Abstract:
A transmission holographic element (1) composed of a first reflection holographic optical element (2) and a second reflection holographic optical element (3). Reflection holographic optical elements (2,3) are adhesively mated such that the distance separating the two elements is no more than a few wavelengths of the incident light beam (9). The incident light beam (9) passes through first element (2) and is reflected off of the second element (3) which reflects the light beam back towards first element (2) which again reflects the light beam through the second optical element (3). In this manner, two discrete reflection holographic elements (2,3) behave as a single transmission holographic element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to holographic optical elements and more particularly to a new method and apparatus in which a transmission holographic optical element is constructed of two reflection holographic optical elements placed together in close proximity. 
     2. Description of Related Technology 
     A hologram created by the interference of a plane reference wave and a divergent spherical wave will, when illuminated by a similar planar wave, create a divergent spherical wave (the virtual image) and, when illuminated by an oppositely directed planar wave will create a convergent spherical wave (the real image). Such a hologram behaves as a lens and can therefore be characterized as an optical focusing device. Since such a hologram is used to control the path of a light beam rather than to display an image it is referred to as a holographic optical element. The main advantages of holographic optical elements are that they are lightweight, compact and thin when compared to conventional lenses. 
     In general, the diffraction efficiency (η) of an optical element is defined as the percentage of incident light that is diffracted into the image field: 
     
         η (%)=(D/I) 100 
    
     where D is the intensity of diffracted, image forming light, and I is the intensity of incident light. 
     In a conventional transmission holographic optical element, the diffraction efficiency is a periodic function of index modulation, and can be expressed approximately by the periodic function: 
     
         η=sin.sup.2 (Δφ/2), 
    
     where Δφ=2π tΔn/λ cos θ, 
     t=is the gelatin thickness, 
     Δn=the refractive index modulation, 
     λ=the wavelength, and, 
     θ=the angle of incidence. 
     For a reflection hologram, diffraction efficiency η=tanh 2  (Δφ/2), which therefore increases with an increase in either the gelatin thickness or modulation. In principal, the saturation value is 1 (i.e., 100%). In reality, the gelatin absorbs some light and so the saturation value is somewhat less (&gt;95%). In the laboratory, refractive index modulation Δn is inferred from measurements of diffraction efficiency. For a transmission hologram, the two vectors representing the incident and diffracted light are nearly parallel and any change in their length (i.e. wavelength) has very little effect on the optimal grating vector, so diffraction efficiency remains high over a broad range of wavelengths. For a reflection hologram, the incident and diffracted light vectors are nearly antiparallel and any change in their length has a very strong effect on the optimal grating vector and thus diffraction efficiency is highly wavelength dependent. 
     A commercial application of such devices is as the combiner within a &#34;HEADS UP DISPLAY&#34;, commonly used on certain types of military aircraft and vision enhancement devices. 
     A conventional transmission holographic optical element is undesireable for use as a combiner because its &#34;see-through&#34; characteristic is poor. 
     A &#34;Heads Up Display&#34; using holographic tuned reflective optical coatings is disclosed in U.S. Pat. No. 4,261,647, issued to Ellis. The reflections in the Ellis patent are strictly specular: angle of incidence equals angle of reflection. This necessitates a space of significant wedge between the elements so that the two encounters of the light beam with either element are not parallel; they must differ in angle by at least the angular bandwidth of the reflective coatings. A wedge space of air introduces two additional air-glass interfaces which can scatter light. A wedge space of glass makes for a very heavy assembly. Moreover, any wedged space makes for a complicated optical design as it introduces difficult to compensate off-axis aberrations. Because the two elements of our sandwich are holographic, angle of incidence need not equal angle of diffraction and the geometry therefore will not require a wedged space. 
     SUMMARY OF THE INVENTION 
     The present invention functions as a transmission holographic element having a narrower spectral bandwidth, a wider field of view and improved immunity to spurious images in comparison to conventional transmission holographic optical elements. 
     The present invention is constructed by placing two reflection holographic optical elements in close proximity. The device is designed such that light is able to pass through a first reflection holographic optical element but is then reflected off the surface of the second holographic optical element. This reflected beam encounters the first holographic optical element where it is again reflected, the light beam finally being reflected back towards and through the second holographic optical element 
     Since the distance between the first and second holographic optical element is small, the light beam can be considered to have passed through a single optical element and have been bent through a single angular deflection. In this way, the two discrete reflection holographic optical elements mimic the behavior of a single transmission holographic optical element, but with improved optical characteristics. 
     An effort is made to maximize diffraction efficiency by attempting to cause Δφ (referring to the diffraction efficiency equation) to become equal to π (this is true for the transmission case, but for a Sandwich Reflection Hologram is less critical because of η saturation). The quantity t/λ cos θ is equal to the path length through the gelatin coating, measured in wavelengths. The gelatin thickness, &#34;t&#34; (5 to 20 microns), is readily controllable, whereas control of the gelatin index modulation Δn (up to about 0.05) is more difficult to achieve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a sandwich reflection holographic optical element constructed in accordance with the principles of the present invention; 
     FIG. 2 is a side elevation view showing the geometries for reconstructing a virtual image at infinity as used in a &#34;Heads Up Display&#34;; 
     FIG. 3 is a side elevation view showing the geometries for recording the halves of a sandwich reflection holographic optical element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the sandwich reflection holographic optical element is shown generally at 1. A first conventional reflection holographic optical element 2 resides in a parallel relationship with a second reflection holographic optical element 3. The reflection holographic optical elements (2,3) are routinely recorded in a number of materials, most commonly dichromated gelatin. Other materials are available, but all are blue sensitive. DCG is about twice as sensitive at 488 nm as at 514 nm and is very nearly insensitive beyond 600 nm. Ideally, one prefers to create and reconstruct at the same wavelength so that the geometry remains unchanged. Unfortunately, suitable laser sources are not available to exactly match some wavelengths. These hologram recording materials are generally coated to a thickness of several wavelengths (5-15 microns) onto rigid substrates of optical glass or plastic. It is common, particularly in the case of dichromated gelatin, to cement a cover glass or plastic element over the hologram recording material to afford some protection against humidity and physical damage. 
     The practice of sandwiching holograms between two glass elements can be adapted to the construction of the sandwich reflection hologram, since each of the two reflection holographic optical elements of the sandwich reflection hologram can be fabricated on its own glass substrate. For example, first reflection holographic optical element 2 may be formed on a glass substrate 4 upon which is deposited the gelatin film 5. Similarly, second holographic optical element 3 may be formed by depositing gelatin film 6 onto glass substrate 7. Subsequently, the pair of substrates 4 and 7 is cemented together with gelatin side 5 facing gelatin side 6, the adhesive residing in region 8, the two elements 3 and 2 being separated by a distance 13(δ). The plate spacing 13(δ) is nominally zero. In practice, since the plates are cemented together the separation 13(δ) is equal to the cement thickness. 
     In practice, contrary to the literal depiction of FIG. 1, the gelatin layers 5 and 6 are usually very much thinner than glass substrates 2 and 7. Typically, the gelatin layers 5 and 6 would have a thickness of ten to twenty microns, whereas the glass substrates 2 and 7 would be several millimeters (several thousand microns) in thickness. 
     Since each constituent reflection holographic optical element 2, 3 is a thick hologram, it will efficiently diffract only that light which is incident within a limited angular and spectral bandwidth. In the sandwich reflection hologram 1, therefore, a ray of light 9 will encounter each holographical optical element 2, 3 twice in its path through the sandwich reflection hologram 1. First, the light ray 9 passes through reflection holographic optical element 2 without diffraction because its angle of incidence 10 lies sufficiently outside the angular bandwidth of element 2, which in this case requires an angle of incidence nearly parallel to light rays 11 or 12 in order to diffract light beam 9. Next, the light ray 9 is diffracted in a backward direction indicated by the path of light ray 11 by the second reflection holographic optical element 3, which has a different angular bandwidth, in this case, for example, diffracting light with an angle of incidence of between 10° and 25°, that is, having an angle of incidence somewhat similar to that of light rays 9 and 11. 
     Diffracted light ray 11 again encounters reflection holographic element 2, but is now approaching element 2 an angle of incidence within the angular bandwidth of element 2, that is, for example, less than 10°, and so is diffracted away from element 2. Diffracted light ray 12 encounters second holographic optical element 3 at an angle of incidence of approximately 0°, which falls outside of the angular bandwidth of element 3, thereby, permitting light ray 12 to pass through second holographic optical element 3. The light beam must pass through the first hologram, be reflected by the second hologram, reflected again by the first hologram, and finally transmitted by the second hologram. 
     Reflections of light waves 9 and 11 begin at the surfaces of the gelatin film layers 6 and 5, respectively, and decrease with increasing depth, insofar as less light is available having been reflected. The physics is similar to that occurring with a multilayered dielectric mirror, except that the layers (grating fringes) are far more numerous and are usually tilted with respect to the substrates 4 and 7. The light beam encounters each hologram twice, at somewhat different angles. The angular bandwidth of each hologram must be such that the diffraction efficiency is high for the desired reflection and low for the desired transmission. In other words the two angles at which the light encounters a hologram must differ by at least one angular bandwidth. Typically, each hologram will have an angular bandwidth of from 5 to 15 degrees (measured in air). Ideally, one must not only control the angular bandwidth, but also make sure that the peak efficiency occurs at the desired angle. 
     Because the two reflection holographic optical elements 2, 3 comprising the sandwich reflection hologram 1 are in intimate proximity, separated only by distance 13(δ) (nominally zero), the net bending of light ray 9 can be considered to occur at a single surface 8. This is an advantage in that the selection of the intermediate wavefront 11 affects only the diffraction efficiency and not the aberration properties of the sandwich reflection hologram 1. Therefore, it is not necessary to precisely control aberrations of the additional intermediate construction wave front used to interferometrically record both reflection holographic elements 2, 3. Only the two construction wave fronts required for a conventional transmission holographic optical element need be precisely controlled. 
     Referring to FIG. 2, the &#34;readout&#34; or reconstruction geometry of a sandwich reflection holographic optical element, as used, for example, in a &#34;heads up display&#34;, is shown. The input transparency plane 14 may be, in a preferred embodiment, a cathode ray tube. Light 21 is emitted from numerous points (15,16,17,18,19) for example on the surface of plane 14, traveling towards sandwich reflection optical element 20. The light 21 is refracted through element 20 and continues traveling until collimated at output plane 22, thereby creating a virtual image at infinity. Output plane 22 could be, for example, the &#34;eye box&#34; of a &#34;heads up display&#34;. In this case, optical element 20 serves as the combiner for the &#34;heads up display&#34;. In one embodiment the path length Rt is 84 MM and the path length Rd is 120 MM. The thickness of optical element 20 is approximately 5 mm. 
     Referring to FIG. 3, the &#34;recording&#34; or construction geometry of a sandwich reflection holographic optical element is depicted. The holographic optical element 23 is actually composed of a first half 24 and a second half 25, analogous to elements 2 and 3 as depicted in FIG. 1. The construction of first half 24 is accomplished by light waves 26 emanating from point source 27 interfering with plane wave 33. The construction of second half 25 is accomplished by light waves 28, which are emitted from point source 29 residing on surface 30, interfering with plane wave 34. Light waves 28 pass through divergent lens 31 and objective lens 32 before striking the surface of element half 25, thereby interfering with plane wave 34. 
     Computer modeling of the sandwich reflection hologram has yielded the following results. Assuming a gelatin thickness of 10 microns and refraction index modulation of 0.03, the intermediate beam between the two reflection components was chosen to be a plane wave. Efficiency versus angle of incidence was computed for two directions in the field of view, namely, center and bottom. The average efficiency of the sandwich reflection hologram was about 65% with an angular bandwidth at the -3 decibel (half power) points being better than nine degrees.