IE20090374A1 - A holographic recording medium - Google Patents

Abstract

A method of recording content surprising the steps of: providing a content storage medium comprising a pre-recorded grating or hologram; and illuminating a pre-recorded grating or hologram with a single recording beam to record content in the grating or hologram. The recording beam may increase the diffraction efficiency of the pre-recorded grating or hologram. Alternatively, the recording beam may form a new grating or hologram in close proximity to a pre-recorded grating or hologram. <Figure 6>

Description

holographic storage system: the invention also relates to a storage swtcm and V» a isna! hologram.

In holographic Jala storage systems the storage of data, either in bit or page format, iirod .es the recording of a grating or hologram and this means the use of two coherent laser beams, the splitting and manipulation of which makes the optical head bulky and complex d amberius I lesselmk. Sergie S. Orlov. Matthew C. Bashaw Holographic data storage systems Proe II I .E. Vol 92 (8). ppl 231 -1280 (2004)). With any optical system that involves a high spatial frequency interference pattern, simplicity and compactness are of particular importance because me macron mechanical stability is needed in both arms of the interferometer in order to maintain a stable interference pattern. Holographic data storage systems depend on the recording of such interference patterns into a recording medium. Efforts are constantly being made to simplify and improve the stability of recording set-ups. Ihe simplest to date would appear to be the colli near approach published by Opt ware (“Ecma international creates TC44 to standardise holographic information storage systems’; http://vvww.optware.co.jp/english/PR_TC44_26 Jan_05.html) where (wo beams are combined before reaching the recording medium, but the optical head is quite complex.

There is therefore a need for a simplified recording system.

Statements of Invention According to the invention there is provided a method of recording content comprising the steps of: providing a content storage medium comprising a pre-recorded grating or hologram; and illuminating the pre-recorded grating or hologram with a single recording beam to record content in the content storage medium.

IE 0 90 3 74 Ihe recording beam may increase ihe diflraction efficiency of ihe pre-recorded grating or hologram, ihc recording beam may increase the diffraction efficiency of the pre-recorded grating or hologram by al least 40 fold I he recording beam may increase the diffraction efficiency oflhc pre-recorded grating by at least It )0 fold.

I he single recording beam may be an on-Bragg beam (the beam may be al the same Bragg angle of the pre-recorded grating or hologram). Alternatively. the single recording beam may be offBragg (the beam may be al a slighl angle to the Bragg angle of the pre-recorded grating). In a further embodiment, the single recording beam may be within the Bragg envelope. Multiple gratings or holograms rnay be recorded using the same pre-recorded grating by varying the offBragg angle of the recording beam during content recording, The recording beam may form a new grating in close proximity to the illuminated pre-recorded grating or hologram. The single beam may be an off-Bragg beam. The single beam may be within the Bragg envelope of the pre-recorded grating. Multiple gratings or holograms may be recorded using the same pre-recorded grating by varying the off-Bragg angle of the recording beam during content recording.

I'he content storage medium may comprise a self developing holographic recording medium. I'he pre-recorded grating or hologram may be recorded in the seif developing holographic recording medium. The pre-recorded grating or hologram may by recorded in the sell' developing holographic recording medium using two recording beams. The pre-recorded grating or hologram may have a spatial frequency of up lo 7.000 lines per mm such as up to 6300 lines per mm. The pre-recorded grating or hologram may have a spatial frequency of between 2.500 to 6.300 lines per mm. The pre-recorded grating or holographic may have a spatial frequency of between 1.000 to 2.500 lines per mm, such as 500 to 1,000 lines per mm, for example 100 to 500 lines per mm or 1 to 100 lines per mm.

The content storage medium may comprise a plurality of pre-recorded gratings or holograms.

I he invention further provides for the use of a self developing holographic recording medium containing a pre-recorded grating or hologram for the storage of content.

We also describe the use of a self developing holographic recording medium containing a prerecorded grating or hologram for the storage (recording) of visually read images and text.

IE 0 9 0 3 74 l hc content may be data (text) or an image. The content may be visible by ey e. (ontenl may be stored by enhancing the pre-recorded grating or hologram. for example the diffraction elticicncy of the pre-recorded grating <»i hologiam may be increased hy illumination with a single beam. The single beam may bv an on-Bragg beam. Alternatively the single beam may be an off-Bragg beam for example a single beam within the Bragg envelope of the prerecorded grating. 1(1 Alternatively, content may be stored by forming a new grating in close proximity lo a prerecorded grating or hologram: for example the new grating may be formed by illumination of the pre-recorded grating with a single beam al a slight angle to the Bragg angle of the pre-recorded grating (an olT-Bragg beam). The single beam may be in the Bragg envelope of the pre-recorded grating. l he recording medium may have a thickness of between I pm and 1 mm. The recording medium may comprise a plurality of pre-recorded gratings or holograms. The pre-recorded gratings or holograms may be multiplexed. The pre-recorded holograms or gratings may be multiplexed in the medium, fhe pre-recorded grating or hologram may comprise a reflection grating, or hologram. Alternatively, the pre-recorded grating or hologram may comprise a transmission grating or hologram. In one embodiment, the pre-recorded grating or hologram may comprise a combination of a reflection and transmission gratings and holograms.

The holographic recording medium may be write once, read many times. The holographic recording medium may contain a security hologram.

The invention further provides for a content storage medium comprising a self developing holographic recording medium containing a pre-recorded grating or hologram, fhe invention also provides for a holographic recording medium comprising a self developing holographic recording medium containing a pre-recorded grating or hologram. The invention further still provides for a security hologram comprising a self developing holographic recording medium containing a pre-recorded grating or hologram. The security hologram may be visible by eye.

IE 0 90 3 7 4 he recording medium may have a thickness of between Odum and 5 mn\ such as a thickness o! between (ί i pin and 2.5 mm, for example a thickness of between Odpm and i mm. The recording medium may contain a plurality of pre-recorded gratings or holograms, ihe prerecorded gratings or holograms may be multiplexed, for example the pre-record cd ho log rants ot gratings imp he multiplexed in the medium. The pre-recorded grating o· hologram max comprise a relleeiion grating or hologram. Alternatively. the pre-recorded gratin.; or hologram max eompri'c u transmission grating or hologram, in one embodiment the rceoiding medium may comprise a combination of reflection and transmission gratings or hologram ^ t he content storage medium may be write once, read many times. The content ’lorage medium may contain a security hologram.

It will be understood that the term content as used herein includes data such m textual data and alpha numerical data: images such as graphical images, videos, video clips photographs, audio recordings, barcodes and the like.

It will he understood that the term “on-Bragg as used herein means a beam that is at the same Bragg angle as one of the beams used to record the pre-recorded grating or hologram !l will he understood that the term “off-Bragg1’ as used herein means a beam that is at a different angle ra that of either of the beams used to record the pre-recorded grating or hologram Si will Ive understood that the term ’‘Bragg envelope as used herein means the range of angles within which a single beam can be successfully used to record a grating of enhanced diffraction efficiency by exploiting an existing low efficiency pre-recorded grating. ll will he understood that the term “close proximity as used herein means that the new grating or hologram is formed within the Bragg envelope of the pre-recorded grating or hologram.

Wc also describe the use of a self developing holographic recording medium containing a prerecorded grating or hologram for the storage of data. Data may be stored by enhancing the prerecorded grating or hologram, for example the diffraction efficiency of the pre-recorded grating or hologram may be increased by illumination with a single beam. The recording medium may have a thickness of between lpm and 1 mm. The recording medium may comprise a plurality of pre-recorded gratings or holograms. The recorded gratings or holograms may be multiplexed. The holograms or gratings may be multiplexed in the medium. The pre-recorded grating or hologram may comprise a reflection grating or hologram. The pre-recorded grating or hologram IE.090 3 74 may comprise a lransmission grating or hologram. Ihe pre-recorded grating or hologram may comprise a combination of a reflection and transmission gratings and holograms, ihe holographic recording medium may be write once, read many times. I he holographic recording medium may contain a security hologram.

W'e also describ: a data storage medium comprising a sell developing holographic reeordinu medium containing a pre-recorded grating or hologram. Ihe recording medium may have a thickness ot between (I. I pm and 5 mm. such as a thickness of between 0.1 pm and 2 5 mm. lot example a thickness of between 0.1 pm and I mm. The recording medium may contain a plurality of pre-recorded gratings or holograms. I he pre-recorded gratings or holograms may be multiplexed, for example the pre-recorded holograms or gratings may be multiplexed in the medium.' he grating or hologram may comprise a reflection grating or hologram. The grating or hologram may comprise a transmission grating or hologram. The grating or hologram max comprise a combination of a reflection and transmission gratings or holograms, fhe data storage medium may be write once, read many times. The data storage medium may contain a security hologram.

We also describe a method of recording data comprising the steps of: providing a data storage medium comprising a pre-recorded grating or hologram: and illuminating the pre-recorded grating or hologram with a single recording beam to record data in the grating or hologram. wherein the recording beam increases the diffraction efficiency of the pre-recorded grating or hologram by at least 40 fold. The recording beam may increase the diffraction efficiency oil he pre-recorded grating by at least 100 fold. The data storage medium may comprise a self developing holographic recording medium. The pre-recorded grating or hologram may be recorded in the self developing holographic recording medium. The pre-recorded grating or hologram may by recorded in the self developing holographic recording medium using two recording beams. The pre-recorded grating or hologram may have a spatial frequency of up to 7.000 lines per mm such as up to 6,300 lines per mm. The pre-recorded grating or hologram may have a spatial frequency of between 2,500 to 6,300 lines per mm. The pre-recorded grating or holographic may have a spatial frequency of between 1,000 to 2,500 lines per mm, such as 500 lo 1.000 lines per mm, for example 100 to 500 lines per mm or 1 to 100 lines per mm. I he data IE 0 9 0 3 74 storage medium may comprise a plurality of pre-rev orded gratings of hologram?·. t he single recording beam may be an on-Bragg beam. I he single recording beam may be mf-Bragg Hie single recording beam may be within the Bragg envelope.

One of the advantages of the system described herein ov· current systems is that ihe two beam holographic recording, requiring extreme stabilily o! the optical system, is done at the point of production ol the content storage medi'im. not ai th.e ponu whet! the content is 'wriitetf* by the end user, thetelbre content storage can be implemented without the need for a complicated onthe-spot holographic recording system. In addition, despite the simplicity of the single beam content recording head, the full range of angular multiplexing is still possible, as is transmission or reflection formal or a combination of both.

Although there will be some reduction in the dynamic range for use of the single beam content recording technique in comparison to a regular two beam recording, the increased dynamic range available due to the large thickness of the gratings is expected to compensate for the reduction in the dynamic range ofthe single beam recording.

Currently, in holographic data storage systems, storage of data either in bit or page formal involves the recording of a grating or hologram involving the use of two coherent laser beams. the splitting and manipulation of which makes the optical head bulky and complex. Our approach, which allows simple one beam recording, with angular multiplexing, would be a significant advance. The content storage techniques described herein could be used for Write Once Read Many mass memory devices, or, in a simpler version, to enable a section or sections of a security hologram to be individually writable. In the content storage application one ofthe benefits of the approach described herein is the simplicity and cost saving associated with the optical head (content writing). The techniques described herein allow for the use of low cost low coherence light sources and enable recording in desktop environments without stabilization. These advantages could allow the developing technology to sidestep many of the problems that have hindered its introduction into the marketplace.

In the security hologram application we describe a technology for which there is no equivalent that we know of on the market. This has potential uses in passports, security cards, biodata recording, individualization of security holograms (inclusion of barcodes, serial numbers, personal data etc. within the hologram) and encryption.

IE Ο 9 Ο 3 74 In an additional embodiment of the invention, the pre-recorded grating or hoi eg ram maj he used to simplify the mass production of holograms. 1 he pre-recorded grating or hologram is first recorded with a laser having a suitable coherence length for holographic iccoiding. ia a methanieally stable environment using a very short exposure and then either the diffraction ctfieiency of a pre-recorded grating or hologram is increased or a new grating or hologram is Svrmvd in close proximity to a pre-recorded grating or hologram under single beam exposure noi'ig luxx coherence light sources in unstable conditions. The reduced need for a mechanically stable. high coherence environment results in a significant cos· reduction and lime caving in the production of high volumes in applications sueh as security holography and holograms for it) packaging It will be appreciated that the applications described herein can be combined in various different combinations.

Brief Description of the Drawings The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:I'ig. 1 is a schematic of a system using the single beam grating enhancement concept for (bit wise) Holographic Data Storage (HDS) recording. (A) illustrates a writing step in a holographic storage disk* where pre - recorded gratings are ‘enhanced' with a single beam; and (B) illustrates a reading slep in a holographic storage ‘disc' where higher diffraction efficiency is obtained from 'enhanced’ gratings; fig. 2 A and B are graphs showing the typical increase in diffraction efficiency with time when a weak (<2%) grating is exposed to a single on-Bragg beam. A standard 2s twobeam recording of a grating is followed by a 25s delay, and then single beam exposure starts al 27s. The diffraction efficiency is observed to increase by more than an order of magnitude: big. 3 is a graph showing the growth of the diffracted beam intensity under single beam exposure conditions for sample layers of different thicknesses; IE 090 3 74 t ig. 4 is a graph showing Bragg curves (the variation of diffraction efficiency with reading beam angle of incidence) for a series of gratings formed using the single beam process using different angles of incidence of the single writing beam, l he giatincs were recorded in different photopolymer layers, but are shown here on one graph for comparison purposes. I he arrows indicate the offset (in degrees) from the Bragg angle of the seed grating (O') There are two identical recordings at each angle, lhc thickness is 130 microns and the spatial frequency is 500lines/mm; lag. 5 is a graph showing that an individual grating from a scries of gratings can be enhanced by illuminating the individual grating with a single beam of light without affecting neighbouring gratings; in this example the spacing between gratings is 2 degrees, ihc spatial frequency is 500 lines/mm and the recording wavelength is 532nm: fig. 0 is a schematic of writing multiplexed data by enhancing seed gratings in a re fleet ion format. fhe single beam enhancement process pushes the diffraction efficiency of an individual grating above a threshold level; fig. 7 is a schematic of reading the multiplexed data in a reflection format. Λ signal above threshold level is obtained for the gratings that have been 'enhanced'; l ig. S is a schematic of writing multiplexed data by enhancing seed gratings in a transmission format. The reconstructed beam (and therefore signal strength) is greater after single beam illumination; fig. 0 shows a schematic of a single beam recording and readout of a single page of data or an image. (Λ) illustrates that a photopolymer layer containing a pre-recorded weak grating produces a weak uniform beam in the first diffracted order of the reading beam: (B) illustrates that a spatial light modulator in the writing beam allows the diffraction efficiency to be enhanced only in some pixels; and (C) illustrates that the reading beam will re-create the pattern in the first order diffracted beam when the spatial light modulator has been removed; fig. 10 is a schematic of the optical set-up actually used to record a page of data with a single recording beam and a seed grating. (Λ) shows the two beam set-up used to record the seed grating, using one converging and one collimated beam; (B) shows the optical IE 0 9 0 3 74 set-up used to record a page of data/ image on an SLM into the recording medium in which I is a special filtered collimated beam: 2 is a beam splitter: 3 is a miiror: 4 is a polarizer: 5 is a Spatial fight Modulator (SLM): 6 is a polarizer: 7 is a lens: 8 is a pinhole: and 9 is a photopolvmer. fhe data ean then be replayed by illumination with a collimated beam. The output is shown in fig. 11: Fig. II i> a photograph of a page of data recorded using single beam recording using a seed grating, fhe setup used is shown in the I ig. 10: f ig. 12 <Λ) is a ploi showing the recording of a diffraction grating using standard twobeam interference without any disturbance: (B) and (C) are plots showing the recordingof a diffraction grating using standard two beam interference in an unstable environment: (1)) is a plot showing short two beam recording to create a seed grating followed (after ?0 seconds delay) by single beam enhancement of a diffraction grating in a stable environment; (E) and (F) are plots showing short two beam recordings to create a seed grating followed (alter 30 seconds delay) by single-beam enhancement of a diffraction grating in an unstable environment (arrows show points at which optical table was struck). f ig. 13 (Λ) is a giaph showing multiplexing of three gratings (upper curve) created using on-Bragg enhancement of seed gratings (lower curve) the angular separation between each grating was 2 degrees: (B) is a graph showing three multiplexed gratings (upper curve) created using on-Bragg enhancement of seed gratings (lowrer curve) the angular separation between each grating was 1.5 degrees; (C) is a graph showing three multiplexed gratings (upper curve) created using on-Bragg enhancement of seed gratings (lower curve) the angular separation between each grating was 1 degree; (D) is a graph showing two overlapped Bragg curves allowing the comparison of the signal read out from gratings formed by the one beam and two beam processes. The upper curve (twobeam) is the read-out from a grating created with the regular two-beam holographic process and the lower curve (one beam) is the signal read from a grating created with the single beam process; and (E) is a graph showing diffraction efficiency versus reading beam angle of incidence fora series of seed gratings in which one (the second from the left) has been 'enhanced' using a single on-Bragg writing beam. The graph shows what happens to neighbouring gratings with a grating separation of ).5° when one grating is illuminated on Bragg. The diffraction efficiency of the neighbouring grating is observed ΙΟ ΙΕ 090 3 74 lo increase somewhat in addition. In this case the recording wavelength was 532nm and ihe spatial frequency was 500iines/mm in a I 30 micron thick ia\er: l ie. 14 (A) and (B) are graphs showing post-exposure with short light source (LED) for gratings with an initial exposure (If) of 3 sec and a post exposure (PE) of 60 see (A) and gratings with an initial exposure (IE) of 2 sec and a post exposure (Pl·.) ot 120 sec (B). these graphs demonstrate that enhancement can he pel formed with low coherence sources: and f ig. 15 is a photograph of a reflection grating created with the single beam process. The circle indicates the area where the retleetion seed grating was recorded. The lower half of the seed grating area (the portion below the dashed line) was then exposed to a single beam of collimated light. Ihe lower half of the circular area (the portion below the dashed line) shows green light being diffracted towards the camera demonstrating that the illuminated portion of the seed grating was successfully enhanced. I'he lack of diffraction from the upper half of the circle (the portion above the dashed line) shows that the unilluminated portion of the seed grating is unchanged.

Detailed Description In one aspect, the invention provides a method for enhancing the diffraction efficiency of a prerecorded weak holographic grating or hologram. In a further aspect, the invention provides a method for creating a new grating hologram in close proximity to a pre-recorded weak holographic grating or hologram. In both cases, the pre-recorded weak holographic grating or hologram is illuminated with a single beam. In the case of enhancing the diffraction efficiency the diffraction efficiency of a pre-recorded weak holographic grating or hologram, the content recording slep is a single beam enhancement process which raises the diffraction efficiency of a pre-recorded grating or hologram, instead of the usual two beam holographic recording. In the ease of forming a new grating or hologram in close proximity to the pre-recorded grating or hologram, ihe content recording step is a single beam illumination of a pre-recorded grating or hologram. Advantageously, the content recording process only requires one recording beam anti interferometric stability is not necessary. This simplifies the content recording process as there is no need to record a holographic grating for each bit of content to be stored. Therefore, problems associated with trying to perform two-bcam holographic recording in a compact content storage system, for example beam manipulation problems and stability problems, arc avoided whilst retaining the advantages associated with holographic data storage. The invention provides for li IE 0 9 0 3 74 one beam holographic content storage with angular multiplexing capability and simple one beam data writing into or in close proximity to pre-recorded holographic gratings such as seeurilv holograms.

In a further aspect, the invention relates to single beam on-Bragg enhancement of the re li active index modulaii<»n in sell-developing holographic recording materials. Low efficiency seed gratings* can be pre-recorded in the storage medium, with multiplexing, high density, multilayer storage and ah the other advantages of holographic recording, but a simple one-beatn system t> all that is required at the content recording stage The diffraction efficiency of a pre-recorded iO holographic grating can be increased by illumination with just one recording beam or a su-v glaring can be created in close proximity to a pre-recorded grating by :1 himinoting the preiccordcd glaring with a single recording beam. The recording beam may be one of the beams used to pro-record the initial low efficiency grating or hologram in the storage medium or it may be any other type of beam with suitable wavelength and angle of incidence. Ihe lecordlng method provides content storage without the challenges normally associated with on-the-spot holographic recording such as low tolerance of vibration in the environment.

Lie. 2,\ shows a 500 lincs/mm recording in which a standard two-beam recording of 2 s duration, of a grating is followed by a 25s delay, and then single beam exposure starts at 27s.

I he diffraction efficiency is observed to increase by a factor of at least It) over (lie original diffraction efficiency.

I ig. 2B shows a layer thickness of 183.3pm in which a standard two-beam recording of 2s duration of a grating is followed by a 25s delay, and then single beam exposure starts al 27s lor a period of 45s. The diffraction efficiency after initial exposure was 0 14%. whereas the dilliuction efficiency after post exposure was 15.3%. The difference in diffraction effieieney achieved corresponds 10 over a 100 foid increase in diffraction efficiency. Referring m I ig 2B. an increase in diffraction efficiency of 109.3 times was achieved. This demonstrates the large diffraction efficiency increases which can be obtained by exposing a pre-recorded grating io a single beam using the methods described herein.

Storage material ranging in thickness from about I micron to above 1 mm has been fabricated. We have found that the single beam recording process described herein is more efficient at greater thickness of storage material. Referring to Fig. 3, which illustrates the diffraction efficiency of layers ranging from 50 microns to 200 microns thick, it can be seen that the thicker .2 IE 0 9 0 3 74 ihe storage material, (lie greater the diffraction efficiency of the final grating. Lower spatial frequencies anti greater layer thicknesses are iikely to lead to even larger increases in diffraction efficiency An increase of eighty times the efficiency of the seed grating lias been observed, flic growth of the diffracted beam under single beam exposure conditions is shown in I-ig. 3 (thickest sample layer).

II a pre-recorded grating is illuminated with a beam of light which is slightly off-Bragg. die Bragg curve of die final grating is shifted in the direction of the offset. This el'tect could he used to reduce the number of seed gratings needed by allowing for several 'data' gratings to he It) formed from one pre-recorded low efficiency seed grating. This effect could abv be used to choose the angular position of the new grating (created in close proximity to die pre-recorded grating by single beam illumination of a pre-recorded grating) by altering the angle of die single recording beam relative to the Bragg angle for the pre-recorded grating, lor an additional dimension of information (content) recording or in order to create specific diffraction effects hi the final hologram. This additional flexibility would increase the content storage capacity of the material to a level comparable to data storage using two beams or may allow for greater tolerances in alignment for single beam writing processes which may facilitate cheaper and simpler recording systems. big. 4 illustrates a series of gratings that are 'read* near their optimum coupling angle or Bragg angle as described above. The gratings were formed using single beam exposure of pre-recorded 'seed* gratings whose original diffraction efficiencies were close to 1.3%. However, in each case the single exposing beam used to increase the diffraction efficiency of the seed grating had a slight angular offset from the original writing angle. The resulting gratings have Bragg curves shifted in the direction of the offset. This could indicate a small tilt in the grating fringes in that direction. As might be expected, the ultimate diffraction efficiency is less under the same exposure conditions when the illuminating beam is not precisely on - Bragg. This is most likely due to the reduced coupling between the single writing beam and the pre-recorded 'seed* grating. At a thickness of 135 microns, spatial frequency 500 lines/mm and wavelength 532nm. an offset of more than 1.5 degrees causes very little increase in the diffraction efficiency of the seed grating. Al high spatial frequency and larger thickness the permitted offset angle may be smaller due to the increased selectivity. This ts important for the minimization of crosstalk and will ultimately determine the number of seed gratings that can be angularly multiplexed into the material. It may also be possible to record a number of enhanced gratings using the same seed grating, and still resolve them as separate data 'bits*. This could mean that a material used in this IE Ο 9 Ο 3 7 4 way would have an M number or storage capacity that is comparable with or not sicniItcantiv lower than liie M number or storage capacity that the marerial has when used in normal wvo beam holographic data storage.

In accordance with Kogelnik’s theory, the width of the Bragg curve is lower for greater thickness and lor higher spatial frequencies. I he graph of fig. 4 gives an example of the relative angular widths. Much narrower peaks (and consequently elosei spacing) could he achieved tot the thicker samples and higher spatial frequencies typically used in content storage l ig. 5 shows dilfraction efficiency plopcd against illurnin.ition angle as the data reading beam scans a range of angles where a series of multiplexed seed’ gratings have been recorded. One > >! the gratings has been ‘enhanced’ without enhancing its neighbours.

The low efficiency ’seed gratings were recorded using a 532 laser while the photopolymer recording medium was rotated by 2 degrees between recordings. One seed grating was then 'enhanced’ by illuminating it with a single beam at the Bragg angle appropriate for dial grating. Λ reading laser scans the medium through a range of angles and the output in the diffracted beam is read with a pholodetcctor so that the diffraction efficiency of each grating is measured Referring to l ig. 5. the individually enhanced grating (m-7pe) shows an increased diffraction efficiency. This demonstrates, to our knowledge for the first lime, that it is possible to use a single beam of light to significantly increase the diffraction efficiency of an individual low efficiency grating without affecting the diffraction efficiency of neighbouring gratings in a series of gratings.

Photopolymer recording materials, such as those of Aprilis and Inphase Technologies, have been researched extensively in the USA, as photopolymers are regarded as the best candidates tor 'Write Once Read Many’ optical data storage. The main disadvantage of most currently available photopolymers is that they suffer from post recording shrinkage. The photopolymer material used herein (for the formulation, see 1. Navdenova, II. Sherif, S. Mintova. S. Martin. V. foal. Holographic recording in nanoparticle-doped photopolymer, SPIE proceedings of the International Conference on Holography. Optical Recording and Processing ol' Information. V 6252. 45-50. 2006) can be characterised by relatively low shrinkage, as recent improvements to the material have allowed us to reduce it to 0.1% for 650 pm layers. However the single beam content storage methods described herein may also work well in other suitable materials. Η I he invention v ill he more clearly understood from the following examples.

Exa1_- / > In thi'' exampie. a set number of weak gratings are pre-recorded in the data storage medium, so that they can be selectively enhanced (or not) according to whether a I or a 0 hit Is to be recoidcd.

Retries a! of the information is carried out in a manner identical to the proved etc for retrieval in standard holographic data storage systems. Λ reading beam of a wavelength to which the medium is insensitive can be used to probe the gratings, or alternatively a low intensity version ofthe writing beam can be used, especially if a UV or white light fixing step is used to render the material insensitive to further exposure. 1 ig. 6 shows a schematic of a system to enhance weak (seed) gratings recorded in the medium.

In some content storage applications this is the data writing step. In the schematic, the single writing beam is incident at the correct angle for on-Bragg illumination of one of the pre-recorded gratings. The efficiency of that grating will therefore increase, giving a stronger signal beam when the grating is later interrogated by the probe beam during data reading (l'ig. 7).

I-ig. 8 shows a similar arrangement, but set up in a transmission grating geometry. where the signal beam would he transmitted through the medium Example 2 - image page wise data storage In the most straightforward single beam page recording system a two-dimensional pattern is used as a mask over the writing beam (in this ease an expanded collimated beam is used) using for example a spatial light modulator. A pre-recorded grating could be preferentially enhanced by the high intensity pixels and the resulting diffraction efficiencies will be proportional to the IE 0 9 0 3 74 intensity in the original image (the grating would have to be at ieast as large in area as the imuuc). I his w ill allow extraction of the image at a later date. ir , It 0 9 0 3 74 In the collimated system, either the mask would have to be in near contact with the photosensitive medium or the image would have to be projected in such a wav that a collimated on-lhagg beam of spatially varying intensity was incident on the photosensitive medium lor exampie using a leteeenuie lens. Fig. 9 shows a schematic of a single beam recording and readout of a single page of data or image. The same possibilities for multiplexing c.xiM in this formal (oo. fhe collimated light passes through in SLM which, through altering the percentage transmission at different pixels, ean control the degree of enhancement in different areas in the recording medium. Ibis allows a patterned 'enhancement' of the seed grating leading tn the recording of a page of data that ean be reconstructed in the first order diffracted beam.

Alternate civ the recording setup can also use a converging or diverging beam of Iight.

I ig. 10 shows the recording setup used to obtain the recording of the image or page nf data with a single recording beam and a seed grating. Fig. 10 (A) shows the two beam set-up used to record the wed grating, which in this case was done with one converging and one collimated beam, fig 19 ill) shows the optical set-up used to record a page of data/ image on an SLM into the recording medium, fhe data was then replayed by illumination with a collimated beam. The result is shown in Fig. 11. Fig. II is a photograph of a reconstructed image of a data page of a checker board pattern, fhe data page was recorded with the set-up shown in fig. 19 and reconstructed using a collimated beam of light, fhe reconstructed checkerboard pattern is seen on the left and the undiffractcd light in the zero order is seen on the right. This shows, for the first time, that a two dimensional page of data ean be recorded as holographic gratings using just one beam of incident light and afterwards 'read* using a reading beam in the same way as in regular holographic data storage.

Example 3 - Data writing in unstable condutions and with low coherence An important advantage of the single beam system is the fact that the second beam needed to produce an interference pattern is produced within the pre-recorded grating inside the recording material. This means that vibrations and disturbances that would normally disturb an interference pattern by causing one part of the optical system to move relative to another do not affect the interference pattern in this ease. Equally the very short path difference (less than the thickness of lhe grating) means that very short coherence length can be tolerated in the tight so vice white still obtaining a high contract interference pattern.

In regular two-beam recording the diffraction efficiency increases as the recording progresses (big 12(A)) As expected the curve is smooth, indicating a steady growth in the refractiv e index modulation ofthe grating and indicating that the interference pattern remained stable throughout the recording. It is well known that due to the micron and sub-micron widths of typical holographic interference fringes in such gratings, even sub-micron environmental vibrations end instabilities cause a blurring' ofthe interference fringes that is catastrophic for the formation ot' '10 (he grating hologram. Vibration isolated optical tables and controlled noise and airflow are routinely used in holographic recording in order to minimize the disturbances that would He detrimental to grating growth.

If a disturbance is deliberately introduced, however, the growth of diffraction eflictency is disturbed d igs. 12(B) and (C)). In the graphs the arrow indicates the point at which ’he vibration isolated optical table was deliberately struck in order to introduce vibration and instability in the setup.

We then com pare this to the situation while single beam recording is carried out: l ig. 12(1)) shows the normal growth of diffraction efficiency with time for single beam exposure of a weak seed grating under normal stable recoiding conditions. The conditions are identical to those in I ig 12(A) and as expected the growth curve is again smooth.

In fig. 12(b) and (1-) a disturbance is again introduced by striking the table during recording.

However, in contrast to the situation with two-beam recording, the use ofthe single beam recording approach means that even in the presence of vibration and instability in the setup the grating grows steadily, l he growth curves in bigs. 5 and 6 arc unaffected by the environmental instability.

These results demonstrated that one-beam recording works well in unstable conditions.

As an indication of the decrease in packing density that may occur for the use of simple one beam writing of data in comparison with regular two beam writing, we have studied the Bragg curves of neighbouring gratings produced by the one beam and two beam systems with decreasing angular separation between the neighbouring peaks. This allows us to estimate, how IE 0 9 0 3 74 iie still being resolvable l^ritgg re^dmg^ close together the' gratings can be recorded wliih process so that a comparison can be made between ihe two beam and one beam systems.

In normal two-beam recording there is a limit to how close to one another (in angular terms) iwo 5 gratings can be recorded before the two peaks become impossible lo resolve during the reading process. Because it depends primarily on the w idth of the peak, the minimum angular separation needed in order to be able to resolve the gratings is a I unction of giaiing thickness, wavelength and the spatial frequency of the grating.

In single beam recording, there are two potential hunting factors One is that, as in two beam recording, ihe re is a limit to how close to one another ί in angular terms) two gi tilings can be iceoided before the two peaks become impossible io resolve during the reading process. The second is that there is a minimum angular separation needed between the seed gratings to avoid the situation where (during the data writing step) enhancement of one causes ail unacceptable increase in the efficiency of its nearest neighbour, rendering it indistinguishable from the enhanced grating. f igs. 13 (Λ) lo (C) show the Bragg curves of seed gratings together with die Bragg curves of the gratings obtained when these seed gratings have been enhanced by a smgie on-Bragg beam, ί he angular separation between the gratings is 2.0 t.S 1 jnd i .0 ° respectively and each goi’ing has been 'enhanced'. hi Fig. 13(C) it is clear that overlap is beginning lo be a problem in the eases of both the two beam recorded seed grating and the enhanced grating. Il is interesting to note that the gratings created with the one beam process are nm broader than those created with the two beam process, big. 13(D) shows two overlapped Bragg curves allowing the comparison of the signal read from gratings formed by the one beam and two beam processes. I'he upper curve (Iwo-beam) is the read-out from a grating created with the regular two-beam holographic process and the lower curve (one beam) is the signal read from a grating created wiili the single beam process, Ihe similarity of the width of the curves indicates that the resolution challenges associated with the reading of multiplexed gratings would be similar for both systems.

As explained above, the selectivity of the writing process is also important as there is a possibility of affecting the efficiency of the neighbouring gratings that are multiplexed at angles close to the grating being enhanced.

IS IE 0 9 0 3 74 I ig. 13(1.) shows the effect that occurs when we enhance a seed grating that has u neighbouring seed grating angularly separated from it by 1.5 2 There is an increase in the diffraction efficiency of die neighbouring grating. 'This will place a limit on the proximity of seed gratings in a system depending on the signal to noise ratio required in the read-out. In this example the thickness is 130 microns and the wavelength 523nm in a 500 lines/mmm grating. In a 11DS system the grating thickness would be greater and the spatial frequency much tiigku which would increase die angular selectivity, making die separation necessary for rcsoinliol· much love:. However, we expect a similar relationship between the angular separation needed in o.\o beam recording and that needed in one beam recordings. These measuiemcuB serve as ..· it) demonstration and a comparison of one beam and two beam technology.

Another advantage of the one-beam data writing system is the capacity to i.rx hwv coherence light sources to enhance existing seed gratings: fog. 1-1 shows the Bragg curve for a grating that has been created by die single beam enhancement of a seed grating where the beam used lo enhance was from an Ll d) ter which the spectrum peak position is 524.29nm and the coherence length is: 80.1 Oum. In log ! b \t grating has iwjn exposed to a single beam from the LED for 60 seconds, in Liguie ·»ί B) it was I ?O seconds.

'Has demonstrates that the coherence length of the source can he as low as St» microns und «he diffraction efficiency is still raised significantly under single beam exposure Hns rnav be because the reference beam that interferes with the incident beam is created within :he photopolymcr layer, so that (here is a very short path difference between the: two inieriering beams.

ExanipJeJ .i Single beam Writable Security holograms The one beam holographic recording approach allows the diffraction efficiency of a pre-recorded grating to he increased significantly by subsequent exposure to a single recording beam incident at or near the Bragg angle. Since text and images that are visible by eye can be added taler by using a single beam of light there are applications in security hologram, production and indiv idualised display holography The recording setup is envisaged to be so simple for the type of content storage described above 35 that it would be possible to utilize a simple vxision in security holography. There arc many l·) reasons why it would be advantageous to be able to combine limited low cos’ data sloiage uitb security holography, not least of which is the grow th in interest in storage of biodata, encryption Levs, and other seeuritv measures. 1E 0 90 3 74 I lie technology described here could provide a method of allowing an end user, say at a passport olliee. bank, or similar, to individualise the security hologram without having to perform two beam holographic recording in order to record unique data. This would allow a cheap one beam system to be built which could have a low coherence source and not be susceptible to vibrations and mechanical disturbance.

Ihe standard overt and covert holographic security measures could be recorded by ii'.e manufacturing company while also preparing a section of the hologram which may contain seed gratings '.unable lor the subsequent recording of content, 'fhe complexity of Mich content pierecorded could range from a simple text mask to allow recording of a person's name and · or photograph etc as a visually readable part of the hologram, to the covert recording of biodata or complex encryption key data in a section of the security hologram. The single beam recordings added by the end user could equally be in the form of holographic diffraction gratings at a range of angle and positions, (lor example suitable for reading by a scanner) fhe techniques described herein prov ide the capability for an end user to form new gratings off-Bragg by altering’ the angle of incidence of the w riting beam, or even limited three dimensional images created using a series of seed gratings that overlap or nearly overlap in area and angular spread.

We envisage that the one-beam recording approach could be used to devise it very simple hologram writing system lor use in security applications, product tracking, and display holography.

Using a one-beam text and image 'writer* consisting of some very simple optical components and a diode laser, the user can write personal information such as date of birth, fingerprints, individualized product information such as barcodes or serial numbers and/or photographs and linages into an existing security hologram.

Identical security holograms could be mass-produced in photopolymer bearing a logo and other generic information, with a section left 'blank' for recording of information by the end user (passport office, bank etc), fhe 'blank' section may contain weak pre-recorded seed gratings whose diffraction efficiencies can be increased or new gratings could be formed in close proximity to the pre-recorded grating by exposure of a pre-recorded grating to a single laser beam, if desired. thereby allowing text and images to be added into the hologram without the need for normal two beam holographic recording. I his conhl allow customized lext and images to be written onto security holograms without the interferometric stability and coherence problems normally associated with holographic recording. |g QQQ J 14 This is distinct from content storage because in this ease visual text and images are written to the recording medium. Text and images are intended to be read by eye (visible bv eye), as holographic images, just as in a regular hologram.

I he advantage for security is that forgery would become almost impossible especially if other features of security holography were used An additional advantage would be the ease with which holographic mierotext/logos could be added by the end user, with for example, the date of issue and company logo easily being incorporated into the text to be recorded.

I he advantages of this approach include the vibration tolerance of the technique; the lack of a reference beam removes the need for interferometric stability and means that a ’writer’ system could be produced cheaply for use in normal desk top environments. Inexpensive liquid crystal screens, masks and/or simple laser scanning could be used to create text or images in the hologram.

An additional advanmge is the fact that the medium can also carry regular holographic images and text; lor additional security, inicrotcxt and other covert holographic security features can be included in the mass-produced hologram and/or the images and text added with the single beam 'writer'.

Example 5 - I ;se <>/ seed gratiniis in the moss production of holograms We have shown that a grating can be recorded with two beams until the diffraction efficiency is lust one percent or lower and it will s’ill respond to a single beam incident at I he Bragg angle by 3ti increasing in diffraction efficiency until the riiffrnciion Ci'fieieney is 70% or higher, thus ’he system described herein is suitable for application in the muss production of low cos; holograms.

In high volume production such as security holograms on packaging, a key cost is the amount of time required to expose each hologram to nn expensive high coherence laser in an inierfcroi net ideally stable environment in ordet io cicatc the image. In our system the lime spend 1E 0 90 3 74 on ibis step would He minimised by including a tun her step using cheap low euheiencc* light sources that would increase the efficiency ofthe grating and/ or superimpose texi and images al a Leer stage of the production process. Ibis could, of course also include the ability to individualize holograms, described in Example ΐ and 4 above, for security and product tracing purposes, i he processes are easily adapted to in-line mass production processes Recording of limited three dimensional holographic images may also he posable vlihiri the Bragg envelope of the seed grating or gratings, as referring to big 4. it can K seen that completely new gratings can be formed at a range of angles depending on the angie of the modem single beam Ex».παβίν 6. ,·Ά :g[e beam enhancement of a reflection hologram l or security and visual display applications the recording must be performed in a reflection format I ig. ' ?' is a photograph of a reflection grating created with the single beam process. The circle indicates the area where the reflection seed giating was recorded, ihe lower half of the seed guatiug was then exposed to a single beam of collimated light. I he lower half o) the circular area indicated shows green light being diffracted towards the camera demonstrating that the iiiumii’ated peiiion iii the seed grating was successfully enhanced. The lack o! dliflaeboo Horn the upper halt of the circle shows that the unillunnnaku portion ofthe seed mg is unchanged I he invention is not limited to the embodiment hereinbefore described. w-i:b iefcrerec to the iucnmpnnying drawings, which may be varied in construction and detail.

Claims (1)

1. Claims IE 09 0 3 74 1. Λ method of recording content comprising the steps of: providing a content storage medium comprising a pre-recorded grating or 5 hologram: and illuminating a pre-recorded grating or hologram with a single recording beam to record content in the grating or hologram 2. Λ method as claimed in claim 1 wherein the recording beam increases the diffraction '10 ellieieney ofthe pre-recorded grating or hologram. Λ method as claimed in claim 2 wherein the recording beam increases the diffraction efficiency ol' the pre-recorded grating or hologram by at least 40-fold. 15 -fi Λ method as claimed in claim 2 or 3 wherein the recording beam increases the diffraction ellieieney ofthe pre-recorded grating or hologram by at least 100-fold. 5. Λ method as claimed in any of claims 2 to 4 wherein the single recoiding beam is an onBraeg beam. 2d 6. Λ method as claimed in any of claims 2 to 4 wherein the single recording beam t< offBragg. 7. A method as claimed tn claim 6 wherein the single recording beam is within the Bragg 25 envelope 3. D d A method as claimed in claim 1 wherein the content is recorded by forming a new grating or hologram in close proximity to a pre-recorded grating or hologram. I ¢). A method as claimed in claim 9 wherein the single beam is an off-Bragg beam IE Ο 9 Ο 3 74 11. A method as ehiimed in claim 10 wherein lhe single beam is within the Bragg envelope 12. Λ method as claimed in claim 10 or II wherein multiple gratings or holograms are recorded using the same pre-recorded grating by varying the oft'-Bragg angle of the 4. 5 recording beam during content recording. 13. A method as claimed in any of chums I io 12 wherein the content storage medium comprises a self developing holographic recording medium 5. 10 14. A method as claimed in claim 13 wherein the pre-recorded grating or hologram. >s recorded in the self developing holographic recording medium. 6. 15. A method as claimed in claim 14 wherein the pre-recorded grating or hologram is recorded in the self developing holographic recording medium using two recording 15 beams. 7. 16. A method as claimed in any of claims I to 15 wherein the pre-recorded grating or hologram has a spatial frequency of up m 7.000 lines per mm. 20 8. 17. A method as claimed in any of claims 1 to 16 wherein the pre-recorded grating o»· hologram has a spatial frequency of up to 6. 500 lines per mm. 9. 18. ?\ method as claimed in any of claims 1 to 17 wherein the pre-recorded grating or hologram has a spatial frequency of between 2.500 to 6,300 lines per mm. 10. 19. \ method as claimed in any ot claims 1 to 17 wherein the pre-recorded grating oi hologram has a spatial frequency of between 1,000 to 2.500 lines per mm. 11. 20. A method as claimed in any of claims I to 17 wherein the pre-recorded gra*mg or 30 hologram has a spatial frequency of between 500 to 1.000 lines per mm. 12. 21. Λ method as claimed in any of claims I to 17 wherein the pre-recorded grating or hologram has a spatial frequency of between 100 to 500 lines per mm. 24 IE 0 9 0 3 74 13. 22 A method as claimed in any ol chains i to I? wherein (he pi\ meorded grating or hologram has a spatial frequency of between I to 160 lines per mm. 14. 23. Λ method as claimed in any of claims 1 to 22 wherein the storage medium comprises a 5 plurality of pre-recorded gratings or holograms 2-1 Use of a seif-developing holograph ie recording medium containing a pie-ieeorded grating or hologram for the storage of content ID 25. Use as claimed in claim 15. 24 wherein the content is dala. 26. Use as claimed in claim 24 or 25 wherein the content is an image, 27. Use as claimed in any of claims 24 to 26 wherein the content is visible by eye 28. Use as claimed in any of claims 24 io 27 wherein the content is stored by enhancing a pre-recorded grating or hologram. 39. t Ac as claimed in claim 28 wherein the di (fraction efficiency of a pre-recorded grating or 2D hologram is increased by illuminahon π the pre recorded grating or hologram with a single beam. oh t >seas claimed in claim 29 wherein the single beam is an on-Bragg beam. 16. 25 31. t ! se as claimed in claim 29 wherein the single beam is an off-Bragg beam. t2. I fse as ..hatmed in claim 3 ΐ wherein the single beam :< in the Bragg envelope. 33 t 'se as claimed in any of claims 24 lo 27 w herein the content is stored t>y forming a new 3(1 grating or hologram in close proximity to a pre-recorded grating or hologram. 34. Use as claimed in eiaim 33 wherein the pre-recorded grating or hologram is illuminated with a simile beam. a.·' claimed in eiaim a4 whcieui ίο*, singu ocam is an οιί-Biagg beam. IE 0 9 0 3 74 36. I -se as claimed in claim 35 wherein the single beam is in the Bragg en\ elope. 37. Use as claimed in any of claims 24 lo 36 wherein the recording medium has a thickness 5 of between Ipmand 1mm. 38. Use as ciaimed in any of claims 24 lo 37 wherein the recording medium comprises a plurality ol pre-recorded gratings or holograms. ! 0 59. I ise as claimed in claim 38 wherein the recorded gratings or holograms are multiplexed. 40 I Ue as ciaimed in claim 39 wherein holograms or gratings are multiplexed in the medium. 15 41. 1 ! se as claimed in any of claims 24 to 40 wherein the pre-recorded grating or hologram comprises a reflection grating or hologram. 47. 1 sc cs claimed in any of claims 24 to 40 wherein the pre-recorded grating or hologram comprises a transmission grating or hologram. 45. ί sc as claimed in any of claims 24 to 40 wherein the pre-recorded grating or hologram ccmpn-.es a combination of a reflection and transmission gratings or hologram 1 -·. 44. Use as ciaimed in any of claims 24 to 43 wherein the holographic recording medium ts 2? wi de once, read many times. 45. Use ax claimed in any of claims 24 to 44 wherein the holographic recording medium contains a security hologram. 17. 30 46. Λ comeiii storage medium comprising a self developing holographic rceotding medium containing a pre-recorded grating or hologram. 47. Λ storage medium as claimed in claim 46 wherein the recording medium has a thickness of between 0.1 uni and 5 mm. IE 0 9 0 3 74 48. Λ storage medium as claimed in ciatm 46 or 47 wherein lhc recording medium has a thickness of between 0. Ιμιη and 2.5 mm. 49. Λ storage medium as claimed in any of claims 46 to 48 wherein the recording medium 5 has a thickness of between 0.1 μηι and I mm. 50. A storage medium as claimed in any of claims 46 lo 49 wherein the iceording medium contains a plurality of pre-recorded gratings or holograms. 10 51. A storage medium as claimed in claim 50 wherein the pre- recorded gratings or holograms are multiplexed. 52. A storage medium as claimed in claim 51 wherein the pre-recorded holograms or gratings are multiplexed in the medium. 55. A storage medium as claimed in any of claims 46 to 52 wherein the grating or hologiam comprises a reflection grating or hologram. 54. \ storage medium as claimed in any of claims 46 to 52 wherein the grating <»i hologram 20 comprises a transmission grating or hologram. 55. A storage medium as claimed in any of claims 46 to 54 wherein the recording medium comprises a combination of reflection and transmission gratings or holograms. 25 56. A storage medium as claimed in any of claims 46 to 55 wherein the storage medium is write once, read many times. 57. A storage medium as claimed trt any of claims 46 lo 56 wherein the storage medium contains a security hologram.