U.S. patent application number 16/535901 was filed with the patent office on 2019-12-05 for system, apparatus and method for using birefringent lenses to create holograms from received electromagnetic radiation.
The applicant listed for this patent is CELLOPTIC, INC.. Invention is credited to Gary BROOKER, Nisan SIEGEL.
Application Number | 20190369556 16/535901 |
Document ID | / |
Family ID | 54359493 |
Filed Date | 2019-12-05 |
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United States Patent
Application |
20190369556 |
Kind Code |
A1 |
BROOKER; Gary ; et
al. |
December 5, 2019 |
SYSTEM, APPARATUS AND METHOD FOR USING BIREFRINGENT LENSES TO
CREATE HOLOGRAMS FROM RECEIVED ELECTROMAGNETIC RADIATION
Abstract
The inventors have discovered a method to improve image quality
in holography and, for the first time, utilize lenses made from
birefringent materials to advantageously split an incoming beam of
either coherent or incoherent light into two coincident beams with
different focal lengths that interfere with one another and thus
create holograms free of electro-optical or pixelated devices. This
discovery has many advantages over current methods to create
holograms in which many components, including multiple lenses,
other electro-optical devices, and/or beam paths are necessary to
create holograms. The current invention provides a purely optical
holographic process which has better performance and holographic
simplicity, in addition to being able to miniaturize holographic
processes more than is currently possible in state of the art
holography systems.
Inventors: |
BROOKER; Gary; (Rockville,
MD) ; SIEGEL; Nisan; (Silver Spring, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CELLOPTIC, INC. |
Rockville |
MD |
US |
|
|
Family ID: |
54359493 |
Appl. No.: |
16/535901 |
Filed: |
August 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15308208 |
Nov 1, 2016 |
10423123 |
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PCT/US15/28477 |
Apr 30, 2015 |
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16535901 |
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61987205 |
May 1, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03H 2001/0447 20130101;
G03H 1/0443 20130101; G02B 1/08 20130101; G03H 1/041 20130101; G03H
2001/0452 20130101; G03H 2001/005 20130101; G03H 2222/31 20130101;
G03H 2223/20 20130101; G03H 2223/17 20130101; G02B 5/3083 20130101;
G02B 5/3016 20130101; G02B 27/283 20130101; G03H 1/06 20130101;
G03H 2001/0216 20130101 |
International
Class: |
G03H 1/04 20060101
G03H001/04; G02B 1/08 20060101 G02B001/08; G02B 5/30 20060101
G02B005/30; G02B 27/28 20060101 G02B027/28; G03H 1/06 20060101
G03H001/06 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with U.S. government support under
grant R44CA192299 awarded by the National Cancer Institute (NCI).
The U.S. government has certain rights in the invention.
Claims
1. An optical apparatus, comprising: a plurality of lenses
including at least one lens having non-quantized anisotropic
electromagnetic properties, wherein the plurality of lenses are
configured to: receive electromagnetic radiation from an object,
wherein the electro magnetic radiation is incoherent light;
transform, by refraction using the at least one lens having
non-quantized anisotropic electromagnetic properties, the received
electromagnetic radiation to generate two or more differentially
modulated electromagnetic waves propagating in a common path; and
provide for the differentially modulated electromagnetic waves to
create electromagnetic interference.
2. The optical apparatus according to claim 1, wherein the at least
one lens having non-quantized anisotropic electromagnetic
properties includes a birefringent lens.
3. The optical apparatus according to claim 2, wherein another lens
is configured to modify the focal length of each of the
differentially modulated electromagnetic waves exiting the
birefringent lens.
4. The optical apparatus according to claim 3, wherein a spacing
factor of the differentially modulated electromagnetic waves is
changeable according to a focal length of said another lens.
5. The optical apparatus according to claim 4, wherein said another
lens is a glass lens.
6. The optical apparatus according to claim 2, wherein the
birefringent lens comprises alpha or beta barium borate
materials.
7. The optical apparatus according to claim 2, wherein the
birefringent lens comprises liquid crystal material encased in flat
or positively or negatively curved birefringent materials.
8. The optical apparatus according to claim 2, wherein the
birefringent lens comprises liquid crystal material encased in flat
or positively or negatively curved non-birefringent materials.
9. The optical apparatus according to claim 1, wherein the
electromagnetic interference forms a hologram representing the
object.
10. The optical apparatus according to claim 9, wherein the
hologram is a FINCH hologram.
11. The optical apparatus according to claim 9, wherein the
hologram is any of a Fresnel hologram, a Fourier hologram, a FINCH
hologram, or an off-axis hologram.
12. The optical apparatus according to claim 1, wherein the
received electromagnetic radiation is from a microscope.
13. The optical apparatus according to claim 1, further comprising
a camera configured to record the interference.
14. The optical apparatus according to claim 1, the apparatus being
further configured to use the electromagnetic interference as an
excitation pattern in scanning holography.
15. The optical apparatus according to claim 1, the apparatus being
further configured to use the electromagnetic interference in an
excitation source in a Structured Illumination (SIM) imaging
system.
16. The optical apparatus according to claim 1, wherein the optical
apparatus is contained within a microscope objective lens.
17. The optical apparatus according to claim 1, further comprising
at least one compensating optic to minimize phase-delay between the
differentially modulated electromagnetic waves relative to the
coherence length of the electromagnetic radiation.
18. The optical apparatus according to claim 1, wherein the
compensating optic is a birefringent compensating optic.
19. A method comprising: receiving electromagnetic radiation from
an object, wherein the electromagnetic radiation is incoherent
light; transforming, by refraction using at least one lens having
non-quantized anisotropic electromagnetic properties, the received
electromagnetic radiation to generate two or more differentially
modulated electromagnetic waves propagating in a common path; and
providing for the differentially modulated electromagnetic waves to
create electromagnetic interference.
20. The method according to claim 19, wherein the at least one lens
having non-quantized anisotropic electromagnetic properties
includes a birefringent lens.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to and claims the benefit of
priority to U.S. Provisional Application Ser. No. 61/987,205, filed
May 1, 2014, the contents of which are, in entirety, hereby
incorporated herein by reference.
BACKGROUND
1. Field
[0003] This invention relates to an apparatus for collecting
Fresnel Incoherent Correlation Holography (FINCH) or other
holography images by use of a birefringent lens or optical element
to alter the phase properties of the received light or other
electromagnetic radiation. The invention also relates to systems
and methods for collecting these holography images.
2. Description of the Related Art
[0004] Holograms are records of the interference patterns created
by two or more light or other radiation waves. In order for the
waves to interfere they must have different phase properties. In
current holography methods the waves that are to be interfered are
passed through different optical paths that impart different phase
properties on each wave. In one class of methods of single-path
holography, the waves are commonly given different phase properties
by being passed through or reflected off of digitized phase
patterns displayed on a spatial light modulator (SLM) or other
optical element. In another class of methods for self-interference
holography, the waves originate from a single wave and are split by
a beam splitter, then reflected off differing mirrors before being
recombined in the last part of the beam path and brought to
interfere. All of these methods produce holograms that may suffer
from significant defects due to slight mismatches in optical path
length, quantization errors or undesired diffraction effects of the
SLM or other optical element. An apparatus, system or method that
allowed all the waves to pass in the same optical path while
receiving different phase properties, without being subject to
unnecessary reflections or quantization errors or undesired
diffraction effects, would be a material advance in the field of
holography.
SUMMARY OF THE INVENTION
[0005] Accordingly, one object of the current invention is to
provide an apparatus with non-quantized anisotropic electromagnetic
properties used to create electromagnetic interference from
received electromagnetic radiation, and a method for its use. The
anisotropic electromagnetic properties may derive from one or more
anisotropic components such as optically birefringent crystalline
or liquid crystalline materials of any kind active at any
wavelength, and may be further adjusted by combination with other
materials. The received electromagnetic radiation may be from
sources such as x-rays, black body radiation, or light of any
wavelength from any source, coherent or incoherent. In the
apparatus, the received electromagnetic radiation is then
transformed by refraction into two or more differentially modulated
waves propagating in a common path, and the modulated
electromagnetic waves create the electromagnetic interference,
which can take the form of a Fresnel, Fourier, Fresnel Incoherent
Correlation Holography (FINCH), off-axis or other hologram. The
interference is recorded by a recording device, and information
about the source of the received radiation can be obtained from the
interference.
[0006] Another object of the current invention is to provide an
apparatus with non-quantized anisotropic electromagnetic properties
used to create electromagnetic interference from received
electromagnetic radiation, and a method for its use. The
anisotropic electromagnetic properties may derive from one or more
anisotropic components such as optically birefringent crystalline
or liquid crystalline materials of any kind active at any
wavelength, and may be further adjusted by combination with other
materials. The received electromagnetic radiation may be from
sources such as x-rays, black body radiation, or light of any
wavelength from any source, coherent or incoherent. In the
apparatus, the received electromagnetic radiation is then
transformed by refraction into two or more differentially modulated
waves propagating in a common path with programmed differences
between the modulations. The modulated electromagnetic waves create
the electromagnetic interference, which can take the form of a
Fresnel, Fourier, Fresnel incoherent Correlation Holography
(FINCH), off-axis or other hologram. The interference is then used
to deliver the programmed information to a subsequent device or
object such as a microscope sample or optical recording medium.
[0007] Another object of this invention is to provide the
advantages listed above in configurations that do not require
external power sources, allowing interference waves (and holograms)
to be obtained in a portable manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1. A diagram depicting conventional imaging lens
wherein the received electromagnetic (EM) radiation from the object
is focused to only one plane of focus.
[0009] FIG. 2. A diagram depicting three configurations for Fresnel
Incoherent Correlation Holography (FINCH) imaging using a spatial
light modulator (SLM) to produce the reference and sample
beams.
[0010] FIG. 3. Schematic of a FINCH fluorescence microscope using
Thin Liquid Crystal Gradient Refractive Index (TLCGRIN) lens.
[0011] FIG. 4. A birefringent lens with two focal lengths f.sub.1
and f.sub.2.
[0012] FIG. 5. Generalized scheme for creating a FINCH
hologram.
[0013] FIG. 6: The differing focal lengths of a birefringent lens
resulting from the differing refractive indices in the transverse
plane of the lens.
[0014] FIG. 7. Wavelength dependent shift in location of optimal
hologram planes.
[0015] FIG. 8. Point hologram raw and processed images captured
from a laser as the EM radiation source, using a FINCH system as in
FIG. 3.
[0016] FIG. 9. Point hologram raw and processed images captured
from a laser as the EM radiation source, using a FINCH system
incorporating a calcite BRL.
[0017] FIG. 10. Schematic of two birefringent lenses used in
tandem.
[0018] FIG. 11. Schematic of a birefringent lens used in
conjunction with a flat birefringent plate.
[0019] FIG. 12. Schematic of a birefringent plate or block used to
create two focal planes from a single spherical glass lens.
DETAILED DESCRIPTION OF THE DRAWINGS
[0020] In classical optical imaging, a beam of light is emitted or
reflected from an object, and is then collected by a lens. In the
simplest case, the light beam is focused by this lens to create an
image at a focal plane. The image is two-dimensional as shown in
FIG. 1 depicting a lens 100 with focal length 105 of f, creating at
a focal plane 106 an image 102 of an object 101, and it is not
possible to discern three-dimensional (3D) information about the
object 101 above or below the plane of focus. Any information above
or below the plane of the object is not translated to the plane of
focus of the lens and is lost.
[0021] While other lenses can be added to the system to improve the
image quality or change the magnification, the 3D information is
still lost. Holographic methods enable the imaging of the 3D
information in a scene. A number of holographic methods exist in
which a sample is illuminated by a laser such that interference of
light reflected or emitted from a sample in combination with a
reference beam creates holograms which fully describe the 3D
properties of an object [Nature 161, 777-778 (1948)]. In classical
holography a coherent source is split into a sample and reference
beam, which then interfere with one another to create a hologram.
While this method cannot be used to measure incoherent light
emissions, such as from a fluorescent sample, scanning holography
has been proposed in which an interference pattern is scanned
across a sample to excite fluorescence and then correlated with a
sample beam to create a hologram [Opt. Lett. 22, 1506-1508 (1997)].
That method is quite complex, and as a multibeam process it suffers
from stringent alignment requirements and is sensitive to
environmental instability because of the need to prevent any
vibration in the system.
[0022] Another method for incoherent holography invented by one of
the present inventors in 2006 [U.S. Pat. No. 8,542,421; Opt. Lett.
32, 912-914 (2007)] is dubbed FINCH for Fresnel Incoherent
Correlation Holography. FINCH creates holograms from an object
emitting incoherent light in a single beam system by
self-interference from two spherical waves originating from the
object. Three example configurations of FINCH using a spatial light
modulator (SLM) are shown in FIG. 2 [adapted from Opt. Exp. 19,
26249-26268 (2011)]. Described in FIG. 2 is 200 FINCH with two
diffractive lenses displayed on the SLM 204, in which one (f.sub.d)
is positive and the other (f.sub.2) is negative. The diffractive
lenses focus the light received from the object 101 through an
intermediate lens 203 into a hologram recorded by a CCD camera 206
at a distance 205 (z.sub.h) away from the SLM. Described in 201 is
FINCH with two diffractive lenses on the SLM 204, in which both
lenses are positive (f.sub.d is the shorter focal length, f.sub.2
the longer). The remainder of this type of FINCH is similar to that
in 200. In 202 is a practical setup that emulates the setup of 201,
with one positive diffractive lens (f.sub.d) displayed on the SLM
204 and one positive glass lens 207 (f.sub.2) placed near to the
SLM. FIG. 2 was adapted from Opt. Exp. 19, 26249-26268 (2011). One
skilled in the art will understand that in the previous paragraph
and throughout this document, the SLMs or other elements that
replace the SLMs are not limited to displaying only one or two
lenses, and that they may display three or more lenses or other
phase patterns as desired for advantageous application to the
holographic process.
[0023] FINCH has shown potential for fluorescence microscopy (J.
Rosen and G. Brooker, "Non-scanning motionless fluorescence
three-dimensional holographic microscopy" Nat. Photonics 2, 190-195
(2008)), and much work has been done to perfect the technique into
a useful high resolution 3D imaging method. The concept that a 3D
image could be obtained from incoherent sources by a holographic
process, without lasers, scanning or axial translation or the need
to capture images at multiple planes of focus to create a 3D image
is appealing. The field has now advanced as a result of additional
work from our group (G. Brooker, N. Siegel, V. Wang, and J. Rosen,
"Optimal resolution in Fresnel incoherent correlation holographic
fluorescence microscopy," Opt. Express 19, 5047-5062 (2011); J.
Rosen, N. Siegel, and G. Brooker, "Theoretical and experimental
demonstration of resolution beyond the Rayleigh limit by FINCH
fluorescence microscopic imaging." Opt. Express 19, 26249-26268
(2011); B. Katz, J. Rosen, R. Kelner, and G. Brooker, "Enhanced
resolution and throughput of Fresnel incoherent correlation
holography (FINCH) using dual diffractive lenses on a spatial light
modulator (SLM)," Opt. Express 20, 9109-9121 (2012); N. Siegel, J.
Rosen, and G. Brooker. "Reconstruction of objects above and below
the objective focal plane with dimensional fidelity by FINCH
fluorescence microscopy," Opt. Express 20, 19822-19835 (2012)) and
other laboratories (P. Bouchal. J. Kapitan, R. Chmelik, and Z.
Buuchal, "Point spread function and two-point resolution in Fresnel
incoherent correlation holography." Opt. Express 19, 15603-15620
(2011); X. Lai, Y. Zhao, X. Lv, Z. Zhou, and S. Zeng, "Fluorescence
holography with improved signal-to-noise ratio by near image plane
recording," Opt. Lett. 37, 2445-2447 (2012); O. Bouchal and Z.
Bouchal, "Wide-field common-path incoherent correlation microscopy
with a perfect overlapping of interfering beams," J. Europ. Opt.
Soc.--Rap. Pub. 8, 13011 (2013)) including the demonstration that
the FINCH optical system is inherently super-resolving (J. Rosen,
N. Siegel, and G. Brooker, "Theoretical and experimental
demonstration of resolution beyond the Rayleigh limit by FINCH
fluorescence microscopic imaging," Opt. Express 19, 26249-26268
(2011); B. Katz, J. Rosen, R. Kelner, and G. Brooker, "Enhanced
resolution and throughput of Fresnel incoherent correlation
holography (FINCH) using dual diffractive lenses on a spatial light
modulator (SLM)," Opt. Express 20, 9109-9121 (2012); N. Siegel, J.
Rosen, and G. Brooker, "Reconstruction of objects above and below
the objective focal plane with dimensional fidelity by FINCH
fluorescence microscopy," Opt. Express 20, 19822-19835 (2012))
Recently it has been shown that the reason for this is that FINCH
overcomes the Lagrange invariant (X. Lai, S. Zeng, X. Lv, J. Yuan,
and L. Fu, "Violation of the Lagrange invariant in an optical
imaging system," Opt. Lett. 38, 1896-1898 (2013) [10]). More
recently FINCH holograms have been created using electrically
modulated transmission liquid crystal optics (G. Brooker, N.
Siegel, Rosen, N. Hashimoto, Makato Kurihara and A. Tanabe,
"In-line FINCH super resolution digital holographic fluorescence
microscopy using a high efficiency transmission liquid crystal GRIN
lens," Opt. Lett. 38(24), 5264-5267 (2013). Additionally, the
inclusion of a Nipkow disk has been used to create confocal FINCH
images, (N. Siegel and G. Brooker, "Improved axial resolution of
FINCH fluorescence microscopy when combined with spinning disk
confocal microscopy," Optics Express Vol. 22, pp 22298-22307 (2014)
and U.S. patent application 62/023,958). The FINCH holographic
process is the subject of U.S. Pat. Nos. 8,009,340; 8,179,578;
8,405,890; 8,542,421 and Japanese patent 8,542,421.
[0024] While FINCH is a considerable advance in incoherent
holography, the SLM method of creating the two interfering beams
still requires two different lenses and those lenses require
perfect alignment. The SLM method used involves displaying one or
more different lens patterns on a spatial light modulator (SLM)
[Opt. Lett. 32, 912 (2007); Opt. Exp. 19, 5047 (2011)] but is prone
to low hologram quality due to lens sampling and to low efficiency
due to higher-order diffracted images. These issues lead to poor
interference, high background and low resolution due to the limited
number of pixels and bit depth of the SLM. Furthermore, since SLM's
are reflective, the optical arrangement requires that the SLM be
positioned on an angle from the optical axis of the imaging system
or arranged on a beam splitter to circumvent mounting it on an
angle. However, angled incidence of the original light beam makes
calibration of the SLM difficult for multiple focal lengths, and
use of a beam splitter significantly reduces the light budget of
the optical system [Opt. Exp. 19, 5047 (2011)].
[0025] FIG. 3 shows a detailed schematic of a more recent method,
which has been to use a glass lens in conjunction with a liquid
crystal Fresnel lens or Gradient Refractive Index (GRIN or TLCGRIN)
lens in a totally transmissive arrangement, reported in Opt. Lett.
38, 5264-5267 (2013). On the left side of the FIG. 300 is depicted
the detailed ray diagram for a FINCH hologram of a point. The light
leaves the object 101, traveling a distance 306 to be collected by
the objective lens 301. The collimated light leaving 301 propagates
the distance 307 to the first of two relay lenses, 302. The light
travels the distance 308 to the second relay lens 303 and then a
further distance 309 to the GRIN assembly 304. The GRIN assembly
304 with two effective focal lengths 312 and 313 creates the two
waves that propagate to the distances 310 and 311, while the
hologram 305 is located at the plane removed from the GRIN assembly
304 by the hologram distance 205. On the right side of the FIG. 301
is depicted the detailed arrangement of the components in the
referenced microscope system. All optics are centered on the
optical propagation axis 314. The dichroic beamsplitter and
emission filter 315 and 316 are necessary for fluorescence
microscopy, while the polarizing beamsplitting cube 317 is used to
polarize the received light at an angle of 45 degrees to the active
axis of the GRIN assembly. The rejected polarization component form
this polarizer is sent to the camera 318 that records a standard
image. The GRIN assembly 304 contains a glass lens 319, and active
GRIN 320 and an inactive GRIN 321. The glass lens focuses all the
light passing through it, while the active GRIN adds additional
focal length to the light that passes parallel to its axis, and the
inactive GRIN serves to compensate for side effects of the light
passing the active GRIN. Thus the two focal lengths 312 and 313 are
created. Distances are corrected to account for the optical path
through the glass of the BS cubes. The final two optics arc the
phase shifting waveplate 322 and the output polarizer 323, which
modulate the overall phase of the hologram and increase
interference efficiency, respectively. The hologram plane 305 is
between the two focal lengths 312 and 313, and a camera 324 is used
to record the hologram. FIG. 3 is adapted from Optics Letters 38,
5264-5267 (2013).
[0026] While the TLCGRIN method is an improvement over the SLM, it
still is limited by the reduced imaging quality of a Fresnel lens
or the limited number of graded regions used to create a liquid
crystal GRIN lens. Furthermore it is challenging to make GRIN
lenses with sufficient aperture and shortness of focal length for
high quality imaging and compactness of a holographic system. In
this GRIN lens system example, the GRIN lens had a 5000 mm focal
length and the glass lens a 300 mm focal length. Furthermore both
the SLM and GRIN lens systems require electrical control of the
devices in addition to compensating lenses to control for
dispersion in the liquid crystal material. This combination of
focal lengths creates a spacing factor between the two focal
lengths of less than 3%, which reduces the axial depth of 3D
objects that can be reliably imaged by the holographic system [Opt.
Exp, 20, 9109 (2012)].
[0027] To address this, the inventors have discovered a unique use
for spherical lenses that can be constructed of birefringent
materials. FIG. 4 shows an example of a lens 400 made from a
birefringent material. Birefringent substances have two distinct
polarization sensitive refractive indices and thus lenses made from
such materials always have two focal lengths f.sub.1 401 and
f.sub.2 402 and produce blurry images when randomly polarized light
is passed through them, since a single sharp plane of focus is not
possible unless the image is viewed through a polarizer. When
randomly polarized light is passed through the lenses, a single
sharp focus cannot be obtained since the multiple refractive
indices of the material cause the lens to display a different focus
for light of p or s polarization, creating two images at distances
403 and 404. Thus these lenses yield a doubled or blurry image 405,
which is generally undesirable in standard optical applications.
For this reason birefringent materials are not typically used to
make optical lenses because of this ordinarily undesirable
property; evidence of this is that birefringent lenses are not
readily commercially available from optical supply houses.
Currently birefringent lenses must be custom made and there are few
reports in the literature of their construction [Proc. of SPIE Vol.
6018, 601812 (2005); Meas. Sci. Technol., 17, 1367 (2006); Optik
118, 335-339 (2007)]. However since birefringent materials such as
calcite, barium borate, lithium niobate and quartz can be readily
worked just like glass, it is possible to readily prepare lenses of
birefringent materials to any lens specification, given a rationale
for making them.
[0028] The inventors have discovered that the simultaneous usage of
the multiple focal lengths of birefringent lenses can be very
advantageous to create very high quality holograms that can reveal
the three dimensional information of objects. The current invention
can be applied to many forms of holography including FINCH and
operates in an electrically independent manner with optical
characteristics that yield unmatched holographic image quality
which exceeds the performance of standard imaging methods.
Furthermore, in addition to holographic imaging applications, the
current invention also enhances and simplifies other forms and uses
of holography and interferometry. For an example, birefringent
lenses were already found in nature long ago in the eye of the
trilobrite, a creature that lived in the sea 450 million years ago.
These eye lenses were called schizochroal and made of birefringent
calcite. One might speculate that lenses made of calcite became
extinct during evolution because of their undesirable optical
properties. Calcite is an optically clear material with two
different refractive indices depending upon the plane of
polarization. Even though it is not a good material to make
standard lenses, its polarizing properties are widely exploited to
make polarizers and polarization sensitive devices such as
Glan-Taylor prisms. Calcite is used because it is optically clear
and its crystal structure can efficiently pass a single axis of
linear polarization. However if lenses are made of calcite, because
of the different refractive indices at the two planes of
polarization, two distinct polarization sensitive focal lengths of
those lenses are observed (see
https:/community.dur.ac.uk/g.d.love/downloadable/china05.pdf).
However mixed polarization light, which is the common form of light
in the environment, a blurred image would result if lenses were
made of birefringent materials. While the trilobrite used calcite
for its lens material, one might wonder if its vision was blurred
or if it could see the two focal planes because its photoreceptors
were cross polarized.
[0029] However, an imaging method that required different aligned
copies of the same image could benefit greatly from just such a
birefringent lens. Incoherent holography, a class of holography
that includes FINCH and other methods [Opt. Lett. 32, 912 (2007);
Nat. Photonics 2, 190 (2008); Opt. Express 19, 5047 (2011); Opt.
Express 19, 26249 (2011); Opt. Express 20, 19822 (2012); Opt. Lett.
38, 3922 (2013); Opt. Lett. 38, 5264-5267 (2013), and U.S. Pat.
Nos. 8,009,340, 8,179,578 and 8,542,421], is a technique for
creating holograms from the interference of two copies of the same
image, or from any singe EM radiation wave that is split into two
copies, and has been demonstrated using polarization-sensitive
optical elements (PSOEs) such as SLMs and liquid crystal Fresnel
and GRIN lenses. These PSOEs, which are not classical refractive
spherical lenses but which may be diffractive or refractive in
operation, serve to split the image beam into two parts with
differing spherical curvatures. In the further description of the
process, we consider light emanating (by emission or reflection or
any other process) from a single infinitesimally small object
point, which creates a "point hologram" that suffices to describe
the system; extended objects larger than this create holograms that
are simply the sums of the holograms of all the differing points
constituting the extended object. It is common to use a broad,
collimated laser beam as a model source of EM radiation in these
systems, since the image of such a beam is a diffraction-limited
spot as from an infinitesimal point source. This aspect enables the
empirical characterization of the best response of any such
system.
[0030] FIG. 5 shows a schematic of the FINCH process highlighting
the role of the PSOE. The PSOE 501 has two different focal lengths,
of which f.sub.d1 is the shorter and f.sub.2 is the longer. Other
optical elements or groups 500, 502 may be used to make specific
alterations in the overall phase, polarization, aberration
correction or magnification or hologram size of the system, but the
beam separation is solely a result of the use of the PSOE. After
emanating from the object and possibly passing other optical
elements, the light wave is split into two waves, of differing
focal lengths by the PSOE. These waves propagate through the same
space in the same direction, and are termed the signal and
reference or f.sub.d1 and f.sub.d2 waves. Currently this is
accomplished in one of two ways: [0031] 1. By polarization: the
received wave hitting the PSOE is polarized at 45 degrees to the
polarization axis of the PSOE. Thus half of the wave with
polarization component projected parallel to the PSOE polarization
axis is given the curvature encoded in the PSOE, while the half of
the wave with polarization component projected perpendicular to the
PSOE polarization axis maintains its original curvature. The result
is the f.sub.d1 and f.sub.d2 waves. [0032] 2. By sampling of the
PSOE: The PSOE is divided into more than one portion, each of which
is encoded with differing spherical phases. The portions may be
interspersed with each other and not contiguous. The received wave
hitting the PSOE is polarized entirely parallel to the PSOE
polarization axis, and the wave emerging from the PSOE has
different portions with differing curvatures added corresponding to
the curvatures encoded in the different portions of the PSOE. If
the PSOE has two portions, the two wave portions emerging from the
PSOE are termed f.sub.d1 and f.sub.d2. However the PSOE can have
more than two portions, in which case there are light waves termed
f.sub.d3, etc.
[0033] Current technologies serving as polarization-sensitive PSOEs
to generate the f.sub.d1 and f.sub.d2 waves include digital spatial
light modulators (SLMs), liquid crystal (LC) Fresnel lenses and LC
gradient refractive index (GRIN) lenses. In some configurations
these components are also used in conjunction with classical
lenses, or more than one of the components may be used in
conjunction with each other.
[0034] After propagating from the PSOEs, the two waves interfere
and create the hologram recorded at the detector (z.sub.h) plane.
The detector may be a CCD, CMOS or other camera or image capture
device as well as a point detector or solid-state device such as an
avalanche photodiode. Optionally the waves may pass through a
variable phase shifter and a polarizer. To reconstruct a point or
image and provide the basis to remove bias and the twin image in
holography, the detector captures two or more raw holograms, in
which the phase of one of the beams is set to differ by a
predetermined amount in subsequent holograms, to allow for the
recovery of the complex hologram that fully captures the phase
characteristics of the original EM source [Optics Letters 22(16),
1269-1270 (1997)]. The collection of raw holograms with such
different phase factors is critical to achieving I optimal result
with FINCH and similar holography methods.
[0035] One of the key parameters in this process is the
relationship between the focal lengths f.sub.d1, and f.sub.d2 and
the hologram recording plane at z.sub.h. Holograms may be recorded
at any point after the PSOE, but the optimal hologram quality is
made possible when the two waves obey a condition of maximal
spatial lap. The condition to ensure maximum overlap between the
f.sub.d1, and f.sub.d2 beams is met when the hologram is recorded
at the plane
z h = 2 f d 1 f d 2 ( f d 1 + f d 2 ) . ( 1 ) ##EQU00001##
This relationship may also be expressed as
z.sub.h=(1+s).times.f.sub.d1=(1-s).times.f.sub.d2, (2)
where the spacing factor s obeys the equality:
s = f d 2 - f d 1 f d 2 + f d 1 . ( 3 ) ##EQU00002##
[0036] As s increases (the distance between f.sub.d1 and f.sub.d2
increases), the point hologram at the optimal z.sub.h plane also
increases in size, as described by the following equation:
R.sub.H=s.times.R.sub.0, (4)
where R.sub.H is the aperture radius of the hologram and R.sub.0 is
the aperture radius of the wave at the PSOE or equivalent. This
size increase renders the point hologram more easily resolvable by
recording devices but decreases the peak intensity of the hologram.
There are other factors [Opt. Express 20, 9109 (2012)] that also
establish upper and lower bounds for s. It is very desirable to
have complete control over s over a wide range in order to be able
to optimize the holographic system for all possible variables. The
s factor does not itself change the resolution of the image coded
by the hologram, but does affect the ease with which the hologram
may be recorded; and further, any arrangement used to change s will
affect other image factors such as magnification and depth of
field.
[0037] Each of the three current technologies mentioned above can
serve to create f.sub.d1 and f.sub.d2, but each also bears
significant disadvantages: [0038] 1. SLMs are easily adjustable to
produce different focal length PSOEs at will, in the form of
digitized Fresnel phase patterns, but suffer from low focusing
efficiency to the desired image, as diffraction from the pixilated
digital SLM causes significant light loss into transverse foci of
higher diffraction orders. Additionally, the PSOEs created on SLMs
suffer from significant variability in focal length as a function
of light wavelength (an effect termed chromatic aberration) which
may degrade performance in hologram formation. [0039] 2. LC Fresnel
lenses are polarization sensitive and do not suffer from
higher-order transverse foci, but may display other axial foci and
certainly suffer from significant chromatic aberration. They are
also not adjustable, and offer only a single nominal focal length.
[0040] 3. LC GRIN lenses have focal lengths adjustable as a
function of applied voltage, and less chromatic aberration than
SLMs or LC Fresnel lenses, but have very long focal lengths that
require them to be paired with regular refractive lenses in order
to achieve reasonable overall focal lengths. Even when combined
with refractive lenses, LC GRIN lenses offer limited possibilities
for spacing factor. Finally, currently used LC GRIN lenses arc
quantized approximations of lenses (because of the practical
limitation of the number of differentially refractive zones
possible) and thus impose spatial distributions of light in the
unfocused beams that can cause reduced interference efficiency and
accuracy of focal length calculation.
[0041] There is a pressing need in this field for the introduction
of a device to create the f.sub.d1 and f.sub.d2 beams without the
disadvantages mentioned above, and with increased flexibility in
the spacing factor s. Birefringent materials possess two or more
refractive indices along different propagation directions in the
material, termed the ordinary and extraordinary axes. These axes
have refractive indices denoted n.sub.o, and n.sub.e, respectively.
Since the focal length of a lens is dependent in part on the
refractive index of the material comprising the lens, these
materials could be used to create spherical lenses that possess two
different polarization-dependent focal lengths, each of which
produces a spherical beam and a focal spot of equal quality to
those of a standard glass lens. FIG. 6 shows a schematic of a
birefringent lens (BRL) focusing light of differing polarization to
different focal planes. FIG. 6a 600 shows a cross-section of a BRL,
with the ordinary 602 and extraordinary 603 refractive indices
projected along the x and y Cartesian axes of the lens. FIG. 6b 601
shows the focal lengths f.sub.be 606 and f.sub.bo 607 of the single
birefringent lens (with radii of curvature R.sub.1 604 and R.sub.2
705 for the two surfaces of the lens) for light polarized parallel
to the extraordinary axis and for light polarized parallel to the
ordinary axis of the lens, respectively. The quality of the beams
and the focal spots of the BRL is much improved over those from
diffractive PSOEs mentioned above. Birefringent Refractive Lenses
offer advantages over PSOEs in several aspects of incoherent
hologram generation, including: [0042] 1. Elimination of the noise
and image artifacts due to unwanted diffraction orders of PSOEs or
the quantization error inherent in digital or binary
representations of lenses. [0043] 2. The possibility of correction
of chromatic, spherical and other aberrations by use of corrective
optics including non-birefringent and birefringent optics. [0044]
3. Precise and flexible tailoring of the spacing factor s by choice
of BRL material, curvature and associated optics. [0045] 4.
Simplification of and size reduction of the optical assembly by
removal of electronic and reflective components.
[0046] This invention covers, in part, the use of a birefringent
refractive lens (BRL), alone or in conjunction with other
refractive lenses or other optical elements, to effect the
splitting of the received wave into two orthogonally polarized
waves with differing spherical curvature to create holograms.
Birefringent crystals have differing refractive indices along their
ordinary and extraordinary crystal axes, and by cutting a lens from
such a material in the proper orientation with these two axes
perpendicular to each other and both lying in the plane of the lens
orthogonal to the direction of light propagation through the lens,
a refractive lens with special properties may be created. These
special properties are that the lens focuses light polarized
parallel to one of its polarization axes (for example, the ordinary
axis, also identified here as the x axis in a Cartesian system to a
given focal plane, while the light polarized parallel to the other
axis (the extraordinary or y-axis) is focused to a different focal
plane (see FIG. 6). This may be easily understood by referring to
the thin lens equation:
1 f = ( n - 1 ) ( 1 R 1 - 1 R 2 ) , or f = 1 ( n - 1 ) ( R 1 R 2 R
2 - R 1 ) = R eff ( n - 1 ) , ( 5 a ) 1 f = ( n - 1 ) ( 1 R ) , or
f = R ( n - 1 ) = R eff ( n - 1 ) , ( 5 b ) R eff = { R 1 R 2 R 2 -
R 1 , for a lens with two curved sides R , for a lens with one
curved sides ( 5 c ) ##EQU00003##
with f being the focal length of the lens, n the refractive index
of the lens material, R.sub.1 and R.sub.2 the radii of curvature of
the two sides of the lens, and R.sub.eff is the "effective" total
curvature of the lens. Equation 5b is for the specific case of a
lens with one flat side (plano-concave or plano-convex) and one
curved side with curvature R. As called out in equation 5c,
R.sub.eff for a lens with two curved sides is exactly equivalent to
R of a plano-concave or plano convex lens. Equivalently to using a
solid birefringent crystal, a birefringent liquid crystal material
may be used to create a BRI, when aligned and placed between two
substrates with curvatures R.sub.1 and R.sub.2. Thus a single BRL,
made from birefringent material with n.sub.o and n.sub.e for the
ordinary and extraordinary refractive indices, has focal length
f.sub.bo for light polarized along its ordinary axis and focal
length f.sub.be for light polarized along its extraordinary axis.
By virtue of the extraordinary axis of the lens being orthogonal to
the direction of light propagation, the extraordinary axis will not
impart a transverse offset to the beam as can happen in other axis
orientations. The two focal lengths of the BRL may be used as the
two focal lengths necessary for the holographic process, and
f.sub.be and f.sub.bo may be substituted for f.sub.d1 and f.sub.d2
in equation 3. By reference to equation 3, then, any single lens
made of a given type of birefringent material will have a constant
spacing factor no matter the physical curvatures of the lens:
s = f be - f bo f be + f bo = n o - n e n o + n e - 2 . ( 6 )
##EQU00004##
However, when used in conjunction with a non-birefringent lens,
each of the focal lengths of the birefringent lens combines with
the single focal length of the non-birefringent lens to result in
two new combined focal lengths, one for each polarization axis of
the birefringent lens. Under the thin-lens approximation and
assuming no distance between the birefringent lens and the standard
lens, the focal lengths f.sub.be.sup.f and f.sub.bo.sup.f of the
combined system are now:
f be ' = f be .times. f r f be + f r , and f bo ' = f bo .times. f
r f bo + f r , ( 7 ) ##EQU00005##
and the combined spacing factor s' of the hologram system can be
increased and decreased from this constant value according to the
following equation:
s ' = f be ' - f bo ' f be ' + f bo ' = f be - f bo f be + f bo + 2
f be f bo f r . ( 8 ) ##EQU00006##
Note the similarity of the right-most part of equation 8 to the
internal part of equation 6, showing the additional factor for
adjustment of the spacing factor. Table 1 contains the refractive
indices, curvatures, focal lengths and inherent spacing factors of
spherical lenses that could be made from several select
birefringent material, calculated from equations 4-6, as well as
corresponding altered focal lengths and altered spacing factors for
systems incorporating these lenses and select glass lenses,
calculated from equations 7 and 8. The collected data demonstrate
the possibility to exercise total control of the spacing factor and
other holography properties based systems.
TABLE-US-00001 TABLE 1 Refractive indices, curvatures, focal
lengths and incoherent hologram parameters of selected birefringent
materials. Birefring. R.sub.1 R.sub.2 f.sub.bo f.sub.be z.sub.h
f.sub.r f.sub.bo' f.sub.be' z.sub.h' material n.sub.o n.sub.e (mm)
(mm) (mm) (mm) s (mm) (mm) (mm) (mm) s' (mm) calcite 1.66 1.49 95
-95 72 98 0.150 83 -166 128 237 0.300 166 calcite 1.66 1.49 190
-190 144 195 0.150 166 N/A 144 195 0.150 166 calcite 1.66 1.49 380
-380 289 391 0.150 332 332 154 179 0.075 166 quartz 1.54 1.55 95
-95 87 86 0.008 87 -173 176 170 0.016 173 quartz 1.54 1.55 190 -190
175 172 0.008 173 N/A 175 172 0.008 173 quartz 1.54 1.55 380 -380
349 344 0.008 346 346 174 172 0.004 173 barium 1.68 1.55 95 -95 70
86 0.101 77 -200 108 150 0.164 126 borate barium 1.68 1.55 190 -190
140 172 0.101 154 N/A 140 172 0.101 154 borate barium 1.68 1.55 380
-380 280 343 0.101 309 100 74 77 0.025 76 borate The first column
refers to the birefringent material of the lens discussed in the
row. n.sub.o and n.sub.e are the ordinary and extraordinary
refractive indices of the birefringent material. R.sub.1 and
R.sub.2 are the radii of curvature of the birefringent lens.
f.sub.bo and f.sub.be are the ordinary and extraordinary focal
lengths of the birefringent lens, as discussed in the text. s is
the inherent spacing factor of the birefringent material, as
discussed in the text. z.sub.h is the optimal hologram distance for
the given combination of birefringent material and lens curvature,
as discussed in the text. f.sub.r is the focal length of an
optional non-birefringent lens used in conjunction with the
birefringent lens for the purpose of altering the spacing factor
and optimal hologram distance. f.sub.bo' and f.sub.be' are the
altered ordinary and extraordinary focal lengths of the
birefringent lens, as discussed in the text. s' is the altered
inherent spacing factor of the birefringent material, as discussed
in the text. z.sub.h' is the altered optimal hologram distance for
the given combination of birefringent material and glass lens, as
discussed in the text.
The implications of equation 8 include that: [0047] 1. The choices
of R.sub.1 and R.sub.2 of the birefringent lens and focal length
f.sub.r of the standard lens allow any spacing factor to be
achieved with a BRL made from any birefringent material. [0048] 2.
Use of a positive lens as the standard lens will reduce s' as
compared to s, while use of a negative lens as the standard lens
will increase s' as compared to s. [0049] 3. Hybrid lenses of any
desired focal length, achromaticity and spacing factor can be made
of materials that are composed of birefringent and non-birefringent
material components cemented together. [0050] 4. While compound
lens compositions of birefringent materials can make a device
achromatic, it should be realized that the wavelength specific
refraction of each lens in a non-achromatic birefringent lens will
proportionally shift the focus of each of the lens focal points
made from a birefringent material. Thus the plane of maximum
interference will be shifted depending on wavelength. Because of
this, a feature enabled by using birefringent lenses is that
wavelength specific holograms can be obtained by hologram detection
at any of those wavelength specific hologram planes even though the
input is polychromatic. FIG. 7 shows an example of the shift in
hologram planes 700, 701, 702 as a function of variance in
wavelength. Dashed lines and double lines represent a blue
wavelength 700, dashed single dot lines and solid lines represent a
green wavelength 701, and dashed double dot lines and triple lines
represent a red wavelength 702. One skilled in the art will realize
that the above equations 5, 7 and 8 may be adjusted for use with
more accurate lens equations and to account for some distance
between the BRL and the glass lens.
[0051] Thus birefringent refractive lenses can be used to
significantly materially improve hologram creation when used in the
following configurations: [0052] 1. As the sole lens or optical
element involved in hologram formation. [0053] 2. In conjunction
with another paired lens or optical clement to alter the spacing
factor of the f.sub.d1 and f.sub.d2 beams, where the other lens or
optical element ay consist of: [0054] a. A single lens or optical
element. [0055] b. A compound lens or optical element. [0056] c. A
sequence of lenses or optical elements. [0057] 3. In conjunction
with another corrective lens or optical element designed to correct
spherical, chromatic or other aberrations in the birefringent
refractive lens, where the corrective lens or optical element may
consist of: [0058] a. Single, compound or multiple standard
non-birefringent corrective lenses or optical elements designed to
correct the aberrations of one or the other focal lengths of the
birefringent refractive lens. [0059] b. Single, compound or
multiple standard non-birefringent corrective lenses or optical
elements designed to correct the average aberration of the two
focal lengths of the birefringent refractive lens. [0060] c. Single
or multiple birefringent corrective lens or optical element
designed to correct the aberrations of one or the other focal
lengths of the birefringent refractive lens, in which the
corrective birefringent lens may be made of a different
birefringent material than the hologram-forming birefringent
refractive lens. [0061] d. Single or multiple birefringent
corrective lens or optical element designed to correct the average
aberration of the two focal lengths of the birefringent refractive
lens, in which the corrective birefringent lens may be made of a
different birefringent material than the hologram-forming
birefringent refractive lens. [0062] e. Single or multiple
birefringent corrective lens or optical element designed to correct
the aberrations of one or the other focal lengths of the
birefringent refractive lens, used in conjunction with standard
non-birefringent lenses or optical elements, in which the
corrective birefringent lens may be made of a different
birefringent material than the hologram-forming birefringent
refractive lens. [0063] f. Single or multiple birefringent
corrective lens or optical element designed to correct the average
aberration of the two focal lengths of the birefringent refractive
lens, used in conjunction with standard non-birefringent lenses or
optical elements, in which the corrective birefringent lens may be
made of a different birefringent material than the hologram-forming
birefringent refractive lens.
[0064] Experimental work has confirmed the improvement seen in a
FINCH system when a current TLCGRIN-based system was compared with
a BRI-based system. FIG. 8 shows FINCH holograms obtained using a
laser as the EM radiation source, from a FINCH system configured
with liquid crystal GRIN lenses and a glass lens to create two
focused beams with a hologram plane between them, as in the prior
art shown in FIG. 3. The top three panels 800, 801, 802 in FIG. 8
show three phase-shifted raw FINCH holograms, which are
significantly distorted from the well-modulated spherical Fresnel
patterns that should characterize the ideal response of a FINCH
system. The bottom three panels in FIG. 8 show, from right to left,
the magnitude 803 of the complex FINCH hologram, the phase 804 of
the complex FINCH hologram, and finally the reconstructed image 805
of the laser beam. The magnitude shows large intensity fluctuations
and both the magnitude and phase show deviations from a perfect
spherical shape. The reconstructed spot shows significant
background signal as well as deviations from a perfect point shape.
FIG. 9 shows the results from a similar system in which the only
difference was the use of a spherical calcite BRL to induce the
differing phase properties between the signal and reference beams
instead of a GRIN lens plus glass lens arrangement; an imaging
relay lens was also used to project the hologram onto the camera
after it passed the BRL. All other factors and settings, including
light source, ancillary optics, polarizers, phase shifting plate
and voltage, and cameras were the same as those used to produce
FIG. 8. In the top row of FIG. 9 are shown three phase shifted raw
holograms 900, 901, 902 as in the top three panels of FIG. 8. The
raw holograms are nearly perfect representations of the desired
spherical Fresnol pattern, and show many more Fresnel rings than
the raw holograms in FIG. 8, a result of the much greater spacing
factors possible when using a calcite BRL instead of the GRIN/glass
system. In the bottom three panels of FIG. 9, we again see, from
right to left, the complex hologram magnitude 903 and phase 904 and
the reconstructed image 905 of the laser. The magnitude and phase
are both perfectly spherical patterns, with the magnitude free from
the significant intensity fluctuations that affect the system
described in FIG. 3 and used to produce FIG. 8. The phase shows a
smooth slope and neat transitions at phase wrapping regions, and
the reconstructed spot is point-like and free from excessive
background levels. The dramatic improvement of FIG. 9 over FIG. 8
is indicative of the overall improvement in holographic imaging
that BRLs can provide over other PSOEs.
[0065] Other systems may be constructed that make use of BRLs. As
shown in FIG. 10, another system 1000 incorporates two BRLs 400 and
1002 used together, whether said BRLs are made front the same
material or not, to achieve further modification of the two waves.
The cross section diagrams 1001 of the two BRLs show how a second
BRL 1002 could be used, with its axes 702 and 703 parallel or
perpendicular to the corresponding axes of the first BRL 400, to
provide chromatic, spherical or other corrections to the first
BRL.
[0066] FIG. 11 shows another system 1100 incorporating a BRL with
two flat sides, hereinafter called a birefringent flat (BRF) 1102,
acting to change the total optical path difference between the two
waves in addition to the BRL that differentially changes the
spherical curvature of the wavefronts of the waves. The cross
sections 1101 show the relative orientations of the ordinary and
extraordinary refractive indices 702 and 703 of the BRL 400 and the
BRF 1102. Optical path length (OPL) is a measure of the distance
traveled by an EM wave, taking into account both the thicknesses of
various media the waves traverse as well as their refractive
indices:
OPL=.SIGMA.d.sub.in.sub.i (9)
where d.sub.i and n.sub.i are the thicknesses and refractive
indices of all media in the path traveled by the wave. The optical
path difference (OPD) of two waves is a measure of the difference
the OPLs the waves traveled. When dealing with incoherent
holography, it is important to keep the total optical path
difference between the two waves low in order to maintain the
conditions necessary for holography interference to occur. The BRL
not only imparts different curvatures to the two waves through the
two focal lengths f.sub.bc 606 and f.sub.bo 607, but also imparts
an overall optical path difference OPD.sub.o between the two waves
that is proportional to the thickness d.sub.BRL of the BRL and the
two refractive indices of the birefringent material:
OPD.sub.o=d.sub.BRL(n.sub.0-N.sub. ) (10)
By using a BRF of the same thickness and cutting angle as the BRL,
but rotated by 90 degrees in the plane orthogonal to the direction
of EM propagation, the OPD.sub.o may be corrected without changing
the relative difference in the spherical curvatures of the two
waves. The wave that projects along the ordinary axis in the BRL
projects along the extraordinary axis of the BRF, and vice versa,
so the non-spherical OPL.sub.o from the BRL is canceled by the BRF.
Tilting the BRL slightly changes the magnitude of this OPD matching
effect.
[0067] Another system shown in FIG. 12 incorporates only a BRF 1200
along with a glass lens 100 to effect the separation of the
received wave from the object 101 into two waves. Waves with
positive spherical curvature entering a medium experience a delay
in achieving their focal point. This delay .DELTA. is proportional
to the thickness t and refractive index n of the medium:
.DELTA. = t ( 1 - 1 n ) ( 11 ) ##EQU00007##
It can readily be seen in the magnified part 1201 of FIG. 12 that a
BRF can delay the wave 1202 parallel to the ordinary axis and the
wave 1203 parallel to the extraordinary axis by different amounts
due to the differing refractive indices, which separates the focal
planes 1204 and 1205 of the two waves and allows for holography
interference 305 to take place.
[0068] Numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *
References