Optical Storage Medium and Optical Device for Writing, Deletion and Reading of Data

Field of technology

The invention concerns an optical storage medium and an optical device for writing, deletion and reading of the optical storage medium data. Such media and devices may be used in all areas of computing technologies requiring the storage of large amounts of data with the possibility to carry out their corrections. Another possible use is the recording, storing and replaying of video and audio files.

The existing state of technology

An optical storage medium and the procedure of its manufacture is known under US Pat. No. 6728154 dated 27-04-2004. This three-dimensional medium consists of a matrix, usually a glass or crystal one, and of ions of metals of rare earths. The recording is realized by exposure to laser of 10 ^s - 10 ¹⁷ W/cm ² for 10 ^"10 sec. Owing to the exposure, the ions of rare earths pass on to a luminescent condition. The information is read by means of the ion luminescence excitation upon the exposure to laser of a corresponding wavelength. The disadvantages of this solution include the need for high energies of the laser radiation along with the short duration, which results in the decrease of the information recording speed, and, furthermore, the necessity of an exact setting of the laser beam position within the recording medium volume without any possibility of anchoring.

Furthermore, a three-dimensional optical storage is known under US Pat. No. 5268862 dated 07-12-1993. It is based on a photochromic material, typically spyrobenzopyran bound in three-dimensional polymeric matrix. The material has two stable conditions - spyropyran and merocyanine, while the transition between the two is initiated by two-photon absorption at the wavelength of 532 nm. In this case, two laser beams are used for the radiation exposure along two axes perpendicular to each other. In this way, the space positioning within the three-dimensional volume of the medium is achieved: the conversion of the photochromic substance only takes place at the intersection of both beams. The other form of the photochromic substance fluoresces under the influence of radiation of 1064 nm wavelength; therefore, after exposure to such a radiation, the recorded information can be read by detecting the fluorescent places. Deletion is carried out by heating the medium as a whole or locally, e.g. by exposure to a 2.12 μm beam. The disadvantages of this method are similar to

the ones mentioned above. Considering the fact that the writing process is a two-photon process, the radiation sources must be capable of high peak power; therefore, they work on relatively low frequencies. The necessity to position two mutually perpendicular beams at an exact place within a three-dimensional medium limits the voxel dimensions to units or even tens of micrometers, while the focusing possibilities reach down to the sub-micrometer levels. Furthermore, there are considerable requirements on the homogeneity and high optical quality of the medium surface, which can be met relatively easily in the case of glass or crystal matrices. With polymers, on the other hand, reaching a similar quality in mass production is either problematical or results in the increase of costs.

Furthermore, volumetric optical storage is known under US Pat. No. 6045888 dated 04-04-2000; this contains a two-photon recording component mixed with a signal component containing the medium which only fluoresces in the "recorded" condition under one-photon absorption of the reading radiation. A component carrying out the conversion of the recording radiation to a shorter wavelength necessary for the realization of recording may be added for improvement. The recording to the medium takes place in "paragraphs" by means of space modulation of the recording beam. The recording medium has a multi-layer structure, in which photo-sensitive and photo-insensitive layers alternate regularly; their selection is carried out either by changing the storage medium position in space or by adjusting the focal length of the reading/writing head. The photo-insensitive layers may represent optical waveguides spreading the radiation of the "reading" wavelength. In such a case, the selection of the layer for reading is realized by focusing the radiation to such a layer. The disadvantages are again similar to those already mentioned: the use of a two-photon medium and the resulting necessity to use powerful radiation sources. The fluorescence used for reading limits the voxel dimensions in order that the signal during the "reading" process can have an acceptable intensity. Although the use of a waveguide for the selection of a reading layer solves the problem with the correct positioning of the "reading" beam, the light spreading through the waveguide has to penetrate through the parting layer of half the wavelength thickness. This results in the fact that the necessary volume of the fluorescent material may only be acquired at the cost of increase of the voxel lateral dimensions resulting in the decrease of the recording density.

It has to be stated that all three-dimensional optical storage devices based on a photochromic medium described in literature so far share a disadvantage related to the physical properties of the principles of storing information in the form of areas with a changed absorptive index or areas where the substance is converted to a luminescent form. The

technology currently enables focusing a light beam into an area of geometric dimensions of 0.5 x 0.5 x 0.5 (in units of wavelength), while the efficiency of the recording beam energy transformation to the photochromic effect is lower than 100%. A typical value of quantum yield for persistently stable photochromic compounds is approximately 0.05 - 0.1 [Yongchao Liang, Alexander S. Dvornikov, Peter M. Rentzepis, A novel non-destructible readout molecular memory. - Optics communication, 223(2003), 61 - 66], while the absorptive index in the spectral maximum does not exceed 1 - 2 cm ^'1 [Satoshi Kawata, Yoshimasa Kawata, Three-dimensional optical data storage using photochromic materials. - Chem. Rev. 2000, 100, 1777 - 1788]. This limits the necessary reading signal level in the minimum voxel volume. Furthermore, it has to be pointed out that during the reading process based on the detection of the passing radiation in the spectral maximum, partial deletion of the information bits occurs with one-photon media, wherefore it is necessary to use light of a wavelength from the absorption band margins [Satoshi Kawata, Yoshimasa Kawata, Three-dimensional optical data storage using photochromic materials. - Chem. Rev. 2000, 100, 1777 - 1788]. For this reasons, the voxel volume must be in practice increased by multiples of tens or even hundreds. To eliminate such insufficiencies, the device must be based on a different approach to information reading.

The most promising method appears to be the confocal microscopy [A. Toriumi, J. M. Herrmann, S. Kawata, Nondestructive readout of a three-dimensional photochromic optical memory with a near-infrared differential phase-contrast microscope. - Optics letters, v. 22 (1997), #8, 555 - 557] based on the detection of a different refractive coefficient in the recorded photochromic area. Despite its high sensitivity and the possibility to use one-photon media (as the detection uses radiation from the very margin of the absorption band, which does not cause the medium to pass on from one condition to another), this method is far from being free of disadvantages. The main problems result from the said high sensitiveness itself, for in addition to the recording phase irregularities, any optical irregularities of the disk including the variable thickness around the half of the wavelength of the light used for reading are also detected. Therefore, practical usability of this method is excluded, as the method requires the manufacture of media - disks - with an optical accuracy that is economically unbearable.

The nature of the invention

The above-mentioned disadvantages can be eliminated by an optical storage medium composed of alternating layers of an optically transparent material, where the first layer is doped with a photochromic substance and the other layer without the photochromic substance has the refraction coefficient of ni; the optically transparent substance doped on a photochromic basis may exist in two forms: form I with the refraction coefficient of H ₂ , form π with the refraction coefficient of n ₃ , while the said coefficients satisfy the relations of ni> n ₂ and H ₁ ^n ₃ , while the transition of the photochromic substance from form I to form II takes place by the effect of light radiation of λi wavelength and the transition of the photochromic substance from form II to form I takes place by the effect of light radiation of X ₂ wavelength.

The device for the multi-layer optical storage medium data writing, deletion and reading consists of a source of light radiation of three separate various wavelengths X ₁ , X ₂ , and X ₃ for the multi-layer optical storage medium data writing, deletion and reading respectively, which passes through a three-channel modulation system for an independent modulation of radiation of various wavelengths, while the light radiation OfX ₁ wavelength for the writing and the light radiation of X ₂ wavelength for the deletion of the optical storage medium data enters the optical storage medium parallel and perpendicular to the optical storage medium layers by means of an optical focusing device, while the optical focusing system ensures the movement of the light radiation OfX ₁ wavelength for the optical storage medium data writing and of the light radiation OfX ₂ wavelength for the optical storage medium data deletion parallel to the optical storage medium layers and the light radiation OfX ₃ wavelength for the optical storage medium data reading is brought to the optical system ensuring the movement of the light radiation of X3 on a plane perpendicular to the optical storage medium layers, for the purpose of sideways exposure of the respective optical storage medium layer without the photochromic substance, while a part of the light radiation of X ₃ wavelength incident on the area of the optical storage medium layer with the photochromic substance wherein the refraction coefficient ns has been changed comes out through such an area with a changed refraction coefficient n ₃ and falls onto the focusing optical system and proceeds further on through the optical divider to the optical photo-decoder, from where the electric signal is transferred for further processing.

With the device for the multi-layer optical storage medium data writing, deletion and reading, parallel reading can be ensured using additional parallel focusing optical systems.

With the device for the multi-layer optical storage medium data writing, deletion and reading, in another version, a matrix of radiation sources with a light radiation optical focusing system with the number of sources corresponding to the number of the optical storage medium layers without the photochromic material can be used as a source of radiation of λ ₃ wavelength for optical storage medium data reading, while the radiation sources are arranged in such a way that each optical storage medium layer without the photochromic material is illuminated by one radiation source, while the switching of the exposure of the individual layers is realized by means of switching the individual sources of light radiation of λ3 wavelength in the radiation source matrix.

The device for the multi-layer optical storage medium data writing, deletion and reading according to the invention utilizes the current drives enabling the movement, such as the rotation, of the optical storage medium.

The optical storage medium and the device for the optical storage medium data writing, deletion and reading solve the construction problem of an optical disk with the maximum layer recording density owing to the independence of the information signal on the initial medium volume, while the intensity of the reading signal is given exclusively by the radiated power of the "reading" signal and is independent on the photochromic substance concentration and volume. The performance improvement of the presented optical storage medium consists in multiplying the storage capacity by the factor proportional to the number of layers doped with the photochromic substance. The presented disc allows the increase of capacity of removable storage media to hundreds of Gb. The efficiency of the presented disc is very high. This is achieved by: a) The possibility to extend the class of media used for active layers (one-photon, two- photon photochromic compounds) b) A practically unlimited number of recording layers only limited by the practical depth of focusing of the optical lens of the reading-deletion-writing system c) The relatively low requirements on the surface optical quality of the medium uniformity d) The elimination of the effect of crosstalks between the individual layers in the reading process.

The advantages of the device according to the invention are related to its basic feature, introducing the structure of a waveguide of photochrome substances with considerably different refraction coefficient between their two stable conditions.

Technically, this result is obtained by creating a waveguide structure in a passive medium demarcated by two layers of photochromic substances wherein information is recorded in the form of voxel fields with a changed refraction index. The passive layer with the waveguide structure is illuminated crosswise by a light source from its side. The light spreads through the waveguide and diffuses on the voxels with the induced change of the refraction coefficient. Optical storage media based on the waveguide structure consisting of a passive substance layer between two layers containing photochromic media whose refraction coefficient changes during the transition from one isomeric form to another have not been found in scientific and technical literature sources.

The feasibility of the proposed device was examined theoretically, based on the solution of an optical problem of light passage through a waveguide with disorders of the refractive index on the borders, and tested experimentally. Within the framework of the said approach, the validity of the basic attributes for the achievement of the technical objective solved by the presented device was confirmed.

The device for the multi-layer optical storage medium data writing, deletion and reading according to the invention utilizes the already existing DVD drives for the movement (rotation) of the optical storage medium; therefore, the industrial use requires no substantial change of the construction of such drives, but only its extension. The industrial use will be achieved by replacing the hitherto single-wavelength reading unit with a three-wavelength unit or with a set of three units, while the basic construction principles of the regulation and recording system will remain unchanged.

The manufacture of the device for data writing-deletion-reading using the multi-layer optical storage medium according to the invention, compared to analogical devices, requires no special top-quality technologies for the production of storage with the optical devices quality. It is presumed that the costs of mass production of optical storage media according to the invention will be comparable to the costs of the production of common CDs and DVDs. The production of recorded multi-layer discs using the technological procedures of mass production of common discs is also possible.

Overview of figures in the drawing

The invention is clarified in more detail in the attached drawings; Fig. 1 illustrates the layer arrangement of a single-sided and a double-sided multi-layer optical storage medium, Fig. 2a shows a schematic diagram of the exposure of the respective optical storage medium layer without the photochromic material and Fig. 2b shows a schematic diagram of the optical storage medium data writing, deletion and reading; Fig. 3 shows a block diagram of the device for the optical storage medium data writing, deletion and reading in the version with the source of light radiation of three wavelengths while Fig. 4 shows a block diagram of the device for the optical storage medium data writing, deletion and reading in the version where the source of light radiation of λ ₃ wavelength for data reading consists of a matrix of sources with the number of sources equal to the number pf the optical disc layers without the photochromic material.

Examples of the invention implementation

In the sample implementation, the optical storage medium has the form of a multi-layer optical disc consisting of γ and α layers doped by a photochromic substance with the refraction coefficient of n ₂ when the photochromic substance is in condition I, and n3 when the photochromic substance is in condition II, and of β layer without the photochromic substance with the refraction coefficient of ni, while the refraction coefficients satisfy the following relations:

ni> τi2 and m<= n3 (1)

The absorption of radiation for data reading with the λ3 wavelength is negligible in all layers. The layers are stacked alternately with periodicity. The β layer thickness is 10 - 100 um, the α and γ layers thickness is 1 - 3 μm. The optical disc may be single-sided or double-sided. A single-sided multi-layer disc has the α, β and γ layers; the additional layers are added as δ or ε in the direction from the α, β and γ layers on one side only, not on both sides. If the disc has the α, β and γ and ε layers, a polymeric substrate with the refraction coefficient of n ₂ may be used as the α layer. A double-sided multi-layer disc contains the α, β and γ layers and the additional layers are added as δ on both sides from the α, β and γ layers on one side and as ε on the other side (Fig. 1). A polymeric substrate may be inserted in the middle of the disc to

ensure mechanical strength. With a higher number of layers, however, the mechanical strength is sufficient even without this measure owing to the polymers of the α and γ layers. The principle of the "read" mode of a multi-layer optical storage medium is based on a waveguide effect, where the illumination from the transverse side by radiation of λ3 wavelength and the divergence angle <= 2θ in the β layer (Fig. 1) result in the total reflection of the light on the boundaries of the medium with different refraction coefficient and, therefore, in the passage of light in the β layer only. The value of the θ angle must satisfy the following relation:

0 <= θ <= (sin VCn ₁ ² - U ₂ ² ))- ^{1 (2} >

A schematic diagram of the passage is given in Fig. 2a.

The illumination of a particular area of the α or γ layer containing the photochromic material by radiation of λi wavelength results in the transformation of the photochromic substance from form I to form π, whereby the refraction coefficient is changed to n3. Such an area may be considered a phase disorder on the border of two neighbouring layers, where the total reflection will be disturbed. In such a case, the light of λ3 wavelength passing through the β waveguide layer without photochromic substance (Fig. 1) will be either broken and will leave the layer, if the transverse dimension of the area with the changed refraction coefficient exceeds the thickness of the layer doped by the photochromic substance, or scattered in the area, if the transverse dimension of the area with the changed refraction coefficient is smaller than the thickness of the layer doped by the photochromic substance, in which case it will also leave the waveguide layer.

In both cases, such an area will be observable in the optical system with high lighting capacity and short focus as lighter points on a uniform non-contrast background consisting of multiple diffusion on incidental irregularities. The value of the optical system's confocal parameter must be a 0.1 -multiple of the waveguide layer thickness. The data deletion is carried out by illumination of the required photochromic substance area (the condition of the n3 refraction coefficient) with a light of %ι wavelength. This initiates the transition of the photochromic substance to form I with the n ₂ refraction coefficient.

The device for the optical storage medium data writing, deletion and reading 7 in a sample implementation in the first alternative shown in Fig. 3 includes, as a source of light radiation 1, a laser based on Nd:YVO ₄ crystal with the conversion to the second and fourth harmonic, so that the laser radiates the wavelengths OfX ₁ (1.064 μm), X ₂ (0.532 um) and X ₃ (0.266 μm).

This light radiation enters the divider 2 consisting of Y-shaped waveguides (fibre optics) or integrated optical links. The divider 2 has three outputs, while at each of these, the light radiation of only one wavelength, λi, X ₂ or λ ₃ respectively, is present. Furthermore, the radiation enters a three-channel electrically managed optical modulation system 3 _^ which is also based on fibre optics or integrated optical elements. This optical modulation system 3, ensures independent modulation of the light radiation of each individual wavelength. The radiation of λi, %i wavelengths proceeds farther on through the coupling element 4 analogical to the Y-shaped waveguide, but connected reversely, and continues, via the divider 5, through the optical fibre to the optical focusing system input 6 with high lighting capacity. The optical focusing system 6 ensures the progression of the light radiance parallel to the optical disc layers, i.e. in the direction of the rotating optical disc's radius 7.

The radiation of λ ₃ wavelength is brought to the optical system input 8, which ensures satisfaction of the geometrical parameters of the beam according to relation (2) and the setting of the beam's position to the multi-layer optical disc layers without the content of the photochromic substance from the lateral direction. The radiation of λ ₃ wavelength is diffused on the optical irregularities in the photochromic layer. A part of the light radiation of %β wavelength falling upon the area of the optical disc layer 7 with the photochromic material wherein the refraction coefficient n3 has been changed comes out from this area with a changed refraction coefficient n3 and falls onto the optical focusing system 6 and proceeds further through the optical divider 5 to the optical photo-decoder 9, which detects the read information based on the signal amplitude; this electric signal is transferred from the optical photo-decoder 9 for farther processing.

Parallel reading may be enabled by using parallel optical focusing systems 6. Fig. 4 shows a diagram of the device for the optical storage medium data writing, deletion and reading 7 in another implementation. With this alternative, the light radiation source 1 for the light radiation OfX ₃ wavelength for the optical storage disc data reading consists of a matrix with an optical system for focusing the light radiance, with the number of sources corresponding to the number of the optical storage disc layers without the photochromic material 111 arranged in such a way that that each optical storage medium 111 layer without the photochromic material is illuminated by one radiation source is always illuminated by one radiation source, while switching of the exposure of the individual layers is realized by means of switching the individual sources of light radiation of λ3 wavelength in the matrix.

Industrial applicability

The optical storage medium and the device for the optical storage medium data writing, deletion and reading according to the invention may be used in all areas of computing technologies requiring storage of large amounts of data with the possibility to carry out their corrections. Another possible use is recording, storing and replaying of video and audio files.