FR2643470A2 - Improvement to the recording and the reading of information in an electrooptical material - Google Patents

Abstract

Device according to Claim 35 of the main patent, characterised in that the electric reading field is modulated at a defined frequency and at a defined amplitude.

Description

IMPROVEMENTS ON RECORDING AND PLAYBACK
INFORMATION IN AN ELECTRO-OPTICAL MATERIAL
The present addition refers to the main patent application filed on July 22, 1988 in the name of the applicant, and relates to a device and a method for recording and reading information in an electro-optical material
In the main patent application, a method and a device have been described capable of freezing the image of an electric field to which such a material is subjected, with a view to applications such as the recording of data or signals, and in particular a process in which the temperature is raised in at least one zone of such a material beyond a transition level from which the orientation of the molecules endowed with electro-optical activity of this material can be modified by external action; an electric field is applied in this zone, the direction and / or intensity of which are determined as a function of optical anisotropy characteristics of the material which it is desired to obtain and, during the application of said field, the temperature in said field is lowered area below the transition temperature to freeze the orientation characteristics of the molecules of the material induced by the field applied in the material, hereinafter called the orienting or polarizing field. The intensity of the electric field applied to the material is modulated in at least one direction of space so as to vary an optical anisotropy characteristic of this material along this direction. For this purpose, it is possible in particular to create a relative displacement, in this direction, of the material and of 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 of the characteristic of resulting optical anisotropy in the material along this direction constitutes a recording of this signal or of these data according to a parameter which can be represented by the variation of anisotropy of the material in this direction.

The creation and the modulation of the anisotropy intended to carry out the writing on the material, can be carried out according to two different modes. The first mode consists in heating locally and applying a global electric field. The second mode consists in heating overall and applying a local electric field.

Writing therefore consists in producing in the material a succession of non-oriented and oriented zones, which constitute the data in the form of bits with two states.
O or 1.

Advantageously, the device for implementing this known writing method comprises local heating means consisting of a laser beam emitter capable of being focused on the support. In this case, it is possible to provide electrodes on either side of the support; the electrodes are made of a material which is both conductive and capable of letting the light of the laser beam pass. The electrodes are for example made up of two plates having a potential difference between them, in the vicinity of which the support passes.

To read such a recording, an electric field is necessary to highlight the variations of said anisotropy characteristic of the material along the direction considered. One can, for this purpose, subject the material to an electric field, hereinafter called detector, developer or reading. An optical detection device measures the variations in the electro-optical activity of the material along said direction to read the information - or the signal which initially modulated the orienting electric field. To do this, we carry out the analysis of a light beam passing through the support (placed under an electric reading field) and some of whose characteristics have been modified by the anisotropy of the material (such as birefringence). detecting the modification due to the applied electric field, of an optical property of the material constituting the strip. Figure 4 of the main patent schematically shows a reading device, comprising. means making it possible to reveal 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. after having crossed the strip, is received by polarimetric type analysis means and comprising a lens focusing the intensity beam - 1o, a polarization splitter, two photodetectors (receiving respective intensities I2 and II) whose outputs are connected to a differential amplifier attacking a circuit for processing electrical signals.

According to a first aspect, the present invention aims to make reading of the information even more efficient, regardless of the optical characteristic measured / detected on reading; in particular, the invention proposes to make the reading insensitive, or at the very least very insensitive, to surface defects of the support (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 originating from the reading.

One can in particular use an amplitude modulation at a determined frequency.

The device according to the invention allows detection of the information with increased reliability, since it makes it easier to distinguish the polarized zones from the non-polarized zones. Indeed, the output signal is at the frequency of the reading field and amplitude modulated by the recorded signal. The read field constitutes the carrier signal amplitude modulated by the recorded signal. In the case of a recorded signal of the analog type, the reading device comprises a circuit for processing the output signal of the type used in the field of radio detection (by rejection of noises outside the detection band). In the case of a recorded signal of the digital type, with two states O or 1, the processing circuit comprises a bandpass filter centered on said frequency or a high pass filter with cutoff frequency substantially lower than said frequency; the output signal comprises a succession of constant signals (bit O) and periodic signals (bit 1) at the modulation frequency of the read field. In addition, the light losses, due for example to a local defect on the material (dust), do not affect the detection of the signal, since the detection does not rely on the average level of the signal (very sensitive to the amount of light) but on the differentiation between constant signal and modulated signal; thus, any local fault on the support will only have the effect of reducing the average level of the signal at this location.

Preferably, the frequency of modulation of the electric reading field is appreciably higher than the frequency corresponding to the rate of recording / reading of the information on the support.

Advantageously, the signal is modulated so that it has some modulation periods for elementary information.

Advantageously, the processing circuit also comprises, in parallel with the bandpass filter, a lowpass filter whose cutoff frequency is significantly lower than the modulation frequency of the read field.

The use of a low-pass filter makes it possible to control the operation of the device; in fact, the level O of the output signal, at the output of the bandpass filter, corresponds either to a bit 0, or to the absence of light; the latter case, on the contrary, gives rise to the output of the low-pass filter, at a zero signal, easily distinguishable from the average level of the signal.

According to a second aspect, the present invention provides 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 the anisotropy characteristics can be carried out in several directions of the space inside the material. We obtain a multidimensional recording. It is thus possible to write at different levels in the thickness of a support of the sheet or strip type. Indeed, by local heating of each given level, under a given orienting field, a recording of the information in volume is carried out, according to the three dimensions of the support. FIG. 8 of the main application shows an exemplary embodiment making it possible to write in the thickness of a strip at several levels. The strip passes between electrodes connected respectively to ground and to the writing potential Several laser transmitters (four in the 'example described) are arranged so as to be able to emit a light beam sequentially in a given direction; semi-transparent mirrors returning the four respective beams in a single direction on the path of which there is a lens which focuses the beam passing through it in an area of the strip located at a given level (in the thickness of the strip ) #. The emitters emit light beams at respective respective wavelengths, such that each beam is focused at a given level in the thickness of the strip. Reading is carried out in the same way, by successively focusing a beam of reading in the direction of alignment of the zones of increasing depth recorded during the writing in each of the zones.

However, this known device, although very advantageous, has limitations. On the one hand, the bits located on the optical reading axis in the thickness of the strip, must be sufficiently distant from each other, so that, during reading, the influence of the bits located above and below the bit on which the read beam is focused, is negligible. On the other hand, even assuming fulfilled the previous condition, it can happen that all the bits located on the read beam path, respectively # below and above the bit read, achieve an effect on the optical detection signal canceling that of the bit read; although statistically unlikely, such a situation would have disastrous consequences on reliability.

The invention, according to a second aspect, overcomes these drawbacks and avoids any crosstalk between the 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 to a given depth, a plurality of means, incorporated into the thickness of the support of electrooptical material, and. able to each produce a local field in a given layer of the support corresponding to a given depth.

According to an advantageous embodiment, the information medium comprises, in the thickness direction, a succession of tri-layer complexes, namely a first conductive layer (forming first electrode), a second electrooptic layer (receiving information), a third conductive layer (forming a second electrode), each tri-layer complex, located at a given depth in the support, 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
electrooptical layer: 1 and 3 microns inactive layer 1 l and 20 microns
For example, the electrodes are made of tin oxide Son02, or itrium oxide Ito.

The writing and / or reading device according to the invention advantageously comprises, on the one hand a source of zero voltage and a source of non-zero voltage, both of which can be connected respectively, for the three-layer complex in which one wishes to write / read, at the first and at the second electrode (so as to create an electric field for writing / reading in the electrooptic layer situated between the electrodes), and on the other hand means capable of selectively heating and locally a portion of said electrooptical layer corresponding to elementary information (or bit). Advantageously, the local heating means consist of a laser beam emitter capable of being focused on said portion of the electrooptic layer.

A third aspect of the invention proposes an advantageous writing mode of the type with global heating and local electric field.

FIG. 9 of the main patent application represents an exemplary embodiment of a device making it possible to create a local electric field by depositing electrical charges located on the surface of the strip and comprising means for global heating of the strip (oven tunnel). The strip comprises a first layer of electrooptical material and a second transparent layer of photoconductive material # trice that is to say, whose conductivity increases with the illumination. On one side of the strip, there is provided a transparent and conductive film constituting an electrode connected to ground. In front of and in contact with the transparent photoconductive layer, is arranged a transparent electrode connected to a potential v. Under the effect of a writing laser beam, a conduction channel is created between the zone in the upper part of the photoconductive layer brought to the potential v, and the zone in the upper part of the strip where the control beam is focused. ; this latter zone is then at potential v and accumulates electrical charges which are locally maintained due to the non-conductivity of the polymer constituting the strip, creating an electric field in the thickness of the strip under the effect of which the electro molecules -active are oriented. The strip is brought, upstream of the writing means, to a temperature such that it remains at a temperature above the critical temperature Tc for a sufficient time after the appearance of the potential v on the area to be polarized, so that the orientation of the molecules takes place, which is frozen during the subsequent cooling of the strip.

This embodiment has the advantage of using a low power laser transmitter.

The invention, according to a third aspect, proposes to improve and perfect this writing mode.

To this end, according to the invention, the method of writing information in a support made of an electro-optical material is characterized in that a material is used having both electro-optical and photoconductive properties.

In the case of a strip-shaped support, the writing / reading device comprises a conductive plate disposed opposite one face of the strip and brought to a non-zero potential, while the other face of the strip is at zero potential; an insulating plate is provided between the conductive plate and the corresponding face of the strip. The writing laser beam passing through the material creates a conduction well between the parallel faces of the strip, and therefore causes the local writing field to be canceled.

The invention will be clearly understood in the light of the following description with reference to the # sin in which: - Figure 1 is a side diagram of a device
reading according to a first aspect of the invention - Figures 2 and 3 are diagrams showing the
electrical device output signals from the
Figure 1 - Figure 4 shows in schematic section a portion
tape provided with field generation means
rooms at different depths - Figure 5 is a side diagram of the device
writing according to a third aspect of the invention;
and - Figure 6 schematically shows a variant of
realization of that shown in figure 4.

FIG. 1 shows a device for reading information carried on a previously recorded tape in one of the ways indicated in the main patent application. The device of Figure 1 is similar to that shown in Figure 4 of the main patent; its constituent elements similar to this last figure bear the same references.

The strip 1 is lit by a reading laser beam 14 emitted by the laser transmitter 131; the lower plate 11 forming the ground electrode and the upper plate 12 forming the electrode is brought to a positive reading potential so as to place the strip in an electric reading field, normal to the plane of the strip. The reading beam 14 ′ after having crossed the strip, is received by the analysis means 16 of the polarimetric type, comprising a lens 17 focusing the beam of intensity Io, a polarization splitter 18 (for example in the form of a Wollaston prism), two photodetectors 19 and 20 (receiving respective intensities I2 and II) whose outputs are connected to a differential amplifier 21 driving a processing circuit 22 of electrical signals. The beam 14 ′ coming 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 the measurement of the rotation of the polarization of the light induced by the electro-optical effect of the material constituting the strip. The analysis means 16 therefore detect a succession (and alternation) of so-called oriented and non-oriented areas such as the hatched area 15. The signal
S detected at the output of amplifier 21 is
S = II - I2 = 10 sin TETA, with TETA the angle of polarization; we have
TETA = A Vlecture, with
A = B writing
where B is a constant representative of the physical parameters of the band.

The read potential is modulated at a given frequency, for example a few tens of kHz.

The device further comprises electronic processing means 220 of the signal from the comparator 21, and comprising a bandpass filter 230, delivering an output signal S1, in parallel on a low pass filter 240 delivering an output signal S2. The bandpass filter is centered substantially on the modulation frequency of the read field, while the cutoff frequency of the low pass filter is significantly lower than said frequency.
Thus, the output signal S1 is of the form represented in FIG. 2 (81 as a function of time), and comprises an alternating succession of portions at constant level (in this case zero) representative of bits 0, and portions of levels modulated at the modulation frequency of the read field, representative of bits 1. The constant level corresponds to an average level of the signal.

The different signal portions SI corresponding respectively to bits 0 and 1, are easily distinguishable from one another, regardless of the surface condition of the strip. Indeed, a local fault (dust, scratch) will only result in reducing the average level of the signal, without affecting the modulation.

The above description refers to a binary record (0 or l). It is possible to apply the modulation of the read field to recordings, intermediate between the binary mode (mentioned above) and the analog mode made up of a set of discrete values of the write signal.

The low-pass filter 240 has an advantageous optional character; it delivers a signal S2 of constant level (corresponding to the average level) since the portions modulated at the modulation frequency are deleted.

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, intervening at time t1, the level of the signal S2 becomes zero, while at the output of the bandpass filter 220, the absence of light does not not different from level zero.

FIG. 4 shows a sectional view of a strip forming an information medium and in accordance with a second aspect of the present invention making it possible to write and read information at different depths in the strip.

The strip consists of a succession in the thickness direction of three-layer complexes referenced a, b, c, d and e. Each complex comprises a first layer forming a first transparent electrode 300, a second layer 301 made of eletrooptical material carrying information, and a third layer 302 forming a second transparent electrode. Each complex is separated from the next by a separation layer 303 made 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
The writing or reading of information on the tape described above is carried out using for example a device comprising the local heating means described in the main patent application in relation to FIG. 8 of said application , and hereinafter recalled
Several laser transmitters, in this case four, referenced 30,31,32,33, are capable of emitting a beam respectively 34,35,36,37 t of semi-transparent mirrors 38,39,40,41, aligned according to an axis oblique to the longitudinal axis of the strip returning the four respective beams in a single direction 42 on the path of which there is a lens 400 which focuses the beam 43 passing through it in an area 44 of the strip located at a given level (in the thickness of the strip). The emitters 30, 31, 32, 33 emit light beams at respective 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 of a given level under field the electrical applied by the electrodes 4 and 5, which modifies the orientation of the molecules of said area. The active molecules of the written areas are therefore oriented in an electric field orthogonal to the plane of the strip.

In order to write or read in a given layer, the laser beam 305 is focused on said layer and the first electrode 300 is connected to a non-zero potential V and the second electrode 302 to ground, which creates an electric field in layer. The electrodes of the layers (or tri-layer complexes) disposed above the layer being written / read are at the same potential as the upper electrode of the latter; similarly, the electrodes of the lower layers are at the potential of the lower electrode of the written / read layer.

This avoids any crosstalk between the layers located at different depths, because writing or reading is done by the simultaneous application of an electric field and heat and this, locally, that is to say only at the desired location corresponding to a bit; the latter for example has dimensions of the order of 1 micron x 1 micron.

FIG. 5 schematically represents from the side a device according to a third aspect of the invention, and intended to allow writing by application of global heat and of a local electric field, in particular by photoconductive effect.

The ban # 1 of electrooptical material runs in a tunnel oven 200 heating the latter uniformly; a laser beam emitter 530, disposed at the outlet of said oven, emits a beam 540 orthogonal to the plane of the strip. Opposite one of the faces (for example upper) of the strip is provided a conductive plate 550 brought to a writing potential, while the other face of the strip is connected to the ground. Between the conductive plate 550 forming an electrode and the corresponding face of the strip, is provided a plate of insulating material 570.

The entire material of the strip is previously oriented or generally polarized by the tunnel oven.

The material of the strip has, in addition to its electro-optical properties, photoconductive properties, that is to say that its conductivity increases with the illumination.

The laser beam 540 creates a conduction well 560 in the strip, which amounts to electrically connecting the parallel faces of the strip; the electric field inside the material, in the area of well 560, is therefore canceled.

Writing according to the method described above is therefore carried out by local cancellation (in a given bit) of the prior global orientation given to the material.

Alternatively, the insulating plate can be replaced by an insulating layer attached to the electrooptic and photoconductive layer.

It should be noted that the laser used can be of low power, and in any hypothesis of power much lower than that necessary to heat in order to write / read used in the other modes.

The foregoing description, relating to any of the aspects of the present invention, refers to a tape. The invention can also be applied to a disc or a card.

In the case of a card, the thickness of the support being generally large compared to that of the electrooptic layer, it is advantageous to provide at least one conductive layer forming an electrode on one of the faces of the electrooptic layer, so as 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 respective aspects. FIG. 6 shows a corresponding exemplary embodiment, namely seen in section (in its thickness), an information medium (in the form of a strip), in several layers, and in which the information can be recorded at several levels in the thickness of the material. A laser transmitter 600 emits a laser beam 601 capable of being focused at a given depth in the material. The strip comprises two multilayer assemblies. The first multilayer assembly comprises an insulating layer 603, an electrooptic and photoconductive layer and an electrode layer 604 (connected to ground); the second assembly comprises a first electrode layer 606, an electrooptic 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 (made of electro-optically inactive material), while the second assembly rests on another separation layer 609.

Opposite the upper face of the strip (that is to say facing the insulating layer 602) is arranged a conductive plate brought to a potential
V. The electrode layers in the strip are likely to be connected to this potential V. In the same way as described in relation to FIG. 5, the laser beam 601 creates a conduction well in a limited portion of the electrooptical layer and photoconductive 607, and thus causes the cancellation of the field inside said portion.

Claims (17)

1 - Device according to claim 35 of the patent  main, characterized in that it comprises  means for modulating the electric field of  reading. 2 - Device according to claim 1, characterized  in that the modulation means are specific to  modulate the field alternately at a frequency  data and the optical beam analysis means  Readers include 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 claims  previous, characterized in that the means  optical analysis also include  parallel on the bandpass filter, a filter  low pass with cutoff frequency  significantly lower than the frequency of  modulation of the reading field. 5 - Process according to claims 13 and 14 of  main patent for reading information  worn on an electro-optical material,  characterized in that the field is modulated  electric 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 we filter the signal in frequency  analysis based on the material. 7 - Method according to one of claims  previous, characterized in that one realizes in  parallel low pass signal filtering  analysis, at a substantially cut-off frequency  lower than the modulation frequency. 8 - Device according to claim 27 of the patent  main, characterized in that it comprises, in addition  means capable of locally heating the material  at a given depth, a plurality of means,  incorporated into the thickness of the material support  electrooptical, and capable of each carrying out a  local field in a given support layer  corresponding to a given depth. 9 - Device according to the preceding claim,  characterized in that the information carrier  includes, in the thickness direction, a  succession of tri-layer complexes, namely a  first conductive layer (forming first  electrode), a second electrooptical layer  (receiving information), a third  conductive layer (forming a second  electrode), each tri-layer 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  - electrooptical layer: 1 and 3 microns - separation layer: l and 20 microns ll - 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 tri-layer complex  in which we want to write / read, at the first  and to the second electrode (so as to create a  electric field of writing / reading in the layer  between the electrodes), and  on the other hand means capable of heating  selectively and locally a portion of said  electrooptical layer corresponding to a  basic information (or bit). 12 - Device according to the preceding claim,  characterized in that the heating means  local consist of a beam transmitter  laser capable of being focused on said  portion of electrooptical layer. 13 - Process according to claim 19 of the patent  main, characterized in that a  material with both properties  electrooptical and photoconductive. 14 - Device for implementing the method according to  the preceding claim, characterized in  that, the support being in the form of a strip, it  includes a conductive plate forming  electrode, arranged opposite one of the faces  of the band and brought to a non-zero potential,  while the other side of the strip is one  zero potential, and in that it further comprises  a layer of electrically insulating material  disposed between said electrode and the face  correspondent of the band.  15 - Device according to the preceding claim,  characterized in that said insulating layer is  attached to the corresponding side of the strip. 16 - Device according to one of claims 1, 8 and  13, characterized in that the information carrier  is in the form of a strip or disc. 17 - Device according to one of claims 1, 8 and  13, characterized in that the information carrier  is a map, comprising at least one layer  support and an electrooptical layer. 18 - Device according to the preceding claim,  characterized in that the electrooptical layer is  separated from the support layer by a layer  conductive forming electrode.