Patent Application
20180188690
The present disclosure relates to method and apparatus for recording and copying holograms. More particularly the present disclosure relates to a method of continuously recording holographic fringes on a photosensitive layer.
Holography is an image-recording process distinct from other image-recording processes; both the phase and amplitude of a wavefront that intercept the recording medium are recorded.
Holography can be used to produce apparent three dimensional images when the recording medium is illuminated from the correct angle. Holography can also produce interference or diffraction gratings which can be used to selectively choose different wavelengths of light or create anti-reflective coatings. Holographic gratings are superior to etched gratings in that they reduce the amount light scattering although at the cost of some reflection efficiency.
In the production of holograms in general, an object to be recorded is irradiated with a first component split from a coherent radiation source (e.g. a laser). Irradiation reflected from the object is directed toward a photosensitive medium (e.g., recording media based on photopolymers, hardened dichromated gelatin, or silver halide). A beam of reflected coherent radiation is commonly termed an object beam. At the same time, a second component beam split from the coherent radiation sources is directed to the photosensitive medium, bypassing the object. A beam of such coherent radiation is commonly referred to as the reference beam. The interference pattern resultant from the interaction between the reference beam and the object beam within the photosensitive medium is latently recorded in the photosensitive medium. When the photosensitive medium is processed and subsequently illuminated and observed at the correct angle (i.e., generally an angle correspondent with the incident angle of the reference beam), the irradiation is diffracted by the interference pattern to reconstruct the wave front that originally reached the recording medium as reflected from the object.
In conventional methods of holographic recording a transparent member (e.g. a glass or crystal rod) is brought in contact with the recording medium. The transparent member typically has a refractive index close to that of the recording media in order to reduce the reflection at the point of contact. An index matching fluid is sometimes also used to provide further efficiency in transmission between the transparent member and recording media. Systems like those described in U.S. Pat. No. 5,504,593 issued to inventor Hotta et Al. describe as system including a curved transparent member and use of index matching fluid to reduce friction between the transparent members and the recording media. Additional advances lead to the apparatus described in U.S. Pat. No. 5,576,853 issued to inventor Molteni et Al. which includes the use of a transparent cylinder, index matching fluid and a mirror.
The index matching fluid lubricates the interface between the transparent member and the recording media. A drawback of this is that the index matching fluid also allows for slippage between the recording media and the transparent cylinder. Slippage can create uneven or doubly recorded images in the recording media. Further drawbacks of current holographic methods include the necessity for an independently mounted mirror which is susceptible to vibrations of the apparatus.
It is with these problems in mind that the current invention has been developed.
Each of
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “first,” “second,” etc., is used with reference to the orientation of the figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
Thin films are used commercially in anti-reflection coatings, mirrors, and optical filters. They can be engineered to control the amount of light reflected or transmitted at a surface for a given wavelength. Thin film interference filters (sometimes called dichroic filters or reflection-type Bragg gratings) work on optical principles similar to a Fabry-Pérot etalon. Such filters take advantage of thin film interference to selectively choose which wavelengths of light are allowed to transmit through the device. These films are created through deposition processes in which material is added to a substrate in a controlled manner.
By way of example, and not by way of limitation, for a filter where the spacing between fringes is d and the refractive index is n and the vacuum wavelength of incident radiation at normal incidence is λ, minimum transmission occurs when 2nd equal to an odd integer multiple of λ/2.
In the implementations depicted herein, the coherent light may be monochromatic (i.e., characterized by a single peak vacuum wavelength) or may be characterized by peaks at two or more different vacuum wavelengths or a broad distribution of vacuum wavelengths. It is noted that monochromatic coherent light is generally characterized by a spectrum having a central peak at a characteristic vacuum wavelength with a narrow distribution of wavelengths about the central peak.
In order to continuously produce holographic images on a sheet of photosensitive material an exemplary system is described without limitation herein; the photosensitive film (103) is loaded onto the loading roller (106) fed underneath the transparent hollow cylinder (104) and on to an offloading roller (107). Likewise the reflective film (102) is loaded underneath the photosensitive film (103) and onto the loading roller (106) fed underneath the transparent hollow cylinder (104) and on to an offloading roller (107). The loading roller (106) and offloading roller (107) are cylindrically shaped and mounted on their longitudinal axis in such a way that they are capable of rotation. The loading roller and offloading roller are positioned with one roller on either side of the transparent cylinder in such a way that a sheet of material fed over top of each of said rollers would come into substantial contact with the transparent cylinder. The transparent cylinder is mounted on its longitudinal axis and is capable of rotation around said axis. The transparent hollow cylinder (104) is coated with a sticky polymer (105). A radiation source (108) is mounted inside the cylinder and is configured to transmit coherent radiation (101) through the transparent hollow cylinder (104) into the photosensitive layer (103). The loading roller (106) and offloading rollers (107) are rotated in synchronized speed with the transparent hollow cylinder (104). The photosensitive layer travels underneath the transparent hollow cylinder (104) as it rotates and the photosensitive layer translates without slipping relative to the cylinder (104) due to friction and stiction forces between a sticky polymer (105) on the cylinder surface and the photosensitive layer (103). By way of example, and not by way of limitation, the sticky polymer (105) may be a cured elastomer, such as cured polydimethylsiloxsane (PDMS).
The web containing the combination of the photosensitive layer and reflective layer is then removed from contact with the sticky polymer on the surface of the cylinder by tension on the web through an offloading roller (107). It should be noted that the sticky polymer increases the friction between the photosensitive layer and the hollow cylinder thereby preventing slippage of the roller or the photosensitive material as it contacts the transparent cylinder. The sticky polymer (105) may also be sufficiently conformable that it makes close contact with both the photosensitive layer (103) and the cylinder (104). The helps avoid an air gap that might lead to undesired reflections due to a large refractive index mismatch between the cylinder and the air gap.
The materials of the photosensitive layer (103), cylinder (104), and sticky polymer (105) may be selected so that they have similar indices of refraction in order to reduce unwanted reflections at the interfaces between the photosensitive layer and the sticky polymer and between the sticky polymer and the cylinder. For the simple case of light travelling from a medium of refractive index n1 to a medium of refractive index n2 at normal incidence, the reflection coefficient R is given by:
By way of example, and not by way of limitation, the indices n1, n2 may be chosen to tune the coefficients of reflection at these interfaces.
The transparent hollow cylinder (104) may be any material used in the art. Without limitation the transparent hollow cylinder may be made out of borosilicate glass or an optical polymer such as Poly(methyl) Methacrylate (PMMA), depending on the illumination wavelength. In many implementations it is desirable that the cylinder (104) be sufficiently transparent to radiation of the illumination wavelength, e.g., greater than 90% transmission at the illumination wavelength. The length of the cylinder (104) may be about the same as the width of the photosensitive film (103) or larger.
The sticky polymer may be formulated to temporarily adhere the photosensitive layer to the cylinder. Alternatively the sticky polymer may be formulated to temporarily adhere the reflective layer to the cylinder. An example of a sticky polymer fit for the current application is polydimethylsiloxane (PDMS) silicone.
The sticky polymer (105) may be applied to the cylinder (104) by any means known in the art in order to ensure that there is adherence of the photosensitive layer to the cylinder as the photosensitive layer traverses under the cylinder. Examples of techniques for applying polymer to the cylinder (104) include, but are not limited to casting using a mold, e.g., as described in U.S. Patent Application Publication Numbers 20140212536 and 20150365301, which are incorporated herein by reference, by laminating, e.g., as described in U.S. Patent Application Publication Number 20130224636, which is incorporated herein by reference, or by application of liquid polymer precursor onto the surface of the cylinder followed by curing. In some implementations, the sticky polymer may be applied directly (400) to the photosensitive layer before it travels under the cylinder. Alternatively, the sticky polymer may be applied to the reflective before it reaches the cylinder (503).
The coherent radiation may be delivered to the photosensitive layer by any means used in the art. Some examples of suitable means for delivering radiation to the photosensitive layer are; a scanning laser head inside the cylinder or a laser beam transmitting from outside of transparent drum to the inside using a mirror to direct the beam to the photosensitive layer. It would be understood by a person of ordinary skill in the art that the beam of radiation could be of any shape; without limitation suitable examples for the currently described invention may be beam points scanned across the length of the drum or a single collimated sheet of radiation directed toward the photosensitive layer. There may be one or more than one source of coherent radiation. Each source may transmit the same wavelength. Likewise multiple coherent radiation sources may be used at different wavelengths in the present invention to create a film with different optical characteristics at different wavelengths i.e. creating a notch filter for different wavelengths in the same layer. By way of example, and not by way of limitation, the vacuum wavelength of the coherent radiation may range, e.g., from the ultraviolet (starting at about 157 nm) to the infrared (up to about 10 microns).
The reflective film used in the current application may be any type suitable for reflection of the high intensity radiation used in holography. Examples of a material suitable for use as a reflective film would be a polished metal foil such as aluminum foil or a metalized polymer such as Aluminum coated Biaxially oriented Polyethylene Terephthalate (BoPET).
The reflective layer may be coupled with the photosensitive layer by any means used in the art. In one example the reflective layer is rolled underneath the photosensitive layer (106) and the combination travel underneath the cylinder (104) through friction forces. In
In certain implementations the photosensitive film may be between about 1 μm and about 10 μm thick to allow for formation of an interference grating perpendicular to the surface of the layer. The photosensitive layer may be any photosensitive composition known in the art. Some examples of suitable photosensitive material, without limitation are; photopolymers, hardened dichromated gelatin, or silver halide.
A holographic master may be used with the transparent drum. A holographic master contains on its surface or within its material an interference pattern that is desired to be transmitted to another material. Without limitation the holographic master may contain images, designs, grating patterns etc., in the form interference patterns. The holographic master may be etched into the surface of the cylinder, formed inside the transparent drum, or mounted to the outside or inside of the transparent drum in such a way that it rotates with the drum. Alternatively the holographic master may be a film which traverses with the photosensitive layer. The holographic master film may be adhered to the photosensitive layer and the drum through use of the sticky polymer. The holographic master may be used to project holographic images in to the photosensitive layer when illuminated by the coherent radiation.
One practical application for interference filters fabricated in accordance with aspects of the present disclosure as described above is as wavelength-selective reflective coatings for a window to protect against undesired transmission of laser radiation through the window. Windows of vehicles, aircraft and buildings are sometimes subject to unwanted laser radiation from commonly-available laser pointers. Such windows may be coated with a film into which has been formed a dichroic filter or reflection-type Bragg grating, e.g., as discussed above. Variations in the refractive index of the film may be engineered to selectively reflect radiation of one or more vacuum wavelengths emitted by common laser pointers, while transmitting other radiation. Fabricating filters into such films as set forth in the present disclosure allows large area filters to be economically fabricated. In particular, fabrication of optical filters as described herein avoids the need for thin film deposition techniques, which often require a vacuum environment.
Aspects of the present disclosure include implementations in which the above-described holographic grating fabrication techniques are slightly modified in order to produce “slanted” volume holographic gratings. In such “slanted” gratings, the refractive index in the developed photosensitive film varies periodically in a direction at an angle to the surface normal of the film.
Aspects of the present disclosure include many variations on the implementations discussed above. For example,
In another implementation illustrated in
A similar implementation depicted in
For some applications, a chirped (i.e., variable pitch) grating may be specifically desired. Aspects of the present disclosure include implementations for fabricating such gratings. By way of example, and not by way of limitation, a chirped grating may be fabricated as illustrated in
Alternatively, a chirped grating may be created using rastering the beam from light source about an axis parallel to the cylinder axis. By way of example,
In an alternative to the implementation that shown in
While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”
