generate nonparallel propagating output beams that exit from the
additional retarder. The system further includes a third retarder
element positioned to receive the nonparallel propagating output
beams and generate substantially coextensive and collinear output
beams that exit from the third retarder. The third retarder element
is a birefringent prism. An optical axis of the retarder element
lies substantially in a plane defined by the source beams. The
retarder element is uniaxial. The optical frequencies of the two
nonparallel propagating source beams differ from one another. The
source includes: a laser generating a single-frequency, polarized
beam; and a Bragg cell positioned to receive a beam derived from
the polarized beam and generate the two nonparallel propagating
source beams having optical frequencies that differ from one
another. The source further includes: a source retarder element
positioned to receive the beam derived from the polarized beam and
transform it into ordinarily-polarized and
extraordinarily-polarized beams, wherein immediately before exiting
the source retarder element, the ordinarily-polarized and
extraordinarily polarized beams generate a composite beam formed by
a pair of overlapping beams, and wherein the Bragg cell is
positioned to receive the composite beam and generate the two
nonparallel propagating source beams having frequencies that differ
from one another. The source further includes: a beam expander
positioned to receive the bears derived from the polarized beam and
expand the size of the polarized beam, and wherein the Bragg cell
is positioned to receive the expanded beam and generate the two
nonparallel propagating source beams having frequencies that differ
from one another. The beam derived from the polarized beam is the
polarized beam. The system further includes a beam contractor
positioned to receive the nonparallel propagating output beams and
contract the size of the nonparallel propagating output beams. The
system is part of a distance measuring interferometry system, which
also includes: an interferometer that directs at least a portion of
one of the output beams along a reference optical path and at least
a portion of the other of the output beams along a variable optical
path and thereafter combines the portions of the output beams into
a signal beam; and a detector for measuring an intensity of the
signal beam. The detector includes a polarizer for producing a
polarized signal beam having a polarization different from the
polarizations of the output beams and the intensity of the signal
beam measured by the detector is an intensity of the polarized
signal beam. The interferometry system further includes measurement
electronics for determining chances in the variable optical path
from the measured intensity.
[0013] In general, in another aspect, the invention features a
system including: a source which during operation generates first
and second source beams propagating along nonparallel directions;
and a retarder element positioned to receive the first and second
source beams and to transform each of the first and second source
beams into an ordinarily-polarized beam and an
extraordinarily-polarized beam, wherein immediately before exiting
the retarder element, the ordinarily-polarized and
extraordinarily-polarized beams generated from the first source
beam differ in optical phase by a first amount and the
ordinarily-polarized and extraordinarily-polarized beams generated
from the second source beam differ in optical phase by a second
amount and wherein the first and second amounts differ by a value
that is substantially equal to .pi. radians (modulo 2.pi.).
[0014] The system may include any of the following features. The
first amount is substantially equal to .pi. radians (modulo .pi.).
The first amount is substantially equal to .pi./2 radians (modulo
.pi.).
[0015] In general, in another aspect, the invention features a
system including: a source which during operation generates first
and second source beams propagating along nonparallel directions;
and a retarder element positioned to receive the first and second
source beams and transform each of the first and second source
beams into overlapping ordinarily-polarized and
extraordinarily-polarized beams, wherein upon exiting the retarder
element the overlapping portions of the ordinarily-polarized and
extraordinarily-polarized beams produced from the first source beam
form a first output beam and the overlapping portions of the
ordinarily-polarized and polarized beams produced from the second
source beam form a second output beam and wherein the first and
second output beams are polarized substantially orthogonal to one
another.
[0016] The system may include any of the following features. An
optical axis of the retarder element lies substantially in a plane
defined by the first and second source beams. The optical axis
makes an angle of about 45.degree. with an axis collinear with the
first source beam.
[0017] In general, in another aspect, the invention features a
system including: a retarder element positioned to receive two
nonparallel propagating input beams and convert them into two
nonparallel propagating output beams that are polarized
substantially orthogonal to one another; and a birefringent prism
positioned to receive the two nonparallel propagating output beams
from the retarder element and convert them into two substantially
parallel optical beams that are polarized substantially orthogonal
to one another.
[0018] The system may include any of the following features. The
retarder element and the birefringent prism are integral with one
another. One of the two nonparallel propagating output beams
propagates within the birefringent prism as an ordinarily polarized
beam and the other of the two nonparallel propagating output beams
propagates within the birefringent prism as an extraordinarily
polarized beam. The birefringent prism is made from a material in
the group consisting of LiNbO.sub.3, KDP, quartz, and TeO.sub.2.
The retarder element is made from a material in the group
consisting of LiNbO.sub.3, KDP, quartz, and TeO.sub.2. The
birefringent prism is a Wollaston prism. The system further
includes a waveplate positioned between the retarder element and
the birefringent prism.
[0019] In general, in another aspect, the invention features a
system including: a source which during operation generates first
and second source beams propagating along nonparallel directions; a
retarder element positioned to receive the first and second source
beams and produce first and second intermediate beams; and a
birefringent prism positioned to receive the first and second
intermediate beams and transform each of the first and second
intermediate beams into ordinarily-polarized and
extraordinarily-polarized beams, wherein the prism has a shape and
a birefringence that causes the ordinarily-polarized beam produced
from the first intermediate beam to produce a first output beam and
the extraordinarily-polarized beam produced from the second
intermediate beam to produce a second output beam, wherein the
first and second output beams exit the prism substantially parallel
to one another and wherein the combined energy of the first and
second output beams is greater than half of the combined energy of
the two source beams. In some embodiments, the polarizations of the
first and second source beams are substantially the same.
[0020] In general, in another aspect, the invention features a
system including: a source which during operation generates two
nonparallel propagating source beams that are polarized
substantially parallel to one another; a retarder plate positioned
to receive the two nonparallel propagazing source beams and produce
two nonnparallel propagating intermediate beams, wherein the
retarder plate has a thickness, birefringence, and orientation that
causes the two nonparallel propagating intermediate beams to be
polarized substantially orthogonal to one another upon exiting the
retarder plate; and a birefringent prism positioned to receive the
two nonparallel propagating intermediate beams and produce two
output beams that are polarized substantially orthogonal to one
another, wherein the prism has a shape and a birefringence that
causes the two output beams to be substantially parallel to one
another.
[0021] The system may have any of the following features. The
system further includes a half waveplate positioned between the
retarder plate and the birefringent prism to change the
polarizations of the two nonparallel propagating intermediate
beams. An optical axis of the retarder plate is substantially
orthogonal to an optical axis of the birefringent prism.
[0022] In general, in another aspect, the invention features a
method including the steps of: generating first and second beams
which propagate along nonparallel directions; separating each of
the first and second beams into overlapping ordinarily-polarized
and extraordinarily-polarized beams; retarding the
extraordinarily-polarized and ordinarily-polarized beams produced
from the first beam relative to one another, wherein the
overlapping portions of the extraordinarily-polarized and
ordinarily-polarized beams produced from the first beam form a
first output beam; and retarding the extraordinarily-polarized and
ordinarily-polarized beams produced from the second beam relative
to one another, wherein the overlapping portions of the
extraordinarily-polarize- d and ordinarily-polarized beams produced
from the second beam form a second output beam, and wherein the
first and second output beams are polarized substantially
orthogonal to one another.
[0023] The method may include any of the following features. The
method further includes the step of making the first and second
output beams propagate parallel to one another. The method further
includes the step of making the first and second output beams
substantially coextensive with one another. The first and second
output beams have optical frequencies that differ from one
another.
[0024] The invention has many advantages. It provides systems and
methods for efficiently generating two substantially coextensive
and collinear beams having orthogonal polarizations. In particular,
the present invention has a conversion efficiency of nominally 100%
for conversion of input intensity into intensities of two
orthogonally polarized exit beam components, and in certain end use
applications the intensity of each of two orthogonally polarized
exit beam components may be adjusted to nominally 50% of the input
intensity.
[0025] The system is also compact and requires relatively few
optics. Additional optics can be included to optimize the overlap
of the orthogonally polarized beams and to compensate for
temperature-dependent changes in the birefringence of the retarder
elements.
[0026] Furthermore, in other embodiments, the invention provides an
apparatus for generating orthogonally polarized beams of different
frequency with a predetermined angle of divergence between them and
a predetermined lateral separation between their energy flux
profiles.
[0027] Also, the invention can provide the source beams for a
heterodyne detection distance measuring interferometry system. Such
systems can provide the precise position and orientation of objects
being processed, such as in semiconductor wafer processing.
Moreover, because of the efficient generation provided by the
invention, a single laser source in the interferometry system can
drive interferometric distance measurements over a large number of
measurement axes. In some embodiments, the invention also uses an
acousto-optic modulator to generate a relatively large frequency
difference (e.g., about 20 MHz) in the orthogonally polarized beam.
This large bandwidth enables relatively fast scan speeds in the
distance measuring apparatus.
[0028] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The structure, operation, and methodology of the invention,
together with other objects and advantages thereof, may best be
understood by reading the detailed description in connection with
the drawings in which each part has an assigned numeral that
identifies it wherever it appears in the various drawings and
wherein:
[0030] FIGS. 1a-1h depict in schematic form the first embodiment
and variants thereof of the present invention from the first
category of embodiments and variants with
[0031] FIG. 1a depicting in schematic form the first
embodiment;
[0032] FIG. 1b depicts in schematic form the first variant of the
first embodiment of the present invention;
[0033] FIG. 1c depicts in schematic form the second variant of the
first embodiment of the present invention;
[0034] FIG. 1d depicts in schematic form the third variant of the
first embodiment of the present invention;
[0035] FIG. 1e depicts in schematic form the fourth variant of the
first embodiment of the present invention;
[0036] FIG. 1f depicts in schematic form the fifth variant of the
first embodiment of the present invention;
[0037] FIG. 1g depicts in schematic form and in greater detail the
embodiment shown FIG. 1a;
[0038] FIG. 1h depicts a cross-sectional view of beams entering a
birefringent prism;
[0039] FIGS. 2a-2c depict in schematic form the second embodiment
and variants thereof of the present invention from the second
category of embodiments and variants with
[0040] FIG. 2a depicting in schematic form the second
embodiment;
[0041] FIG. 2b depicts in schematic form the first variant of the
second embodiment of the present invention;
[0042] FIG. 2c depicts in schematic form the second variant of the
second embodiment of the present invention;
[0043] FIGS. 3a-3b depict in schematic form the third embodiment
and variant thereof of the present invention from the third
category of embodiments and variants with
[0044] FIG. 3a depicting in schematic form the third
embodiment;
[0045] FIG. 3b depicts in schematic form the first variant of the
third embodiment of the present invention;
[0046] FIG. 4 depicts in schematic form the fourth embodiment of
the present invention from the fourth category of embodiments and
variants;
[0047] FIG. 5 depicts in schematic form an embodiment of a light
source for use with the embodiments in the FIGS. 1-4;
[0048] FIG. 6 depicts in schematic form a distance measuring
interferometry system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0049] The present invention relates to apparatus for providing
light beams of orthogonal states of polarization and of different
frequency for use in precision metrology applications such as in
the measurement of length or length changes using interferometric
techniques. A number of different embodiments of the invention are
disclosed in the form of optical devices for efficiently
transforming an input optical beam comprising two components having
differing frequency profiles, the same states of linear
polarization, and directions of propagation differing by a small
predetermined angle from a light source to an output beam having
two principal, typically parallel, output beams of differing states
of polarization, one principal output beam comprising substantially
the same frequency components as one of the input beam components
and a second principal output beam comprising substantially the
same frequency components as one other of the input beam
components. The frequency profiles of the input beam components are
typically displaced one from the other but may beneficially have
the same frequency profiles. The energy flux profiles of the
principal output beams may be spatially separated, partially
coextensive, or substantially coextensive in accordance with the
details of particular device embodiments and the requirements of
the metrology or other contemplated application. In addition,
thermal compensation may be made available through the use of
thermal compensating elements.
[0050] Referring to the drawings in detail, and initially to FIG.
1a, FIG. 1a depicts, in diagrammatic form, the first embodiment of
the present invention. The primary optical element in the first
embodiment is a phase retardation plate 60, shown in FIG. 1a,
typically made of a birefringent crystal, e.g. LiNbO.sub.3,
potassium dihydrogen phosphate (KDP), or paratellurite (TeO.sub.2),
or another birefringent material, e.g. liquid crystals. The input
beam 41, incident on a phase retardation plate 60, comprises two
components, 42 and 44, with the two components having substantially
the same but different frequencies, the same state of linear
polarization, and directions of propagation differing by a small
angle a, as shown in FIG. 1a, from a light source 40, comprising,
for example, a single frequency laser and an acousto-optical
modulator. The directions of polarizations of beams 42 and 44 are
substantially at 45.degree. to the plane of FIG. 1a, the direction
of the polarization of a beam relative to the plane of FIG. 1a
being indicated in FIG. 1a as the angle in parentheses located next
to the light beam number. For a given light beam, the angle is
positive for a right-handed rotation from the x axis about a z
axis, the x axis being contained in the plane of FIG. 1a, the y
axis pointing out of the plane of FIG. 1a, and the z axis pointing
in the direction of propagation of the given light beam. The
optical axis 61 of the phase retardation plate 60 is orientated at
an angle .alpha..sub.1 with respect to a normal to the entrance
facet of phase retardation plate 60 as illustrated in FIG. 1a and
orthogonal to a normal to the plane of FIG. 1a.
[0051] Upon entering crystal 60, component 42 becomes first and
second components 62 and 63, respectively, of an internal beam,
wherein internal beam components 62 and 63 are ordinarily
(90.degree.) and extraordinarily (0.degree.) polarized,
respectively. Similarly, the second component 44 of the input beam
becomes on entering phase retardation plate 60 third and fourth
components 64 and 65, respectively, of the internal beam, wherein
internal beam components 64 and 65 are ordinarily (90.degree.) and
extraordinarily (0.degree.) polarized, respectively.
[0052] Since beam components 42 and 44 are incident on retardation
plate 60 at different angles relative to optic axis 61,
extraordinarily polarized beam components 63 and 65 propagate
within retardation plate 60 with phase velocities corresponding to
different indices of refraction.
[0053] In general, the index of refraction n for an extraordinarily
polarized beam propagating at an angle .theta. with respect to an
optical axis of a birefringent crystal is 1 1 n 2 = cos 2 n o 2 +
sin 2 n e 2 ( 1 )
[0054] where n.sub.o and n.sub.e are the ordinary and extraordinary
principal indices of refraction of the birefringent crystal. The
different indices of refraction for beam components 63 and 65 can
be determined from Eq. 1. The index of refraction for ordinarily
polarized beam components 62 and 64 is n.sub.o. According to these
indices of refraction, the optic axis orientation (angle
.alpha..sub.1) and the thickness d.sub.1 of retardation plate 60
are chosen such chat retardation plate 60 introduces a phase shift
of p.pi. radians in beam 63 relative to beam 62 and a phase shift
of (p+1).pi. radians in beam 65 relative to beam 64, p being an
integer. Typically the angle .alpha..sub.1 is set at a value
substantially equal to 45.degree..
[0055] For example, for a divergence of about 3.5 mrad between beam
components 42 and 44, each having visible wavelengths and being
incident substantially normal to a LiNbO, retardation plate 60
having an optic axis orientation of .alpha..sub.1=45.degree., a
suitable thickness d.sub.1 is about 2.4 mm, or odd multiples
thereof. For a retardation plate made of a material more
birefringent than LiNbO.sub.3, such as TeO.sub.2, a suitable
thickness can be smaller. In the case of TeO.sub.2 it would be
about 1.4 mm.
[0056] Beams 62, 63, 64, and 65 exit phase retardation plate 60 as
beams 72, 73, 74, and 75, respectively. As a consequence of the
phase shifts introduced in beams 62, 63, 64, and 65, the
polarization of portions of beams 72 and 73 that overlap one
another is substantially at 45.degree. to the plane of FIG. 1a and
is substantially orthogonal to the polarization of portions of
beams 74 and 75 chat overlap one another, which is substantially
135.degree. to the plane of FIG. 1a. The directions of propagation
of beams 72 and 73 are parallel and the directions of propagation
of beams 74 and 75 are parallel, the entrance and exiting faces of
retardation plate 60 being substantially parallel.
[0057] There is a small lateral shear S.sub.b between beams 72 and
73 and between beams 74 and 75 as shown in FIG. 1a, the two lateral
shears being substantially the same. The lateral shears are
primarily a consequence of the difference in the direction of the
respective energy flux vectors and the wave front vectors for
extraordinarily polarized beams 63 and 65 in phase retardation
plate 60. The lateral shears between beams 72 and 73 and between
beams 74 and 75 depicted in FIG. 1a are exaggerated for the purpose
of clearly illustrating the effects. Typically, the lateral shear
S.sub.b is substantially smaller than the spot size of beam
components 72, 73, 74, and 75, which is typically not smaller than
about 1 mm. For example, if either of beam components 42 and 44 are
incident on a 2.4 mm thick LiNbO, retardation plate 60 at
approximately an angle of normal incidence and .alpha..sub.1 is
approximately 45.degree., the lateral shear S.sub.b between beam
components 72 and 73, or 74 and 75 is approximately 90 microns.
Thus beams 72 and 73 and beams 74 and 75 substantially overlap in
most cases.
[0058] Referring again to FIG. 1a, beams 72, 73, 74, and 75 enter a
birefringent prism 120 made of a negative uniaxial crystal, e.g.
LiNbO.sub.3 or KDP. The optical axis of birefringent prism 120 is
orientated at an angle of 45.degree. to the plane of FIG. 1a.
Alternatively, for a birefringent prism 120 made of a positive
uniaxial crystal, e.g. quartz or TeO.sub.2, the optical axis of
birefringent prism 120 is orientated at an angle of 135.degree. to
the plane of FIG. 1a.
[0059] Upon entering birefringent prism 120, each of beams 72, 73,
74, and 75 separate into an ordinarily polarized beams (polarized
at about 135.degree.) and an extraordinarily polarized beams
(polarized at about 45.degree.). Because beams 72 and 73
substantially overlap one another and because of the phase
difference between these two beams introduced by retardation plate
60, the ordinarily polarized beams from beams 72 and 73
destructively interfere with one another, substantially canceling
out each other. Thus, the ordinarily polarized beams from 72 and 73
are not shown in FIG. 1a. Conversely, the extraordinarily polarized
beams from beams 72 and 73 constructively interfere with one
another and emerge from prism 120 as beams 92 and 93, respectively,
which substantially overlap and have polarizations of 45.degree..
Similarly, the extraordinarily polarized beams from beams 74 and 75
destructively interfere with one another, substantially canceling
out each other. Thus, the extraordinarily polarized beams from
beams 74 and 75 are not shown in FIG. 1a. The ordinarily polarized
beams from beams 74 and 75 constructively interfere with one
another and emerge from prism 120 as beams 94 and 95, respectively,
which substantially overlap and have polarizations of
-45.degree..
[0060] The apex angle .alpha..sub.2 of birefringent prism 120 is
selected so that beams 92 and 93 exit birefringent prism 120
parallel to beams 94 and 95. This is possible because beams 92 and
93 emerge from beams propagating as extraordinarily polarized beams
in prism 120 and beams 94 and 95 emerge from beams propagating as
ordinarily polarized beams in prism 120. As a result, the system
produces a pair of substantially equal-intensity output beams, beam
96 (formed from the superposition of beams 92 and 93) and beam 97
(formed from the superposition of beams 94 and 95), beams 42 and 44
being of substantially equal intensities, that propagate parallel
to one another and have orthogonal polarizations (45.degree. and
-45.degree., respectively). There is a small non zero lateral shear
between beams 92 and 94, S.sub.a, which is exaggerated in FIG. 1a.
Typically this shear is less than about 100 microns. Beams 92 and
93 have the same frequency profile as the first input beam
component 42 and beams 94 and 95 have the same frequency profile as
the second input beam component 44, which is different from that of
beams 92 and 93 if, for example, beams 42 and 44 emerge from an
acousto-optic modulator within light source 40.
[0061] In some cases, such as when the lateral shear S.sub.b is not
negligible, the destructive interference between portions of beams
72 and 73 that propagate as ordinarily polarized beams within prism
120 is not complete. Similarly, the destructive interference
between portions of beams 74 and 75 that propagate as
extraordinarily polarized beams within prism 120 can also be
incomplete. However, even in these cases, birefringent prism 120
insures that beam 96 (which emerges from extraordinarily-polarized
beams) has a polarization orthogonal to the polarization of beam 97
(which emerges from ordinarily-polarized beams). As shown in FIG.
1g, portions of beams 72 and 73 that propagate as ordinarily
polarized beams within prism 120 and do not completely cancel out
because of destructive interference emerge a-s spurious beams 92a
and 93a, which diverge away from beams 96 and 97. Similarly,
portions of beams 74 and 75 that propagate as extraordinarily
polarized beams within prism 120 and do not completely cancel out
because of destructive interference emerge as spurious beams 94a
and 95a, which also diverge away from beams 96 and 97. Because of
the divergence, a spatial filter can be used to separate the
spurious beams from output beams 96 and 97.
[0062] The retardation imparted by retardation plate 60 minimizes
the energy in spurious beams 96a (the combination of beams 92a and
93a) and 97a (the combination of beams 94a and 95a) by the
destructive interference of beams 92a and 93a and beams 94a and
95a, respectively. And, the retardation imparted by retardation
plate 60 maximizes the energy in beams 96 (the combination of beams
92 and 93) and 97 (the combination of beams 94 and 95) by the
constructive interference of beams 92 and 93 and beams 94 and 95,
respectively. FIG. 1h shows a cross-section of beams 72, 73, 74,
and 75 before entering birefringent prism 120 (intersecting
reference plane 77 in FIG. 1g). For purposes of illustration, the
beams have exaggerated lateral shears S.sub.a and S.sub.b. The
overlapping portions of beams 72 and 73 produce superposition beam
78 having a polarization of 45.degree. and the non-overlapping
portions, 86 and 87, retain the polarizations of beams 72 and 73,
respectively. Similarly, the overlap ping portions of beams 74 and
75 produce superposition beam 79 having a polarization of
135.degree. and the non-overlapping portions, 88 and 89, retain the
polarizations of beams 74 and 75, respectively. Superposition beams
78 and 79 contribute entirely to output beams 96 and 97,
respectively, after passing through prism 120. In contrast,
non-overlapping portions 86, 87, 88, 89 contribute equally to
output beams 96 and 97 and spurious beams 92a, 93a, 94a, and 95a
after passing through prism 120. Thus, for a retardation plate
imparting the correct phase differences, the energy in the spurious
beams decreases with lateral shear S.sub.b.
[0063] In some cases, small amounts of the overlapping portions of
beams 72 and 73, and beams 74 and 75, also contribute to the
spurious beams if the respective retardations imparted by
retardation plate 60 are not homogeneous across the respective beam
profiles or differ from the correct amount (i.e., a phase shift of
p.pi. radians in beam 63 relative to beam 62 where p is an integer
and a phase shift of (p+1).pi. radians in beam 65 relative to beam
64). However, even in such cases, birefringent prism 120 insures
that output beams 96 and 97 have orthogonal polarizations. The
efficiency of the transformation of input beam 41 into output beams
96 and 97 may be defined differently for different end use
applications. One end use application may simply consider only the
efficiency of transformation into an output beam without concern
for degree of overlap or coextensiveness of output beam components
whereas in another end use application, the degree of overlap or
coextensiveness of output beam components is important such as in
the creation of heterodyne interference signals. The efficiency of
the transformation with respect to the creation of a heterodyne
signal is dependent on a series of factors comprising the size of
the lateral shear S.sub.b, the size of lateral shear S.sub.a, and
the size of the small angle .delta. relative to the size of the
beam divergence of the input beam components 42 and 44, the size of
a beam divergence being related to the diameter of the beam.
[0064] The size of lateral shear S.sub.a effects the degree of
overlap of the amplitudes of light beams with differing frequencies
leading to the heterodyne signal. The size of a beam divergence
relative to the size of the small angle .delta. effects the size of
the variation in the .pi. phase shift introduced between the
relative phase of beams 64 and 65 and the relative phase of beams
62 and 63 that can be achieved by the phase retardation plate 60.
Expressions for the efficiency are obtained for the case where the
amplitudes of beams 72, 73, 74, and 75 are constant across the
respective wavefronts to illustrate in the simple form the
properties of the invention without departing from the spirit and
scone of the invention.
[0065] One expression for the efficiency
E.function..function.(S.sub.a,S.s- ub.b) is 2 Eff ( S a = 0 , S b )
= 1 ( l b1 cos 2 + l b2 ) - S b 2 R 1 ( sin l b1 cos 2 + 1 2 sin l
b2 ) where ( 2 ) l b1 = cos - 1 ( S b 2 R ) ( 3 ) l b2 = cos - 1 (
S b 4 R ) ( 4 ) = ( S b 2 R ) ( 0 2 R ) ( 5 ) S b = ( r - ) d 1 ( 6
)
[0066] R is the radius of the output beam components,
.lambda..sub.0 is the wavelength of the light beam in vacuum,
d.sub.1 is the thickness of phase retardation plate 60 as shown in
FIG. 1a, and r is the angle between the energy flux vector of a
beam and the optical axis of phase retardation plate 60 and having
the angle .theta. between the normal to the wave front of the beam
and the optical axis of phase retardation plate 60.
[0067] A second expression for the efficiency
E.function..function.(S.sub.- a,S.sub.b) is 3 Eff ( S a , S b = 0 )
= 2 ( l a - S a 2 R sin l a ) ( 7 ) l a = cos - 1 ( S a 2 R ) ( 8
)
[0068] The angle r is related to .theta. and the principal indices
of refraction n.sub.o and n.sub.e of phase retardation plate 60
according to the formula 4 tan r = n o 2 n e 2 tan ( 9 )
[0069] [cf. Section 7 of Chapter 10 by J. Bennett and H. Bennett,
"Handbook of Optics (McGraw-Hill, New York) 1978]. By mathematical
manipulation of Eq. (9), the difference (r-.theta.) can be
expressed as 5 tan ( r - ) = ( n o 2 - n e 2 ) tan n e 2 + n o 2
tan 2 ( 10 )
[0070] By using these or related equations, tolerable upper limits
for S.sub.a and S.sub.b can be determined.
[0071] A number of different embodiments of the apparatus of the
invention in addition to the first embodiment (shown in FIG. 1a)
are described. While they differ in some details, the disclosed
embodiments otherwise share many common elements and the additional
embodiments naturally fall into four categories, the first category
comprising the first embodiment and variants thereof. The second
category includes embodiments wherein the effects of the lateral
shear S.sub.a on the efficiency of transformation are either
reduced or substantially eliminated, the third category includes
embodiments wherein the effects of the lateral shear S.sub.b on the
efficiency of transformation are either reduced or substantially
eliminated, and the fourth category includes embodiments wherein
the efficiency of conversion and the relative phase of the
principal output beam components relative to the relative phase of
the different frequency components of the input beam are
temperature compensated for changes in temperature of birefringent
elements of an embodiment.
[0072] Reference is now made to FIG. 1b, which depicts in
diagrammatic form the first variant of the first embodiment of the
present invention. The first variant of the first embodiment is
from the first category as described in the preceding paragraph.
The apparatus of the first variant of the first embodiment in FIG.
1b comprises many of the same elements as the first embodiment in
FIG. 1a, the elements of the first variant of the first embodiment
performing like operations as like denoted elements in the first
embodiment.
[0073] The description of light beam 41 for the first variant of
the first embodiment is the same as that for the description of
light beam 41 for the first embodiment. Further, the description of
light beam components 72, 73, 74, and 75 and the progenitors of
light beam components 72, 73, 74, and 75 for the first variant of
the first embodiment is the same as that for the description of
light beam components 72, 73, 74, and 75 and the progenitors of
light beam components 72, 73, 74, and 75 for the first
embodiment.
[0074] Referring to FIG. 1b, beams 72, 73, 74, and 75 enter
half-wave phase retardation plate 90 of the usual type and exit 90
as beams 82, 83, 84, and 85, respectively. Phase retardation plate
90 rotates the planes of polarization of beams 72, 73, 74, and 75
such that the polarization of one of the combined beams is
substantially polarized in the plane of FIG. 1a and the
polarization of the other combined beam is substantially polarized
perpendicular to the plane of FIG. 1b, the polarization of combined
beams 82 and 83 being substantially orthogonal with respect to the
polarization of combined beams 84 and 85.
[0075] Light beams 82, 83, 84, and 85 enter birefringent prism 220.
Beams 192 and 193 emerge from extraordinarily polarized beams
within prism 220 and combine to form output beam 196 and beams 194
and 195 emerge from ordinarily polarized beams within prism 220 and
combine to form output beam 197. The optical axis of birefringent
prism 220 is orientated at an angle of 90.degree. to the plane of
FIG. 1b and is made of a negative uniaxial crystal, e.g.
LiNbO.sub.3 or KDP. Alternatively, the optical axis of birefringent
prism 220 is orientated at 0.degree. to the plane of FIG. 1b and is
made of a positive uniaxial crystal, e.g. quartz or TeO.sub.2. The
optical axis 221 of birefringent prism 220 is shown in FIG. 1b for
a negative uniaxial crystal. The apex angle .alpha..sub.3 of
birefringent prism 220, shown in FIG. 1b, is selected so that beams
192, 193, 194, and 195 have parallel directions of propagation.
Output beams 196 and 197 thus propagate parallel to one another,
have orthogonal polarizations, and are substantially coextensive.
There is a non-zero lateral shear between beams 192 and 194,
S.sub.a, which is exaggerated in FIG. 1b. Beams 192 and 193 have
the same frequency profile as the first input beam component 42 and
beams 194 and 195 have the same frequency profile as the second
input beam component 44 which typically is displaced from that of
beams 192 and 193.
[0076] The remaining description of the first variant of the first
embodiment is the same as corresponding portions of the description
given for the first embodiment, except that the polarizations of
output beams 196 and 197 are at 90.degree. and 0.degree.,
respectively. The difference in polarization is a consequence of
the orientation of optic axis 221 in prism 220.
[0077] Reference is now made to FIG. 1c, which depicts in
diagrammatic form the second variant of the first embodiment of the
present invention. The second variant of the first embodiment is
from the first category, the same category as that of the firs:
embodiment. The apparatus of the second variant of the first
embodiment in FIG. 1c comprises many of the same elements as the
first variant of the first embodiment in FIG. 1b, the elements of
the second variant of the first embodiment performing like
operations as like denoted elements in the first variant of the
first embodiment.
[0078] The description of light beam 41 for the second variant of
the first embodiment is the same as that for the description of
light beam 41 for the first embodiment. The primary optical element
in the second variant of the first embodiment is a phase
retardation plate 160 typically made of a birefringent crystal,
e.g. LiNbO.sub.3, KDP, or TeO.sub.2, shown in FIG. 1c. The optical
axis 161 of the phase retardation plate 160 is orientated at an
angle .alpha..sub.4 with respect to a normal to the entrance facet
of phase retardation plate 160 as illustrated in FIG. 1c and
orthogonal to a normal to the plane of FIG. 1c.
[0079] Upon entering crystal 160, component 42 becomes first and
second components 162 and 163, respectively, of an internal beam,
wherein internal beam components 162 and 163 are ordinarily
(90.degree.) and extraordinarily (0.degree.) polarized,
respectively. Similarly, the second component 44 of the input beam
becomes on entering phase retardation plate 160 a third and fourth
components 164 and 165, respectively, of the internal beam, wherein
internal beam components 164 and 165 are ordinarily (90.degree.)
and extraordinarily (0.degree.) polarized, respectively.
[0080] Phase retardation plate 160 introduces a phase shift of
[p+({fraction (1/2)})].pi. radians in beam 163 relative to beam 162
where p is an integer and a phase shift of [p+({fraction
(3/2)})].pi. radians in beam 165 relative to beam 164 by having the
angle .alpha..sub.4 set at a value between 0.degree. and
90.degree., typically set at substantially 45.degree., and
adjusting the thickness d.sub.2 of phase retardation plate 160.
Beams 162, 163, 164, and 165 exit phase retardation plate 160 as
beams 172, 173, 174, and 175, respectively. As a consequence of the
phase shifts introduced in beams 162, 163, 164, and 165, the
polarization of combined beams 172 and 173 is either substantially
right-hand or left-hand circularly polarized and the polarization
of combined beams 174 and 175 is either substant ally left-hand or
right-hand circularly polarized. The directions of propagation of
beams 172 and 173 are parallel and the directions of propagation of
beams 174 and 175 are parallel, the entrance and exiting faces of
crystal 160 being parallel.
[0081] Referring to FIG. 1c, beams 172, 173, 174, and 175 enter
quarter-wave phase retardation plate 190 of the usual type and exit
190 as beams 182, 183, 184, and 185, respectively. Phase
retardation plate 190 is orientated to convert a circularly
polarized beam into a linearly polarized beam such that the
superposition of beams 182 and 183 is substantially linearly
polarized perpendicular to the plane of FIG. 1c and the
superposition of beam 184 and 185 is substantially linearly
polarized in the plane of FIG. 1c, the linear polarization of
combined beams 182 and 183 thus being substantially orthogonal to
the linear polarization of combined beams 184 and 185. The
individual polarizations of beams 182, 183, 184, and 185 are
circularly polarized and thus parenthetical indications of linear
polarization for these beams are not present in FIG. 1c.
[0082] Light beams 182, 183, 184, and 185 enter birefringent prism
220 and exit as beams 192, 193, 194, and 195, respectively. The
remaining description of the second variant of the first embodiment
is the same as corresponding portions of the description given for
the first variant of the first embodiment.
[0083] Reference is now made to FIG. 1d, which depicts in
diagrammatic form the third variant of the first embodiment of the
present invention. The third variant of the first embodiment is
from the first category of embodiments, the same category as that
of the first embodiment. The apparatus of the third variant of the
first embodiment in FIG. 1d comprises many of the same elements as
the first embodiment in FIG. 1a, the elements of the third variant
of the first embodiment performing like operations as like denoted
elements in the first embodiment.
[0084] The description of light beam 41 for the third variant of
the first embodiment is the same as that for the description of
light beam 41 for the first embodiment. A principal optical element
in the third variant of the first embodiment is a phase retardation
plate 60, the same as phase retardation plate 60 of the first
embodiment.
[0085] The optical elements in the third variant of the first
embodiment different from the elements of the first embodiment are
birefringent prisms 130 and 132, the combination of the two
birefringent prisms being, for example, of the Wollaston prism
type. The two orthogonal optical axes of the two components of the
Wollaston prism, 130 and 132, are orientated at angles
.+-.45.degree. to the plane of FIG. 1d. The angle .alpha..sub.5 of
the Wollaston prism comprising components 130 and 132 is chosen so
that the output beams 292, 293, 294, and 295 have parallel
directions of propagation. Beams 292 and 293 are from components of
beams 72 and 73, respectively, in the same manner as beams 92 and
93 are from components of beams 72 and 73 as described in the first
embodiment. Beams 294 and 295 are from components of 74 and 75,
respectively, in the same manner as beams 94 and 95 are from
components of beams 74 and 75 as described in the first embodiment.
Beams 292 and 293 combine to form output beam 296 and beams 294 and
295 combine to form output beam 297.
[0086] The principal difference between the third variant of the
first embodiment and the first embodiment is that the directions of
propagation of the principal output beams of the third variant of
the first embodiment are substantially parallel to the direction of
propagation of the input beam, whereas the directions of
propagation of the principal output beams of the first embodiment
differ from the direction of propagation of the input beam by some
non-zero angle.
[0087] The remaining description of the third variant of the first
embodiment is the same as corresponding portions of descriptions
given for the first embodiment.
[0088] Reference is now made to FIG. 1e, which depicts in
diagrammatic form the fourth variant of the firs: embodiment of the
present invention. The fourth variant of the first embodiment is
from the first category of embodiments, the same category as that
of the first embodiment. The apparatus of the fourth variant of he
first embodiment in FIG. 1e comprises many of the same elements as
the first variant of the first embodiment in FIG. 1b, the elements
of the fourth variant of the first embodiment performing like
operations as like denoted elements in the first variant of the
first embodiment.
[0089] The description of light beam 41 for the fourth variant of
the first variant is the same as that for the description of light
beam 41 for the first variant of the first embodiment. A principal
optical element in the fourth variant of the first embodiment is a
phase retardation plate 60, the same as phase retardation plate 60
of the first variant of the first embodiment.
[0090] The optical elements in the fourth variant of the first
embodiment different from the elements of first variant of the
first embodiment are birefringent prisms 230 and 232, the
combination of the two prisms being, for example, of the standard
Wollaston prism type. The two orthogonal optical axes 231 and 233
of the two birefringent components of the Wollaston prism, 230 and
232, respectively, are orientated parallel to and orthogonal to the
plane of FIG. 1e or vice versa depending on the properties of the
birefringent crystals comprising birefringent prisms 230 and 232.
The optical axes 231 and 233 are shown in FIG. 1e for birefringent
prisms 230 and 232 comprising negative uniaxial crystals. The angle
.alpha..sub.6 of the Wollaston prism comprising components 230 and
232 is chosen so that the output beams 292, 293, 294, and 295 have
parallel directions of propagation. Beams 292 and 293 are from
components of beams 82 and 83, respectively, in the same manner as
beams 192 and 193 are from beams 82 and 83 as described in the
first variant of the first embodiment. Beams 294 and 295 are from
components of beams 84 and 85, respectively, in the same manner as
beams 194 and 195 are from components of beams 84 and 85 as
described in the first variant of the first embodiment.
[0091] The remaining description of the fourth variant of the first
embodiment is the same as corresponding portions of descriptions
given for the first variant of the first embodiment, except that
the polarizations of output beams 296 and 297 are at 90.degree. and
0.degree., respectively. The difference in polarization is a
consequence of the orientation of optic axes 231 and 233 in prisms
230 and 232.
[0092] The principal difference between the fourth variant of the
first embodiment and the first variant of the first embodiment is
that the directions of propagation of the principal output beams of
the fourth variant of the first embodiment are substantially
parallel to the direction of propagation of the input beam, whereas
the directions of propagation of the principal output beams of the
first variant of the first embodiment differ from the direction of
propagation of the input beam by some non-zero angle.
[0093] Reference is now made to FIG. 1f, which depicts in
diagrammatic form the fifth variant of the first embodiment of the
present invention. The fifth variant of the first embodiment is
from the first category of embodiments, the same category as that
of the first embodiment. The apparatus of the fifth variant of the
first embodiment depicted in FIG. 1f comprises many of the same
elements as the second variant of the first embodiment depicted in
FIG. 1c, the elements of the fifth variant of the first embodiment
performing like operations as like denoted elements in the second
variant of the first embodiment.
[0094] The description of light beam 41 for the fifth variant of
the first embodiment is the same as that for the description of
light beam 41 for the second variant of the first embodiment. A
principal optical element in the fifth variant of the first
embodiment is a phase retardation plate 160, the same as phase
retardation plate 160 of the second variant of the first
embodiment.
[0095] The optical elements in the fifth variant of the first
embodiment different from the elements of the second variant of the
first embodiment are birefringent prisms 230 and 232, the
combination of birefringent prisms 230 and 232 being, for example,
of the standard Wollaston prism type. The two orthogonal optical
axes 231 and 233 of the two birefringent components, 230 and 232,
respectively, of the Wollaston prism are orientated parallel to and
orthogonal to the plane of FIG. 1f. The optical axes 231 and 233
are shown in FIG. 1f for birefringent prisms 230 and 232 comprising
negative uniaxial crystals. The angle .alpha..sub.6 of Wollaston
prism comprising components 230 and 232 is chosen so that the
output beams 292, 293, 294, and 295 have parallel directions of
propagation. Beams 292 and 293 are from components of beams 182 and
183, respectively, in the same manner as beams 192 and 193 are from
beams 182 and 183 as described in the second variant of the first
embodiment. Beams 294 and 295 are from components of beams 184 and
185, respectively, in the same manner as beams 194 and 195 are from
components of beams 184 and 185 as described in the second variant
of the first embodiment.
[0096] The remaining description of the fifth variant of the first
embodiment is the same as corresponding portions of descriptions
given for the second variant of the first embodiment.
[0097] The principal difference between the fifth variant of the
first embodiment and the second variant of the first embodiment is
that the directions of propagation of the principal output beams of
the fifth variant of the first embodiment are substantially
parallel to the direction of propagation of the input beam, whereas
the directions of propagation of the principal output beams of the
second variant of the first embodiment differ from the direction of
propagation of the input beam by some non-zero angle.
[0098] Reference is now made to FIG. 2a, which depicts in
diagrammatic form the second embodiment of the present invention.
The second embodiment is from the second category of embodiments
wherein the effects of lateral shear of the type S.sub.a on the
efficiency of transformation are either reduced or substantially
eliminated. The apparatus of the second embodiment in FIG. 2a
comprises many of the same elements as the first variant of the
first embodiment in FIG. 1b, the elements of the second embodiment
performing like operations as like denoted elements in the first
variant of the first embodiment.
[0099] The description of light beam 41 for the second embodiment
is the same as that for the description of light beam 41 for the
first embodiment. A principal optical element in the second
embodiment different from the elements of the first variant of the
first embodiment is a phase retardation plate 260 shown in FIG. 2a,
typically made of the same birefringent material as phase
retardation plate 60. The optical axis 261 of the phase retardation
plate 260 is orientated at an angle .alpha..sub.7 with respect to a
normal to the entrance facet of phase retardation plate 260 as
illustrated in FIG. 2a that is orthogonal to a normal to the plane
of FIG. 2a. The angle between optical axis 261 and optical axis 61
typically is nominally 90.degree.. With the prescribed orientation
of the optical axis 261, the lateral shear between the beam
comprising the combination of beams 84 and 85 and the beam
comprising the combination of beams 82 and 83 is reduced as the
respective beams propagate through phase retardation plate 260. The
reduction is a consequence of the lateral shear produced by the
difference between the directions of the energy flux vector and the
wave front vector for each of the extraordinarily polarized
components of beams 84 and 85 in phase retardation plate 260. The
thickness d.sub.3 of phase retardation plate 260 is chosen so that
the net lateral displacement S.sub.a after prism 220 between the
principal output beams, i.e. beams 192 and 193 and beams 194 and
195 (as shown in FIGS. 1b and 1c) has a predetermined value,
typically zero. The apex angle .alpha..sub.3 of prism 220 is chosen
so that the principal output beams have a predetermined angle of
divergence or convergence, typically zero.
[0100] The remaining description of the second embodiment is the
same as corresponding portions of the description given for the
first variant of the first embodiment.
[0101] Reference is now made to FIG. 2b, which depicts in
diagrammatic form the first variant of the second embodiment of the
present invention. The first variant of the second embodiment is
from the second category of embodiments wherein the effects of
lateral shear of the type S.sub.a on the efficiency of
transformation are either reduced or substantially eliminated. The
apparatus of the first variant of the second embodiment in FIG. 2b
comprises many of the same elements as the first variant of the
first embodiment in FIG. 1b, the elements of the first variant of
the second embodiment performing like operations as like denoted
elements in the first variant of the first embodiment.
[0102] The description of light beam 41 for the first variant of
the second embodiment is the same as that for the description of
light beam 41 for the second embodiment. Principal optical elements
in the first variant of the second embodiment different from the
elements of the first variant of the first embodiment are
birefringent prisms 320 and 420 shown in FIG. 2b, birefringent
prisms 320 and 420 typically made of the same birefringent material
such as birefringent prism 220 of the first variant of the first
embodiment.
[0103] The optical axis 321 of birefringent prism 320 is orientated
at an angle of 90.degree. to the plane of FIG. 2b, the same as for
birefringent prism 220 of the first variant of the first embodiment
and the optical axis 421 of birefringent prism 420 is orientated
parallel to the plane of FIG. 2b and substantially perpendicular to
the direction of propagation of the optical beams propagating in
birefringent prism 420, birefringent prisms 320 and 420 comprising
negative uniaxial crystals. The apex angle .alpha..sub.8 of
birefringent prism 320 and the apex angle .alpha..sub.9 of
birefringent prism 420 are chosen such that the principal output
beams, beams 192, 193, 194, and 195 (as shown in FIGS. 1b and 1c)
following prism 420, have a predetermined spatial separation,
typically zero, and have a predetermined angle of divergence or
convergence, typically zero.
[0104] The remaining description of the first variant of the second
embodiment is the same as corresponding portions of the description
given for the second embodiment.
[0105] Reference is now made to FIG. 2c, which depicts in
diagrammatic form the second variant of the second embodiment of
the present invention. The second variant of the second embodiment
is from the second category of embodiments wherein the effects of
lateral shear of the type S.sub.a on the efficiency of
transformation are either reduced or substantially eliminated. The
apparatus of the second variant of the second embodiment in FIG. 2c
comprises many of the same elements as the fourth variant of the
first embodiment in FIG. 1e, the elements of the second variant of
the second embodiment performing like operations as like denoted
elements in the fourth variant of the first embodiment.
[0106] The description of light beam 41 for the second variant of
the second embodiment is the same as that for the description of
light beam 41 for the second embodiment. Principal optical elements
in the second variant of the second embodiment different from the
elements of the fourth variant of the first embodiment are
Wollaston prisms comprising birefringent prisms 330 and 332 and
birefringent prisms 430 and 432. The optical axes of the two
components of Wollaston prism comprising prisms 330 and 332 are
orientated the same as optical axes of the two components of
Wollaston prism comprising prisms 230 and 232 of the fourth variant
of the first embodiment, corresponding components of Wollaston
prism comprising prisms 230 and 232 and of Wollaston prism
comprising prisms 330 and 332 comprising the same birefringent
material. The components of Wollaston prism comprising prisms 430
and 432 comprise the same birefringent material as the components
of Wollaston prism comprising prisms 230 and 232. The angle
.alpha..sub.10 of Wollaston prism comprising prisms 330 and 332 and
the angle all of Wollaston prism comprising prisms 430 and 432 are
chosen such that the principal output beams, beams 192, 193, 194,
195 (as shown in FIGS. 1b and 1c) following prisms 430 and 432,
have a predetermined spatial separation, typically zero, and have a
predetermined angle of divergence or convergence, typically
zero.
[0107] The remaining description of the second variant of the
second embodiment is the same as corresponding portions of the
description given for the second embodiment.
[0108] It will be apparent to those skilled in the art that the
S.sub.a type lateral shear compensating/reduction feature of the
second embodiment and variants thereof can be incorporated into
different ones of the disclosed embodiments and variants thereof of
the present invention without departing from the scope and spirit
of the present invention.
[0109] Reference is now made to FIG. 3a, which depicts in
diagrammatic form the third embodiment of the present invention.
The third embodiment is from the third category of embodiments
wherein the effects of lateral shear of the type S.sub.b on the
efficiency of transformation are either reduced or substantially
eliminated. The apparatus of the third embodiment in FIG. 3a
comprises many of the same elements as the first variant of the
first embodiment in FIG. 1b, the elements of the third embodiment
performing like operations as like denoted elements in the first
variant of the first embodiment.
[0110] The description of the light beams for the third embodiment
is similar to the description of the light beams in the first
embodiment. A principal optical element in the third embodiment
different from the elements of the first variant of the first
embodiment is a phase retardation plate 260 shown in FIG. 3a,
typically made of the same birefringent material as phase
retardation plate 60. The optical axis 261 of the phase retardation
plate 260 is orientated at an angle .alpha..sub.12 with respect to
a normal to the entrance facet of phase retardation plate 260 as
illustrated in FIG. 3a, orthogonal to a normal to the plane of FIG.
3a, and the angle between optical axis 261 and optical axis 61 of
phase retardation plate 60 typically is nominally 90.degree..
[0111] Light source 40a generates an input beam 30 having a
polarization of 45.degree.. Upon propagating through retardation
plate 260, beam 30 separates into extraordinarily polarized beam 37
and ordinarily polarized beam 36, which exit retardation plate 260
as beam 33 and beam 32, respectively. Retardation plate 260 thereby
introduces a lateral shear between beams 32 and 33 as a result of
the difference between the directions of the energy flux vector and
the wave front vector for the extraordinarily polarized beam. These
beams then propagate though an acousto-optic Braga cell 35, which
diffracts half of beams 32 and 33 into frequency-shifted beams 44a
and 45a. The undiffracted halves of beams 32 and 33 emerge from
Bragg cell 35 as beams 42a and 43a. Reams 42a, 43a, 44a, and 45a
enter retardation plate 60 forming beams 62, 63, 64, and 65 as in
the embodiment described in FIG. 1a. The thickness d.sub.4 of phase
retardation plate 260 is chosen so that the lateral shear
introduced by phase retardation plate 260 exactly compensates for
the lateral shear S.sub.b produced by retardation plate 60. Thus,
beams 72 and 73 and beams 74 and 75 completely overlap one
another.
[0112] The third embodiment in addition to substantially
compensating for the lateral shear S.sub.b produced by retardation
plate 60 also substantially eliminates the first order sensitivity
of the relative phases of output beams 72 and 73 and output beams
74 and 75 to changes in the orientation of the input beam 30 in the
plane of FIG. 3a.
[0113] The remaining discussion of the third embodiment is the same
as corresponding portions of the description given for the first
variant of the first embodiment.
[0114] Reference is now made to FIG. 3b, which depicts in
diagrammatic form the first variant of the third embodiment of the
present invention. The first variant of the third embodiment is
from the third category of embodiments wherein the effects of
lateral shear of the type S.sub.b on the efficiency of
transformation are either reduced or substantially eliminated. The
apparatus of the first variant of the third embodiment in FIG. 3b
comprises many of the same elements as the first variant of the
first embodiment in FIG. 1b, the elements of the first variant of
the third embodiment performing like operations as like denoted
elements in the first variant of the first embodiment.
[0115] The description of light beam 30 for the first variant of
the third embodiment is the same as that for the description of
light beam 30 for the third embodiment. A set of optical elements
in the firs: variant of the third embodiment different from the
elements of the first variant of the first embodiment are prisms
110, 210, 310, and 410 shown in FIG. 3b, typically of the non
birefringent type. Prisms 110 and 210 are used as beam expanders in
the plane of FIG. 3b before Bragg cell 35 and prisms 310 and 410
are used as beam contractors in the plane of FIG. 3b after phase
retardation plate 60. The net result is a reduction in the lateral
shear of the S.sub.b type by a factor equal to the reduction factor
of the beam contraction produced by prisms 310 and 410, the beam
expansion factor of the beam expansion produced by prisms 110 and
210 being the reciprocal of the reduction factor of the beam
contraction produced by prisms 310 and 410.
[0116] The remaining discussion of the third embodiment is the same
as corresponding portions of the description given for the first
variant of the first embodiment.
[0117] It will be apparent to those skilled in the art that the
S.sub.b type lateral shear compensating/reduction feature of the
third embodiment and variant thereof can be incorporated into
different ones of the disclosed embodiments and variants thereof of
the present invention without departing from the scope and spirit
of the present invention.
[0118] Reference is now made to FIG. 4, which depicts in
diagrammatic form the fourth embodiment of the present invention.
The fourth embodiment is from the fourth category of embodiments
wherein the relative phase of the principal output beam components
relative to the relative phase of the different frequency
components of the input beam are temperature compensated for
changes in temperature of the phase retardation plate(s) in the
apparatus. The apparatus of the fourth embodiment in FIG. 4
comprises many of the same elements as the first embodiment in FIG.
1a, the elements of the fourth embodiment performing like
operations as like denoted elements in the first embodiment.
[0119] The description of light beam 41 for the fourth embodiment
is the same as that for the description of light beam 41 for the
first embodiment. A principal optical element in the fourth
embodiment different from the elements of the first variant of the
first embodiment is a chase retardation plate 140 shown in FIG. 4,
which is made of the same birefringent material as phase
retardation plate 60. In other embodiments, it is possible that
retardation plate 140 is made of a material different from
retardation plate 60. The optical axis of a phase retardation plate
140 is orientated orthogonal to the optical axis of phase
retardation plate 60 and substantially orthogonal to the directions
of propagation of the beams in phase retardation plate 140. The
thickness d.sub.5 of phase retardation plate 140 is chosen so that
changes in the phase differences between beams 72 and 73 and beams
74 and 75 that arise due to temperature changes in retardation
plate 60 are compensated by the phase differences between beams 72
and 73 and beams 74 and 75 imparted by retardation plate 140. For
relatively small angles of .delta. (e.g., on the order of 3.5
mrad), this condition is satisfied when d.sub.5 satisfies Equation
(11) 6 T [ ( n o - n ) d 1 2 0 + ( n o - n e ) d 5 2 0 ] = 0 ( 11
)
[0120] where T is temperature, .lambda..sub.0 is the wavelength of
beam 41, and n is the index of refraction given by Eq. (1) for the
extraordinarily polarized beams in retardation plates 60.
[0121] The remaining discussion of the fourth embodiment is the
same as corresponding portions of the description given for the
first embodiment.
[0122] It will be apparent to those skilled in the art that the
temperature compensating feature of the fourth embodiment can be
incorporated into different ones of the disclosed embodiments and
variants thereof of the present invention without departing from
the scope and spirit of the present invention.
[0123] It will also be apparent to those skilled in the art that in
alternative variants different orientations of the optic axis in
retardation plate 140 are possible. For example, retardation plate
140 can be replaced with a second retardation plate that is
identical to phase retardation plate 60 except that it is rotated
by an angle of 90.degree. about the direction of propagation of
input beam 41 so that its optic axis is contained in a plane
perpendicular to the plane of FIG. 4. For the alternative variant
of the example, it will be apparent to those skilled in the art
that the relative phases of output beams 392 and 393 and output
beams 394 and 395 are sensitive in first order to changes in the
orientation of input beam 41 in a plane orthogonal to the plane of
FIG. 4. It will be also evident to those skilled in the art that
when the alternative variant of the example is employed twice in
the third embodiment, once to compensate for thermal effects of
birefringent plate 60 and once to compensate for the thermal
effects of birefringent plate 260 (see FIG. 3a), the sensitivity of
the relative phases of output beams corresponding to beams 72 and
73 and output beams 74 and 75 to changes in the orientation of
input beam 30 in a plane orthogonal to the plane of FIG. 3a is
substantially eliminated, the optical axes of the two thermal
compensating regarding plates being substantially orthogonal.
[0124] It will be further evident to those skilled in the art that
each of the embodiments and variants thereof of the present
invention may be configured to receive source beams with converging
directions of propagation and produce either output beams with
diverging directions of propagation or outrun beams with parallel
directions of propagation according to the requirements of the end
use application without departing from the spirit and scope of the
present invention.
[0125] As would be known to those of skill in the art, light source
40 for producing diverging input beams 42 and 44 (as shown in FIG.
1a) can include many different embodiments. One such embodiment is
shown in FIG. 5. A laser 10 provides a beam 12 of optical energy,
which has a single, stabilized frequency and is linearly polarized.
Laser 10 can be any of a variety of lasers. For example, it can be
a gas laser, e.g. a HeNe, stabilized in any of a variety of
conventional techniques known to those skilled in the art to
produce beam 12, see for example, T. Baer et al., "Frequency
stabilization of a 0.633 .mu.m He--Ne-longitudinal Zeeman laser,"
Applied Optics, 19(18), 3173-3177 (1980); Burgwald et al., U.S.
Pat. No. 3,889,207, issued Jun. 10, 1975; and Sandstrom et al.,
U.S. Pat. No. 3,662,279, issued May 9, 1972. Alternatively, light
source 10 can be a diode laser frequency stabilized by one of a
variety of conventional techniques known to those skilled in the
art to produce beam 12, see for example, T. Okoshi and K. Kikuchi,
"Frequency Stabilization of Semiconductor Lasers for
Heterodyne-Type Optical Communication Systems," Electronic Letters,
16(5), 179-181 (1980) and S. Yamaqguchi and M. Suzuki,
"Simultaneous Stabilization of the Frequency and Power of an AlGaAs
Semiconductor Laser by Use of the Optogalvanic Effect of Krypton,"
IEEE J. Quantum Electronics, QE-19(10), 1514-1519 (1983).
[0126] The specific device used for source 10 will determine the
diameter and divergence of beam 12. For some sources, e.g. a diode
laser, it is necessary to use conventional beam shaping optics 14,
e.g. a conventional microscope objective, to provide input beam 18
with suitable diameter and divergence for the elements that follow.
When laser 10 is a HeNe laser, for example, beam shaping optics 14
may not be required. The elements 10 and 14 are shown in dashed box
16 which represents the source of the input beam 18 and, for this
embodiment and others with analogous features, includes well-known
alignment means for spatially and angularly positioning input beam
18. Such alignment means could, for example, comprise precision
micro-manipulators and steering mirrors. The input beam 18 has one
stabilized frequency .function..sub.Land is linearly polarized. The
polarization orientation, by way of example, is typically
45.degree. to the plane of FIG. 5.
[0127] An electrical oscillator 22 provides a frequency stabilized
electrical signal 21 of frequency .function..sub.o to a
conventional power amplifier 23. The electrical output 24 of the
power amplifier 32 drives an array of conventional piezoelectric
transducers 34 affixed to a Bragg cell 20. The array of
piezoelectric transducers 25 generates an acoustic wave 26 within
Bragg cell 20. Conventional techniques known to those skilled in
the art of acousto-optical modulation are used to absorb, by
absorber 27, the acoustic wave 26 char passes through to the walls
of the acousto-optical Bragg cell 20. The acoustic wave diffracts a
portion of input beam 18 into a diffracted and frequency-shifted
beam 29, which diverges within Bragg from beam 28, the remaining
portion of input beam 18. Upon exiting Bragg cell 20, beams 28 and
29 form beams 42 and 44, respectively, which are two diverging
beams having substantially identical polarizations and different
frequencies. As described above, beams 42 and 44 propagate into at
least one retardation plate and a birefringent prism (and possibly
additional optics) to produce two substantially coextensive and
collinear beams having substantially orthogonal polarizations and
different frequencies.
[0128] The system described above can be used in a wide range of
interferometric measuring systems, one such example being the
distance measuring interferometry system 501 shown in FIG. 6. The
system described above (depicted as system 500) produces two
substantially coextensive and collinear, frequency shifted beams
502 and 504 having substantially orthogonal polarizations. In
particular, beam 502 is polarized within the plane of FIG. 6 (i.e.,
0.degree.) and has a frequency .function..sub.1 and beam 504 is
polarized substantially orthogonal to the plane of FIG. 6 (i.e.,
90.degree.) and has a frequency .function..sub.2. Beams 502 and 504
are incident on a polarizing beam splitter 506, which reflects beam
504 to a first retroreflector 508 and transmits beam 502 through a
quarter wave plate 505 and onto a stage mirror 507. The stage
mirror is movable along the propagation direction of beam 502 and
reflects beam 502 back through quarter wave plate 505 and back to
polarizing beam splitter 506. The double pass through quarter wave
plate 505 and the reflection from stage mirror 507 rotate the
polarization of beam 502 to 90.degree. so that beam splitter 506
reflects beam 502 toward a second retroreflector 509, which in turn
reflects the beam back to beam splitter 506. Then, beam splitter
506 reflects beam 502 back through quarter waveplate 505 and toward
stage mirror 507. Once again stage mirror reflects beam 502 back
through the quarter waveplate toward the beam splitter. The second
double pass through quarter waveplate 505 and the reflection from
stage mirror 502 return the polarization of beam 502 to 0.degree..
Thus, beam splitter 506 now transmits multiply-reflected beam 502
and recombines it with beam 504, which is reflected by beam
splitter 506 after being reflected back to beam splitter 506 by
retroreflector 508. The recombined beams then enter into a mixing
polarizer 510 (e.g., a polarizer oriented at 45.degree.) and the
intensity of the resultant optical signal 511 is measured by
detector 512. The frequency of the intensity signal measured by
detector 512 is equal to the difference in frequency between beams
502 and 504 plus a term associated with the speed of movable stage
mirror 507. Detector 512 sends a signal 514 based on the intensity
measurement to electronics 516, which also receives a signal 518
from system 500 indicative of the frequency difference and relative
phase of beams 502 and 504. From signals 514 and 518 electronics
516 determines changes in distance to stage mirror 507. In some
applications, the stage mirror is mounted onto wafer processing
stage so that the interferometry apparatus 501 measures the precise
position of a wafer being processed.
[0129] As is well known in the art, apparatus 501 can be modified
in many ways. In particular, the apparatus can include similar sets
of additional optics to provide distance measurements along
multiple axes. The efficiency provided by system 500 for producing
beams 502 and 504 insures that there is sufficient energy to make
these measurements along multiple axes.
OTHER EMBODIMENTS
[0130] Other embodiments are also in the scope of the invention.
For example, elements such as the retardation plates and
birefringent prisms can be integral with one another rather than
being separated from one another. Furthermore, rather than using
retardation plates having parallel entry and exit faces, the
invention can include any retarder element made from a birefringent
material. For example, the invention can include a retarder element
made from a birefringent material having a varying thickness. In
this case, one can translate the position of the retarder element
so that beams propagating through the retarder element propagate
through an optimum thickness. Also, in other embodiments the shape
and orientation of the retarder element can produce the desired
retardances by causing internal reflections of the beams being
retarded within the retarder element.
[0131] Other aspects, advantages, and modifications are within the
scope of the following claims.
* * * * *