 |
Claims  |
|
|
We claim:
1. A method of producing an interference generated phase contrast image,
comprising the steps of:
generating a beam of collimated coherent light;
splitting the beam of collimated coherent light into a first beam and
second beam with a first beam splitter;
splitting the first beam into third beam and fourth beam with a second beam
splitter;
directing the third beam to a transparent phase object substantially
located on a plane;
passing a first portion the third beam through the transparent phase object
into a photorefractive holographic means, for producing and recording a
hologram of the transparent phase object and for generating a phase
conjugate beam of the transparent phase object from the beam of collimated
coherent light;
splitting the phase conjugate beam into a first conjugate beam and a second
conjugate beam with the second beam splitter;
splitting the second conjugate beam into a third conjugate beam and a
fourth conjugate beam with the first beam splitter,
directing the third conjugate beam with the first beam splitter to
intercept a ground glass to produce a first spot viewed from above;
reflecting a second portion of the third beam from the transparent phase
object to form a specularly reflected beam;
adjusting the entry of the specularly reflected beam into the second beam
splitter with an adjustment means;
splitting the specularly reflected beam into a first and second reflected
beam with the second beam splitter;
splitting the second reflected beam into a third reflected beam and a
fourth reflected beam with the first beam splitter;
directing the third reflected beam with the first beam splitter to
intercept the ground glass to produce a second spot viewed from above;
adjusting the position of the second spot with the adjustment means so that
it falls coincident on the first spot, resulting in the first conjugate
beam and the first reflected beam forming coincident beams;
intercepting the coincident beams with a image forming optical system;
recording a first image formed by the image forming system;
shifting the relative distance between the hologram and the transparent
phase object a fraction a distance incommensurate with a wave length;
recording a second image formed by the image forming means;
subtracting the first image from the second image to obtain an interference
generated phase contrast image.
2. The method of claim 1, wherein the step of shifting the relative
distance between the hologram and the transparent phase object includes
shifting the transparent phase object an incremental lateral distance; and
wherein the step of subtracting the first image from the second image
results in obtaining a combination interference generated phase contrast,
gradient contrast image.
3. The method of claim 1, further comprising, prior to the step of
intercepting the coincident beams with an image forming optical system,
the steps of:
positioning a lens between the first beam splitter and the ground glass,
projecting an interference pattern of fringes on the lens; and
utilizing the adjusting means until the pattern is symmetric about a
central node.
4. The method of claim 1 wherein at least one of the first and second beam
splitters is a 45.degree. uncoated pellicle beam splitter.
5. The method of claim 1, further comprising, after the step of generating
the beam of collimated coherent light, the step of rotating the
polarization of the beam to lie in the plane of the second beam splitter.
6. The method of claim 1, wherein the adjustment means comprises at least
one mechanically adjuster or piezoelectric adjuster.
7. The method of claim 1, wherein the optical imaging system comprises a
microscope and a video camera.
8. The method of claim 1 wherein the recording means comprises a frame
grabber with pixelated digital storage of the image and the subtracting
means includes a digital process.
9. The method of claim 1, wherein the hologram is a photorefractive
hologram of a barium titanate crystal.
10. The method of claim 1 further comprising the step of temporarily
recording the hologram.
11. A method of producing a phase gradient contrast image, comprising the
steps of:
generating a beam of collimated coherent light;
splitting the beam of collimated coherent light into a first beam and
second beam with a first beam splitter;
splitting the first beam into third beam and fourth beam with a second beam
splitter;
directing the third beam to a transparent phase object substantially
located on a plane;
passing a first portion the third beam through the transparent phase object
into a photorefractive holographic means, for producing and recording a
hologram of the transparent phase object and for generating a phase
conjugate beam of the transparent phase object from the beam of collimated
coherent light;
splitting the phase conjugate beam into a first conjugate beam and a second
conjugate beam with the second beam splitter;
splitting the second conjugate beam into a third conjugate beam and a
fourth conjugate beam with the first beam splitter,
directing the third conjugate beam with the first beam splitter to
intercept a ground glass to produce a first spot viewed from above;
reflecting a second portion of the third beam from the transparent phase
object to form a specularly reflected beam;
adjusting the entry of the specularly reflected beam into the second beam
splitter with an adjustment means;
splitting the specularly reflected beam into a first and second reflected
beam with the second beam splitter;
splitting the second reflected beam into a third reflected beam and a
fourth reflected beam with the first beam splitter;
directing the third reflected beam with the first beam splitter to
intercept the ground glass to produce a second spot viewed from above;
adjusting the position of the second spot with the adjustment means so that
it falls coincident on the first spot, resulting in the first conjugate
beam and the first reflected beam forming coincident beams;
intercepting the coincident beams with a image forming optical system;
recording a first image formed by the image forming system;
shifting the transparent phase object an incremental lateral distance;
recording a second image formed by the image forming means;
subtracting the first image from the second image to obtain a phase
gradient contrast image.
12. A method of producing a combination interference generated contrast
phase contrast, gradient contrast image, comprising the steps of:
generating a beam of collimated coherent light;
splitting the beam of collimated coherent light into a first beam and
second beam with a first beam splitter;
splitting the first beam into third beam and fourth beam with a second beam
splitter;
directing the third beam to a transparent phase object substantially
located on a plane;
passing a first portion the third beam through the transparent phase object
into a photorefractive holographic means, for producing and recording a
hologram of the transparent phase object and for generating a phase
conjugate beam of the transparent phase object from the beam of collimated
coherent light;
splitting the phase conjugate beam into a first conjugate beam and a second
conjugate beam with the second beam splitter;
splitting the second conjugate beam into a third conjugate beam and a
fourth conjugate beam with the first beam splitter,
directing the third conjugate beam with the first beam splitter to
intercept a ground glass to produce a first spot viewed from above;
reflecting a second portion of the third beam from the transparent phase
object to form a specularly reflected beam;
adjusting the entry of the specularly reflected beam into the second beam
splitter with an adjustment means;
splitting the specularly reflected beam into a first and second reflected
beam with the second beam splitter;
splitting the second reflected beam into a third reflected beam and a
fourth reflected beam with the first beam splitter;
directing the third reflected beam with the first beam splitter to
intercept the ground glass to produce a second spot viewed from above;
adjusting the position of the second spot with the adjustment means so that
it falls coincident on the first spot, resulting in the first conjugate
beam and the first reflected beam forming coincident beams;
intercepting the coincident beams with a image forming optical system;
recording a first image formed by the image forming system;
shifting the relative distance between the hologram and the transparent
phase object a fraction a distance incommensurate with a wave length;
shifting the transparent phase object an incremental lateral distance;
recording a second image formed by the image forming means;
subtracting the first image from the second image to obtain a combination
interference generated phase contrast, phase gradient contrast image.
13. A method of producing an interference generated image comprising the
steps of:
generating a beam of collimated coherent light;
directing the beam along an axis through a transparent phase object;
passing a first portion the beam through the transparent phase object into
a photorefractive holographic means, for producing and recording a
hologram of the transparent phase object and for generating a phase
conjugate beam of the transparent phase object from the beam of collimated
coherent light;
reflecting a second portion of the beam from the transparent phase object
to form a specularly reflected beam;
adjusting the specularly reflected beam and the phase conjugate beam to
form coincident beams;
forming and recording an image of the coincident beams;
shifting the transparent plate object axially to form a shifted beam;
forming and recording a second image of the shifted beam;
subtracting the first image from the second image to obtain an interference
generated image.
14. A method of producing a phase gradient contrast image comprising the
steps of:
generating a beam of collimated coherent light;
directing the beam along an axis through a transparent phase object;
passing a first portion the beam through the transparent phase object into
a photorefractive holographic means, for producing and recording a
hologram of the transparent phase object and for generating a phase
conjugate beam of the transparent phase object from the beam of collimated
coherent light;
reflecting a second portion of the beam from the transparent phase object
to form a specularly reflected beam;
adjusting the specularly reflected beam and the phase conjugate beam to
form coincident beams;
forming and recording an image of the coincident beams;
shifting the transparent plate object transverse to the axis to form a
shifted beam;
forming and recording a second image of the shifted beam;
subtracting the first image from the second image to obtain an interference
generated image.
15. A holographic microscope system for phase imaging of a transparent
phase object, comprising:
a source for a beam of collimated coherent light;
a first beam splitter receiving and splitting the light beam into first and
second beams;
a stage for supporting the transparent phase object, wherein the second
beam enters the transparent phase object at the stage, and wherein a first
portion of the second beam is reflected by the transparent phase object to
form a specularly reflected beam;
means for adjusting the position of the stage in at least two orthogonal
directions;
a photorefractive hologram recording crystal receiving a second portion the
second beam and forming a photorefractive hologram, the crystal generating
a phase conjugate beam;
means for aligning the phase conjugate beam and the specularly reflected
beam;
an objective for receiving an image of the beam; and
a processor for recording and comparing at least two images.
16. A holographic microscope system according to claim 15, wherein the
aligning means comprises a second beam splitter directing the phase
conjugate beam and the specularly reflected beam to a ground glass,
whereupon spots are formed on the ground glass from the phase conjugate
beam and the specularly reflected beam, wherein movement of the stage by
the adjusting means results in the spots falling coincident on the ground
glass. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for producing an
intensity contrast from phase detail in a transparent phase object. More
particularly, the invention relates to a method and apparatus for
producing an interference generated phase contrast image, or a phase
gradient contrast image, or a combination interference generated phase
contrast, phase gradient contrast image. The invention is particularly
suited for the microscope imaging of phase shift photomasks.
DESCRIPTION OF THE PRIOR ART
Phase modulating photomasks are used to project intensity patterns in
semiconductor wafer printers. These masks, because of their complexity and
cost, require careful microscopic inspection of their phase shifting
structures before use and additional qualifying inspections during use.
Inspection of these masks and the more conventional intensity masks is
also important with respect to phase modulating defects appearing as
intensity spots in the projected image.
These phase structures and phase defects cannot, however, be effectively
imaged by ordinary optical means such as conventional bright field
microscopy. While certain microscope phase imaging techniques such as
Zernicke phase contrast, interference microscopy and differential
interference contrast microscopy, can produce representation of phase
structure, these methods are generally inadequate with respect to
sensitivity and resolution for adequate inspection. Scanning laser
microscopy is also applicable to these imaging tasks but again can be
inadequate in phase sensitivity and resolution to meet inspection needs.
Prior related coherent light methods of microscopic imaging phase objects
to produce images called phase gradient contrast images are described in
U.S. Pat. Nos. 4,721,362 to Brody et al. and 4,921,333 to Brody et al.,
assigned to the U.S. Government and hereby incorporated by reference
herein. These methods produce images of gradients in a specified
direction, of phase deformation introduced into fronts of constant phase
as the result of the transmission of a coherent beam through a phase
object. The deformations result from variations in the optical path
lengths of the transmitted beam; these are produced by variations of
thickness and variations of refractive index. The phase gradient contrast
images produced will show lateral gradients in these phase modulating
structures.
Phase gradient methods are useful for examining the structure of phase
shift photomasks and other phase structures. However, since they do not
create an image representing phase shift directly, but rather show
gradients in phase, the images are some times difficult to interpret in
terms of phase shift. For example, larger areas which shift phase
uniformly with boundaries outside the microscope field cannot be directly
distinguished from non-phase shifting areas, both showing the same uniform
intensity. To distinguish such phase shifting regions from non-phase
shifting regions, a direct visualization of phase with respect to some
reference level is required. Such images are called phase contrast images.
Direct visualization of phase resulting in phase contrast rather than phase
gradient contrast is possible using the incoherent light method of
Zernicke phase contrast or methods of interference generated phase
contrast but these lack adequate sensitivity for many purposes. Zernicke
phase contrast also introduces certain artifacts into the image which can
confuse interpretation of these images.
In the prior art, with regard to both the coherent imaging methods of the
aforementioned patents, and conventional incoherent light methods, there
is no simple direct method of producing combination images of phase
contrast and phase gradient contrast. Such combination images would be
useful with the phase gradient components of the image identifying small
defect regions. The combination images show features involving spatially
abrupt phase modulation like edges and small point defects as well as
large areas of shifted phase.
Additionally, the introduction of phase contrast to phase gradient contrast
images separates a uniform background from a phase shifted area, providing
contrast between the two. Such contrast is particularly useful in
delineating, for example, small biological objects such as single cells
including bacteria for the purpose of automated counting.
A special problem with the earlier methods for phase gradient contrast
microscope imaging found in U.S. Pat. Nos. 4,721,362 and 4,921,333 can
result from fringes produced by the interference of specularly reflecting
light surface so that the specularly reflected light from a phase plate
and the beam are transmitted back through the plate. The result is an
obscuration of phase gradient detail. This can be avoided by increasing
the tilt of the phase plate with respect to the beam back propagating into
the microscope objective until the specularly reflected light does not
enter the objective pupil. This method, though effective in removing
fringes, has disadvantages. First, a compensating tilt in the microscope
axis is needed to bring the full field simultaneously in focus. Provision
for such tilting is not made in normal microscope design. Second, use of
large tilts generally make it necessary to use longer working distance
objectives of lower numerical aperture and thus lower resolving power.
Another problem arises because the phase gradient methods of the
aforementioned patents involve only the transmitted beam. Although the
effects of surface relief, on transmission, show up in such images, these
cannot be separated from phase modulating effects within the bulk of the
plate.
Accordingly, it is an object of the present invention to provide a method
and apparatus for producing an interference generated phase contrast image
of a phase shift photomask and other phase objects in which relative phase
variation are produced by variations in thickness.
It is another object of the present invention to produce with the same
apparatus an image which shows gradients in phase introduced by
transmission as phase gradient contrast images and phase gradient contrast
images in which coherent artifacts are absent.
It is still another object to produce either combination phase gradient
contrast-interference generated phase contrast images or separate phase
gradient contrast and interference generated phase contrast images.
A further object is to produce phase gradient laser images without fringes
while maintaining the microscope axis parallel to the beam direction and
perpendicular the phase plate surface, and also maintaining the plate
perpendicular to the beam.
SUMMARY OF THE INVENTION
According to the invention, a method of producing an interference
generated, phase contrast image, includes the steps of generating a
polarized beam of collimated coherent light, splitting the beam of
collimated coherent light into a first beam and second beam with a first
beam splitter, splitting the first beam into third beam and fourth beam
with a second beam splitter.
The third beam is directed to a transparent phase object substantially
located on a plane, whereupon a first portion of the third beam is passed
through the transparent phase object into a photorefractive holographic
means, for producing and recording a hologram of the transparent phase
object and for generating a phase conjugate beam of the transparent phase
object from the beam of collimated coherent light.
The phase conjugate beam is split into a first conjugate beam and a second
conjugate beam with the second beam splitter, and the second conjugate
beam is split into a third conjugate beam and a fourth conjugate beam with
the first beam splitter. The third conjugate beam is directed with the
first beam splitter to intercept a ground glass to produce a first spot
viewed from above. A second portion of the third beam is reflected from
the transparent phase object to form a specularly reflected beam, and the
entry of the specularly reflected beam into the second beam splitter is
adjusted with a phase plate adjustment means. The specularly reflected
beam is split into a first and second reflected beam with the second beam
splitter, and the second reflected beam is split into a third reflected
beam and a fourth reflected beam with the first beam splitter. The third
reflected beam is directed with the first beam splitter to intercept the
ground glass to produce a second spot viewed from above. The position of
the second spot is adjusted with the plate adjustment means so that it
falls coincident on the first spot, resulting in the first conjugate beam
and the first reflected beam forming coincident beams.
The now coincident phase conjugate beam passing through the second beam
splitter and specularly reflected beam passing through the second beam
splitter are intercepted with a image forming optical system and a first
image formed by the image forming system is recorded. The relative
distance between the hologram and the transparent phase object is shifted
a fraction of a distance incommensurate with a wave length, whereupon a
second image formed by the image forming means is recorded. The first
image is subtracted from the second image to obtain an interference
generated phase contrast image.
According to another aspect, the step of shifting the relative distance
between the hologram and the transparent phase object includes shifting
the transparent phase object an incremental lateral distance, and the step
of subtracting the first image from the second image results in obtaining
a combination interference generated phase contrast, phase gradient
contrast image.
According to another aspect, prior to the step of intercepting the
coincident beams with an image forming optical system, a lens is
positioned between the first beam splitter and the ground glass, and an
interference pattern of fringes is projected by the lens. The adjusting
means is utilized until the pattern is symmetric about a central node.
Preferably, the first and second beam splitters are 45.degree. uncoated
pellicle beam splitters.
According to another aspect, after the step of generating the polarized
beam of collimated coherent light, the polarization of the beam is rotated
to lie in the plane of the beam splitter.
According to yet another aspect, the thickness of the beam splitter is
adjusted for maximum reflectivity at the wavelength of the laser beam.
Preferably, the phase plate adjustment means includes at least one
mechanical adjuster or piezoelectric adjuster, and the optical imaging
system includes a microscope and a video camera.
Also preferably, recording means includes a frame grabber with pixelated
digital storage of the image and the subtracting means includes a digital
process.
It is further preferred that the hologram is a photorefractive hologram of
a barium titanate crystal in which preferably, the hologram is temporarily
recorded.
According to another embodiment, a method of producing a phase gradient
image, may include the steps of generating a beam of collimated coherent
light, splitting the beam of collimated coherent light into a first beam
and second beam with a first beam splitter, splitting the first beam into
third beam and fourth beam with a second beam splitter.
The third beam is directed to a transparent phase object substantially
located on a plane, whereupon a first portion of the third beam is passed
through the transparent phase object into a photorefractive holographic
means, for producing and recording a hologram of the transparent phase
object and for generating a phase conjugate beam of the transparent phase
object from the beam of collimated coherent light. The phase conjugate
beam is split into a first conjugate beam and a second conjugate beam with
the second beam splitter, and the second conjugate beam is split into a
third conjugate beam and a fourth conjugate beam with the first beam
splitter. The third conjugate beam is directed with the first beam
splitter to intercept a ground glass to produce a first spot viewed from
above. A second portion of the third beam is reflected from the
transparent phase object to form a specularly reflected beam, and the
entry of the specularly reflected beam into the second beam splitter is
adjusted with an adjustment means. The specularly reflected beam is split
into first and second reflected beams with the second beam splitter, and
the second reflected beam is split into a third reflected beam and a
fourth reflected beam with the first beam splitter.
The third reflected beam is directed with the first beam splitter to
intercept the ground glass to produce a second spot viewed from above. The
position of the second spot is adjusted with the adjustment means so that
it falls coincident on the first spot, resulting in the first conjugate
beam and the first reflected beam forming coincident beams. The now
coincident phase conjugate beam passing through the second beam splitter
and the specularly reflected beam passing through the second beam splitter
are intercepted with an image forming optical system and a first image
formed by the image forming system is recorded. The transparent phase
object is shifted an incremental lateral distance, whereupon a second
image formed by the image forming means is recorded. The first image is
subtracted from the second image to obtain phase gradient contrast image.
According to yet another preferred embodiment, a method of producing a
combination interference generated phase contrast, phase gradient contrast
image, includes the steps of generating a beam of collimated coherent
light, splitting the beam of collimated coherent light into a first beam
and second beam with a first beam splitter, splitting the first beam into
third beam and fourth beam with a second beam splitter. The third beam is
directed to a transparent phase object substantially located on a plane,
whereupon a first portion the third beam is passed through the transparent
phase object into a photorefractive holographic means, for producing and
recording a hologram of the transparent phase object and for generating a
phase conjugate beam of the transparent phase object from the beam of
collimated coherent light.
The phase conjugate beam is split into a first conjugate beam and a second
conjugate beam with the second beam splitter, and the second conjugate
beam is split into a third conjugate beam and a fourth conjugate beam with
the first beam splitter. The third conjugate beam is directed with the
first beam splitter to intercept a ground glass to produce a first spot
viewed from above. A second portion of the third beam is reflected from
the transparent phase object to form a specularly reflected beam, and the
entry of the specularly reflected beam into the second beam splitter is
adjusted with an adjustment means. The specularly reflected beam is split
into a first and second reflected beam with the second beam splitter, and
the second reflected beam is split into a third reflected beam and a
fourth reflected beam with the first beam splitter. The third reflected
beam is directed with the first beam splitter to intercept the ground
glass to produce a second spot viewed from above. The position of the
second spot is adjusted with the adjustment means so that it falls
coincident on the first spot, resulting in the first conjugate beam and
the first reflected beam forming coincident beams. The now coincident
phase conjugate beam passing through the second beam splitter and the
specularly reflected beam passing through the second beam splitter are
intercepted with a image forming optical system and a first image formed
by the image forming system is recorded.
The relative distance between the hologram and the transparent phase object
is shifted a fraction of a distance incommensurate with a wave length, and
the transparent phase object is shifted an incremental lateral distance,
whereupon a second image formed by the image forming means is recorded.
The first image is subtracted from the second image to obtain a
combination interference generated phase contrast, phase gradient contrast
image.
According to a further embodiment, a method of producing a combination
interference generated phase contrast, phase gradient contrast image,
includes the steps of generating a beam of collimated coherent light,
directing the beam through a transparent phase object, passing a first
portion of the beam through the transparent phase object into a
photorefractive holographic means, for producing and recording a hologram
of the transparent phase object and for generating a phase conjugate beam
of the transparent phase object from the beam of collimated coherent
light, reflecting a second portion of the beam from the transparent phase
object to form a specularly reflected beam, adjusting the specularly
reflected beam and the phase conjugate beam to form coincident beams,
forming and recording an image of the coincident beams, shifting the
transparent plate object laterally and axially to form a modified beam,
forming and recording a second image of the modified beam, and subtracting
the first image from the second image to obtain an interference generated
phase contrast, phase gradient contrast image.
According to yet another embodiment, a method of producing a combination
interference generated phase contrast, phase gradient contrast image,
includes the steps of forming and recording a first image of a beam of
collimated coherent light projected through a transparent phase object,
shifting the transparent phase object in two orthogonal directions, i.e.,
in vertical and lateral directions, forming and recording a second image
of a beam of collimated coherent light projected through the shifted
transparent phase object, and subtracting the first image from the second
image to obtain an interference generated phase contrast, phase gradient
contrast image.
The invention also includes a microscope for phase imaging of a transparent
phase object including a source for a beam of collimated coherent light, a
first beam splitter receiving and splitting the light beam into first and
second beams, and a second beam splitter receiving and splitting the
second beam into third and fourth beams. A stage is provided for
supporting the transparent phase object, and the third beam enters the
transparent phase object at the stage. A first portion of the third beam
is reflected by the transparent phase object to form a specularly
reflected beam. The microscope also includes means for adjusting the
angular tilt of the stage, means for shifting the position of the stage in
at least two orthogonal directions, i.e., in vertical and lateral
directions, and a photorefractive hologram recording crystal receiving a
second portion the third beam and forming a photorefractive hologram, the
crystal generating a phase conjugate beam. A ground glass plate forms
spots from the phase conjugate beam and the specularly reflected beam. The
microscope includes an objective for receiving an image of the beam, and a
processor for recording and comparing at least two images.
The above and other objects, features and advantages of the present
invention will become apparent from the following description and the
appended claims taken in conjunction with accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a two beam splitter arrangement of the components of the
preferred embodiment.
FIG. 2 is a photograph of a video display showing a phase shift photomask
in interference generated phase contrast.
FIG. 3 is a photograph of the video display of a phase shift photomask in
phase gradient contrast.
FIG. 4 is a photograph of the video display of a phase shift photomask in
combined interference generated phase contrast and phase gradient
contrast.
FIG. 5 is a photograph of video display of an exfoliated cheek cell in
combined interference generated phase contrast, phase gradient contrast.
FIG. 6 shows another embodiment of the invention having a single beam
splitter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiment is an inspection microscope for visualizing in
interference generated phase contrast, or in phase gradient contrast, or
in combination interference generated phase contrast, phase gradient
contrast, phase shifting structure and defects in a transparent phase
object, such as phase shifting masks. The invention is also suitable for
imaging a variety of transparent biological applications such as unstained
tissue sections and individual cells, as well as other phase objects. The
output images are in digital form; they are thus useful not only for
display but also for quantitative measurement of the imaged quantities.
With reference to FIG. 1, a holographic microscope system for phase imaging
is shown generally as 10. A light source such as laser 12 may produce a
polarized collimated laser output beam 14. Laser beam 14 is split into two
beams, first beam 16 and second beam 18, by a first beam splitter 20.
After passing through first beam splitter 20, first beam 16 passes through
to a second beam splitter 22, whereupon first beam 16 is split into a
third beam 24 and a fourth beam 26. Second beam 18 and fourth beam 26 are
not used. Preferably, first beam splitter 20 and second beam splitter 22
are 45.degree. uncoated pellicle beam splitters. A quarter waveplate 28 is
provided to rotate the polarization of beam 14 for maximum reflection off
uncoated pellicle beam splitter 22.
Third beam 24 is reflected downwardly by second beam splitter 22 through a
transparent phase object 30, such as a phase shift reticle mask plate.
Transparent phase object 30 is mounted on a stage 32, which allows the
position of the transparent phase object to be shifted in two orthogonal
directions. A first portion 24A of third beam 24 exits from transparent
phase object 30 into a photorefractive hologram recording crystal 34 where
it forms photorefractive hologram 36 after a relatively short formation
time. Preferably, crystal 34 is a single crystal of barium titanate with a
crystal axis specifically oriented with respect to the laser beam
polarization and direction of incidence, as known in the art. A second
portion 24B of third beam 24 is specularly reflected by transparent phase
object 30 to form a specularly reflected beam 38.
Hologram 36 generates a phase conjugate beam 40. The phase conjugate beam
travels in the opposite direction of third beam 24 and back through
transparent phase object 30, where the phase distortions introduced by
transparent phase object 30 are removed from phase conjugate beam 40. More
specifically, the fronts of the constant phase of this retropropagating
phase conjugate beam 40 are identical in form to those of the third beam
24 passing into the plate. However, with the direction of propagation
reversed, the phase effects are corrected. Phase conjugate beam 40 enters
second beam splitter 22 and is split into a first conjugate beam 42 and a
second conjugate beam 44. First conjugate beam 42 enters an objective 62
of a microscope 64. Second conjugate beam 44 enters first beam splitter 20
and is split into a third conjugate beam 46 and a fourth conjugate beam
48. Fourth conjugate beam 48 is passed back into laser 12. Third conjugate
beam 46 is reflected by first beam splitter 20 to a ground glass plate 50
and forms a stationary spot 52 viewed from above. Specularly reflected
beam 38 is similarly reflected by second and first beam splitters 22, 20
to ground glass plate 50 to form a spot 54, again as viewed from above.
Coarse mechanical adjusters 56 and fine piezoelectric actuators 58 are
provided to tilt stage 32 and/or transparent phase object 30 in any of all
of three dimensions to adjust the direction of specularly reflected beam
38. Adjustment of beam 38 such that spot 54 is coincident with stationary
spot 52 produces coincident beams 60. The same fine piezoelectric
actuators 58 can be used to vertically displace stage 32 in the direction
of coincident beams 60. The transparent phase object 30 can also be
laterally displaced a small distance in a specified direction in a short
amount of time by fine piezoelectric actuators 59.
When third conjugate beam 46 and specularly reflected beam 38 are aligned
so as to be coincident, the interference between the fronts of constant
phase from third conjugate beam 46 and specularly reflected beam 38
produce intensity patterns. The intensity patterns are imaged with a
microscope objective. Specifically, coincident beams 60 enter objective 62
of microscope 64 to form an aerial image 66. If the beams are correctly
aligned, and the variation in phase resulting from the reflection are
within one wavelength, image 66 is an interference generated phase
contrast image resulting from the interference of specularly reflected
beam 38 and phase conjugate beam 40. A relay lens 68 is used to project
aerial image 66 onto a video sensing plate 70 of a video camera 72. The
video output of camera 72 is transmitted to an image processor 74, where
the output may be digitized and stored, for example, in image storage
planes 76, 78. A computer 84 controls frame grabbing accession by storage
planes 76 and 78. Computer 84 creates and transmits a synchronized and
delayed signal to electronic drivers (not shown) for piezoelectric
actuators 58 and 59. Images from processor 84 are displayed on video
monitor 86.
The interference generated phase contrast image is, however, typically in
poor contrast and characterized by coherent artifacts and internal
produced interference fringes. A high contrast artifact free image is
obtained by an electronic processing method including the steps of 1)
electronically capturing this initial image and digitizing it and storing
it in a pixelated memory plane, and 2) shifting the mask a specified
distance in the beam direction, i.e., an axial shift. This produces a
change in the image, for example, a bright region relative to the
background resulting from constructive interference becomes a dark region
as the result of destructive interference. This new image is similarly
captured, digitized and stored. Processing then subtracts the fist image
from the second and converts the difference to image form; this image is
then displayed on video monitor 86.
Adjustment of the specularly reflected beam 38 to coincide with third
conjugate beam 46 by superimposition of spots 52 and 54 results in general
alignment of the beams. A more exact alignment is, however, generally
required and is obtained by positioning a low power microscope objective
80 between first beam splitter 20 and ground glass plate 50, the objective
projecting an interference fringe pattern. Coarse mechanical adjusters 56
and fine piezoelectric actuators 58 are used to tilt stage 32 until the
fringe pattern is symmetric about a central node. The fringe pattern may
also be viewed on a ground glass plate 82 placed at the position of the
aerial image.
The holographic microscope system 10 as described can be used for producing
a phase gradient contrast image. As in the interference generated phase
contrast, a first image, resulting from | | |
