US3821722A - Method of effecting cyclic reversible holographic recording in a thermo plastic material - Google Patents

Abstract

A method of effecting cyclic, reversible, holographic recording in a thermoplastic material including the steps of sensitizing the thermoplastic material by placing the same by a corona charge at a positive or negative potential and thereafter performing a holographic exposure and developing and fixing by cooling the resultant hologram. By increasing the number of recording cycles, the corona discharge voltage required for sensitizing is increased and the thermoplastic material is therefore placed at progressively higher positive or negative potentials.

Description

United Stats-1 a V/ METHOD OF EFFECTING CYCLIC,

REVERSIBLE, HOLOGRAPHIC RECORDING IN A THERMO-PLASTIC MATERIAL [75] Inventor: Joachim Ost, Martinried, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany [22] Filed: May 16, 1973 [21] Appl. No.: 360,768

[ll] 3,821,722 June 28, 1974 3.245.053 4/1966 Brown 340/173 TP Primary Examiner-Terrell W. Fears Attorney, Agent, or Firm--Hill, Gross, Simpson, Van Santen. Steadman. Chiara & Simpson 5 7 1 ABSTRACT A method of effecting cyclic, reversible, holographic recording in a thermoplastic material including the steps of sensitizing the thermoplastic material by placing the same by a corona charge at a positive or negative potential and thereafter performing a holographic exposure and developing and fixing by cooling the resultant hologram. By increasing the number of recording cycles, the corona discharge voltage required for sensitizing is increased and the thermoplastic material is therefore placed at progressively higher positive or negative potentials.

1 Claim, 1 Drawing Figure OR IN: sac/ware METHOD OF EFFECTING CYCLIC, REVERSIBLE, I-IOLOGRAPHIC RECORDING IN A THERMO-PLASTIC MATERIAL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for effecting cyclic, reversible, holographic recording in a thermoplastic material, and is more particularly concerned with such holographic recording in which, prior to each new recording cycle, the thermoplastic material is placed adjacent a corona charge at a positive or negative potential and thus sensitized. Afterward holographic exposure is carried out and the resultant hologram is developed by heating and fixed by cooling.

2. Description of the Prior Art Holographic recording techniques in which a photochemical process is not required are already well known in the art. For example, reference may be taken to the book by Kiemle and Ross entitled Einfuhrung in die Technik der Holographie, Akademische Verlagsgesellschaft, Frankfurt am Main, 1969, Page 217. The storage medium employed in this type of process is a thermoplastic synthetic material which has been applied to a layer of transparent, photoconducting material on an electrically conductive, clear, special glass substrate. To effect sensitizing, the external surface of the synthetic material is provided with a uniformly distributed, positive or negative charge by the use of a corona charging mechanism which influences a negative or positive charge at the glass-photoconductor interface. At the time of exposure, the photoconductor becomes conductive at points where light is incident, whereas at unexposed locations it continues to insulate. At the conductive locations, the charges pass through the photoconductor up to the interface with the synthetic material layer. After exposure, the arrangement is rendered insensitive to light by a secondary charge.

In order to develop the hologram, the synthetic material is heated up to the softening point. In so doing, locations carrying positive charges are attracted by the opposite negative charges, and vice versa, so that a surface relief formation is produced corresponding to the charge distribution and therefore to the exposure, and this surface relief is fixed by cooling. In this manner, a surface phase hologram is produced.

The hologram can be erased again if the layer is heated to above the softening point. Surface forces here, again, generate a smooth layer surface and the synthetic layer can again be used for recording.

Because of the possibilities of carrying out record/- read out/erase cycles and of effecting the development in only a short time, i.e., in the order of seconds, at the recording location, thermoplastic materials are particu larly significant where holographic storage is concerned. Also, the photosensitivity of the thermoplastic materials of this type is at least as good as that of conventional photographic emulsions.

In executing holographic recording cycles of this kind in thermoplastic materials, however, it is only possible in practice to carry out a limited number of such cycles because the signal to noise ratio, and therefore the efficiency, becomes poor from cycle to cycle and ultimately ceases to satisfy certain minimum requirements.

The reduction in the signal to noise ratio, and therefore the reduction in image quality and in efficiency, may be traced to several factors. For example, the properties of the thermoplastic material deteriorates because of the thermaloading applied during development and erase phases. Also, deterioration occurs because of the residual charges left over from preceding recording cycles. Again, the electrostatic charges result in the absorption of dust at the layer surface, which dust, during the development and erase processes, when the layer softens, becomes embedded in the layer. The embedded dust particles act as condensation nuclei in the neighborhood of which the layer breaks up completely after a certain number of cycles.

Furthermore, with increasing number of cycles, increased wrinkling occurs in the layer. This is also due to the fact that the surface tension at the time of erasing is insufficient to restore a smooth surface from the relief structure of the previously recorded hologram.

SUMMARY OF THE INVENTION Consequently, the object of the present invention is to provide a method for the cyclic, reversible, holographic recording of a thermoplastic material of the type initially described, in which with an increasing number of cycles the signal to noise ratio is substantially better and therefore the image quality is substantially better than is the case in previously. known meth ods.

To achieve this object, the present invention is based on the proposal that with an increasing number of re cording cycles, the corona charge voltage required for sensitizing is increased and that the thermoplastic material is thus placed in a progressively higher positive or negative potentials.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantages of the invention, its organization, construction and operation will be best understood from the following detailed description of a preferred embodiment of the invention taken in conjunction with the single FIGURE which illustrates a section through a recording layer suitable for practicing the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, a layer carrier 3, which may, for example, comprise a transparent synthetic foil, is carried upon a grounded substrate 4. At its free surface, the layer carrier 3 supports a layer 2 which may comprise, for example, a mixture of a photoconductive and a thermoplastic material.

A corona charge device 1 is provided above the layer 2 and rendersit possible to produce at the free surface of the layer 2 a charge, for example a positive charge. This charge sensitizes the thermoplastic material, the sensitizing being carried out in darkness. Subsequently, the thermoplastic material 2 is holographically exposed and a charge profile corresponding to the optical interference pattern results wherein the charges migrate to the' exposed locations. The development of the hologram is carried out thereafter by heating. By means of electrostatic forces, deformation of the free surface of the layer 2 is produced, this deformation being determined by the illumination energy. By rapid cooling, a stable hologram is created which, by heating to a temperature higher than the developing temperature, can be erased again.

With the formation of the holographic relief image at the free surface of the layer 2, the magnitude of the electrostatic surface tension plays a significant part. In accordance with the method of the present invention, the layer 2 is sensitized prior to the first record/erase cycle using a relatively low corona charge voltage. This charge voltage is correspondingly increased as the number of cycles progresses, so that the thermoplastic material is placed at progressively higher positive or negative potentials.

Recording as disclosed above produces the result that the signal to noise ratio, i.e., the relative image quality, drops off substantially more slowly during read out, as the number of cycles progresses, so that consequently substantially more record/erase cycles are possible.

Although I have described my invention by reference to a particular embodiment thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of my invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.

I claim:

1. A method of carrying out cyclic, reversible, holographic recording in a thermoplastic material, comprising the steps of: creating a corona charge with a corona discharge voltage; sensitizing thermoplastic material with the corona charge prior to each new recording cycle; progressively increasing the corona discharge volt age to increase the corona charge for sensitizing prior to each new recording cycle; holographically exposing the sensitized thermoplastic material; heating the exposed material to produce a surface relief; and fixing the surface relief by cooling the thermoplastic material.

Claims (1)

1. A method of carrying out cyclic, reversible, holographic recording in a thermoplastic material, comprising the steps of: creating a corona charge with a corona discharge voltage; sensitizing thermoplastic material with the corona charge prior to each new recording cycle; progressively increasing the corona discharge voltage to increase the corona charge for sensitizing prior to each new recording cycle; holographically exposing the sensitized thermoplastic material; heating the exposed material to produce a surface relief; and fixing the surface relief by cooling the thermoplastic material.

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