ADVANCED TRAINING IN RECORDING AND READING INFORMATION IN AN ELECTRO-OPTIC MATERIAL

Abstract

Device according to claim 35 of the main patent, characterized in that the reading electric field is modulated at a determined frequency and a determined amplitude.

Description

IMPROVEMENTS IN RECORDING AND PLAYBACK INFORMATION IN AN ELECTRO-OPTIPLUE MATERIAL This addition refers to the main patent application filed on July 22, 1988, in the name of the applicant, and concerns a device and a method for recording and reading information in an electro-optical material. In the main patent application, a method and device for freezing the image of an electric field to which such a material is subjected is described, for applications such as data or signal recording, and in particular a method in which the temperature is raised in at least one area of ​​such a material above a transition level beyond which the orientation of the electro-optically active molecules of this material can be modified by an external action; an electric field is applied in this area, the direction and / or intensity of which is determined according to optical anisotropy characteristics of the material that one wishes to obtain, and, during the application of said field, the temperature in said area is lowered below the transition temperature to freeze the orientation characteristics of the material's molecules induced by the applied field in the material, hereafter referred to as the orienting or polarizing field.The intensity of the electric field applied to the material is modulated in at least one spatial direction so as to vary an optical anisotropy characteristic of that material along that direction. This can be achieved, in particular, by creating a relative displacement, along that direction, of the material and the source of the electric field. If the modulation of the electric field is produced by a signal or data to be recorded, the variations in the resulting optical anisotropy characteristic of the material along that direction constitute a recording of that signal or data as a function of a parameter that can be represented by the variation of the material's anisotropy in that direction. The creation and modulation of anisotropy, used to create writing on the material, can be carried out in two different ways. The first method involves localized heating and the application of a global electric field. The second method involves global heating and the application of a localized electric field. Writing therefore consists of creating in the material a succession of unoriented and oriented regions, which constitute the data in the form of two-state bits. O or 1. Advantageously, the device for implementing this known writing process includes local heating means consisting of a laser beam emitter capable of being focused onto the substrate. In this case, electrodes can be provided on either side of the substrate; the electrodes are made of a material that is both conductive and capable of transmitting the light from the laser beam. The electrodes are, for example, made of two plates with a potential difference between them, in the vicinity of which the substrate passes. To read such a recording, an electric field is necessary to highlight the variations in the material's anisotropy characteristic along the considered direction. For this purpose, the material can be subjected to an electric field, referred to hereafter as a detector, developer, or reading field. An optical detection device measures the variations in the material's electro-optical activity along this direction to read the information—or the signal—that initially modulated the orienting electric field. This is achieved by analyzing a light beam passing through the medium (placed under a reading electric field) whose characteristics have been modified by the material's anisotropy (such as birefringence). The reading process relies on detecting the modification, due to the applied electric field, of an optical property of the material constituting the recording.Figure 4 of the main patent schematically represents a reading device, comprising means for revealing the anisotropy of the material, in the form of a strip, previously oriented using a field and a writing laser beam, both substantially orthogonal to the plane of the film. The reading beam, after passing through the strip, is received by polarimetric type analysis means comprising a lens focusing the beam of intensity -10, a polarization splitter, two photodetectors (receiving respective intensities I2 and I1) whose outputs are connected to a differential amplifier driving an electrical signal processing circuit. According to a first aspect, the present invention aims to make the reading of information even more efficient, regardless of the optical characteristic measured / detected during reading; in particular, the invention aims to make the reading insensitive, or at least very insensitive, to surface defects of the medium (such as scratches or dust). To this end, according to the invention, the electric reading field is modulated and the effect of this modulation is detected in the optical signal from the reading. One can, in particular, use amplitude modulation at a specific frequency. The device according to the invention enables information detection with increased reliability, since it makes it easier to distinguish polarized from unpolarized areas. Indeed, the output signal is at the frequency of the reading field and amplitude-modulated by the recorded signal. The reading field constitutes the carrier signal amplitude-modulated by the recorded signal. In the case of an analog recorded signal, the reading device includes an output signal processing circuit of the type used in radio detection (by rejecting noise outside the detection band).In the case of a recorded digital signal with two states (O or 1), the processing circuit includes a bandpass filter centered on that frequency or a high-pass filter with a cutoff frequency significantly lower than that frequency. The output signal comprises a succession of constant signals (bit 0) and periodic signals (bit 1) at the modulation frequency of the readout field. Furthermore, light loss, due for example to a local defect in the material (dust), does not affect signal detection, because detection is not based on the average signal level (which is very sensitive to the amount of light) but on the differentiation between the constant signal and the modulated signal. Thus, any local defect in the medium will only have the effect of reducing the average signal level at that location. Preferably, the modulation frequency of the electric field reading is substantially higher than the frequency corresponding to the recording / reading rate of the information on the medium. Advantageously, the signal is modulated in such a way that it exhibits a few modulation periods for an elementary piece of information. Advantageously, the processing circuit also includes, in parallel with the bandpass filter, a lowpass filter whose cutoff frequency is substantially lower than the modulation frequency of the reading field. The use of a low-pass filter allows control of the operation of the device; indeed, the level O of the output signal, at the output of the band-pass filter, corresponds either to a bit 0, or to the absence of light; this latter case gives rise, on the contrary, at the output of the low-pass filter, to a zero signal, easily distinguishable from the average level of the signal. According to a second aspect, the present invention proposes a method and a device for writing and reading information in volume, in the thickness of an electro-optical material. As mentioned in the main patent application, the modulation of anisotropic characteristics can be achieved in several spatial directions within the material. This results in multidimensional recording. It is thus possible to write at different levels within the thickness of a sheet or strip substrate. Indeed, by locally heating each given level under a specific orienting field, a volumetric recording of information is achieved, along the three dimensions of the substrate. Figure 8 of the main application shows an example of an embodiment enabling multi-level writing within the thickness of a strip.The tape runs between electrodes connected respectively to ground and write potential. Several laser emitters (four in the example described) are arranged so that they can sequentially emit a light beam in a given direction; semi-transparent mirrors reflect the four respective beams in a single direction, along whose path is a lens that focuses the beam passing through it onto a specific area of ​​the tape located at a given level (within the tape's thickness). The emitters emit light beams at different wavelengths, such that each beam is focused at a specific level within the tape's thickness. Reading is performed in the same way, by successively focusing a reading beam in the direction of alignment of the areas of increasing depth recorded during writing in each of the areas. However, this device—although very advantageous—has limitations. Firstly, the bits located on the optical reading axis within the thickness of the strip must be sufficiently far apart so that, during reading, the influence of the bits located above and below the bit on which the reading beam is focused is negligible. Secondly, even assuming the previous condition is met, it can happen that all the bits located on the reading beam path, respectively below and above the bit being read, exert an effect on the optical detection signal that cancels out the effect of the bit being read; although statistically unlikely, such a situation would have disastrous consequences for reliability. The invention, according to a second aspect, remedies these drawbacks and avoids any crosstalk between layers located at different depths. To this end, the device according to the invention comprises, in addition to means capable of locally heating the material at a given depth, a plurality of means, incorporated in the thickness of the electrooptic material support, and capable of producing each a local field in a given layer of the support corresponding to a given depth. In accordance with an advantageous embodiment, the information carrier comprises, in the thickness direction, a succession of tri-layer complexes, namely a first conductive layer (forming the first electrode), a second electro-optical layer (receiving the information), a third conductive layer (forming the second electrode), each tri-layer complex, located at a given depth in the carrier, being separated from the next by an inactive separation layer. Preferably, the thickness of the layers is between the following values Electrode layer: 100 and 5000 Angstroms Electro-optical layer: 1 and 3 microns; inactive layer: 1 and 20 microns For example, the electrodes are made of tin oxide SonO2, or of itrium oxide Ito. The writing and / or reading device according to the invention advantageously comprises, on the one hand, a zero-voltage source and a non-zero-voltage source, both capable of being connected, respectively, for the three-layer complex in which writing / reading is desired, to the first and second electrodes (so as to create a writing / reading electric field in the electro-optical layer located between the electrodes), and on the other hand, means for selectively and locally heating a portion of said electro-optical layer corresponding to an elementary piece of information (or bit). Advantageously, the local heating means consist of a laser beam emitter capable of being focused on said portion of the electro-optical layer. A third aspect of the invention proposes an advantageous method of writing of the type with global heating and local electric field. Figure 9 of the main patent application illustrates an example of a device for creating a local electric field by depositing localized electric charges on the surface of the strip, and includes means for heating the strip globally (tunnel furnace). The strip comprises a first layer of electrooptical material and a second transparent layer of photoconductive material, i.e., whose conductivity increases with illumination. On one side of the strip, a conductive and transparent film is provided, forming an electrode connected to ground. Facing and in contact with the transparent photoconductive layer is a transparent electrode connected to a potential v.Under the influence of a laser writing beam, a conduction channel is created between the upper part of the photoconductive layer, held at potential v, and the upper part of the strip where the control beam is focused. This latter area is then at potential v and accumulates electrical charges that are maintained locally due to the non-conductivity of the polymer constituting the strip. This creates an electric field within the thickness of the strip, under the influence of which the electroactive molecules orient themselves. Upstream of the writing mechanism, the strip is heated to a temperature such that it remains above the critical temperature Tc for a sufficient duration after the potential v appears on the area to be polarized, allowing the molecules to orient themselves. This orientation is fixed during the subsequent cooling of the strip. This method of implementation has the advantage of using a low-power laser emitter. The invention, according to a third aspect, aims to improve and perfect this method of writing. To this end, according to the invention, the method of writing information into a medium made of an electro-optical material is characterized in that a material exhibiting both electro-optical and photoconductive properties is used. In the case of a strip-shaped medium, the writing / reading device comprises a conductive pad positioned opposite one face of the strip and held at a non-zero potential, while the other face of the strip is at zero potential; an insulating pad is placed between the conductive pad and the corresponding face of the strip. The laser writing beam passing through the material creates a conduction well between the parallel faces of the strip, thus causing the local cancellation of the electric writing field. The invention will be better understood in light of the following description with reference to the drawing in which: - Figure 1 is a side view diagram of a device Reading according to a first aspect of the invention - Figures 2 and 3 are diagrams showing the electrical output signals of the device Figure 1 - Figure 4 shows a schematic cross-section of a portion of a band equipped with means for generating fields premises at different depths - Figure 5 is a side view diagram of the device writing according to a third aspect of the invention; and - Figure 6 schematically shows a variant of realization of the one shown in figure 4. Figure 1 shows a device for reading information stored on a previously recorded tape, in one of the ways indicated in the main patent application. The device in Figure 1 is similar to that shown in Figure 4 of the main patent; its constituent elements, similar to those in the latter figure, bear the same reference numerals. The strip 1 is illuminated by a reading laser beam 14 emitted by the laser emitter 131; the lower plate 11, forming a ground electrode, and the upper plate 12, forming an electrode, are held at a positive reading potential Vreading so as to place the strip in a reading electric field, normal to the plane of the strip. The reading beam 14', after passing through the strip, is received by the polarimetric analysis means 16, comprising a lens 17 focusing the beam of intensity I0, a polarization splitter 18 (for example, in the form of a Wollaston prism), two photodetectors 19 and 20 (receiving respective intensities I2 and I1) whose outputs are connected to a differential amplifier 21 driving an electrical signal processing circuit 22. The beam 14' from the film is oblique with respect to the normal to the film, and likewise for the optical axis of the optical elements 17, 18, 19, and 20.The reading is based on the detection and measurement of the rotation of the polarization of the light induced by the electro-optical effect of the material constituting the band. The analysis means 16 therefore detect a succession (and alternation) of so-called oriented and non-oriented zones such as the hatched zone 15. The signal. S detected at the output of amplifier 21 is S = II - I2 = 10 sin TETA, with TETA the polarization angle; we have TETA = A Vlecture, with A = B writing where B is a constant representing the physical parameters of the band. The potential Vlecture is modulated at a given frequency, for example a few tens of kHz. The device further includes electronic signal processing means 220 for the signal from the comparator 21, comprising a bandpass filter 230, delivering an output signal S1, in parallel with a low-pass filter 240 delivering an output signal S2. The bandpass filter is centered substantially on the modulation frequency of the readout field, while the cutoff frequency of the low-pass filter is substantially lower than said frequency Thus, the output signal S1 is of the form represented in Figure 2 (81 as a function of time), and comprises an alternating succession of portions at constant level (in this case zero) representing bits 0, and portions of levels modulated at the modulation frequency of the reading field, representing bits 1. The constant level corresponds to an average level of the signal. The different portions of the SI signal, corresponding respectively to bits 0 and 1, are easily distinguishable from one another, regardless of the surface condition of the tape. Indeed, a local defect (dust, scratch) will only decrease the average signal level, without affecting the modulation. The preceding description refers to a binary recording (0 or 1). It is possible to apply playback field modulation to recordings intermediate between binary mode (mentioned previously) and analog mode, consisting of a set of discrete values ​​of the writing signal. The 240 low-pass filter has an advantageous optional characteristic; it delivers a constant level S2 signal (corresponding to the average level) since the portions modulated at the modulation frequency are suppressed. The signal S2 is non-zero as long as a certain amount of light is detected; in the event, for example, of failure of the detection means or of the laser, occurring at time t1, the level of the signal S2 becomes zero, whereas at the output of the bandpass filter 220, the absence of light is indistinguishable from the zero level. Figure 4 shows a cross-sectional view of a strip forming an information carrier and conforming to a second aspect of the present invention allowing information to be written and read at different depths in the strip. The tape consists of a succession, along its thickness, of three-layer complexes labeled a, b, c, d, and e. Each complex comprises a first layer forming the first transparent electrode 300, a second layer 301 of electro-optical material carrying the information, and a third layer 302 forming the second transparent electrode. Each complex is separated from the next by a separating layer 303 of inactive material. The respective thicknesses of the different layers are, for example: electrode layer: 1000 Angstroms electrooptical layer: 1 micron Separation layer: 4 microns Writing or reading information on the above-described tape is carried out using, for example, a device comprising the local heating means described in the main patent application in relation to Figure 8 of said application, and recalled below. Several laser emitters, in this case four, referenced 30, 31, 32, 33, are capable of emitting a beam respectively 34, 35, 36, 37 from the semi-transparent mirrors 38, 39, 40, 41, aligned along an oblique axis relative to the longitudinal axis of the band, reflecting the four respective beams along a single direction 42 on the path of which is a lens 400 which focuses the beam 43 passing through it into a zone 44 of the band located at a given level (in the thickness of the band). The emitters 30, 31, 32, 33 emit light beams at different respective wavelengths, and such that each beam is focused at a given level 44, 45, 46 or 47. The beam 43 focused by the lens 400 heats an area at a given level under the electric field applied by the electrodes 4 and 5, which changes the orientation of the molecules in said area.The active molecules in the written areas are therefore oriented in an electric field orthogonal to the plane of the band. To write or read from a given layer, the laser beam 305 is focused onto that layer, and the first electrode 300 is connected to a non-zero potential V and the second electrode 302 to ground, creating an electric field within the layer. The electrodes of the layers (or three-layer complexes) located above the layer being written / read are at the same potential as the upper electrode of that layer; similarly, the electrodes of the lower layers are at the potential of the lower electrode of the layer being written / read. This avoids any crosstalk between layers located at different depths, because writing or reading is done by the simultaneous application of an electric field and heat, and this is done locally, that is to say only at the location sought corresponding to a bit; the latter has dimensions on the order of 1 micron x 1 micron. Figure 5 schematically represents, from the side, a device according to a third aspect of the invention, intended to allow writing by application of global heat and a local electric field, in particular by photoconductive effect. The electro-optical strip 1 passes through a tunnel furnace 200, heating it uniformly; a laser beam emitter 530, located at the furnace's exit, emits a beam 540 perpendicular to the plane of the strip. Opposite one face (for example, the upper face) of the strip is a conductive plate 550 held at a potential Vwriting, while the other face of the strip is connected to ground. Between the conductive plate 550, which acts as an electrode, and the corresponding face of the strip, is a plate of insulating material 570. The entire material of the strip is pre-oriented or globally polarized by the tunnel furnace. The material constituting the strip has, in addition to its electro-optical properties, photoconductive properties, that is to say that its conductivity increases with illumination. The 540 laser beam creates a 560 conduction well in the strip, which amounts to electrically connecting the parallel faces of the strip; the electric field inside the material, in the area of ​​the 560 well, is therefore canceled. Writing according to the previously described process is therefore achieved by local cancellation (in a given bit) of the prior global orientation given to the material. Alternatively, the insulating wafer can be replaced by an insulating layer applied over the electro-optical and photoconductive layer. It should be noted that the laser used may be of low power, and in any case of power much lower than that required to heat in order to write / read used in other modes. The preceding description, relating to any aspect of the present invention, refers to a tape. The invention is also applicable to a disk or a card. In the case of a card, since the thickness of the substrate is generally large compared to that of the electro-optical layer, it is advantageous to provide at least one conductive layer forming an electrode on one of the faces of the electro-optical layer, in order to reduce the total thickness of the layers subjected to the electric field. It is possible to combine and associate the features of the invention according to its second and third aspects, respectively. Figure 6 shows a corresponding embodiment, namely, a multi-layered information carrier (in the form of a strip) viewed in cross-section (through its thickness), in which information can be recorded at several levels within the material's thickness. A laser emitter 600 emits a laser beam 601 that can be focused to a given depth within the material. The strip comprises two multi-layered assemblies.The first multilayer assembly comprises an insulating layer 603, an electro-optical and photoconductive layer and an electrode layer 604 (connected to ground); the second assembly comprises a first electrode layer 606, an electro-optical and photoconductive layer 607, a second electrode layer 608 connected to ground; these two multilayer assemblies are separated from each other by a separation layer 605 (of electro-optically inactive material), while the second assembly rests on another separation layer 609. Opposite the upper surface of the strip (i.e., facing the insulating layer 602) is a conductive plate held at a potential V. The electrode layers in the band are likely to be connected to this potential V. In the same way as described in relation to Figure 5, the laser beam 601 creates a conduction well in a limited portion of the electro-optical and photoconductive layer 607, and thus causes the field to cancel within said portion.

Claims

DEMANDS 1 - Device according to claim 35 of the patent principal, characterized in that it comprises means of modulating the electric field reading. 2 - Device according to claim 1, characterized in that the modulation methods are specific to modulate the field alternately at a frequency data and optical beam analysis methods The reading system includes a frequency filter. 3 - Device according to claim 2, characterized in that the frequency of the electric field of reading is significantly higher than the frequency corresponding to the cadence recording / reading information on the support. 4 - Device according to one of the claims previous ones, characterized in that the means optical analysis also includes, in parallel to the bandpass filter, a filter low-pass filter whose cutoff frequency is significantly lower than the frequency of modulation of the reading field. 5 - Method according to claims 13 and 14 of main patent for reading information carried on an electro-optical material, characterized in that the field is modulated electrical reading and we detect the effects of this modulation in the signal from the reading. 6 - Method according to the preceding claim, characterized in that the field is modulated alternately in amplitude and to perform the detection involves frequency filtering the signal analysis derived from the material. 7 - Method according to one of the claims previous ones, characterized in that one realizes in parallel low-pass filtering of the signal analysis, at a cutoff frequency substantially lower than the modulation frequency. 8 - Device according to claim 27 of the patent principal, characterized in that it comprises, in addition means capable of heating the material locally at a given depth, a plurality of means, incorporated into the thickness of the material support electro-optical, and capable of each producing an local field in a given layer of the support corresponding to a given depth. 9 - Device according to the preceding claim, characterized in that the information support includes, in the direction of thickness, a A succession of three-layer complexes, namely a first conductive layer (forming the first electrode), a second electro-optical layer (receiver of the information), a third conductive layer (forming a second electrode), each trilayer complex, located at a given depth in the support, being separated from the next by an inactive layer of separation. 10 - Device according to the preceding claim, characterized in that the thickness of the layers is between the following values - Electrode layer: 100 and 5000 Angstroms - Electro-optical layer: 1 and 3 microns - Separation layer: 1 and 20 microns - Device according to claim 8, characterized in that it includes, on the one hand, a source of zero voltage and a non-zero voltage source, connected respectively, for the three-layer complex in which one wishes to write / read, at the first and to the second electrode (so as to create a electric field of writing / reading in the layer electro-optical structure located between the electrodes), and on the other hand, means capable of heating selectively and locally a portion of said electrooptical layer corresponding to a elementary information (or bit). 12 - Device according to the preceding claim, characterized in that the heating methods local units consist of a beam emitter laser capable of being focused on said portion of electrooptic layer. 13 - Method according to claim 19 of the patent principal, characterized in that one uses a material exhibiting both properties electro-optical and photoconductive. 14 - Device for implementing the process according to the preceding claim, characterized in this that, the support being in the form of a strip, it includes a conductive plate forming electrode, positioned opposite one of the faces of the band and brought to a non-zero potential, while the other side of the strip is at one zero potential, and in that it further includes a layer of electrically insulating material arranged between said electrode and the face corresponding to the band. 15 - Device according to the preceding claim, characterized in that said insulating layer is reported on the corresponding side of the strip. 16 - Device according to any one of claims 1, 8 and 13, characterized in that the information medium is in the shape of a strip or a disc. 17 - Device according to any one of claims 1, 8 and 13, characterized in that the information medium is a map, comprising at least one layer support and an electro-optical layer. 18 - Device according to the preceding claim, characterized in that the electro-optical layer is separated from the support layer by a layer conductive forming electrode.