Doubly modulated on-axis thick hologram optical element

An on-axis thick phase holographic optical element for use as a lens is fabricated by incorporating two off-axis holograms of two point sources located on opposite sides of the plate made by use of reference beams having a common angle with respect to the photographic media and complementary curvatures in the two cases. The element may be formed either by forming the two holograms in a single photographic emulsion, incoherently relative to one another, using a double exposure technique, or by forming the holograms on two physically separated media and then joining them to one another with their emulsion sides in contact. The resultant elements enjoy the low dispersion and aberrations like a conventional on-axis thin holographic optical element and the high diffraction efficiency like a thick hologram and additionally provide an extremely high ratio of diffracted to undiffracted light energy in the on-axis image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to planar optical diffraction elements for use as 
lenses or the like, and to a method of making the elements employing thick 
holographic techniques. 
2. Prior Art 
Planar optical elements incorporating diffraction patterns are useful as 
lenses, for performing Fourier transformation and spatial filtering in 
coherent optical processing apparatus, imaging and various like purposes. 
They are commonly formed by holographic methods involving the coherent 
recordation (on a photographic media) of the interference pattern between 
a wavefront emanating from a light source, and a coherently generated 
reference wavefront. 
In the past, two main techniques have been available for forming these 
holographic optical elements (HOE). One method employed object and 
reference wavefronts arrayed on-axis relative to the optical element to 
produce a thin two-dimensional fringe pattern in the plane of the optical 
element and the other used a reference beam off-axis relative to the 
object beam to produce a holographic record through the thickness of the 
photographic media. The on-axis HOE provides a low dispersion and minimal 
optical aberrations when compared to the off-axis thick HOE's. On the 
other hand the thick HOE's provide a relatively high ratio of diffracted 
to undiffracted light. In the past, either a thick or thin HOE has been 
chosen, depending upon the nature of its intended use. 
The present invention is broadly directed toward a unique form of 
diffraction optical element which combines the most advantageous 
characteristics of conventional thin and thick HOE's and provides a better 
ratio of diffracted light energy to undiffracted light energy in the 
on-axis image than either of these prior art elements. The invention also 
relates to a method of forming this unique HOE. 
SUMMARY OF THE INVENTION 
The holographic optical elements of the present invention broadly comprise 
planar lens-like optical elements having two grating patterns recorded 
therein. The grating patterns are formed through the thickness of the 
media and may be formed on a single media or on two separate medias which 
are joined together into a single unitary structure. The two patterns both 
operate upon an incident wavefront, either simultaneously or sequentially, 
depending upon the nature of the element, and provide equal and opposite 
aberrations which largely cancel one another to produce a low overall 
aberration. 
The dispersions produced by the two patterns are also equal and in opposite 
directions and result in a very low net dispersion. 
The holographic optical elements of the present invention can be formed in 
a single photographic emulsion or in two separate photographic emulsions 
which are then joined together to form a unitary structure. In the case of 
the use of the single photographic emulsion two thick holograms of two 
point sources disposed on opposite sides of a photographic media are 
formed sequentially employing a double exposure technique. The reference 
beams employed with the two exposures must make the same angle with 
respect to the photographic media in both cases and have complementary 
curvatures, i.e., two plane waves may be employed for the two reference 
beams or one wave may be concave and the other wave convex in a 
complementary manner. The light wavefronts used to produce the two 
halograms are incoherent relative to one another. The diffraction 
mechanism of such HOE's formed in a single layer is the cross-coupling of 
the incoherently added thick holograms. 
The resultant holographic optical element (which will be termed a simple 
DOMOTHOE, from doubly modulated thick optical element) can act as either a 
positive or a negative optical element, i.e., it will either condense or 
disperse an incident wavefront depending upon the angle of incidence of 
the wavefront with respect to the element. 
The other method of forming holographic optical elements of the present 
invention involves the formation of two separate off-axis holograms of 
point sources on separate photographic media. The reference beams employed 
in the formation of the two holograms must have the same angle relative to 
the photographic media and have complementary curvatures; preferably, both 
are plane waves. In the formation of these two holograms the object and 
reference beams have the same attitude with respect to the photographic 
media in both cases; they are both incident either upon the emulsion side 
or upon the backing side of the photographic media. After development, the 
emulsion sides of the two holograms are joined into intimate contact with 
one another to form a unitary structure. 
The diffraction mechanism of this complex optical element (which will be 
termed a complex DOMOTHOE), is sequential modulation. Each modulation adds 
aberrations and dispersions, but the aberrations and dispersions are equal 
and opposite and result in relatively low aberration and dispersion. 
The complex holographic optical elements of the present invention can be 
constructed in either positive or negative form. The positive form may be 
achieved by employing object and reference beams in the formation of the 
two separate holograms which are incident upon the backing side of the 
photographic media. When the emulsion sides are joined together to form 
the unitary structure the combined structure acts like a condensing lens. 
Alternatively, if both the object and reference wavefronts are incident 
upon the emulsion side of the photographic media during the formation of 
the two holograms a negative element having the characteristics of a 
double concave lens will result from the composite structure. 
The focal power of the devices of the present invention are functions of 
the focal length of the two individual holograms which form the device. 
These focal lengths can be selected to minimize aberrations in a given 
application. 
Optical elements formed in accordance with the present invention have been 
found to be extremely useful and efficient for imaging purposes, Fourier 
transformation and spatial filters.

Referring to the drawings, the fabrication of a complex DOMOTHOE, generally 
indicated at 10 in FIG. 1, involves the initial formation of two holograms 
12 and 14 and their assembly into a unitary structure by joining the 
emulsion sides of the two individual holograms to one another as by 
cementing or clamping to form the single structure 10. 
The complex DOMOTHOE illustrated in FIG. 1 has its diffraction patterns 
arranged to act as a positive optical element. The negative element, and 
its method of formation, are illustrated in FIGS. 2a, 2b and 2c. 
The hologram 12 forming one-half of the complex DOMOTHOE structure is 
formed using an exposure step schematically diagrammed in FIG. 1a. A 
photographic plate consisting of a thick emulsion coating 16 formed on a 
transparent support element 18 is used in the formation of the hologram. 
The photographic media may be of the type conventionally employed to form 
thick holograms. Plate 18 may be a rigid glass structure or flexible film 
structure. 
The photographic media is exposed to light from a point source 20 located 
along the central axis of the photographic media on the support plate 
side. The distance of the point source 20 from the emulsion coating side 
16 will be termed F.sub.p1. The photographic media is also illuminated by 
a reference beam 22 of off-axis light coherent with light from the point 
source 20, both beams being preferably derived from a common laser source 
employing a beam splitting technique. The reference beam 22 is planar in 
the preferred embodiment of the invention and makes an angle .theta..sub.p 
with respect to the central axis of the photographic plate. This hologram 
is subsequently developed in a conventional manner to form the holographic 
element 12. 
The second hologram 14 is recorded by using a point source 24 located on 
the central axis of a photographic plate having an emulsion side 26 and a 
backing side 28. Like the point source 20, the point 24 is located on the 
backing side of the photographic plate and is displaced at distance 
F.sub.p2 which may be the same or different than F.sub.p1. The reference 
beam 30 employed in the formation of the second hologram makes an equal 
and opposite angle .theta..sub.p with the central axis and is also a plane 
wave. If the reference beam 22 had some curvature with respect to the 
photographic plate it would be necessary for the reference beam 30 to have 
a complementary curvature. 
After the two resulting plates are developed using conventional hologram 
processing techniques suitable for holography to form the plates 12 and 
14, the plates are cemented together or otherwise fixed together with 
their emulsion sides in contact to one another to form a structure 10. The 
structure 10 will act as a positive holographic element and a point source 
located at F.sub.p1 will be focused at F.sub.p2 and vice versa. 
FIGS. 2a, b and c illustrate the processes for forming a negative complex 
DOMOTHOE generally indicated at 34 by forming two holograms 36 and 38 and 
joining them with their emulsion sides in contact with one another. The 
steps employed in recording the two holograms are illustrated in FIGS. 2a 
and 2b and are substantially identical to the processes used to form the 
two holograms making up the positive complex DOMOTHOE, as illustrated in 
FIGS. 1 and 2, except that the object and reference beams are incident 
upon the emulsion side of the photographic media rather than the support 
side. The resulting DOMOTHOE 34 will act as a negative optical element or 
as a double concave lens. FIG. 2c illustrates the optical performance of 
the negative complex DOMOTHOE 34. 
A simple DOMOTHOE, generally indicated at 50, which can be employed as 
either a positive or negative optical element is formed by the incoherent, 
double exposure, addition of two hologram recording steps as illustrated 
in FIGS. 3a and 3b. The first hologram is formed by exposing a 
photographic media consisting of an emulsion side 52 on a transparent 
backing side 54 with coherent beams from an on-axis point source 56 and a 
reference beam 58. The reference beam 58 is preferably planar and makes an 
angle .theta..sub.s with respect to the central axis. During one of the 
steps the point source is located on the emulsion side of the photographic 
plate at focal distance F.sub.1. In the second step the photographic 
plate, before development, is exposed to a light beam from a point source 
60 located at a distance F.sub.2, on the backing side 54 of the 
photographic media. The reference beam employed in the second exposure 62, 
makes an equal and opposite angle .theta..sub.s with respect to the media 
and has a plane wavefront or a curvature complementary to the curvature of 
the reference beam 58. 
As illustrated in FIG. 3a the resulting element can act as either a 
positive or a negative element, like a double convex or double concave 
lens. 
FIG. 4 illustrates the use of a positive DOMOTHOE 64, produced by the 
method of the present invention, as either a complex or simple structure, 
to focus an image of an object 66 a screen 68. The optical element of the 
present invention can also be used as coherent processing elements.