Method and apparatus for copying holographic disks

A multi-faceted holographic disk can be copied in a one-step process by using a source disk, actually a sandwich of two thin film layers of photosensitive material. The first thin film layer is capable of producing multiple reference beams simultaneously. The second thin film layers is a "master" copy of the disk to be copied. Both layers are produced using known off-axis holographic techniques. A target disk, having an unexposed film of photosensitive material is located ajacent the second thin film layer. The source disk is illuminated with coherent light, preferably in the form of a conical beam with an apparent point of origin on an axis through the common centers of the source and target disks. Several optical elements capable of producing the conical beam are illustrated.

TECHNICAL FIELD 
The present invention relates to optical systems and more particularly to a 
method and apparatus for copying holographic disks. 
BACKGROUND OF THE INVENTION 
A known type of optical scanner uses a rotating holographic disk as a beam 
deflecting element. The disk includes a circular glass substrate which 
supports an annular thin film divided into individual beam deflecting 
generally sectorial areas or facets. The thin film is an exposed and 
developed photosensitive material, such as a silver halide emulsion or a 
dichromated gelatin. The original of each facet in the annulus is exposed 
using known off-axis, holographic techniques. According to these 
techniques, two beams of coherent light, a reference beam and an object 
beam, are directed at a film of unexposed, photosensitive material. The 
overlapping beams create optical interference patterns in the layer of 
photosensitive material. These interference patterns are fixed by 
developing the material using conventional techniques suitable for the 
particular photosensitive material employed. If the material is then 
illuminated with a reconstruction beam which is the conjugate of the 
original reference beam, the thin film will diffract or bend part of the 
reconstruction beam to produce the conjugate of the original object beam. 
For purposes of this description, a conjugate of a light beam may be 
defined as follows. All the rays of a conjugate beam are opposite to the 
rays of the original beam. That is, if rays in the original beam diverge 
from a single point, then rays in the conjugate of this beam will travel 
in the opposite direction and converge to that same point. 
If the thin film is moved relative to the reconstruction beam, the 
conjugate of the object beam will sweep through an arc. The particular 
path followed by the conjugate beam is a function of the relative 
orientation of the original reference beam and original object beam at the 
time of exposure of the initial facet. By using different angles and 
orientations of original reference beams and original object beams, 
reconstructed object beams following different paths can be generated by 
different facets. Arrays of beam-folding mirrors can be located in the 
paths of the reconstructed object beams to redirect the object beams into 
complex, omni-directional scan patterns. 
Multi-faceted holographic disks of the type described above can be made by 
exposing individual, oversized sheets of photosensitive material and by 
developing these sheets separately. Individual facets of the desired size 
and shape can be cut from the sheets and bonded to the clear glass 
substrate with a suitable adhesive material. 
The steps of individually exposing a separate sheet of photosensitive 
material for each facet, cutting the facet to the correct dimensions, 
positioning the facet in the correct place on the clear glass substrate 
and bonding the facet to that substrate are obviously time consuming, 
labor intensive and subject to errors. These factors make this "cut and 
paste" type of disk fabrication unsuitable for anything other than 
extremely limited quantities of disks. A disk made by the "cut and paste" 
method described above is normally used only as a master disk. 
To provide large quantities of holographic disks, the master disk itself 
may be used in a multi-step copying process, sometimes referred to as a 
step and repeat process. In a step and repeat process, an annular thin 
film of photosensitive material (a target disk) is placed face to face 
with the thin film of the master disk. All facets, except one, on the 
master disk are masked from any ambient light. The unmasked facet is 
illuminated using a collimated reference beam directed at the surface of 
the master disk at the same angle as the reference beam originally used in 
producing the facet on the master disk. When this reference beam is 
transmitted through the exposed facet on the master disk, the exposed 
facet will separate the beam into a zero order component, which is 
basically an extension of the reference beam, and a first order component, 
which follows the path of the original object beam. The zero and first 
order components of the beam will interfere in the previously undeveloped 
thin film to form an interference pattern in the target film. The area of 
the target film in which the interference pattern is formed corresponds to 
the area of the unmasked facet on the master disk. 
Once a facet has been generated in the target disk using these steps, the 
exposed area is masked from any light and a different facet on the master 
disk is unmasked. The interference pattern recorded in this next facet is 
copied into the target film using a reference beam having the same 
orientation as the reference beam originally used to generate this next 
facet. 
These steps are repeated one facet at a time for each facet in the master 
disk until the interference pattern of each facet has been copied into the 
target film. At this point, the target film is developed in a single 
processing operation. 
While this "step and repeat" method is superior to the cut and paste method 
for manufacturing holographic disks in large quantities, it still has 
disadvantages. The step and repeat method takes an undesirable amount of 
time since each facet must be exposed in a separate operation, the facet 
area masked must be changed for each exposure operation, and; the 
orientation of the reference beam may have to be changed for successive 
exposure operations. All of these operations require intervention by a 
human operator or, as an alternative, a highly automated system capable of 
performing such operations. The costs of developing an automated system 
diminish the attractiveness of that alternative. 
SUMMARY OF THE INVENTION 
The present invention is an apparatus and method which permits a 
multi-faceted holographic disk to be copied in a one-step process. 
The apparatus includes a source disk having a first layer of light 
transmitting material capable of producing multiple, collimated reference 
beams simultaneously and a second adjacent layer of light transmitting 
material. The second layer comprises the multi-faceted holographic disk to 
be copied or replicated. The apparatus further includes a coherent light 
source which is positioned on a normal from the center of the source disk. 
The light source illuminates a substantial portion or all of the first 
layer of the source disk with coherent light. Means are provided for 
positioning a target disk, having a third layer of unexposed 
photosensitive material adjacent the second layer of the source disk. When 
the coherent light source is energized, the image which is produced in the 
target disk replicates that portion of the second layer in the source disk 
which is illuminated by the multiple reference beams generated by the 
first layer of the source disk.

TECHNICAL DESCRIPTION 
Referring to the drawings, FIG. 1 is a partial top view of a multi-faceted 
holographic disk 10 comprising a circular glass substrate 12 which 
supports a thin film 14 of material. The thin film is divided into a 
plurality of sectorial facets, such as facet 16, which have been generated 
using known off-axis holographic techniques. Only a few of the facets 
which actually exist are shown. In practice, facets occupy the entire 
annular area of the thin film. The disk 10 is preferably driven directly 
by an electric motor (not show:) having an output shaft aligned with the 
center of hub area 18 on the disk. 
The disk structure shown in FIG. 1 is not per se part of the present 
invention and is not to be described in detail. Further details 
of such a disk may be found in U.S. Pat. No. 4,415,224, which is assigned 
to the assignee of the present invention. 
FIG. 2 is a simplified, partially schematic drawing of an apparatus for 
practicing the present invention. The apparatus includes a source disk 18, 
which is actually a sandwich of two different layers of thin film, as will 
be described in more detail later. The first layer is a 
reference-beam-generating layer capable of producing multiple reference 
beams simultaneously while the second layer contains the facets to be 
copied. A target disk 20 is aligned with the source disk 18 on a common 
axis 22. The apparatus further includes a light source 24 capable of 
illuminating either all or a substantial portion of the upper surface of 
source disk 18 with coherent light. In a preferred embodiment, light 
source 24 produces a conical light beam, represented by rays 26, which 
limits light produced by source 24 to an annular area on the surface of 
the source disk 18. When viewed along axis 22, the light pattern would 
have an annular cross section. 
As mentioned above, the source disk 18 is a sandwich of two thin film 
layers. Both layers have the same pattern of facets. The facets in the two 
layers serve different purposes, however. The facets in the first or upper 
layer are used to generate multiple reference beams simultaneously with 
each reference beam having a predetermined angle relative to the facet 
surface. 
Each facet in the first layer is produced by exposing a photosensitive 
material to two overlapping beams of coherent light using off-axis 
techniques. This is illustrated in FIG. 3 for one particular facet. The 
first layer in the source disk is a thin film 28 adhered to the lower 
surface of glass substrate 30. A step and repeat process of the type 
described earlier may be used to expose each facet in the film 28. A 
reference beam 32 which appears to originate at a point 34 on the axis 22 
is directed at the film 28 in a particular facet area. A collimated object 
beam 36 is directed at the film 28 and overlaps the reference beam 32 in 
the selected facet area. While any one facet area in the film 28 is being 
exposed to the reference and object beams, all other facet areas are 
masked from any light. For each facet, the reference beam 32 appears to 
originate at the same point 34 in the axis 22. The object beams may, 
however, be directed at the film 28 at different angles relative to the 
surface of the film. When the film 28 has been completely exposed and 
developed, illumination of a given facet area by a reconstruction beam 
originating at point 34 will produce a beam 38 which is directed along the 
same path as the original object beam 36. The beam 38 serves as a 
reference beam which can be used in a one step copying process. 
Although not shown in FIG. 3, illumination of a facet area by a 
reconstruction beam produces both a first order beam represented by beam 
38 and a zero order beam (not shown) which is an extension of the 
reconstruction beam originating at point 34. The steps of exposing and 
developing the first layer 28 would be controlled in accordance with known 
film processing techniques to maximize the efficiency of the film 28; that 
is, to make the first order beam as strong as possible relative to the 
zero order beam. 
The second thin film layer 44 in the source disk 18 contains the 
holographic disk which is to be copied. This layer can be made using 
conventional off-axis holographic techniques and some form of the cut and 
paste process described earlier. 
FIG. 4 shows the light beams which would be used to originally expose one 
facet on the second layer 44, which is adhered to a glass substrate 46. In 
a preferred embodiment of the invention, the object beam is a diverging 
beam 40 having an apparent point source 45 while the reference beam is a 
collimated beam 43 which overlaps the object beam in a limited facet area 
in the thin film 44. The angles of the object beam 40 and reference beam 
42 relative to the surface of the film 44 determine the path which will be 
followed by a reconstructed conjugate of the object beam 40 when the film 
44 is later illuminated by a collimated reconstruction beam 42. The 
reconstructed conjugate will appear to originate at the disk and to be 
focused at point 45. 
It is important to note that, for a given facet, the orientation of the 
reference beam 43 used in exposing a facet in the second layer 44 disk 
should be the same as the orientation of the object beam 36 used in 
exposing the corresponding facet in the first layer. The reason for this 
is explained with reference to FIG. 5 which shows limited areas of the 
source disk 18 and of the target disk 20 as they are positioned for the 
actual copying operation. FIG. 5 shows that the source disk 18 is actually 
a sandwich in which the reference-beam-generating first layer 28 and the 
second layer 44 are placed face to face preferably with an interposed film 
50 of index matching adhesive. The index matching adhesive reduces 
reflection losses at the interface between the layers 28 and 44. The 
target disk 20 includes a glass substrate 52 which supports an unexposed 
layer 54 of photosensitive material. The thin film 54 of unexposed 
material is positioned adjacent the bottom surface of the source disk 18, 
preferably with a layer 56 of index matching fluid for reducing reflection 
losses. 
In the actual copying process, the source disk 18 is illuminated with a 
coherent light beam 58 which appears to originate on the axis of the disk 
18. When the rays in beam 58 strike the thin film layer 28 in disk 18, 
each facet area in the film layer 28 will diffract a substantial portion 
of the impinging optical energy to form multiple reference beams which 
will impinge on the layer 44 of the source disk 18. A substantial portion 
of the optical energy in the beam 58 will be directed into the thin film 
44 along the path indicated by arrow 60. Thin film 44 will generate both a 
zero order beam directed along the axis 60 and a first order beam (not 
shown) which will be centered on an axis dependent upon the optical 
geometry employed in making the particular facet area in film 44. The zero 
order and first order beams overlap and interfere in the thin film layer 
54 of the target disk to produce interference patterns in the given facet 
area. Every facet in the target disk can be exposed simultaneously since 
multiple reference beams are generated simultaneously. 
After the one-step exposure process, the target disk 20 can be developed 
using conventional techniques. When target disk 20 is later illuminated 
with a collimated reconstruction beam which impinges on the surface of the 
thin film 54 at the angle of incidence of the conjugate of the object beam 
42 used in making the source disk, a scan line 62 is generated which is a 
conjugate of the reference beam 40 used in making the source disk. As 
shown in FIG. 5, the generated beam 62 is preferably a converging beam. 
The conical beam produced by light source 24 can be generated using a 
number of different optical elements. FIGS. 6-8 show three suitable 
elements. Referring first to FIG. 6, the collimated laser beam 64 produced 
by a conventional laser is directed at a conventional beam expander 66 
which increases the diameter of the laser beam while maintaining its 
collimation. The expanded beam 68 is directed at a conical prism 70 which 
appears triangular but is actually itself conical when viewed in three 
dimensions. The prism 70 refracts the optical energy in expanded beam 68 
to form a cone of light represented by rays 72. 
FIG. 7 discloses another type of conical element. In FIG. 7, the original 
laser beam 64A, beam expander 66A, expanded beam 68A and the conical rays 
72A are intended to be identical to the correspondingly numbered elements 
in FIG. 6. The only element that differs is the conical prism 74. Prism 74 
has an inverted conical surface 76 which is the optical equivalent of the 
normal conical surface on element 70. 
FIG. 8 shows another embodiment in which the light beams and the beam 
expander are intended to be identical to those shown in FIGS. 6 and 7. 
This identity is indicated by the use of the same numbering scheme for the 
corresponding elements but with the suffix "B" attached to each number. In 
FIG. 8, a conical reflector 76 is used to generate the conical light 
pattern 72B. 
While there have been described what are considered to be preferred 
embodiments of the invention, variations and modifications in those 
embodiments will occur to those skilled in the art once they learn of the 
invention through the foregoing technical description. For example, while 
the description contemplates a one step copying process, it is entirely 
possible that substantial benefits could still be realized if the process 
were used to copy disks in two or more steps since the number of copy 
steps would still be considerably less than the number of steps required 
for a facet-by-facet copying process. Therefore, it is intended that the 
appended claims shall be construed to include not only the preferred 
embodiments but all such variations and modifications as fall within the 
true spirit and scope of the invention.