WO2007023324A2

WO2007023324A2 - High capacity and high speed data storage system

Info

Publication number: WO2007023324A2
Application number: PCT/HU2006/000064
Authority: WO
Inventors: Sándor Lako
Original assignee: Lako Sandor
Priority date: 2005-08-08
Filing date: 2006-08-08
Publication date: 2007-03-01
Also published as: WO2007023324A3

Abstract

The invention belongs to data storage on DVD. The technical result is the increased image sharpness in disk. It is achieved by disposing an image at an angle with respect to wave propagation plane and usage of Bessel-Gauss beams.

Description

HIGH CAPACITY AND HIGH SPEED DATA STORAGE SYSTEM

The object of the invention is a method and apparatus that can provide a further increase in storage capacity and data rate while ensuring compatibility with existing and forthcoming DVD technologies and fulfilling the requirements of industrial-scale reproduction.

Because of space limitations, extensive description of the technology can not be provided here even for replaceable storage media. To narrow the circle, we can safely state that neither silicon nor magnetic based storage devices can compete with the DVD because of their respective cost efficiency. In the following sections, the most important technologies applying optics as the key area will be discussed from the viewpoint of our invention.

It seems practical to differentiate between surface and volumetric technologies. Surface technologies that is coding on one or maximum on a few surfaces, mostly similar to current technologies, generally try to minimize the information coding surface element. The linear dimension of the surface element, however, is a parameter difficult to change, and the limit of data reading and mainly writing is in reverse ratio to this size, which gives little hope to significantly increase data speed. The reduction of the surface element can be achieved optically through reducing and/or increasing the numeric aperture.

The wavelength of solid-state light sources is extremely difficult to reduce below the 405 nm value currently used in the Blu-ray technology. Apart from the traditional tools of increasing the numeric aperture, the technologies moving to the direction of increasing numeric aperture also have to use the effect of evanescent space so that we can achieve NA values higher than the unit. Due to the integration of the evanescent space into the data carrier, which requires extreme preciseness and the elimination of all mechanical interfering effect, this research tendency is highly risky because it jeopardizes the changeability of optical data medium, which is their unique benefit compared to other technologies.

The magneto-optical solutions allow resolution below diffraction limit through the phase switching of the Curie-point which has better localizability than that of the electromagnetic field. The phase switching with controllable size runs on a few per cent of the diffraction limited focal point as a result of which storage capacity significantly increases but this technology has much less yield in the field of data speed. On the one hand, the rate of the linear size reduction is lower than that of the surface element. Besides, the key problem of these procedures is the poor signal- to-noise ratio, the improvement of which can be achieved by the utilization done during the reading of the above threshold effect (Magnetic-Super-Resolution, MSR). As in the case of achieving good signal-to-noise ratio, reading also depends on the threshold effect, apart from the size of the coding surface element, its duration also limits the reading speed. Apart from the reduction of the addressed surface element, using some kind of grey scale is also an opportunity to increase the information content of the surface element. Its gain, however, is limited because secure detection is uncertain even with a few bits, and the related literature allows us to conclude an especially slow writing speed.

Apart from their significant benefits, such as high data speed and data density, we can state in general about volumetric technologies that their mechanical/optical complication, special data medium move their application costs to an order of magnitude different from the surface technologies. These systems are usually burdened with compatibility problems too, and apart from fast and suitable diffraction efficiency of holographic data recording, they require performance higher by order of magnitude than required by diode lasers used in the current optical technologies.

There are of course exceptions almost to all above statements but there is no technology that can offer a solution to the described problems while keeping the mentioned benefits.

One of the promising technologies named C3D is a solution using fluorescent detection. It basically is a system of discs placed on each other where the use of fluorescence is to reduce the noise level of detection to the desired level. Data speed basically equals to the speed of a single- layer disc but the capacity can significantly be increased. Apart from this drawback, this quasi- volume (based on multi-layer structure) data recording technology has the benefit of easily developable compatibility.

With the combination of two-photon fluorescence and confocal detection, data storage volume can significantly be reduced by the sensitivity of the system makes the application of the data medium unlikely. And its price, due to the ultra-short pulse laser systems, excludes its everyday application. The holographic methods using authentic volumetric data storage compared to present day regular optical drivers are much more complicated systems. The volume multiplexed system, though the solution that came up 30 years ago, its cost-effective and robust production has been unsolved up to know. The holographic technology has a joyful domestic relation, too. The current and former researchers of BME (Budapest University of Technology and Economics) have successfully been working in the field of holographic data recording. Their names are related to the development of two prototypes. The holographic safety engineering data card isn't compatible with the currently operating systems and its multimedia application is not practical. The other development, a device using polarization holograms fixed in multi-layer structure aims at a wider scale application. Of the currently researched holographic technologies, this latter one is the device showing the most positive features. The system is much simpler and has higher tolerance limits than other holographic technologies and doesn't contain any moving parts apart from the spinning of the data recording disc and the addressing solution of the data carrier layers. But compared to current systems, it still has complicated optical system, it is relatively costly to manufacture the carrier of one-two hundred layers, addressing the data carrier layers is complicated and increasing its data speed by multiplexing is limited so it motivates further developments in the field of holographic recording, too. As far as volumetric data recording systems are concerned, it should be emphasised that they pose very high requirements with respect to light sources, which makes the applied lasers costly, while systems applying cheap laser diodes are in reality only quasi-volumetric methods that read/write only a single layer at a time.

The preferred implementation of our invention applies laser beams with non-conventional topology. Based on relevant literary sources, in this section we give an overview of methods for producing Bessel beams. Bessel-Gauss beams [F. Gori, G. Guttari, C. Padovani, "Bessel-Gauss beams", Opt. Comm. 64 (6), 491-495 (1987)], or other truncated Bessel beams [L. Vicari, "Truncation of non-diffracting beams" Opt. Comm. 70, 263-266 (1989)] as quasi non-diffracting beams came to the focus of scientific study at the end of the 1980's, mainly because of the especially high focal depth of focusing achievable with such beams. In experimental setups they were first produced by truncating a relatively easily producible laser beam utilizing an annular aperture and focusing the resulting beam (Durnin method). Subsequently, proposals for producing the beams in a laser resonator have also been published. However, a laser resonator generating a beam with Bessel-like topology has not yet been created. The fundamental reason behind the failure to provide such a resonator is that published proposals apply such special optical elements [A. A. Tovar, "Production and propagation of cylindrically polarized Laguerre- Gaussian laser beams", JOSA A 15, 10 2705-2711 (1998)] that cannot be produced in a quality necessary for laser operation.

So far, the application of Bessel-type beams has been oriented towards exploiting their peculiar focusing characteristics. The main appeal of this beam type was originally its unusually high focal depth, but that has been lately complemented by the great variety of realizable polarization states. The topology which is characteristic for Bessel-type beams permits a polarization state which can render focusing sharper than ever [R. Dorn, S. Quabis, G. Leuchs, "Sharper focus for radially polarized light-beam", PRL 91, 23 233901 (2003)].

On grounds of the above, it would be evident to call for the application of this beam type in the field of micromachining, especially for technologies utilizing non-linear or threshold effects, or in certain subfields of multiphoton fluorescence microscopy. However, these applications are seriously hindered by the fact that - at least in case the aim is to reap the advantages of high focal depth — they require significantly higher pulse energies and/or peak power (higher even by orders of magnitude) than setups applying conventional beams with much shorter beam neck focused to a spot of similar half value width. Because these Bessel-type beams can only be derived from conventional beams at the cost of significant energy losses, the development of Bessel-Gauss lasers should be a preferred research direction from the aspect of data recording but also that of micromachining and microscopy. A substantial advance has been made in the field of special intensity-distribution laser beams with the introduction of surface-emitting fibre lasers [Ofer Saphira et al, "Surface emitting fiber-laser" Opt. Express 14 3929-3935 (2006)]. However, these lasers are not mode-synchronized, and polarization properties of the output are not optimal.

From the aspect of the inventive recognition special emphasis should be laid on the work done in the field of superluminal propagation by Zsolt Bor and his colleagues [Z. Horvath et al., "Acceleration of femtosecond light pulses to superluminal velocities by Gouy phase shift" Appl. Phys. B 63 481-484 (1996)], especially their calculations motivating me to examine Gouy phase anomaly. For the deeper understanding of the phenomenon I found it especially fruitful to decompose the angular spectrum of the Gauss beam into Bessel beams. In the case of pulses propagating as Bessel components, a pulse propagating in the direction of the optical axis with superluminal group velocity appeared in case of each component. This has lead me to the recognition that for Bessel beams individual spatial portions of the optical axis having an extension commensurable with the half value width of the focus, and more generally, similar spatial portions of an axis pointing into a direction significantly different from directions within the angular spectrum are not in action connection even in the case when a relatively small separation distance, dependent on their normal-to-axis extension and the angular deviation of the latter axis, is introduced between them. This, on the one hand, means that the positions of constituents of a pattern similar to the above mentioned one, consisting of uniform shapes and being disposed on the optical axis, can be definitely recovered from the diffracted beam, for instance with the application of a relatively simple arrangement described below, in the section dealing with the disclosure of the invention. On the other hand, by reversing this operation it is in principle possible to modulate individual spatial portions of the optical axis separately from one another. Laser beams with conventional topology do not have this property, which, although does not exclude their application for multiplexing, but still affects the stability of the system negatively.

The elaboration on, and the recognition of the technological applicability of another idea, namely that obstructing the central maximum of Bessel beams causes the maximum to reappear in the course of the propagation of the beam [Belyi et al., "Properties of parametric frequency conversion with Bessel light beam" Opt. Comm. 162 169-176 (1999)], also plays a central role in the invention. According to my calculations complete regeneration occurs at the above mentioned "separation distance" from the location of obstruction.

DISCLOSURE OF THE INVENTION

The invention will be presented with reference to the following drawings:

Fig. 1 illustrates the action connections of volume portions applied for coding individual bits, Fig. 2 shows the schematic view of an optical setup applicable for multiplexed data recording utilizing a conventional optical setup,

Fig. 3 shows an arrangement comprising a holographic imaging element, applicable for multiplexed data recording and retrieval,

Fig. 4 shows an implementation of a confocal detector setup applied for the multiplexed retrieval of an aligned bit pattern.

Fig. 5 shows the simplest possible arrangements for a light source emitting a Bessel-Gauss beam, with Fig. 5a showing an arrangement utilizing a high-aperture mirror, Fig. 5b showing an arrangement utilizing a high-aperture lens, and Fig. 5c showing a laser setup utilizing the cylindrical surface of an optical fibre as a coupling-out element.

Fig. 6 illustrates a pulse-regime Bessel-Gauss laser resonator, with Fig. 6a showing an implementation of a Bessel-Gauss laser resonator with mode-synchronized output where the mode synchronizing element is disposed inside the laser-active material, and Fig. 6b showing an implementation of a Bessel-Gauss laser resonator comprising Q-switched output.

Fig. 7 shows implementations of the data storage medium, Fig. 7a showing a PDLC storage medium, and Fig. 7b illustrating a multilayer medium.

Detailed description of the invention with reference to the accompanying drawings

I have recognised that multiplexed optical data storage does not necessarily require the application of a holographic recording method, because superimposed layers of the multi-layer storage medium can be written and read simultaneously with the application of a suitable light source and imaging optical setup. Conditions for a possible simultaneous writing/reading arrangement are illustrated in Fig. 1. Simultaneous writing and reading can be achieved by means of an optical setup where there is significant angular separation between the axis 1 of the inscribed bit pattern and the propagation direction 2 of the Fourier-components of the light beam both during writing and reading. This requirement provides that the bits to be recorded do not interact with one another in case small-size volume elements 3 and a large enough separation distance 4 are applied. The separation distance can be taken to be proportional to the normal-to- axis extension of the volume elements applied for data storage and to the cosine of the smallest angle between the Fourier-components of the beam and the axis of the bit pattern, by way of analogy with diffraction on an oblique slit and in accordance with the principle of superposition. On the other hand, I have recognised that it is possible to simultaneously write to and read from spatial portions located at different distances from the surface of the storage medium by means of an imaging element that is capable of simultaneously handling beams from multiple light sources (or, alternatively, beams generated by splitting the beam of a single light source). This latter setup is shown in Fig. 3.

Fig. 2 shows an optical setup adapted for imaging a bit pattern. According to my recognition, an off-axis imaging transformation constrained only to the extent that it should provide a small enough image of the bit pattern to be recorded (with the pattern playing the role of the object of the imaging), for instance, through a multiple-beam interference effect utilizing a group of apertures that in themselves do not meet this requirement, can in principle be applied for providing multiplexed high-density data recording/storage. A possible implementation of the beam carrying readily retrievable information about the bit pattern can be a beam originating from the bit pattern irradiated by a coherent or incoherent light beam and being apertured by an aperture 3 at least at a relatively large distance from the bit pattern and in the vicinity of the pattern's axis, which forms the image 4 of the bit pattern through an imaging in which the bit pattern acts as object (the imaging indicated by 2f-2f in the drawing). The bit pattern to be recorded can be displayed by means of a liquid crystal modulator 5, which has liquid crystal molecules 9 introduced into a transparent multilayer arrangement consisting of electrode 6 and dielectric 7 layers, through a small bore disposed perpendicularly with respect to the layers. With the application of two voltage levels this modulator arrangement can display any bit pattern, in the sense that locations of the pattern representing Os and Is have different effect on the incident beam. The bit pattern can be displayed by other means, such as utilizing an electro-optical modulator. In the context of the present invention the term "image" is used in a general sense, meaning that some characteristic feature of light is significantly different in locations representing Os from locations representing Is. The most efficient illumination of this arrangement (the illumination for which the loss from input light power is the smallest) is a conical, or alternatively, (multiply) apertured conical, coherent or incoherent illumination. Such an illumination can be provided with little loss and sufficient intensity by means of imaging a long and straight light-emitting volume, or the output of a Bessel-beam laser resonator, onto the bit pattern. From the aspect of bit pattern illumination the application of the above mentioned, Bessel-type beams is preferred. The beams should be imaged such that the imaging conforms to the bit pattern in a way that the smallest cylindrically shaped volume containing the pattern should contain the focal volume of the zero-order Bessel beam, or in case of partially azimuthal polarization, that of the first order Besser beam. This provides high contrast for the imaging, because according to my calculations the entire energy flow crosses this cylindrical surface. In case a Bessel-Gauss beam or other multiple-beam interference effect is applied, as the bit pattern has a length shorter than the bounded focal line, its axial position should conform to the centre of the focal volume for the uniform illumination of the pattern. In case of high-aperture imaging radial polarization plays an important role in minimising the addressed volume. It can be achieved by means of a layer having polarization-dependent optical characteristics being disposed on an optical element, or through polarization rotation. The arrangement according to our invention provides that a bit pattern can be recorded at a given position of the storage medium, with the number of potential recording positions equalling those of one-dimensional methods.

In Fig. 3 an arrangement utilizing a holographic optical element is presented. Again, the bit pattern 1 is imaged to the storage medium 2 as a real image. The holographic imaging element 3 performs the role of focusing incident laser beams 4 coming at different angles onto different layers 5 and to lateral positions slightly differing from one another if necessary (e.g. for the sake of hologram quality and for reducing heat load) but keeping these lateral positions fixed relative to one another, with focusing carried out in a diffraction-limited way and with high aperture, which cannot be achieved with conventional optical elements in case multiple macroscopic laser sources are applied. In this arrangement information is stored during recording by a laser source 4 of a given direction writing to a given layer 5, with individual bits represented by the polarization state or the lack/existence of light entering the imaging element. The holographic imaging optic is capable of effectively generating a radially polarized output 6 from an input beam that is linearly polarized in a suitable direction. With this setup it is preferable to apply a focusing with the smallest possible focal depth instead of a Bessel-type focusing. Also, the holographic imaging element can be applied to provide imaging correction for layers located at different distances from the surface of the storage medium. Fig. 4 shows a confocal optical setup adapted for retrieving the information carried by a bit pattern from light emitted in an active or passive manner by the illuminated pattern. Linear illumination required for volumetric data storage may be provided by conventional laser beams. However, for instance in case using of an edge emitting laser diode focused by cylinder lenses gets displaced, caused an occasional shift in the position of the storage disc, this displacement is not oriented in the direction given by the focal line. Because the magnitude of this displacement falls in the range of the axial position shift of the disc, effective confocal detection of the bit pattern cannot be provided even for disc position shifts of a few micrometers. Thus, as it will become clear later, it is expedient to apply a bit block 1 oriented perpendicularly to the surface, and for illuminating said bit block 1 a multiple-beam interference effect, or in borderline cases, a Bessel-Gauss beam can be expediently applied because in these cases the optical axis of illumination may coincide with the direction of the bit block. As far as the image of the bit block, appearing on the spatial filter applied for confocal detection, is concerned, for the effective operation of the filter it is important to apply a high-aperture imaging 3 (in specific cases, utilizing multiple elements) in the direction perpendicular to the bit block, or utilize the multiple- beam interference of information-carrying beams crossing at a relatively high angle, which results in a high effective aperture. The minimal required numeric aperture in the direction of the bit block is determined only by the requirement that the images of individual volume elements of the bit pattern do not overlap on the detector 7. A particular advantage of our system applied for reading normally oriented bit blocks is that, as far as the light source/storage medium/detector setup is concerned, the orientation and position of the optical axis of illumination, the orientation and position of the axis of the bit block, and the optimal position of the spatial filter 4 applied for confocal detection are independent of the axial position shift 5 of the storage disc, which is the most difficult to control of all uncertainty factors concerning disc position. In order to improve the quality of imaging on the slit or, in case an imaging system generating a sequence of multiple real images is applied, on the multiple-slit system, optical correction elements 6 or alternatively holographic elements can also be utilized. From the aspect of detection Bessel-type beams have the special advantage that the maximum of the zero-order Bessel function is surrounded by a dark annular area. This effect, which does not occur in confocal detection systems utilizing conventional light sources, enhances the contrast of detection. The system has a relatively high tolerance (a few hundredths of millimeters) for position shifts (displacements) occurring in the direction of the optical axis. This shift does not reduce data safety due to the redundancies of the CCD detector and to the markers disposed at both ends of the bit pattern. It is preferable to apply a reflective optical arrangement. The practical application of our data storage method can be seriously hindered by the difficulties arising from the complicated generation of Bessel-type beams. However, as it has been mentioned in relation to the recording of bit patterns, this beam type has a peculiar characteristics, namely that the entire energy flow crosses a narrow cylindrical volume surrounding the focal volume. This property makes it possible to design a laser resonator generating a robust Bessel-Gauss beam. The proposed laser arrangements are shown in Fig. 5. Resonator setups illustrated in Figs. 5a and 5b are rotationally symmetrical to symmetry axis 3. According to our recognition, a number of conventional laser resonators applied for generating Gauss beams have a stable Bessel-Gauss mode besides operating in a conventional way, with a beam path parallel with the optical axis. For instance, Fig. 5b shows the axial section of the Bessel-Gauss mode of a lens-stabilized plane-plane mirror setup. The stability conditions can be computed easily using the ABCD matrix method well known for those skilled in the art. Similar results can be obtained for a resonator comprising a slightly modified reflective optical arrangement (Fig. 5a). A key feature of realizing a Bessel-Gauss beam output is that an optical imaging element/elements 1 (which can be either reflective, refractive or holographic elements, or elements with surface diffraction patterns ) should be applied in the resonator such that the non-paraxial beam path 2 realized by these elements should be confined to the resonator (this requires optical elements with sufficiently high aperture). An important type of possible resonator arrangements of this type exploits significant spherical aberration of the applied optical imaging. However, we have recognised that stable Bessel-Gauss modes usually co-occur with stable linear off-axis modes. Thus, in usual cases there are special conditions needed for the excitability of the Bessel-Gauss mode. This is obvious in the case where no blocking is applied or no loss is introduced with respect to the paraxial beam path. In a resonator with a specially selected pumped volume it can be provided that the gain of the Gauss-mode operation be significantly lower than that of the Bessel-Gauss mode, while the Bessel-Gauss mode is prevented from collapsing into non-paraxial, linear beam paths. Based on the characteristic feature of Bessel- Gauss beams, namely that the entire energy flow crosses a narrow cylindrical surface surrounding the focal volume, population inversion should be generated in the vicinity of the focal area of the Bessel-Gauss mode, in a volume significantly smaller than the mode volume portion of the paraxial or linear off-axis beam paths falling into the laser-active material can provide that the gain of the Bessel-Gauss mode rises above the laser threshold while the gains of other modes remain below it. The cornerstone for pumping optimization is that population inversion should be generated in the entire focal depth, or else the diffraction losses of the Bessel-Gauss mode may increase by multiple orders of magnitude. By means of adjusting the stability ranges of the resonator in special cases it can even be provided that, although the Bessel-Gauss mode is stable in the axial section, in a direction perpendicular to that the mode is not stable. In this case the resonator has a Bessel-Gauss mode but the linear off-axis beam paths disappear and only the Gauss mode should be eliminated, which can be provided by simple means.

Mode competition can be controlled trivially by suitably sized and positioned slits and absorbents to achieve a relatively higher gain for the Bessel-Gauss mode. A similarly obvious solution is the annular configuration of the laser-active volume.

The buildup of the Bessel-Gauss mode can be further fostered if the refractive or suitably coated reflective elements have relatively smaller reflection loss for the Bessel-Gauss mode. Such a (main or supplementary) role can be performed also by birefringent elements. The birefringent element can be the laser-active material 4 itself, the effect of which should be taken into account in the design process of the resonator.

Different reflection losses pertaining to different polarization states of the Bessel-Gauss mode can be utilized to produce radially or azimuthally polarized beams, which can be brought about in the simplest way by applying thin-film optical coatings having different reflection characteristics for S- and P-polarized light, or, in case of refractive optics it could be sufficient to utilize the polarization-dependent reflection occurring in the proximity of the Brewster angle. It should be noted here that the application of diffracting surfaces is only expedient for producing azimuthal polarization, because dark (non-illuminated) rings are formed on the end mirror only in this polarization state.

With the help of birefringent elements beam manipulation aimed at producing beams of special polarization characteristics can be carried out not only inside the resonator but also utilizing an element disposed outside it. Thus, the usually radially polarized Bessel-Gauss output can be transformed e.g. into an azimuthally polarized beam utilizing a birefringent plate of suitable thickness and orientation.

In Fig. 5c an implementation of the long and straight light emitting volume mentioned among suggested light source types in relation to Fig. 2 is illustrated. The laser resonator shown in Fig. 5 c is implemented without any conventional coupling-out element (the figure shows only that portion of the beam path 9 which is of interest here, other elements being illustrated by a rectangle 5). At a given section of the beam path the beam is coupled into an optical fibre 6 which comprises a grating 7 implanted/etched/ablated on its surface, with the grating 7 being adapted for coupling out the beam and producing emitted beam 10. Preferably, an end mirror 8 is evaporated on the other end of the fibre. This out-coupling fibre section can also be comprised in a unidirectional ring laser, of course without the mirror at the fibre's end. With this arrangement it is also possible to provide light emission at the cylindrical surface of the fibre in a short-pulse regime, and polarization characteristics of the emitted light may also be better than in the case of conventional solutions. In case the beam coupled into the fibre is circularly polarized, this polarization state can be transformed to nearly radial for the output beam by means of a suitably anisotropic element.

Fig. 6 shows axial section views of possible implementations of a pulsed Bessel-Gauss laser. For the purposes of the present invention the application of pulsed-regime operation is justified by the improved signal-to-noise ratio of multiphoton processes of the storage medium (to be described later) and the better controllability of heat propagation characteristics. An important property of Bessel-Gauss resonators is that the geometry of the resonator can be generally described by a group delay dispersion (GDD) value (determined also by the wavelength- dependent refractive index of refractive elements), which is of crucial importance from the aspect of short-pulse operation. Dispersion compensation inside the resonator should be designed taking into account this GDD value. For implementing dispersion compensation e.g. chirped mirrors or cylindrically symmetrical refractive surfaces can be applied. Angular dispersion, occurring as an inherent property of the resonator, makes it possible to radically simplify the setup of ultrashort-pulse lasers. This opens up possibilities of miniaturization for our invention, and the application scope of the invention may also be extended to other fields, such as multiphoton microscopy and micromachining. For short-pulse operation an indispensable element is the mode-synchronization unit 1. This can be implemented by exploiting a characteristic feature of Bessel-Gauss beams 2, namely that the entire energy flow crosses the cylindrical surface of a narrow cylindrical volume portion 3 surrounding the focal volume, because mode synchronization using saturable absorbents requires high power density, and in our case mode coupling should be effective for the entire beam. Thus, our mode synchronization unit comprises a spatial portion that covers a focal volume of the Bessel-Gauss mode, and expediently also provides a saturable absorption effect at the wavelength of the laser, at least in the spatial portion in question. To obtain a simple laser arrangement the mode synchronization unit and the gain medium can be implemented as a single element in case a solid state active material 5 is applied, with saturable absorber material 4, for instance quantum dots, being introduced through a small bore into the focal volume generated in the active material. The question of Q-switching arises primarily in relation to the implementation of data recording. With a high repetition rate Q-switched Bessel-Gauss laser it is recommended to apply a saturable absorber material 7 as well, but with longer regeneration time than in the case when short pulses are produced. The location of the absorber material is not a critical factor, and thus it can be disposed at a greater distance from the focal volume if that is required for achieving the desired repetition rate. It should be mentioned at this point that the pulsed laser according to our invention, combined with the imaging optics described referring to Fig. 2 and with a modulator element, can be capable of radically increasing the speed of multiphoton micromachining processes as it is capable of operating on a three-dimensional block, while conventional laser apparatuses can operate only at a single point at the same time. It should also be noted that, combined with the detection arrangement illustrated in Fig. 4, our pulsed laser can be applied in the field of real-time multiphoton fluorescence microscopy. The inventive Bessel-Gauss laser, when applied together with a suitable detection apparatus, is capable of simultaneously scanning multiple points, thereby increasing the temporal resolution of real time scanning microscopy and/or the scanned volume.

Fig. 7 shows two types of storage material adapted for being utilized for our data storage method. The sharp maximum characteristic of the zero-order Bessel-Gauss beam appears in case of strong focusing of a radially polarized beam as a strong maximum of the field strength component parallel with the optical axis. It is important to utilize a data storage material that undergoes modulation by the radially polarized Bessel-Gauss beam in an area that is as small as possible and is abruptly bounded. This can be achieved by providing a material for the storage medium that is anisotropic and its interaction with the field strength component parallel with the optical axis is greater than with components normal to the axis. According to our recognition the writable storage medium applicable for the purposes of the invention can be implemented as a system (Fig. 7a) of liquid crystal droplets 3 doped with photosensitive molecules 1 dispersed in a polymer matrix 2 (PDLC), having a typical thickness of a few tenths of millimeters but at most 1-2 millimeters, with the directors of liquid crystal droplets being oriented in a direction perpendicular to the surface of the storage medium. The storage material may be provided in a disc-like or other shape, conforming to the applied positioning method. Bounding surfaces of the storage medium are covered with a protective layer 5 providing increased mechanical stability. The doped liquid crystal droplets of the data storage material may react more intensively to the axial field strength component, preferably having a far greater absorption coefficient for this field direction than for the direction perpendicular to it. For inscribing information it is preferable to utilize a threshold phenomenon or a sufficiently fast nonlinear effect. Such a suitable threshold phenomenon can be the thermally induced phase transition of the liquid crystal. The material of the liquid crystal droplets is chosen such that it has phase transition temperature values ensuring that at room temperature (with a reasonable margin) the droplets have an ordered phase, blocking the reactions of dissolved molecules contained in them. During recording, however, the material in its lower-viscosity state (resulting from getting illuminated at locations conforming to the bit pattern) should allow a reaction, e.g. photochemical reaction induced either by the laser beam inscribing the phase transition pattern conforming of the bit pattern or another illumination 6 (typically at a wavelength different from the laser causing the phase transition) to proceed. The localisation of photochemical reagents is carried out by the PDLC structure that does not allow to pass reagents from one droplet to another. In order to provide as small data storage volume elements as possible, and also for the sake of exact recording localization a so-called nano-PDLC structure (comprising sub-micron droplets) should be applied. The PDLC structure may provide refractive index matching for certain propagation directions at different wavelengths in a plannable manner, and may also be a means for achieving slightly/partially polarized fluorescence emission. By applying a voltage to a conductive layer 7 disposed on the two sides of the data storage disc, refractive index matching may be different for reading and writing. An important consideration for designing the thermal properties of the medium is that heat accumulated at a given location during recording should not cause without the intention of inscribing a bit pattern such a transition in the liquid crystal droplet that could enable a photoreaction to take place. The application of a Q-switched laser pulse helps reduce the heat load of the storage medium. The photoreaction may be directed towards synthesising new molecules, towards molecular isomerisation, or to an interaction with the molecules of the liquid crystal matrix. During recording our aim is to change optical properties of the medium at the wavelength applied for reading. It is preferable to apply molecules producing multiphoton absorption and effective emission, or to synthesize or activate such molecules during data inscription. The multilayer data storage medium structure shown in Fig. 7b can be produced on an industrial scale. Although - because of the required careful positioning of bit blocks 8 and due to the complex multilayer structure itself - the manufacturing process will likely be more complex than the production of conventional storage media, other suggested solutions for multiplexed data recording at comparably high data rates presently lay far more remote from industrial-scale production. Information is stored in data storage layers 9 by the existence or lack of a dye spot 10 showing multiphoton fluorescence, with the layers 9 being separated from each other by optically inactive layers 11 having a thickness equalling the separation distance. Summary of the advantages of the inventive data storage system

The inventive data storage system is capable of simultaneously writing and reading 32, 64 or even 128 bits (depending on the applied processor technology) at a single position of the storage medium, significantly increasing the speed of both data recording and retrieval, and even opens up possibilities for direct data delivery to optical processors. With the application of the invention the storage capacity of optical storage media can be increased to a few hundred Gbytes. Our system can be compatible with both present-day and forthcoming DVD standards, and is capable of reading a storage medium that can be produced on an industrial scale. By exploiting analogous processes, certain elements of the system can be utilized in the fields of confocal fluorescence microscopy and multiphoton micromachining. The Bessel-Gauss laser resonator, which is the light source of preference of the present invention, may provide a solution for generating ultrashort pulses in a manner simpler than existing methods. Our system records information on the storage medium in a self-adjusting way and thus no special layer structure is required. During data retrieval the system is especially stable/robust with respect to the axial position uncertainty of the medium.

Claims

1. Method for information storage, characterised by that information is stored by imaging a bit block into a data storage medium in such a way that the principal direction of the bit block is set at a substantial angle with respect to the propagation direction of the imaging plane wave.

2. Method for information storage, characterised by that information is recorded in the storage medium by imaging a bit sequence into the data storage medium in such a way that focal volumes utilized for storing individual bits are produced by the imaging element at different depth positions of the data storage medium, and by that the relative positions of said volumes are fixed.

3. Implementation of an appliance based on the method of Claim I₅ characterised by that the bit pattern to be recorded is produced with the application of a light modulator comprising modulating volumes arranged into a block.

4. Implementation of an appliance based on the method of Claim 1, characterised by that the bit pattern to be recorded is illuminated by a Bessel-type beam or simultaneously by multiple coherent beams.

5. Method for reading a column storing information, characterised by that a Bessel-type beam is utilized for the illumination of the pattern.

6. Implementation of an apparatus based on the method of Claim 5, characterised by that it utilizes confocal detection, and by that a linear grating is utilized during confocal filtering.

7. Implementation of an appliance based on the method of Claim 2, characterised by that individual bits of the bit image produced in the storage medium by the single-step recording have different lateral position.

8. Method for micromachining, characterised by that machining is carried out utilizing a modulator consisting of volumes arranged into a block, said volumes having a modulatable optical parameter, with said modulator being adapted for creating a pattern, the machining being carried out by imaging said pattern into a data storage medium in such a way that the principal direction of the bit block is set at a substantial angle with respect to the propagation direction of the imaging plane wave.

9. Method for implementing confocal microscopy, characterised by that a Bessel-type beam is applied for illumination, and a linear slit is applied for detection.

10. Method for producing a Bessel-Gauss laser beam, characterised by that, in order to prevent the truncation of the non-paraxial mode, the resonator comprises suitably dimensioned optical elements and also comprises an optical imaging element or elements permitting the appearance of stable non-paraxial beam paths.

11. Implementation of an appliance based on the method of Claim 7, characterised by that the relatively higher gain per time unit of the non-paraxial Bessel-Gauss beam is provided by introducing a loss with respect to the paraxial beam path or by eliminating the paraxial beam path.

12. Implementation of an appliance based on the method of Claim 7, characterised by that the gain per time unit of the Bessel-Gauss beam is increased compared to the gain per time unit of other beam paths with the application of a population-inverted volume that is smaller than the mode volume portion of other beam paths falling into the laser-active material and is better conforming to the mode volume of the Bessel-Gauss mode.

13. Implementation of an appliance based on the method of Claim 7, characterised by that the laser resonator comprises an optical element with non-perpendicular angle of incidence, and thereby significantly different losses are introduced with respect to selected and non-selected polarization states by means of the coatings of optical elements, or the birefringence of elements, or a near-Brewster angle of incidence, or by any combination of these.

14. Implementation of the appliance according to Claim 10, characterised by that the beam of desired polarization characteristics is produced by means of a birefringent element disposed outside the output of the resonator.

15. Data storage medium, characterised by that the strength of the interaction between the medium and incident light highly depends on the polarization of incident light.

16. Data storage medium, characterised by that during data recording the thermally induced phase transition of a liquid crystal is utilized for blocking or permitting reactions.

17. The data storage medium according to Claim 12, characterised by that the reaction is a photoreaction.

18. Multilayer data storage medium, wherein volume elements applied for data storage are arranged into a block.

19. Implementation of a laser apparatus based on the method of Claim 7, characterised by that in a Bessel-Gauss focal volume of the apparatus a saturable absorbent is applied to provide pulse- regime operation.

20. Implementation of the apparatus of Claim 7, characterised by that a resonator setup having a stable Bessel-Gauss mode and unstable linear non-paraxial modes is applied.

21. Method for producing coherent radiation with special beam characteristics, characterised by that energy is coupled out from the laser resonator by means of a diffraction pattern formed on the cylindrical surface of an optical fibre disposed in the light path.