Abstract:
The disclosure relates to a magneto-optical reading device for multi-track magnetic tapes, including a flat large incident beam directed onto the active part of a magneto-optical read head and reflected by this head onto an array of sensors, characterized in that it includes a correction device able to move the zone of incidence of the beam on the read head in order to keep the beam optimally positioned on the active part of the read head as the latter suffers progressive wear. The invention increases the useful working life of such a reading device.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to devices able to read magneto-optically and simultaneously all the parallel tracks recorded on a magnetic tape. It is notably applicable to magnetic tape systems for recording high-density digital data, known under the name SDCR (Static Digital Cassette Recording). 
   2. Description of the Related Art 
   The SDCR high-density magnetic tape reading/recording system was designed in the central R&amp;D laboratory of the THOMSON-CSF company with the goal of producing a high-density magnetic tape recording system operating simultaneously on a set of parallel tracks, without using classic rotating heads. For this purpose the SDCR uses a highly original recording device incorporating a magnetic head having a set of integrated poles laid out matrix-fashion, and a Kerr effect magneto-optical read head of substantially monolithic structure. The write and read heads therefore enable parallel recording and reading of extremely high data density, despite the fact that the fabrication of the SDCR is very simple compared with rival systems. 
   More particularly, the Kerr effect read head includes, in a manner now known to professionals of the art, a prism comprising a stack of suitable materials. It is illuminated by a laser producing a flat polarized incident beam whose thickness is adapted to the dimension of the bits written on the magnetic tracks, and whose width is sufficient to cover the whole breadth of the magnetic tape. After reflection on the sensitive layer of the head, the polarization of the optical beam changes according to the direction of the magnetization induced by each recorded track of the magnetic tape. This polarization change is transformed into an intensity change, by a polarizer for example. The beam is then received on an array of sensors, of CCD (charge-coupled device) type for example. Each of the cells of this CCD then delivers a signal representative of the information recorded on each of the tape tracks. 
   The system described succinctly above has been divulged in more detail in a set of patents describing the global architecture of the system and certain special aspects of its fabrication. We can notably mention the following French patents filed by the applicant:
     8917313 (28 Dec. 1989), publication no. 2 656 723,   84 07761 (18 May 1984), publication no. 2 564 674,   93 01407 (7 Feb. 1993), publication no. 2 701 332,   90 00546 (18 Jan. 1990), publication no. 2 657 100,   92 11146 (18 Sep. 1992), publication no. 2 696 037,   8614974 (28 Oct. 1986), publication no. 2 605 783,   87 14818 (4 May 1987), publication no. 2 622 335,   88 05592 (27 Apr. 1988), publication no. 2 630 853,   96 08393 (5 Jul. 1996), publication no. 2 750 787.   

   This system has enabled satisfactory prototypes to be built. Experiments on these prototypes have however revealed a number of problems whose resolution would enhance both the performance and working life of devices made according to this system. 
   One of these problems concerns the wear of the magnetic read head. 
   Although this read head is made from particularly abrasion-resistant materials, it is still subject to wear caused by the movement of the magnetic tape over it, which leads to progressive degradation of the head&#39;s performance and ultimately to its failure. This is moreover common to all systems in which there is contact between the read head and the magnetic tape, in particular the systems with rotating heads used today. 
   SUMMARY OF THE INVENTION 
   To overcome this disadvantage, or at least to significantly slow down this phenomenon, the invention proposes a magneto-optical reading device for multi-track magnetic tapes, including a flat large incident beam directed onto the active part of a magneto-optical read head and reflected by this head onto an array of sensors, characterized in that it includes correction means able to move the zone of incidence of said beam on said read head in order to keep said beam optimally positioned on said active part of said read head as the latter suffers progressive wear. 
   According to a characteristic of the invention, said correction means include a first flat parallel-faced plate positioned on the path of said incident beam and rotatable around a first axis parallel to the plane of said incident beam and perpendicular to its axis. 
   According to another characteristic, said correction means also include a second flat parallel-faced plate positioned on the path of the reflected beam returned by said read head and rotatable around a second axis parallel to the plane of said reflected beam and perpendicular to its axis, this second plate being provided to correct the displacement of the reflected beam associated with the displacement of said incident beam caused by the rotation of said first plate. 
   According to another characteristic of the invention, said second plate also includes a third rotation axis perpendicular to the plane of said reflected beam so as to be able to move this reflected beam in its own plane in order to keep the spot beams of this beam modulated by said magnetic tape tracks on the corresponding sensors of said detection array, and thereby compensate for any wandering movements of the magnetic tape. 
   According to another characteristic of the invention, the device includes a controlled system that drives said correction means to assure permanent and optimal compensation for wear of said read head. 
   Other characteristics and advantages of the invention will become clear on reading the following description of a preferred embodiment, taken only as non-limitative example, with reference to the attached drawing which shows schematically a read head according to the invention in operation with a magnetic tape. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is an exemplary embodiment of a magneto-optical reading device of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the FIGURE, a magnetic tape  101  carrying a set of parallel tracks  102  moves over a read head  103 . The structure of this head is known and is therefore shown only schematically and transparently to show the area of contact of the head with the magnetic tape. 
   The active part  104 , magnetized by the tape, is a very narrow straight strip (about 2 to 3 microns wide). In the FIGURE it is shown at the extreme left-hand edge of the read head  103 . 
   This active part is illuminated by a laser (not shown in the FIGURE) whose polarized light beam is shaped by a known optical system (not shown) such that its dimensions match those of the active part  104 . 
   This beam  105 , oriented to illuminate the active part  104 , is reflected from this active part  104  with its polarization modulated according to the orientation of the bits in the magnetic tracks  102 , to form a return beam  106  comprising a series of modulated spot beams which are directed onto an array  107  of sensors, of CCD type for example. Each sensor  108  in this array receives one of the spot beams of the beam  106  which has been modulated by one of the tracks  102 . The intensity received by each of these cells depends on the modification of the beam&#39;s polarization caused by the magnetic state of the track being read. Polarizing plates (not shown, but known to professionals of the art) are used to transform this polarization change into an intensity change that can be detected by the cells  108 . 
   The interest of this system is that the reading of the tracks is not individualized at the actual head  103 , unlike magneto-resistive heads which necessitate a longitudinal segmentation adapted to the tracks and a tracking system to follow the tracks on the magnetic tape and compensate the lateral wandering of the tape, both of which are very difficult to implement. 
   Individualization of the tracks is therefore transferred to the array of sensors  107 , which is a well-known and commercially-available device. 
   Compensation for tape wandering is achieved using a flat parallel-faced plate  109  that can rotate about an axis  110  perpendicular to the plane of the beam  106 . The movement ω x  thus obtained enables this beam  106  to be moved in its own plane while remaining parallel to itself, according to the well-known optical properties of flat parallel-faced plates. A servo system, of known type, is used to keep the image of the tracks  102  on the cells  108  to which they are assigned, such that each of these cells always delivers the signal recorded on the corresponding track with the maximal level possible—and despite movements of the magnetic tape in its own plane. 
   To operate correctly the system requires contact of the magnetic tape  101  with the head  103 . This inevitably leads to wear of the head  103  over time, especially in very high bit-rate systems in which the tape may be spooled at very high speed. Although this wear is very slight in absolute terms, owing to the very small dimensions of the active part  104 , its relative value is not negligible. Consequently means of compensating for this wear is essential if the device is to have a reasonable working lifetime. 
   In the example shown in the FIGURE, the active part  104  will be displaced by a distance Z after a few hundred hours of operation, to a position  111  which is situated off the plane of the incident beam  105 . 
   To be able to move this incident beam onto the zone  111 , the invention provides for a second flat parallel-faced plate  112  that can rotate about an axis  113  parallel to the plane of the incident beam  105  and perpendicular to the beam&#39;s axis. In this manner, and according to the well-known optical action of flat parallel-faced plates, the movement ω z′  of this flat parallel-faced plate moves the incident beam  105  in a direction perpendicular to its plane beam while maintaining it parallel to itself. This beam can therefore be moved to the new position  111  of the active part of the read head. 
   The return beam  116  reflected by this zone at its new position  111  is of course shifted relative to the initial reflected beam  106  and therefore no longer lands in the right position on the sensor array  107 . 
   To compensate this shift of the return beam, the invention also proposes to turn the flat parallel-faced plate  109  about an axis  114  parallel to the plane of the return beam  106  and perpendicular to its axis. The movement ω z″  of the plate  109  around this axis  114  will be substantially symmetrical to the movement ω z′  of the plate  112  around the axis  113  and will compensate the displacement of the return beam associated with the displacement of the incident beam, such that this reflected beam always lands at the right position on the sensor array  107 . 
   These displacements of the incident and reflected beams, which need to be made only extremely slowly, can be obtained by micromotors, piezoelectric for example (not shown in the FIGURE) and controlled by a controlled system (not shown) which will react in response to a detected error signal. For this purpose we could use, for example, a dedicated track of the magnetic tape that acts as a synchronization track. We could also, for example, monitor the average output signal power of the sensor cells  108  in order to achieve optimal and continuous compensation for the wear of the read head. 
   For reasons of visibility, in the FIGURE the flat parallel-faced plates  109  and  112  are shown only in their rest positions when the magnetic tape is centered and the read head has not yet suffered any wear. In this situation, the beams  105  and  106  are those effectively transmitted by the flat parallel-faced plates, whereas the beams  115  and  116  are those that would be transmitted when these plates have turned on the axes  113  and  114  through the angles necessary to obtain the required deviations. The paths of these beams in the plates, as illustrated in the FIGURE, are therefore not the real ones; they are shown schematically to enable the invention to be understood. 
   The invention can be extended to all means enabling an automatic compensation for wear of the head, such as for example a micromotor that moves the head itself—or just its sensitive part—so as to keep the reading beam on the active part of the head.