Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2003-03-20
2004-08-10
Dunn, Drew (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S029000, C359S030000, C385S037000, C430S001000
Reexamination Certificate
active
06775037
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
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STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
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REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.
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BACKGROUND OF THE INVENTION
A Hologram is recorded by the interference of two coherent wavefronts. When a hologram is illuminated by one of the beams used in its construction process, the light diffracted by the hologram reconstructs the other wavefront completely, including phase and amplitude information. As such, if one of the wavefronts is originated from a three-dimensional object, an image of this 3D object can be reconstructed from its hologram. The term “hologram” is not only used to describe a device that reproduces the image of a 3D object, it is now commonly used to describe any device that can diffract light into a multitude of colors. These diffractive devices are used in graphic design for wrapping papers, package covers, labels to authenticate products and many other applications. It is difficult to pinpoint when the commercialization of such holograms began. In searching patent literature, an early U.S. Pat. No. 3,567,561 issued in 1971 described the use of composite grating structures as surface ornaments. Holograms displayed on credit cards are probably the earliest commercial holograms used on a large scale.
There are many methods for producing holograms. The 3D holograms on credit cards are called rainbow holograms (see U.S. Pat. No. 3,633,989). This type of hologram can produce 3D images with only horizontal parallax and can be reconstructed with a white light source. Ornamental surface type holograms are made by the interference of two parallel laser beams or two diverging laser beams with the same divergence cone. When a graphic pattern is composed of many hologram segments having different angles of rotation and different periods, each segment in the pattern has to be recorded sequentially on the same recording surface. For this reason a technique similar to a dot matrix printer was developed and used grating dots to construct graphic patterns. A patent was granted in Taiwan (Taiwan Patent 263565 issued in 1984) for one such system (see also U.S. Pat. No. 6,043,913). This Taiwan patent could be one of the earlier patents describing a dot matrix grating system for producing holograms.
FIG. 1
shows the optical system for recording a dot matrix grating according to Taiwan Patent 263565. An incoming laser beam
101
is split into two beams
105
and
106
by beam splitter
104
. These two beams are recombined at recording plane
108
. Since only one lens
102
is used to focus the laser beam on surface
108
, only one beam,
105
or
106
, can be focused perfectly on surface
108
. As a result, the diameter of the focused spots on surface
108
have to be sufficiently large so that both focused beams are within the depth of focus of lens
102
. The period of the fringes within the overlapping beams is given by
T
=
λ
sin
ϑ
.
The fringe period can be adjusted by changing the angle of the prism mirror
107
. The orientation of the interference fringes is set by rotating the optical assembly consisting of prism
104
and prism mirror
107
. There are a number of problems related to this early design:
(1) The required depth of focus results in a very large beam spot on the recording surface,
(2) The laser used in this system must have long coherent length because the optical path length of the two beams are not equal,
(3) The beams on the recording plane are circular in shape with non-uniform beam profiles.
(4) The fringes are not continuous across adjacent grating dots.
For these reasons, the resolution of the early dot matrix system was limited to about 400 dots per inch and not very efficient in diffracting light.
FIG. 2
shows a more recent system for recording dot matrix holograms. A laser beam
201
is directed by a mirror
202
to a beam splitter
203
, with output beams
204
and
205
. Beam
205
is directed to a prism mirror
206
. This system uses additional prism mirrors
207
,
208
,
209
and
210
to equalize the optical path length to reduce the coherence requirement of the laser source. A lens
213
is also used to simultaneously focus beams
204
and
205
on the recording surface
214
. The spot diameter on the recording surface is given by &dgr;=&lgr;F/d, where &lgr; is the wavelength of the laser light, F is the focal length of lens
213
and d is the diameter of the laser beams. Suppose that a spot diameter &dgr;=10 &mgr;m is needed for the system and &lgr;=0.5 &mgr;m, the ratio of
F
d
is equal to 20. The period of the fringes is equal to
T
=
λ
2
sin
θ
,
because both beams subtend an angle &thgr; with respect to the optical axis. In
FIG. 2
it can be seen that the focal length F and the diameter of the lens
213
is also related by
tan
θ
=
D
2
F
.
To obtain T=1 &mgr;m , the diffraction angle is equal to 14.5 degree. This angle determines that the lens
213
must be an f-2 lens. In this dot matrix system, prisms
211
and
212
can be moved up and down in unison to change the interference angle &thgr; and hence the period of the fringes. In spite of the improvements in this more recent system over the system shown in
FIG. 1
, the problems related to beam shape, beam non-uniformity, and fringe continuity remained unsolved.
U.S. Pat. No. 5,291,317 proposed an optical system, which further resolved some of the aforementioned difficulties.
FIG. 3
shows the optical system according to U.S. Pat. No. 5,291,317. A laser beam
301
illuminates a mask
302
and a grating
303
. The lens
307
produces a de-magnified image of the grating on the, recording surface
308
. The mask
302
defines an aperture so that the shape of the grating dot on the recording surface
308
can be rectangular, hexagonal or circular in shape. The laser beam
301
has been expanded so that its intensity profile, between its perimeters
304
and
305
, on the grating
303
is nearly uniform. The grating is mounted on a rotary stage so that its fringes can be rotated under computer control. This system is simple in concept but with a fundamental optical restriction on the lens
307
. Suppose that the lens
307
has focal length F=10 mm and it is used to de-magnify the grating image by a factor
10
. In order to record a grating dot with a fringe spacing of 1 &mgr;m, with a laser wavelength of &lgr;=0.5 &mgr;m the period of the grating
303
is 10 &mgr;m. The diffraction angle of this grating according to relationship sin &thgr;=&lgr;/T is equal to 2.86 degree. To achieve the 10× reduction, the grating is approximately 100 mm from the lens. Therefore, 1
st
order beam will be at a distance 5 mm from the center of the lens
307
. This means that ratio of the focal length to the diameter of the lens (f-numer) is about 1. This lens is difficult to design, if not impractical. This difficulty can not be avoided by using smaller de-magnification. For example, reducing the de-magnification to 5 will reduce the distance between the grating
303
and the lens
307
to 50 mm. However, the diffraction angle will increase from 2.86 degrees to 5.74 degrees. The result is still that we need a lens aperture equal to the focal length of the lens. However, when a laser is used as the light source and a spatial filter
306
is used to block the 0
th
order wave from the grating
303
, the intensity variation on plane
308
has twice the spatial frequency of grating
303
. This phenomenon can be explained as follows. Suppose that the complex amplitude of the phase grating image on the recording plane is given by
i
(
x
)
=
1
+
sin
(
2
π
x
T
)
.
After the aperture
306
stops the 0
th
order of grating
303
, the intensity variation on the recording plane
308
is equal to
I
(
x
)
=
[
sin
(
2
π
Boutsikaris Leo
Dunn Drew
K Laser Technology Inc.
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