Magnetooptical reproducing head and apparatus for reproducing magnetically recorded data from multiple tracks.

The invention relates to optical reading on a magnetic recording medium.

The general principle of the magneto-optical effect (Kerr or Faraday effect) is as follows: a thin layer of suitable magnetic material, illuminated by a light beam, changes the properties of the beam in a manner that depends on the magnetic polarization of the layer.

Therefore, writing information on a magnetic tape is a magnetic writing, but reading is optical.

The change of the light beam can be of various nature: changing the reflection coefficient, introducing a variable optical phase delay, or variation of the polarization plane of polarized light. This last phenomenon is the easiest to use.

The magneto-optical pickup used to read magnetic information written in very small areas. So we should be able to register on a magnetic tape a very high density of information in digital form, each small region defining a bit of information. And in particular it should be register parallel tracks each having a succession of digital information.

However, in practice it is not easy to read by an optical beam the information recorded on magnetic tape. a magneto-optical transducer is used, which is the reading head before which scrolls the tape. This transducer comprises a magnetic thin film which can give rise to a magneto-optical effect. The strip carrying magnetic information is scrolled in the immediate vicinity of the head and flow induced changes in the thin layer. It is on this thin layer and not on what the band led a reading laser beam. Attempts have been made in this direction to try to play VCR tapes containing recorded tracks obliquely relative to the tape axis (this is the classic video tape recording); Each track represents a television scan line and the information recorded on this track represent the luminance signal or chrominance for that line. The oblique tracks have a limited length (bandwidth in the oblique direction of the track) and should be read in succession as and when they pass before the band. The reading resolution is the optical resolution of the illumination beam and the reading means of the modulation of the return beam.

Another avenue of research was opened with attempts to register and read the magnetic stripe tracks parallel to the tape direction, this being made possible by the invention of magneto-optical heads and thin-film gap localized linear, such as those described in the patent application EP 0 436 424. If the tape passes a magnetic circuit gap, it generates a flow in the circuit, which depends on the local magnetic polarization of the band, and this flow is directed towards the thin magneto-optic layer which is illuminated by a light beam. The resolution of the reading in of the tape direction is defined by the width of the gap, which is the thickness of a thin layer, therefore typically hundreds or thousands of angstroms. This resolution is much better than the optical resolution. In the direction transverse to the air gap, the resolution is limited by the optic.

To play a tape with parallel tracks with this type of head, so we proposed to carry then the gap of the head as a very narrow line, parallel to the plane of the web and perpendicular to the travel direction of the band. The layers of magnetic circuits of the transducer are etched according to the pattern of tracks to be read, that is to say that delimits areas of magnetic circuits separated from each other and spaced so that each elementary magnetic circuit thus delimited (and ends in a small portion of the common line gap) is always placed in front of the runway must read. All tracks are parallel to the tape direction. Each elementary area of ​​the magneto-optical layer etched is illuminated by a laser beam or a respective portion of the laser beam, and the signal returned from each area is detected separately, for example by the individual pixels of an array of photosensitive sensors transfer load receiving the light beam modulated by the portions of magneto-optical layer. Each pixel then corresponds to a track, and successively receives the information entered on the track; we can therefore read simultaneously by a linear array of sensors different successions of information corresponding to different tracks.

The invention relates to the heads thin film narrow linear gap, and it aims to make it easier to read bytes when the magnetic tracks are very narrow and very close to each other and therefore risk of a share of 'cause crosstalk between tracks and the other from shifting relative to the reading head during the movement of the strip. The invention also relates to an apparatus for reading magnetic data, provided with means for catching fluctuations of the recording medium to a magneto-optical pickup narrow linear gap.

According to a first aspect, the invention is primarily concerned only to magnetic heads for reading information recorded on a magnetic tape in the form of parallel tracks disposed one relative to the other with a pitch less than about 15 microns, and why the problem of alignment between tracks and to read the magnetic circuits that read becomes especially crucial.

According to the invention, it is proposed that for these heads the magneto-optic layer is continuous along a linear head gap over a width corresponding to a large number of tracks (preferably over the entire bandwidth to read), and it is illuminated by a light beam focused on a line parallel to said gap and adjacent to this air gap.

In the reading device, the head will be arranged so that the direction of the linear gap is transverse to the direction of the tracks, and transverse to the travel direction of the recording medium (tape or other) . The linear air gap is also parallel to the plane of the recording medium.

In the following, the "transverse" word is generally used to mean either perpendicular or oblique. The obliquity of the gap relative to the tracks to read is desirable for example when the magnetic information is recorded "with azimuth", that is to say with an oblique angle between the magnetic polarization and the direction of the track.

The recording tracks are generally parallel to the direction of travel of the recording medium, but it will be seen that the head according to the invention has the advantage of allowing the realization of a reading apparatus capable of also reading magnetic strips which tracks extend obliquely to this direction; in this case, the head is arranged such that the linear gap is transverse to both relative to the direction of travel and with respect to the track direction.

Although it was considered until now that the heads magneto-optical multitrack intended to play tracks that pass simultaneously in front of the air gap must operate with an etching of the magneto-optical layer and even different layers forming the circuits magnetic, otherwise the discrimination of closely spaced tracks parallel to the tape direction was impossible, it appeared, however, that not only this print was not necessary, but more the lack of engraving could facilitate the reading of these tracks.

It must be understood that, by its very principle, the magneto-optical reading done indirectly through a transducer, tends to lose the ability to discriminate with respect to each other defining tracks different information. The transducer tends to create crosstalk between these tracks, crosstalk which hardly exist in the case of a direct reading on the tape. The etching of the magnetic layers, when possible, or the mutual distance of the tracks beyond several tens of micrometers apart, were the solutions proposed to address this problem.

According to the invention, it builds structures to narrow linear gap, coupled with an illumination of the magneto-optical layer close this gap, to remove a print head which proves both unnecessary disadvantageous from a technological point of view, and disadvantageous as discussed regarding the catching side fluctuations of the band.

Thus the invention provides a read head for reading on a recording medium (usually a tape) Magnetic parallel magnetic strips arranged in a not less than about 15 micrometers, the head comprising a magneto for this transducer -optical comprising a magneto-optical thin layer near which can scroll through the recording medium, and a narrow linear gap adjacent to this thin layer of general direction parallel to the support plane and transverse to the direction of travel, characterized in that the magneto-optic layer extending continuously along the linear gap over a length corresponding to a large number of parallel tracks (preferably a length corresponding to the entire width of the tape), and in that means are provided for directing a light beam onto this layer along a line parallel to the gap and in the immediate vicinity thereof.

The beam is preferably focused on this line by suitable optics, such as a cylindrical lens extending across the width of the head, that is to say in practice the entire width of the tape to be read.

The portion of sunlit layer, causing the magneto-optical effect is very close to the narrow linear gap; the magnetic flux generated in a portion of magnetooptic layer under the effect of the magnetic information recorded in the immediate vicinity of this band portion, remains independent of the magnetic fluxes which circulate in other neighboring magneto-optic layer portions and correspond to other information stored, despite the extreme proximity of the magnetic tracks. This would not be true if the illumination of the layer was not made in the immediate vicinity of the air gap, the magnetic flux lines in the dispersing layer As one moves away from the air gap since non engraved magnetic circuit is common to all tracks; Moreover, although some crosstalk exists between adjacent tracks, we know that correct crosstalk.

To minimize crosstalk between tracks that tend to naturally create a magneto-optical continuous layer, it further provides that the layer is magnetically anisotropic and its transverse axis of difficult magnetization (preferably perpendicular) to the direction of the linear gap in the plane of the layer. The easy axis is instead parallel to the gap. The magnetic permeability is greater with the difficult axis as along the easy axis, thus spreading the magnetization area in a direction perpendicular to the air gap, which limits the crosstalk between tracks.

The invention is applicable both in the case of planar read heads and non-planar. In the planar technique, the magnetic layers and non-magnetic which form the common magnetic circuit narrow linear gap are deposited in thin layers on a plane face parallel to the surface against which the magnetic tape scroll. In the non-planar technique, the magnetic layers and non-magnetic thin layers are deposited in a plane transverse to the plane of the strip; it scrolls against the wafer layers, obliquely or perpendicularly to the main surface on which were deposited magnetic layers.

The invention is more generally applicable when technology Moves a magneto-optical thin film to a linear narrow gap whose extension direction is parallel to the web plane and transverse to the direction of travel of the band. In an extreme case even, the invention applies when the magnetic circuit of the head comprises a single magneto-optical thin film whose edge extends in a line against which scrolls the web; this structure is possible when the information recorded on the tape are magnetic polarizations oriented perpendicular to the plane of the strip; in this case, the narrow linear gap is formed between the linear end of the magneto-optic layer and the magnetic tape itself.

The invention also relates to an optical reading apparatus of magnetic record carriers with narrow tracks (preferably less than 10 to 15 microns) and close (no spacing preferably less than 10 to 15 microns), the apparatus having a read head with a narrow linear gap, a magneto-optic layer extending continuously along the gap and in the immediate vicinity thereof, the read head being adapted to read the information recorded in the recording medium in the form of parallel tracks, means to scroll through the media in a plane parallel to the narrow gap and in an oblique or perpendicular direction relative to the general direction of the gap, means to focus a beam light on the magneto-optical layer along a line parallel to the gap and adjacent to the air gap, so as to modulate the beam by the magnetic flux flowing through the magneto-optic layer, and optoelectronic means for separately detecting variations in beam modulation due to scroll each track support to the playhead.

The return beam modulated by the continuous magneto-optic layer is preferably focused onto a photosensitive linear detector which may be a photosensitive array. So we can say that an image of the modulation magnetic bias along the length of the magneto-optical layer is projected onto the photosensitive detector. It is therefore also an image of the magnetic information carried by the strip and situated under the linear gap at a given instant that is projected onto the photosensitive detector. The linear detector thus sees at each moment an image of parallel information tracks. The length of the tracks extends in effect in a transverse direction with respect to the narrow gap.

If the tracks of the band are parallel to the running direction (generally) can be assigned to each track respective one or more sensors.

It is preferably provided that means are provided for shifting the relative position of the image of the tracks transmitted by the modulated beam relative to the detector so that a portion of beam modulated by the web of a respective track information falls on the or the elementary sensors corresponding to this track. These means can be mechanical or electrical; they act on the position of the array with respect to the head or the position of the beam relative to the strip, but not on the position of the head relative to the tape; for example the beam may be deflected or deformed by a mirror or a parallel plate whose rotation is controlled electrically. Or the position of the bar is changed by a motor.

The beam deflection may be carried out automatically from the signals read by the array of sensors, or by reference to one or more reference tracks, or even from a correlation between the recorded signals on adjacent tracks, the correlation may show whether there is a stable or increasing crosstalk between channels. It is desirable in this case that the photosensitive sensor has at least two pixels per track.

According to the invention, the same read head can read magnetic stripes having a pitch of tracks and bands with no tracks entirely different, which was impossible in the prior art. It is sufficient that the sensor has a minimum number of pixels corresponding to what is necessary for the not the lowest.

The object of the invention is therefore also very generally a magneto-optical reading apparatus for reading a magnetic recording medium having multiple parallel tracks, the apparatus comprising a magneto-optical transducer with a narrow linear gap and continuous magnetic layer extending along this narrow linear gap, with means for directing a light beam onto the continuous layer according to a line extending close to the gap, means for directing the beam modulated by the layer onto a photosensitive linear detector able to provide differentiated signals resulting from the reading of the separate magnetic tracks, the apparatus being characterized in that it comprises means for relatively moving with respect to one another linear detector and the image projected by the modulated beam on the detector, magnetic information located below the linear air gap to monitor tracks without action on the position of the transducer.

If the detector is a linear array of photosensitive pixels, it is therefore understood that these displacement means consist of adjustable overall deflection means of the modulated beam and / or adjustable displacement of the strip, in both cases without transverse displacement of the transducer.

This playback apparatus is obviously particularly advantageous in the case where the tracks are spaced less than 10 to 15 micrometers from each other.

The device is also particularly advantageous when the tracks of the recording medium are parallel to the direction of travel, but a feature of the invention is that this condition is not mandatory: the unit can play oblique tracks scrolling simultaneously in the air gap, the air gap being transverse to the tracks (and perpendicular or oblique to the direction of travel); a reconstruction of the tracks is then made electronically by processing the detector output signals for assigning each time to a particular track the signals from sensors that receive the image of the track.

Finally, thanks to the continuous layer of the transducer and the tracking means we can now read the magnetic strips which information is recorded on strictly contiguous tracks (without empty space between two guard adajacentes tracks); and this is true even if the adjacent tracks are recorded without azimuths of different magnetic polarization.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the accompanying drawings in which:<ul><li>1 shows the general principle of a magneto-optical reading apparatus;</li><li>2 shows a magneto-optical read head;</li><li>Figure 3 and Figure 4 show plan and edge view, respectively, the magnetic circuit of a head of the type of Figure 2 but multitrack;</li><li>FIG 5 and FIG 6 show plan and edge view respectively, the magnetic circuit of Multitrack read head according to the invention;</li><li>7 schematically shows the playback device with moving means of a photosensitive linear array;</li><li>Figure 8 shows schematically the possibility of beam deflection to reset the playing tracks;</li><li>9 schematically shows the principle of optical correction offset adjustment by tracking and the magnification of the beam; </li><li>Figures 10 to 14 schematically represent various kinds of magneto-optical heads of technologies in which the invention is applicable.</li></ul>

A magneto-optical reading system can be represented schematically as in Figure 1: a light source 10 (preferably a semiconductor laser diode) emits a beam 12 which is collimated by a collimator 14 and focused by a lens 16 on the head 18, reading and in particular on a magneto-optical thin film located close to the magnetic tape 20 to read. The beam is reflected by the thin layer (the system may also operate in transmission rather than reflection), and the polarization plane of the reflected light undergoes a rotation which depends on the magnetic polarization of the thin layer. This magnetic polarization itself depends on the direction of magnetic polarization of the band portion is about to pass the reading head. The reflected beam is focused by a lens 22 on a photosensitive device 24, passing through a polarization analyzer 26. The electrical signal supplied by the photosensitive device depends on the polarization of the light, thus the magnetic polarization of the strip every moment. The signal may result from the difference between the pixels of two photosensitive arrays respectively receiving the two polarization components of the beam.

2 shows more specifically the construction of the head, in a particular embodiment.

The magneto-optical head shown schematically in Figure 1 is made of magnetic layers and non-magnetic superposed, defining a magnetic circuit whose very narrow air gap (a few angstroms or thousands of angstroms) is width the thickness d a non-magnetic layer, said gap extending linearly at the end of the head. In the example of Figure 1, the head comprises a magneto-optical thin film 32, a thin layer of air gap 34, and a layer 30 of magnetic circuit closing, high permeance. These layers are cross-sectional views. They are deposited on a transparent substrate 36. They may be coated with a protective layer 38. All these layers terminate at the end of the head, before which can be scrolled to the recording medium (magnetic tape in principle) 20 . the air gap 34 ', defined at the end of the head in contact with the strip 20, is constituted by the portion of the nonmagnetic layer 34, where this layer is flush with the end of the head. This gap 34 'extends linearly in a direction generally parallel to the plane of the tape, and transverse (typically perpendicular) to the tape direction. The magnetic circuit constituted by the layers 30, 32, 34 closes in the magnetic tape 20 where it is in contact with the air gap 34 '. The magnetic flux circulates in the magnetic circuit, and in particular in the magneto-optical layer 32 with a direction related to the local magnetic polarization of the strip. The light beam 12 illuminates the magneto-optical layer, is reflected by this layer along with its polarization is modulated depending on the direction of the magnetic flux flowing through the layer. The modulated beam is directed to reading means capable of detecting the beam polarization variations. The laser beam 12 is introduced into the transparent substrate, for example by a face 42 parallel to the plane of the thin layer 32, it is totally reflected by a polished face 40 to the layer 32; prisms such as 44 may be provided to allow entry or exit of the beam in the substrate.

With this type of head very narrow linear gap structure, it can achieve read heads fully integrated multitrack, to read magnetic information tracks running parallel to each other along the tape running direction : for this is carried out side by side etching of the magnetic layers and non-magnetic, juxtaposed individual magnetic circuits each comprising a portion of magneto-optical layer 10, a portion of non-magnetic layer 12 and a cover layer portion 14 . the linear gaps of these magnetic circuits are aligned along a generally transverse to the movement of the strip. so you can read different tracks by detecting the modulation of the light beam by each of the individual magnetic circuits. The tracks can be read simultaneously if desired, and if the reading means of the modulated beam so permit.

Figure 3 shows a view in the plane of the layers, diagramming the separation of magnetic circuits to read multiple tracks; and Figure 4 shows a corresponding sectional view, end view of the head.

In Figure 3, the magnetic circuits of the head are etched so as to form respective lead elements, each placed in front of a respective track on the tape. The magneto-optical layer 32 is etched to define particular 32.1 individual zones; 32.2, 32.3, etc. which delimit the individual magnetic circuit and all terminate in a line on the wafer 50 of the head. Is shown by the hatched lines of magnetic data tracks written on the tape 20 which passes in front of the heads (each head to a track) in an oblique plane or perpendicular to the plane of the magneto-optical layer.

In Figure 4, the portion we see the head with the etched layers overlays defining the individual magnetic circuits the gaps which are all aligned: 32.1 superposition, 34'.1, 30.1 for the first circuit, 32.2, 34'.2 30.2 for the second, etc .. These layers are deposited on the substrate 36 and are covered with a protective layer 38. again, there is shown by dashed lines magnetic tracks that pass before the individual heads in quasi- contact with the edge of the linear gap 34 '.

So the head is an integrated multi-head; the laser beam 12 illuminates all the separate magneto-optical layer areas; the beam modulated by each zone is returned to a linear array of photosensitive sensors such as a sensor corresponds a respective single head and therefore a respective track of the tape 20.

According to the invention, it is proposed to retain, for a multi-head for reading closely spaced tracks, the structure of non-etched planar layers which is normally reserved for a single track head or a head for reading tracks sufficiently distant from each others.

It was found in fact that in the case of magneto-optical layers very thin, we could discriminate parallel tracks very close, even without etching of the magneto-optical layer, particularly when the light beam illuminates the magneto-optical layer very close to the linear gap situated in front of the passage of the magnetic tape.

Typically, it is proposed that for read heads for parallel tracks of spacing pitch less than 15 micrometers (preferably less than 10 or even 5 micrometers), the illumination of the magneto-optic layer by the beam is made on a distance which is substantially of the same order (less than 10 to 15 micrometers for example) of air gap edge against which the web travels.

5 shows schematically, in a view similar to that of Figure 3, the DC magnetic circuit structure according to the invention. The layer 32 is continuous over the entire width of scanning of the magnetic tape.

The magneto-optical layer is sufficiently thin that the magnetic flux lines due to the polarization of a track of the magnetic tape, remain localized without excessive crosstalk with the flux lines that would come from neighboring tracks, at least near immediate vicinity of the air gap 34 '. the magneto-optic layer therefore is illuminated in a narrow linear area 52 very close to the edge 50 of the diaper, that is to say very close to the linear gap 34 'which is in contact over its entire length with the band. Preferably, it is a cylindrical lens that focusses the illumination beam along this line, for example a thickness of several micrometers and a distance of a few micrometers of layer 50 of the board.

6 shows, in a view similar to Figure 4, the magneto-optical layer continues to head structure according to the invention.

preferably will arrange to give a strong magnetic anisotropy to the magneto-optical layer, giving it a difficult axis of magnetization in the plane of the layer perpendicular to the direction of the gap. The magnetic anisotropic permeability will be greater in a direction away from the air gap in a direction parallel to the gap. The magnetizing field due to the information recorded on a runway strip is thus spread more easily in a direction away from the air gap in a direction parallel to the gap, reducing crosstalk between tracks very relatives.

It follows, contrary to what one might think in the prior art, a sufficiently small transverse distribution of information to read by very narrow and very close tracks.

This magnetic anisotropy, which is sort of a pre-magnetization of the magneto-optical layer present even when there is no magnetic information to be read on a tape, is carried out either by an external field parallel to the direction of linear air gap or by an additional ferromagnetic layer inducing an anisotropy magnetic coupling, or by intrinsic property of the magneto-optical layer (deposited under a magnetic field for example).

To give some further explanation, we can say that with an isotropic layer the magnetization would decrease significantly a factor 1 / e (e = 2,7) at a distance of 1 / π times the track width. With an anisotropic layer, this decrease occurs at a distance in the report increased permeability between the hard axis and the easy axis.

The layer 32 is continuous, there is no problem of positioning with respect to respective tracks of the magnetic tape, while in the prior art it was necessary exact positioning of the mulitpiste head relative to the tracks. Now it is only the position of the photosensitive sensors compared to the image of tracks of the magnetic tape to watch. This means you can reposition playing tracks by simply deflecting the modulated return beam or by simply moving the photosensitive array with respect to the image conveyed by the return beam if the cross fluctuations of the band movement shift the image, it will act on the relative position of the return beam modulated with respect to the reading module, but not on the position of the head relative to the tape. In particular, a correlation between neighboring pixels of a photosensitive array may show such a shift of the reading. We can then correct the situation by beam deflection or shift of the bar. The correlation between neighboring pixels can be done either from the signals from the useful leads or from signals from reference tracks. Thus, a servo track playback, based on the signals from the detector itself.

The displacement means of the photosensitive array or beam deflection can be diverse: electromagnetic (motor, electromagnet) or piezoelectric particular.

In Figure 7, beam position correction is ensured by a longitudinal movement of a holder 56 carrying the bar 24 of photosensitive sensors, by means of a motor M. For simplicity, there is shown the head 18 with its edge 50, seen in the X direction of travel of the belt 20 (direction perpendicular to the plane of the paper), and immediately above, the return laser beam 54 modulated after reflection on the linear zone 52 of layer 32 and directed on a bar linear photosensitive. This representation simplified for ease of understanding, ignores the fact that the beam paths can be more complex between Zone 52 and the bar, and do not take into account the optical means on the one hand to project on the array an image of the modulation of the beam 54 and on the other hand to separate the beam of the polarization components (since the modulation is a variation of polarization).

Under these conditions, if the strip is transverse fluctuations in the Y direction, that is to say in its own plane but perpendicular to the direction of travel, the motor M will be ordered to make a corresponding lateral displacement of the strip up 'to replace the pixels that are assigned to a specific track of the tape at the location where the beam portion reaches effectively modulated by the track. The displacement of the bar is in the longitudinal direction thereof. A correlation between the signals from the pixels assigned to adjacent tracks to be aware of any possible error position to act accordingly on the engine.

There are in principle one pixel per track to be read, but can also be several when such correlation should be performed.

Figure 8 depicts very schematically another method of tracking correction: the read head is fixed, the photosensitive array is fixed relative to the head, but a parallel plate 60, rotatable through a motor or an electromagnet not shown, laterally shifting the image projected by the return beam 54, in a direction tending to retain always the same pixel the image portion corresponding to a given track; This restores the error due to transverse fluctuations of the band.

Parallel blade could also be a deformable optical electric control (lens or blade associated with piezoelectric means), to accomplish the same goal. Also, a rotating mirror or a deformable mirror, electromagnetic or piezoelectric drive, inserted in the path of the return beam, may restore the slopes positioning errors.

In addition, an important aspect of the invention is the ability to correct deformities of the band, particularly because of its aging: a band tends to stretch out by dint of being used; in this case, the slopes tend to approach. A correction is possible by providing an adjustable optical magnification on the return path of the beam, this light to play tracks distributed to a step that may vary (over time or from one band to the other). It also can play tapes recorded in different standards and not track widths gradissement a value is assigned to each standard.

This possibility is schematically represented in Figure 9 by an optical means 70 (symbolized by a lens) having two possible correction direction to take account of both a lateral shift and a variable track spacing relative to the pixels receive the beam modulated by these tracks. Of course, a combination of means of acting on the beam 54 described above is also possible, or a combination of an action on the beam and a displacement of the optoelectronic reading means 24. In general, the invention allows for the desired correction (lateral offset and / or magnification) by simple action on the return beam or by moving the playback bar, but without transverse movement of the read head relative to the tape.

Generally the means for relative displacement of the image and the detector may comprise slaving means for maintaining the image of a determined track on the sensors which are assigned to it, and these means can use the signals from the detector linear itself.

This reading will be understood from the foregoing that the original principle of adjustment or automatic correction tracking is applicable regardless of the width and the pitch of the tracks. It is related only to the fact that the magneto-optic layer is continuous.

The foregoing is considered to simplify the narrow gap of the head extends in a direction perpendicular to the tape direction. But in the case of records with azimuthal polarization (obliquity of the magnetic polarization with respect to the track), the head is placed with a corresponding orientation of the gap obliquely to the running direction.

Similarly, it was considered that the tracks are generally parallel to the direction of travel, but can be oblique, while still marching in parallel under the gap. The narrow gap is oriented transversely to the strip running direction and relative to the direction of the tracks.

The invention applies to various types of multitrack heads realization technologies. The description above referred to a technology ticular, non-planar type.

In non planar technology, magnetic and non-magnetic layers are deposited on a major surface which is not the face to which the tape travels. Therefore, these layers extend transversely to the plane of the strip. In planar technology, the layers are formed on one side of the head before which scrolls the web; they thus extend essentially in a plane parallel to the strip, although some portions of these layers (and particularly the active portion magnetooptical illuminated by the beam 12) are optionally in oblique or perpendicular to the main side planes.

Figures 10 to 14 schematically shows some general principles of magneto-optical heads which the invention is applicable.

10 is a simple reminder of the principle of non-planar head of Figure 2: the head comprises a substrate on which are deposited two magnetic layers separated by a nonmagnetic layer. One of the magnetic layers is the magneto-optical active layer, it can be directly on the substrate (which must be transparent to let the light beam).

Figure 11 is a variant SM magnetic substrate has a non-magnetic layer 34 itself topped by a magneto-optical layer 32; the illumination does not occur with the substrate because it is normally not transparent to the optical wavelengths used (visible light in principle).

12 shows a particular case wherein the magnetic circuit is constituted by a single magneto-optic layer extending on a nonmagnetic substrate, this layer being transverse to the strip. The linear air gap is formed by the gap 34 'between the edge of the layer and the strip itself. This solution is possible when the tape door magnetic domains oriented perpendicular to the plane of the strip; because in this case they induce in the magnetooptic layer a corresponding magnetic orientation without need of a sealing layer with high permeance. The lighting may be from the substrate side (which must then be transparent) or on the other side.

13 shows a schematic planar solution with two portions of thin magnetic layers 32 ', 32' '(one at least capable of exhibiting a magneto-optic effect) deposited side by side on a main face of the head ( transparent substrate, the light being effected by following the magnetooptical layer) and separated by a gap in very narrow non-magnetic material extending linearly over the entire width of the tape to be read.

Finally, Figure 14 shows another planar solution, wherein the portion of magneto-optical layer truly active and will be illuminated by the beam is oblique or even perpendicular to the main surface of the head, although this layer is formed by depositing on the main surface of the head.