A system for reducing noise holograms

Abstract: A hologram recording assembly (10) includes a trans­ parent, absorbent member (12) supported on a substrate (14). The transparent member (12) is positioned to be directly incident to an exposure beam (A₁, A₂). The transparent member (12) is out of moisture equilibrium during an exposure time. Consequently, the transparent member (12) increases or decreases in thickness (S) depending on a moisture difference between it and the environment (28). As the thickness (S) changes, a phase shift (A₁°, A₂°) in the light reflections within the recording assembly (10) blurs spurious holograms while leaving the desired hologram intact.

Description: This invention generally relates to a system and method for reducing noise holograms and, more specifically, to an improved system and method for inducing phase shifts of reflective light during construction of the primary hologram. High quality holographic optical elements are used in defraction optics display systems, such as head up displays (HUDs), for advanced aircraft, helmet mounted displays, laser protective devices, narrow band reflective filters, and holographic high gain screens for simulators. These are only a few of the many uses of high quality reflective holograms. One of the critical problems in defraction optic display systems utilizing a replicated hologram has been a degradation of the holographic images as a result of the effects of spurious reflection in transmission hologram recordings that are frequently generated during the holographic replication process. Perhaps the most damaging spurious noise holograms are generated by reflections from surfaces which are interfaces of materials of different indexes of refraction, such as air/glass interfaces of the transparent surfaces of a recording cover plate, a hologram substrate, the recording medium itself, and optical elements used to generate the recording beams. These reflections can combine with the primary holographic beams at the recording film to form spurious reflection holograms, when the beams are in the opposite direction, and spurious transmission holograms when the beams are in the same direction. A subsequent display system using this hologram will be degraded by ghost images from the spurious reflection hologram recordings, and rainbow-like flair patterns from the spurious transmission hologram recordings. The prior art has attempted to solve these problems in several different ways. A simple approach has been to form the hologram with energy beams that strike the recording material surfaces at Brewster's angle, at which reflections are at a minimum. This method has very limited application because in general the incident angle is set by the desired holographic function, and is not a variable that one can select at will. Another approach has been to reduce the spurious reflections by attempting to match the indexes of refraction at the different surface interfaces with an index matching fluid, such as a mineral oil. The entire recording module is immersed in an index of refraction matching oil bath. The shape of the container is then designed so that surface reflections are directed away from the recording material. For many optical configurations, such a shape does not exist. Furthermore, the phase instability of the optical paths in oil degrade the desired hologram. Also, the problems of stabilizing the oil after each plate change, the need to frequently clean the oil of impurities, and the delays in handling the volume of oil needed all make this method unsuitable for production. More sophisticated approaches are disclosed in U.S. Patent Nos. 4,458,977, 4,458,978, and 4,456,328, in which spurious holograms caused by glass/air interfaces are eliminated by moving an outside cover plate to change the phase of the reflected rays relative to the primary beams during the recording period. Thereby, spurious holograms are not formed. For these approaches that use a moving cover plate, the rate of movement or phase change is a function of the exposure time which depends on the sensitivity of the recording medium. The total movement must provide a phase change of at least one half wavelength in the reflected spurious hologram beams to nullify or "smear out" any spurious interference patterns that would otherwise be recorded. While those systems have advantages over the prior art, they still have certain disadvantages. For example, the drivers to move the cover plate take time to adjust, calibrate, and test. The complete apparatus in complex to set up, needs long stabilization time, and has many modes of potential failure to decrease yield. Another attempt to suppress spurious holograms is disclosed in U.S. Patent No. 3,601,017. Therein, an immersion liquid is applied to either the surface of the recording medium or the surface of a transparent support member, which surface is remote from the direction of light incidence. The thickness of the immersion agent layer is varied over time during an exposure by evaporation or, when it is not so readily evaporated, by generating acoustic or surface waves which, during their reflection, directionally modulate the wave field in the liquid. Preferably, the transparent immersion liquid is O-xylene. This system also has drawbacks, including the fact that the evaporating liquid is not a good optical surface. Further, the variation in thickness would not appear to be highly regulated, particularly when using generated waves. A need still exists in the technology to provide an improved system and method for producing a high quality hologram that has minimal spurious holograms and at the same time can be produced in an economical and efficient manner. According to the invention a system is provided for recording a hologram with reduced spurious interference patterns, said system including: a holographic medium comprising: a substrate transparent to exposure beams, a first layer of holographic recording medium on a first side of said substrate, and a second layer on a side of said substrate, the thickness of said second layer being variable; means for altering the thickness of said second layer during an exposure time of said first layer; said system being characterized by first means for producing an exposure beam incident on the first layer from the side of the second layer; and second means for reflecting back said exposure beam after it has passed through said holographic medium towards said first layer so that an interference pattern is formed by the incident and the reflected beam. According to the invention a method for recording a hologram with reduced spurious interference patterns comprises the steps of: providing a holographic medium comprising: a substrate transparent to exposure beams, a first layer of holographic recording medium on a first side of said substrate, and a second layer on a side of said substrate, the thickness of said second layer being variable; producing an exposure beam incident on the first layer from the side of the second layer; reflecting back said exposure beam after it has passed through said holographic medium towards said first layer so that an interference pattern is formed by the incident and the reflected beam; and altering the thickness of said second layer during an exposure time of said first layer. SUMMARY OF THE INVENTION The present invention seeks to overcome the disadvantages found in the prior art by providing a recording assembly that includes a mirror positioned at the bottom of the recording assembly, and a reservoir of index matching fluid is adjacent to a reflective surface of the mirror. A recording medium is then placed on a side of the reservoir opposite the mirror. A substrate supports the recording medium, as well as an absorbent, transparent member on a side of the substrate opposite the recording medium. The absorbent member is out of moisture equilibrium with the surrounding environment. Thereby, the absorbent, transparent member can either absorb moisture from the environment or release it into the environment, depending upon the moisture differential between the transparent member and the environment. During the time that the absorbent, transparent member is either absorbing or releasing moisture, preferably at a constant rate, the recording assembly can be recording an interference pattern in the recording medium. By steadily increasing or decreasing the thickness of the absorbent member, the interference fringes or patterns which constitute the spurious holograms are blurred to an extent sufficient to practically remove the interference caused by the spurious hologram. At the same time, the desired hologram is not significantly blurred and remains sufficiently clear for the needed purpose. Specifically, the blurring is caused by the motion of the interference pattern in space as a result of one reflection surface moving. The process of blurring the spurious holograms continues during the recording time period and, during such time, the rate of expansion or contraction of the absorbent member remains substantially constant. The objects of the present invention can best be seen from an examination of the specification, claims, and drawings hereto. FIG. 1 depicts the recording of a hologram according to the present invention wherein an absorbent or transparent member in the recording assembly is static; FIG. 2 discloses a recording assembly according to the present invention wherein an absorbent member has been reduced or increased in thickness during the recording of a hologram; FIG. 3(a) is a graph of increasing thickness of the absorbent member over time according to the present invention; FIG. 3(b) is a graph of decreasing thickness of the absorbent member over time according to the present invention; and FIG. 4 depicts an exposure system for the hologram assembly according to the present invention. The following description is provided to enable any person skilled in the art to which the present invention pertains, or with which it is most nearly connected, to make and use the same, and sets forth the best mode contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide an improved system for reducing noise holograms. FIG. 1 is a side view of a recording assembly 10 which is constructed according to the present invention. While the present embodiment shows the recording assembly 10 with a flat configuration, it should be understood that other configurations, such as a curved one, are contemplated. The recording assembly 10 is a module that includes at the bottommost portion thereof, when viewed from FIG. 1, a reflective element or mirror 20. The mirror 20 has at its top surface thereof, when viewed from FIG. 1, a reflective surface 22 that serves to reflect light rays which are designated as A₁, A₂. A reservoir of an index matching fluid 18 is disposed immediately above and in contact with the reflective surface 22. The fluid 18 can be of any suitable and well-known matching fluid commonly used in holographic recording modules, such as xylene or mineral oil. For example, the fluid 18 may be an optical grade mineral oil or a microscope immersion oil. In this particular embodiment, with the recording assembly having an overall thickness of about 12,7 to 19,05 mm (0.50 to 0.75 inches), the reservoir of index matching fluid 18 has a thickness of approximately 5,08 to 6,35 mm (0.20 to 0.25 inches). With a thickness of less than 0,254 mm (0.01 inches) for the fluid 18, controlled recording is difficult. More than 6,35 mm (0.25 inches) may present problems of index variations. A recording medium or film 16 is positioned immediately adjacent a top side of the reservoir of index matching fluid 18. The recording medium 16 includes a top surface 24 and a bottom surface 26, and is made of any suitable, well-known recording film material for holograms. In this particular embodiment, the recording material 16 is made of dichromated gelatin. The thickness of the recording medium 16 in this particular embodiment is approximately 0,0254 mm (0.001 inches) but can vary. A substrate 14 supports the recording medium 16 at the upper surface 24 thereof. While the substrate 14 is made of glass in this particular embodiment, it can be appreciated by those skilled in the art that other materials, such as semiconductor materials, can also be used, and which are dependent upon the wavelength of the radiation. The thickness of the substrate 14 is approximately 0.25 inches in this embodiment, but can vary. An absorbent or transparent member 12 is supported on a side of the substrate 14 which is opposite the recording medium 16. The absorbent member 12 is, during the recording period, out of moisture equilibrium with a surrounding environment 28. In this particular embodiment, the absorbent member 12 is made of undichromated gelatin and has a thickness of about 10 to 50 micrometers. The present invention contemplates that the absorbent member 12 may not be transparent and may contain a dye therein, such as very fine carbon that is water soluble and miscible with the gelatin (or other material) in a solvent or a wavelength dependent dye such as methyl orange, which might be used for a green-blue exposure range. With a dye, spurious reflections are decreased from absorption by the dye material, which reflections particularly occur at an air interface 13 of the member 12. In the event that a dye is in the absorbent member 12, a greater reduction of noise or spurious holograms is achieved than without a dye. This is due to the following. Primary light beams, such as A₁, A₂ (FIG. 1) go through the member 12 once, and may then exit the assembly as beams A₁′, A₂′. In contrast, a noise beam, such as A₁″ (FIG. 1), effectively goes through the member 12 three times. If, for example, absorption loss from the recording medium 16 and mirror 20 are ignored, and a reflection factor at the surface 13 of about 4% is assumed, the noise beam A₁″ will be about 3.2% of the primary beam A₁ without a dye. With the addition of a dye, the primary beam A₁ may need to be twice as bright to overcome the 50% absorption factor of the dye, but then the noise beam A₁″ is then reduced to about 0.8% of the primary beam at the medium 16. As is known in the art, a gelatin film such as the absorbent member 12 can shrink, and the shrinkage appears to occur in four distinct mechanisms. One mechanism is a loss of water. Shrinkage of the gelatin due to loss of water is nearly equivalent to the volume of water lost. The water can occur as interstitial or as fairly loosely bound water. In either case, the bulk of the water comes off fairly rapidly (less than one day at 60°C). As can be appreciated, because of the natural origin of the basic gelatin material, the chemical composition and structure of the gelatin can vary from one sample to another. Accordingly, the rate at which a gelatin layer (i.e., the absorbent member 12) can lose water, or even absorb water, can differ from one type of gelatin to another. FIG. 2 depicts the manner in which the present invention takes advantage of the ability of gelatin to absorb and lose water. The absorbent member 12, which is made of gelatin, is shown in hatched lines having a thickness S. The thickness S is representative of the thickness of the absorbent member 12 in a static state. In such a state, the absorbent member 12 is neither absorbing nor losing moisture in a net amount with respect to the environment 28. Consequently, the thickness of the absorbent member 12 remains stable or static. However, when the absorbent member 12 is out of moisture equilibrium with the environment 28, the absorbent member 12 may swell by the absorption of water and its thickness would increase by an amount designated by ΔS. The change in thickness of ΔS is represented in FIG. 2 by the phantom lines above the cross-hatched lines that represent the absorbent member 12 in a static state. Thus, as the absorbent member 12 absorbs water and leaves its static state, it increases in thickness by an amount ΔS. On the other hand, when the absorbent member 12 loses water to the environment, the absorbent member 12 will be represented by the aggregate of the phantom lines and the cross-hatched lines in FIG. 2. Thus, when the absorbent member loses water to the environment 28, the absorbent member becomes reduced in thickness by an amount represented by ΔS. When the absorbent member 12 finally reaches moisture equilibrium, it will then be in the static state which is represented by the cross-hatched lines. The present invention utilizes the changes in thickness through a distance ΔS to produce a phase shift of a light moving through the recording assembly 10. In particular, a phase shift of the light passing through the recording assembly 10 in an amount of approximately one-half wavelength will substantially eliminate the otherwise present spurious holograms, while leaving the desired hologram with sufficient clarity. The phase shift is depicted in FIG. 2 by rays A₁°, A₂°. (The production of phase shift and consequent blurring of noise holograms in similar contexts, which is applicable herein, is generally described in U.S. Patent No. 4,458,977, 4,458,978, and 4,456,328.) With the above size parameters in this particular embodiment, a change in thickness of about 1.5% will produce the needed phase shift. The change may range from about 0.5% for thick members and to about 2.5% for thin members. FIGS. 3(a), (b) graphically illustrate, for purposes of examples, changes in a thickness S in an absorbent member 12. The figures only seek to depict what might be considered model situations. In FIG. 3(a), a moisture curve M is shown for an absorbent member 12 absorbing moisture. FIG. 3(b) shows another moisture curve M′, which is representative of an absorbent member 12 shrinking as it loses moisture. In FIG. 3(a), between a time t₁ and a time t₂, the thickness of the absorbent member 12 increases from a thickness S₁ to a thickness S₂ in a substantially linear fashion. This substantially linear occurrence on a portion of the moisture curve M is denoted by the reference letter E. To the left side of the curve E, when viewed from FIG. 3(a), the absorbent member 12 is just beginning to increase in thickness, and thus the curve is significantly less linear than the portion E. Similarly, the portion of the curve M on the right side of the curve E, when viewed from FIG. 3(a), represents a time during which the absorbent member 12 is becoming saturated or in moisture equilibrium with the environment 28. Again, the right side of the curve M is significantly less linear than the portion E. Referring to FIG. 3(b), between a time t₁′ and a time t₂′, a thickness of an absorbent member 12 decreases from a thickness S₁′ to a thickness S₂′. This represents a substantially linear decrease in thickness over time, and is indicated as E′ in a portion of the moisture curve M′. As in the case shown in FIG. 3(a), the portions of the moisture curve M′ on either side of the portion E′ represent nonlinear changes of the thickness S over time. In either instance represented by FIGS. 3(a), (b), the exposure of the recording medium 16 is limited to the time represented by the curves E, E′, respectively. During those time periods, since the thickness S of the absorbent member 12 is linearly changing over time, the phase shift of the reflections in the recording assembly 10 is generally linear over time. Consequently, the blurring or smearing out of the spurious holograms is substantially linear over time. If, however, the exposure were to occur outside of the portions E, E′, the thickness change would not be linear, and thus the phase shift would not be linear. In turn, this would cause the spurious holograms to be unevenly blurred or smeared out, and thus actually result in the creation of spurious holograms. As was noted above, FIGS. 3(a), (b) are merely illustrations of what might be considered a model absorbent member 12. Experimentally, a 300 bloom strength, porkskin type A gelatin was used as the absorbent member 12. Test results indicated that an increase in relative humidity from 0 to 5% produces a swelling rate that is nearly linear over a 30-minute period of time. This is generally equivalent to 0.05 wavelengths of argon laser light (5145 Å) per minute when the absorbent member 12 is 50 microns thick. An increase of relative humidity from 0 to 22% produced a swelling rate of approximately 0.22 wavelengths per minute for the same member 12. A decrease in relative humidity of this particular member did not give a substantially linear shrinkage rate from 22 to 0% relative humidity. It can be appreciated by those skilled in the art that the material out of which the absorbent member 12 is constructed can be selected to provide a greater tendency or lesser tendency to absorb and lose moisture. Such a characteristic may be important for the particular recording assembly into which the absorbent member 12 is incorporated, and the length of time over which the exposure is expected to occur. With very short exposure times, it may be necessary to utilize an absorbent member 12 that quickly absorbs or loses moisture. On the other hand, with a long exposure time, it may be necessary to utilize a different material for the absorbent member 12 that more slowly absorbs or loses moisture. In operation, and as shown in FIG. 4, a light source 40 produces the light rays A₁, A₂ which are shown in FIGS. 1, 2. The light rays impinge upon the recording assembly 10 which is placed in a sealed housing 30. The housing 30 controls the relative humidity of the environment 28 surrounding the recording assembly 10. In this particular embodiment, a nitrogen gas source 36 can introduce nitrogen through an inlet 32 of the housing 30. The nitrogen gas is allowed to pass over the recording assembly 10 and thereby draw moisture out of the environment 28 through an outlet 34. As moisture is drawn out of the housing 30, a moisture gradient is set up such that the absorbent member 12 tends to lose moisture to the environment 28. When the absorbent member 12 must absorb moisture, a moisture source 38 can pump moisture through the inlet 32 and into the environment 28 to again create a moisture gradient. With a higher relative moisture in the environment 28, the absorbent member 12 can absorb moisture. The result is that the absorbent member 12 in the recording assembly 10 can be forced to absorb water or lose water, as desired. In so doing, the absorbent member 12 will change in thickness in a substantially linear relationship over a specific period of time, and during that time the exposure of the recording medium 16 can occur. The linear change in thickness produces a linear phase shift in reflections in the recording assembly 10 and substantially blurs out spurious holograms while leaving the desired hologram intact. The above only represents particular embodiments of the present invention, and it is contemplated that various modifications to the above can be effected.

Claims: A system for recording a hologram with reduced spurious interference patterns, said system including: a holographic medium (12, 14, 16) comprising: a substrate (14) transparent to exposure beams (A₁, A₂), a first layer (16) of holographic recording medium on a first side of said substrate (14), and a second layer (12) on a side of said substrate (14), the thickness (S) of said second layer (12) being variable; means (30 - 38) for altering the thickness (S) of said second layer (12) during an exposure time (t) of said first layer (16); said system being characterized by first means (40) for producing an exposure beam (A₁, A₂) incident on the first layer (16) from the side of the second layer (12); and second means (20) for reflecting back said exposure beam (A₁, A₂) after it has passed through said holographic medium (12, 14, 16) towards said first layer (16) so that an interference pattern is formed by the incident and the reflected beam. The system of claim 1, characterized in that the thickness of the second layer (12) varies in dependence upon any moisture differential between the moisture content of the second layer (12) and that of a surrounding environment (28). The system of claim 1 or claim 2, characterized in that said means (30 - 38) for altering the thickness (S) of said second layer (12) communicate a fluid to said second layer (12) to alter said thickness (S) during said exposure time (t). The system of any of claims 1 - 3, characterized by third means (18) for matching indexes of refraction of beams (A₁, A₂), said third means (18) being intermediate said second means (20) and said first layer (16). The system of any of claims 1 - 4, characterized in that said first layer (16) and said second layer (12) are on oppositely facing surfaces of said substrate (14). The system of any of claims 1 through 5, characterized in that said second means (20) comprise a reflection element (20) positioned adjacent one surface of said substrate (14) and adjacent said first layer (16), said second layer (12) being positioned on an opposite facing surface of said substrate (14). The system of any of claims 1 through 6, characterized by a dye means in said second layer (12) for decreasing reflection of beams (A₁, A₂) during said exposure time (t). The system of any of claims 1 through 7, characterized in that said substrate (14) is made of glass. The system of any of claims 1 through 8, characterized in that said second layer (12) is made of gelatin. The system of any of claims 1 through 9, characterized in that said first layer (16) is dichromated gelatin. A method for recording a hologram with reduced spurious interference patterns, comprising the steps of: providing a holographic medium (12, 14, 16) comprising: a substrate (14) transparent to exposure beams (A₁, A₂), a first layer (16) of holographic recording medium on a first side of said substrate (14), and a second layer (12) on a side of said substrate (14); producing an exposure beam (A₁, A₂) incident on the first layer (16) from the side of the second layer (12) ; reflecting back said exposure beam after it has passed through said holographic medium (12, 14, 16) towards said first layer (16) so that an interference pattern is formed by the incident and the reflected beam; and altering the thickness (S) of said second layer (12) during an exposure time (t) of said first layer (16). The method of claim 11, wherein said exposure time (t) is defined from a period of time (t₁ through t₂), and at a first time (t₁) said second layer (12) is out of moisture equilibrium with a surrounding environment (28). The method of claim 11 or claim 12, wherein the thickness (S) of said second layer (12) increases or decreases during said exposure time (t).

Inventors: WREEDE JOHN E, WREEDE, JOHN E.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1989
Main classification: G03H1
IPC codes: G03H1