OPTICALLY Reproducing method and device

Abstract: An optically recording medium in which by projecting a light beam for reading out signals onto a magneto-optical recording medium having a multilayer film including the record retaining layer and the reproducing layer which is coupled magnetically to the record retaining layer, the signals recorded magnetically in a record retaining layer are transferred to a reproducing layer and the signals are read out through the conversion of the signal to optical signals by the magneto-optic effect. Another optically recording medium in which by projecting a light beam for reading out signals onto an optically recording medium in which phase pits are formed and their reflection factors change with temperature, the reflection factors are varied partially in a scanning spot, and the phase pits are read out. The track pitch (p) of a pit RP, which is a recorded signal, in the direction perpendicular to the scanning direction of a laser beam is a half or less of the diameter D of the spot of the laser beam. Thereby the track density is increased, and the recording capacity of the medium can be increased.

Description: Technical Field The present invention relates to reproducing devices and methods with optical recording media. BACKGROUND ART An optical recording medium may be roughly classified into a read-only medium, such as a so-called compact disc, and a medium on which signals may be recorded, such as a magneto-optical disc. With these recording media, it is desired to raise the recording density further because digital video signals as recording signals are in need of a data volume several to ten times or more the data volume for digital audio signals, and also because there is a demand for further reducing the size of a product such as a player by reducing the size of the recording medium such as a disc even when recording the digital audio signal. Meanwhile, the recording density of the optical recording medium is governed by the line density along the scanning direction of the recording tracks and the track density conforming to an interval between adjacent tracks in a direction perpendicular to the scanning direction. The opto-physical limits of the line and track densities are governed by the wavelength λ of a light source and the numerical aperture NA of an objective lens. In general, a readout limit of 2NA/λ is generally accepted as for a spatial frequency during signal reproduction. For this reason, for realizing high density in an optical recording medium, it is thought to be necessary to shorten the wavelength λ of the light source of the reproducing optical system, such as a semiconductor laser, and to increase the numerical aperture NA of the objective lens. However, since there is also a limit in improving the wavelength λ of the light source or the numerical aperture NA of the objective lens, researches are under way for improving the structure of the recording medium and the reading method to increase the recording density. The density in a direction at right angles to the laser scanning direction (recording track direction) and the pitch arraying distance (so-called track density and a track pitch as a reciprocal thereof) are hereinafter scrutinized. The spot diameter produced on irradiation of the recording medium with the laser light (diameter of a so-called first dark ring) is given by 1.22λ/NA where λ is a wavelength of the laser light and NA the numerical aperture of an objective lens. For NA = 0.5 and λ = 780 nm (0.78 µm), the spot diameter is 1.9 µm. Although a usual limit of 1.5 to 1.6 µm is set to a pit arraying interval in the direction at right angles to the laser scanning direction (so-called track pitch) because of crosstalk from neighboring tracks, it is desired to improve the limit further. In view of the above described status of the art, it is an object of the present invention to provide a reproducing device having a read-out laser beam and a magneto-optical recording medium improving the so-called track density (recording density in a direction at right angles to the laser light sweeping direction) and a reproducing method using such a magneto-optical recording medium, wherein the above mentioned crosstalk may be diminished by a simple arrangement. EP-A-0 492 553 being a document according to Art. 54 (3) EPC describes magneto-optical recording medium which comprises on a transparent substrate a magneto-optical recording layer having an exchange interacted multi-layer structure which consists of a first magnetic film serving as a reproduction layer, a second magnetic film serving as an intermediate layer and a third magnetic film serving as a record holding layer, these films being magnetically coupled to one another. The Curie points and coercitive forces of the respective films are set so that the magnetic pattern formed in the first magnetic film is partially expanded, shrunk or inverted depending on the temperature distribution in a reproduction laser beam spot. A reproduction method described in EP-A-0 492 553 uses a specific temperature distribution exhibited in the scanning read-out laser beam's spot to mask a portion within the laser beam's spot so that a magnetic domain pattern recorded in this portion apparently disappears, leaving a single magnetic domain pattern to exist in the region of the beam's spot thereby improving the reproducing resolution (fig. 4 and page 5, line 34 to page 6, line 25 of EP-A-0 492 553). EP-A-0 492 581 also being a document according to Art. 54 (3) EPC describes a further magneto-optical recording medium having on a transparent substrate at least a reproducing layer, an intermediate layer and a record hold layer, wherein the direction of magnetization of the reproducing layer is turned in the direction of an initializing magnetic field. For reproducing information recorded on the magneto-optical recording medium, the reproducing layer is irradiated with a laser beam and a reproducing magnetic field ia applied to the same simultaneously to heat the reproducing layer in a temperature distribution so that a region remaining in an initialized state, a region to which information signals are transferred and a region magnetized in the direction of the reproducing magnetic field are formed. That is, the reproducing method of EP-A-0 492 581 also uses a masking effect within the region of the read-out laser beam spot so that the track recording density can be greatly increased (refer e.g. to figs. 5 to 7 and column 7, line 14 to column 8, line 9 of EP-A-0 492 581). Neither EP-A-0 492 553 nor EP-A-0 492 581 are stipulating that a recording density in a direction at right angles to the laser light sweeping direction is reduced to be not more than one half the spot diameter of the read-out light. EP-A-0 305 979 discloses a known optical recording and reproducing system using an optical recording medium. The recording medium is so constructed that the track gaps are reduced to about the half of an optical spot diameter so that high density is attained. To achieve these small track gaps the optical recording medium of the known optical recording and reproducing system uses localised characteristic change, e.g. the change of the real amplitude reflection coefficient due to the radiation of the optical spot. To achieve a stable tracking control, the known recording and reproduction system uses preformed pits (so-called prepits) in the optical recording medium, said prepits being formed in and between the tracks and having a discrimination pattern to discriminate information pits from said prepits for tracking control and for timing control of the recording and reproducing operations. DISCLOSURE OF THE INVENTION According to a first aspect of the present invention, the above object is solved by a reproducing device having a read-out laser beam and a magneto-optical recording disc comprising a multi-layer film as a recording layer, said multi-layer film having at least a reproducing layer and a recording holding layer magnetically coupled to each other, said recording holding layer having signals magnetically recorded therein previously, said reproducing layer having magnetic domains, said recording layer being an exchange-coupled multi-layer film, said multi-layer film further comprising an intermediate layer, said intermediate layer having a lower Curie temperature than said recording holding layer and being deposited between said reproducing layer and said recording holding layer on a transparent substrate, wherein said magnetic domains recorded in said recording holding layer are transcribed to said reproducing layer prior to reproduction by means of said intermediate layer at an ambient temperature below a predetermined Temperature, said reproducing device further comprising means for radiating the read-out laser beam to a reproducing surface of said magneto-optical recording disc and forming a light spot, means for directing laser light reflected from said surface of said magneto-optical recording disc to a photo detector, and a magnetic head for impressing a reproducing magnetic field, wherein a magnetic domain forming a recording pit in said reproducing layer and falling within said laser light spot and having opposite direction of magnetisation than the direction of said reproducing magnetic field is read by means of a magneto-optical effect, characterized in that the pit arraying pitch of recording signals along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam, wherein upon reproduction a region of raised temperature is temporarily formed in said reproducing layer by the radiation of said read-out laser beam, said region being lightly shifted against the scanning direction of the read-out laser beam towards the rear side of the light spot, said raised temperature being higher than said ambient temperature, wherein recording information within said shifted region is temperature being higher than said ambient temperature, wherein recording information within said shifted region is erased by being aligned with the direction of the reproducing magnetic field so that said shifted recording-erased region is masking an adjacent pit (claim 1). According to a second aspect of the present invention, the above object is solved by a method of reproducing information signals by means of a read-out laser beam from a magneto-optical recording disc having a structure in accordance with claim 1, wherein following steps are carried out: radiating the read-out laser beam on the reproducing layer of said magneto-optical recording disc, thereby heating the recording disc to a temperature higher than said ambient temperature within a region of the read-out laser beam which is slightly shifted against the scanning direction of the read-out laser beam towards the rear side of the light spot produced by the read-out laser beam, aligning the magnetic domains in the reproducing layer within said shifted region with the direction of a reproducing magnetic field and thereby erasing recording information of pits in said reproducing layer within said shifted region of higher temperature, and reading the recording information from said optical recording medium within the light spot of the read-out laser beam by converting said magnetic recording signals within said light spot under photo-magnetic effects into optical signals while using said recording-erased region in said reproducing layer as a mask for masking adjacent pits (claim 2). According to a third aspect of the present invention, the above object is solved by a reproducing device having a read-out laser beam and a magneto-optical recording disc comprising a multi-layer film as a recording layer, said multi-layer film having at least a reproducing layer and a recording holding layer magnetically coupled to each other, said recording holding layer having signals magnetically recorded therein previously, said reproducing layer having magnetic domains, said recording layer being a relief type recording layer and further comprises a reproduction assistant layer and an intermediate layer being stacked sequentially between said reproducing layer and said recording holding layer on a transparent substrate, said intermediate layer having a Curie temperature lower than the Curie temperature of the reproducing layer and approximately equal to the Curie temperature of said recording holding layer, and wherein the magnetic domains of said reproducing layer and the reproduction assistant layer are aligned before reproduction in one direction by applying an initialising magnetic field, said reproducing device further comprising means for radiating the read-out laser beam to a reproducing surface of said magneto-optical disc and forming a light spot thereon means for directing laser light reflected from said surface of said magneto-optical disc to a photo detector, and a magnetic head for applying said initialising magnetic field in said one direction and impressing a reproducing magnetic field opposite to said one direction, wherein only a magnetic domain forming a recording pit in said reproducing layer by having a magnetisation in said one direction and falling within said laser light spot upon being transcribed from said recording holding laser to said producing layer is read by means of a magneto-optical effect when said reproducing magnetic field is applied, characterized in that the pit arraying pitch of recording signals along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam, wherein upon reproduction by scanning with the read-out laser beam a region whose temperature exceeds a predetermined reproducing temperature is shifted slightly against the scanning direction from the read-out laser light spot towards the rear side thereof so that upon application of the reproducing magnetic field the magnetic walls of the reproduction assistant layer are caused to disappear and the information of the recording holding layer is transcribed to the reproducing layer and so that a further region within said light spot which does not yet reach said reproducing temperature is masked and only said region with the temperature above said reproducing temperature becomes the signal detecting region (claim 3). According to still another aspect of the present invention, the above object is solved by a method of reproducing information signals by means of a read-out laser beam from a magneto-optical recording disc having a structure in accordance with claim 3, wherein following steps are carried out: radiating the read-out laser beam on the reproducing layer of said magneto-optical recording disc, thereby heating the recording disc to a temperature higher than a predetermined reproducing temperature within a region which is slightly shifted against the scanning direction of the read-out laser beam towards the rear side of the light spot produced by the read-out laser beam and thereby lowering coercitivity of the reproduction assistant layer within said region applying a reproducing magnetic field and thereby effecting disappearance of the magnetic walls of the reproduction assistant layer and transcribing the information of the recording holding layer to the reproducing layer within said region, and reading the recording information thus transcribed within said region while using a region of said reproducing layer within the light spot which does not yet reach the reproducing temperature as a mask for masking adjacent pits (claim 4). Finally, according to a fifth aspect of the present invention, the above object is solved by a reproducing device having a read-out laser beam and a magneto-optical recording disc comprising a recording layer, an exchanged-coupled magnetic multi-layer film including a reproducing layer and a recording holding layer, wherein signals are magnetically recorded in the recording holding layer, and magnetic domains in the reproducing layer are aligned, said reproducing device further comprising means for radiating the read-out laser beam to a reproducing surface of said magneto-optical recording disc and forming a light spot thereon, means for directing laser light reflected from said surface of said magneto-optical recording disc to a photo detector and a magnetic head for impressing a reproducing magnetic field, wherein magnetic domains forming recording pits in said reproducing layer and falling within said laser light spot and having opposite magnetisation as the direction of said reproducing magnetic field are read by means of a magneto-optical effect, characterized in that the pit arraying pitch of recording signals along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam, said magneto-optical recording disc being a mixed type of the erasure and relief type, wherein the read-out laser beam is radiated to said magneto-optical recording disc in an initialised state thereof in which the recording pits are extinct at room temperature for heating the recording medium and thereby forming a recording relieved region at a position slightly deviated rearward from the laser beam spot against the scanning direction of the laser beam and at the same time forming a recording erased region at a higher temperature within the recording relieved region so that only a recording pit within an area of the laser beam spot not masked by the recording erased region within the recording relieved region becomes a signal detection region (claim 5). BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a plan view schematically showing a magneto-optical disc as an embodiment of the optical recording medium according to the present invention and a laser light spot on the recording medium. Fig.2 is a schematic view showing a schematic arrangement of an optical system of a reproducing apparatus. Fig.3 is a view showing a carrier level of playback signals with respect to the reproducing laser power. Figs.4A and 48 are a plan view and a cross-sectional view, respectively, showing the state in which the laser light is radiated on an erasure type magneto-optical recording medium. Figs.5A and 5B are a plan view and a cross-sectional view, respectively, showing the state in which the laser light is radiated on a relief type magneto-optical recording medium. Fig.6 is, a view for illustrating the track servo by a multiple light source beam. Referring to the drawings, certain embodiments of an optical recording medium used in the present invention will be explained. First, an embodiment in which the present invention is applied to a magneto-optical recording medium as a recordable medium, and then an embodiment in which the present invention is applied to a variable reflectance optical recording medium as a medium which may at least be reproduced, will be explained. The magneto-optical recording medium is comprised of a transparent or light-transmitting substrate of e.g. polycarbonate, on a major surface of which a magnetic layer having an axis of easy magnetization in a direction perpendicular to the film surface and exhibiting superior photomagnetic effects, such as a thin layer of a rare earth - transition metal alloy, are deposited together with a dielectric layer and a surface protective layer etc. A laser light, for example, is radiated from the side of the transparent substrate for recording and/or reproducing signals. Signals are recorded on the magneto-optical recording medium by so-called thermo-magnetic recording in which the magnetic layer is locally heated by e.g. laser light radiation close to the Curie temperature to reduce the coercivity to zero in this region and a recording magnetic field is applied to this region from outside for magnetization in the direction of the recording magnetic field. The recorded signals may be reproduced by taking advantage of the magneto-optical effect, such as the so-called magnetic Kerr effect or Faraday effect, in which the plane of polarization of the linearly polarized light, such as laser light, is rotated according to the direction of the magnetization of the magnetic layer. In the embodiment shown in Fig.1, a medium for high density reproduction of the mixed type of the erasure and relief types as later explained is employed as a magneto-optical recording medium. This magneto-optical recording medium has, as a recording layer, an exchange-coupled magnetic multi-layer film at least including a reproducing layer and a recording holding layer magnetically coupled to each other. For reproducing the magneto-optical recording medium in which signals are magnetically recorded on the recording holding layer and magnetic domains in the reproducing layer are aligned, a laser light is radiated to the reproducing layer for heating the reproducing layer and signals magnetically recorded on the recording holding layer are converted by photomagnetic effects into optical signals as the signals are transcribed to the reproducing layer. In the high density reproducing technique, shown in Fig.1, a magneto-optical recording medium is employed, in which, by virtue of temperature distribution produced by laser radiation, a low-temperature area maintained in an initialized state within a beam spot SP, a high-temperature relief-recorded region FL in which magnetization in the recording holding layer has been transcribed to the surface reproducing layer, and a recording-erased region ER which has magnetization aligned in the direction of the impressed magnetic field for erasure by being further raised in temperature. That is, the laser light is radiated to the recording medium which is in the initialized state in which the information recording pits are erased, for heating the recording medium for producing the elliptical relief-recorded region FL at a position slightly deviated rearward from the laser light spot SP in the scanning direction of the laser light. The recording-erased region ER at a higher temperature is formed within the recording-relieved region FL so that only a recording pit RP within an area not masked by the recording-erased region ER in the recording-relieved region FL, that is a signal detecting region DT, within the light spot SP, is read. The signal detecting region DT may have a shorter pit interval along the laser light scanning direction, that is, a higher line recording density, because a distance d along the laser light scanning direction is narrower than the diameter of the light spot SP. On the other hand, the signal detecting region DT has a small distance in a direction at right angles to the laser light scanning direction so that crosstalk from neighboring tracks may be reduced, so that the pitch between neighboring tracks (so-called track pitch) p may be narrower or denser to raise the track density and hence the recording density. In the embodiment of Fig.1, as sizes of various parts of the recording pit RP, the pitch interval in the direction at right angles to the laser light scanning direction, or so-called track pitch, is p , the shortest interval between pits arrayed along the laser light scanning direction or track direction, or the shortest pit recording period (reciprocal of the line recording density) is q and a pit width in the direction at right angles to the laser light scanning direction (radial direction of the disc) is h . Although the recording pits RP are arrayed in Fig.1 at the above-mentioned shortest recording period q , the arraying distance (as well as the pit length along the laser light scanning direction) may naturally be changed in conformity to recording data. A reproducing apparatus for a magneto-optical disc, which is the magneto-optical recording medium shown in Fig.1 arranged as a disc, is schematically shown in Fig.2. Referring to Fig.2, a laser light radiated from a semiconductor laser 11 as a laser light source is collimated by a collimator lens 12 before being transmitted via a beam splitter 13 to an objective lens 14. The magneto-optical disc 15 is the magneto-optical recording medium explained in connection with Fig.1. The laser beam radiated via objective lens 14 is reflected by the above-mentioned magnetic multi-layer film for photomagnetic recording so as to be incident on and reflected by beam splitter 13 via objective lens 14 before being converged by a converging lens 16 so as to be incident on a photodetector, such as a photodiode 17. A magnetic head 18 for impressing a reproducing magnetic field H read is arranged on the opposite surface of the magneto-optical disc 15 with respect to the laser light radiation position (the upper surface of the disc in the drawing). Fig.3 shows the carrier level for playback signals (plotted on the ordinate) with respect to the playback laser power (plotted on the abscissa) for the case in which, when the wavelength λ of the laser light is 0.78µm and the numerical aperture of the object lens 14 is 0.5, that is when the spot diameter D on the recording medium is 1.22λ/NA = 1.9 µm, the track pitch p is selected to be 0.8 µm, which is not more than one-half the spot diameter D = 1.9 µm, as an example. In Fig.3, a curve a indicates a main carrier component and a curve b indicates a crosstalk component. A hatched area in Fig.3 indicates a range for which the carrier to noise (crosstalk) ratio or so-called C/N ratio of 50 dB may be obtained. The reproducing power for this case is about 2.2 to 2.8 mW, which is in a range of ±10% on either sides of about 2.5 mW as a center. If various conditions, such as required in ordinary reproducing apparatus, are taken into account, the C/N ratio of 50 dB is sufficient for achieving effective signal reproduction. Besides, the margin of ±10% or more which may be set for the reproducing laser power is sufficient to take account of the fluctuations in sizes or characteristics of various devices such as semiconductor laser 11. In this manner, effective signal reproduction may be achieved with the track pitch p = 0.8 µm which is not more than one-half the spot diameter (D = 1.8 µm), while the fluctuations in the semiconductor laser 11 or the like devices may be tolerated to permit mass production of reproducing apparatus. Moreover, should effective reproduction be possible with the C/N ratio of 40 dB, the reproducing power may have a value in a wider range of about 2 to 3.5 mW. Besides, if the margin of fluctuations of ± 10% is adopted, signal reproduction may be achieved with a narrower track pitch p to render it possible to design the actual products in this line. The magneto-optical recording medium, which is capable of high- density reproduction as described above, is hereinafter explained. The present Applicant has previously proposed in e.g. Japanese Laid-Open Patent No.1-143041 (1989) and Japanese Laid-Open Patent No.1-143042 (1989) a method for reproducing information signals for a magneto-optical recording medium wherein information bits (magnetic domains) are enlarged, diminished or reduced to zero during reproduction for improving the playback resolution. The essential point of the technology consists in that the recording magnetic layer is an exchange-coupled multi-layer film composed of a reproducing layer, an intermediate layer and a recording layer, and in that the magnetic domain of the reproducing layer heated by the playback light beam during reproduction is enlarged, diminished or erased at a zone of higher temperatures for diminishing the inter-bit interference during reproduction to render it possible to reproduce signals having a period lower than the light diffraction threshold. There is also proposed in the application documents of JP Patent Application No.1-229395 (1989) published as JP-A-3 093 058 on 18.04.91 a technology in which the recording layer of the magneto-optical recording medium is formed by a multi-layer film including a reproducing layer and a recording holding layer magnetically coupled to each other, the direction of magnetization is aligned in advance to an erased state, the reproducing layer is heated to a temperature higher than a predetermined temperature by radiation of the laser light during reproduction, and in which magnetic signals written on the recording holding layer only in this heated state are read out while being transcribed on the reproducing layer to eliminate signal crosstalk to improve the line recording density and the track density. The high density reproducing technology may be roughly classified into an erasure type and a relief type, shown schematically in Figs.4 and 5, respectively. Referring first to Figs.4A and 4B, the erasure type high density reproduction technique is explained. With the erasure type, the recording medium, on which information recording pits RP are exhibited at room temperature, is heated by radiation of a laser light LB to produce a recording-erased region ER within the beam spot SP of the radiated laser light LB, and the recording pit RP within a remaining region RD within the light spot SP is read, by way of achieving reproduction with improved line density. In sum, this technique consists in that, when reading the recorded pit RP within the light spot SP, the recording-erased region ER is used as a mask to narrow the width d of the read-out region RD to provide for reproduction with increased density along the scanning direction of the laser light (track direction), that is, with increased line recording density, as well as to increase the density along the direction at right angles to he scanning direction, that is the track density. The recording medium for erasure type high density reproduction has an exchange-coupled magnetic multi-layer film structure composed of an amorphous rare earth for photomagnetic recording (Gd, Tb) - iron group (Fe, Co) ferrimagnetic film. In an example shown at B in Fig.4, the recording medium has a structure in which a reproducing layer as a first magnetic film 61, an interrupting layer (intermediate layer) as a second magnetic layer 62 and a recording holding layer as a third magnetic layer 63, are deposited in this order on a major surface (the lower surface in the drawing) of a transparent substrate formed e.g. of polycarbonate, not shown. The first magnetic layer (reproducing layer) 61 is e.g. a GdFeCo layer with a Curie temperature T c1 > 400°C, while the second magnetic layer (interrupting layer or an intermediate layer) 62 is e.g. a TbFeCoAl film having a Curie temperature T c2 of 120°C and the third magnetic layer (recording holding layer) is e.g. a TbFeCo layer with a Curie temperature T c3 of 300°C. Meanwhile, arrow marks entered in the magnetic films 61 to 63 shown at B in Fig.4 represent the direction of magnetization of the magnetic domains. H read represents the direction of the reproducing magnetic field. The reproducing operation is briefly explained. At an ambient temperature below a predetermined temperature T OP , the recording magnetic domain of the recording holding layer 63 is transcribed to the reproducing layer 61 by means of the interrupting,layer 62. If the laser light LB is radiated on the recording medium for raising the medium temperature, changes in the medium temperature are produced with a time delay with the scanning of the laser light, so that a region at a temperature higher than the predetermined temperature T OP , that is the recording-erased region ER, is shifted slightly towards the rear side of the light spot SP along the laser scanning direction. At the temperature higher than the predetermined temperature T OP , the magnetic domains of the reproducing layer 61 are aligned in the direction of the reproducing magnetic field H read , so that recording is erased on the medium surface. In this manner, the recording of the region ER at a temperature higher than the predetermined temperature T OP is masked to achieve high density reproduction. With the relief type high density reproducing technique, shown in Fig.5, the recording medium in a state in which information recording pits RP are erased at ambient temperature (initialized state) is irradiated with a laser light and thereby heated to form a signal detecting region DT, as a recording-relieved region, within the beam spot SP of the laser light, and only the recording pit RP within this signal detecting region DT is read for improving the playback line density. The recording medium for such high density relief reproduction has a magnetic multi-layer structure according to magnetostatic coupling or magnetic exchange coupling. In an example shown in Fig.5, a reproducing layer 71 as a first magnetic layer, a reproduction assistant layer 72 as a second magnetic layer, an intermediate layer 73 as a third magnetic layer 73 and a recording holding layer 74 as a fourth magnetic layer are stacked sequentially on a major surface (the lower surface in Fig.3) of a transparent substrate, not shown, formed e.g. of polycarbonate. The first magnetic layer (reproducing layer) 71 is formed e.g. of GdFeCo and has a Curie temperature T c1 > 300°C, the second magnetic layer (reproduction assistant layer) 72 is formed e.g. of TbFeCoAl and has a Curie temperature T c2 of approximately 120°C and the third magnetic layer (intermediate layer) 73 is formed e.g. of GdFeCo and has a Curie temperature T c3 of approximately 250°C and the fourth magnetic layer (recording holding layer) 74 is formed e.g. of TbFeCo and has a Curie temperature T c4 of approximately 250°C. Arrows entered in the magnetic layers 71, 72, 73 and 74 indicate the direction of magnetization of magnetic domains. H read represents the direction of the reproducing magnetic field. The operation during reproduction is explained briefly. The domains of the reproducing layer 71 and the reproduction assistant layer 72 are aligned before reproduction in one direction (in an upward direction in Fig.5) by the initializing magnetic field. A reproducing magnetic field H read is applied in an inverse direction while a laser light LB is radiated. With scanning by the laser light, temperature changes in the medium are produced with a delay, so that the region whose temperature exceeds a predetermined reproducing temperature T RP (recording-relieved region) is shifted slightly from the light spot SP towards the rear side along the scanning direction. With the temperature above the predetermined reproducing temperature T RP , coercivity of the reproduction assistant layer 72 is lowered, so that, when the reproducing magnetic field H read is applied, the magnetic wall is caused to disappear so that the information of the recording holding layer 74 is transcribed to the reproducing layer 71. Thus a region within the light spot SP which does not yet reach the reproducing temperature T RP is masked and the remaining region within the light spot SP becomes the signal detecting region as recording-relieved region DT to permit high density reproduction. There is also devised a high density reproducing technique consisting in a combination of the erasure type and the relief type, as shown in Fig.1. With this technique, the laser light is radiated to the recording medium in an initialized state thereof in which recording pits are extinct at room temperature for heating the recording medium for forming a recording-relieved region FL at a position slightly deviated towards the rear of the beam spot of the radiating laser light, while simultaneously forming a recording-erased region of a higher temperature within the recording relieved region FL. In the specification and the drawings of our co-pending JP Patent Application No.H 3-418110 (1991), there is proposed a signal reproducing method for a magneto-optical recording medium wherein a magneto-optical recording medium having at least a reproducing layer, an intermediate layer and a recording holding layer is employed, a laser light is radiated and a reproducing magnetic field is applied to the reproducing layer, a temperature distribution generated by the laser radiation is utilized to produce a region where an initialized state is maintained, a region to which the information of the recording holding layer is transcribed and a region the domains of which are aligned in the direction of the reproducing magnetic field, in a field of view of the lens, to produce a state equivalent to optically masking the field of view of the lens to increase the line recording density and the track density as well as to assure satisfactory frequency characteristics at the time of reproduction, in which there is no risk that the region of transcription of the information of the recording holding layer be diminished or enlarged even if the reproducing power is fluctuated. According to the above-described high density reproducing technique employing such magneto-optical recording medium, only the read region RD, which is a partial region within the recording-relieved region, or the recording pit RP within the signal detecting region DT, is read within the light spot SP. Since the size of the read region RD or the signal detection region DT is smaller than the size of the light spot SP, the distance between adjacent pits in the directions along and at right angles to the laser light scanning direction may be reduced to raise the recording density to increase the recording capacity of the recording medium. Meanwhile, as a tracking servo for achieving tracking during signal reproduction with such a high track density, a so-called sampling servo system or a technique of enlarging the track pitch in a servo-equivalent manner by a multiple light source beam is effective. The tracking servo by the multiple light source beam, which is a technique proposed by the present Applicant in the specification and drawings of our copending JP Patent Application No. H-2-142521, is explained briefly by referring to Fig.6. In Fig.6, so-called guide grooves 31 are formed on the recording medium at an array pitch s which may, for example, be larger than the diameter of a light spot SP. Recording pits RP, arrayed at an array pitch p in a direction at right angles to the laser scanning direction, is equal to, for example, not more than one half the arraying pitch s of the guide grooves 31. A plurality of, for example, three light beams are radiated to the recording medium from laser light sources, not shown, with three light spots SP a , SP b and SP c of these light beams being associated with three recording pit rows, or so-called recording tracks, arrayed at the above-mentioned arraying pitch p . If the central spot SP 0 of these three light spots is positioned on the guide groove 31, and tracking is performed only with this light spot, the remaining two spots SP a , SP b are guided for scanning the recording pit rows on both sides of the guide groove 31. In general, the arraying pitch p may be set to s/2n. That is, as a general rule, plural light beams are radiated from laser light sources for generating plural light spots SP, offset from one another by 1/2n times the arraying pitch s of the guide grooves, on the recording medium, and an n number of signals (recording pits RP), the spatial phase of which is offset by 1/2n times the arraying pitch s of the guide grooves, are formed on the basis of these beam pots SP. These n number of signals are multiplied to generate tracking control signals having the spatial frequency equal to 1/n times the arraying pitch s of the guide grooves, and the tracking control is performed on the basis of these tracking control signals.

Claims: A reproducing device having a read-out laser beam and a magneto-optical recording disc (15) comprising a multi-layer film (61, 62, 63, 71-74) as a recording layer, said multi-layer film having at least a reproducing layer (61, 71) and a recording holding layer (63, 74) magnetically coupled to each other, said recording holding layer (63, 74) having signals magnetically recorded therein previously, said reproducing layer (61, 71) having magnetic domains, said recording layer being an exchange-coupled multi-layer film, said multi-layer film further comprising an intermediate layer (62), said intermediate layer (62) having a lower Curie temperature (T c2 ) than said recording holding layer (63) and being deposited between said reproducing layer (61) and said recording holding layer (63) on a transparent substrate, wherein said magnetic domains recorded in said recording holding layer (63) are transcribed to said reproducing layer (61) prior to reproduction by means of said intermediate layer (62) at an ambient temperature below a predetermined Temperature (T OP ), said reproducing device further comprising means (12, 13, 14) for radiating the read-out laser beam to a reproducing surface of said magneto-optical recording disc (15) and forming a light spot (SP), means (13, 14, 16) for directing laser light reflected from said surface of said magneto-optical recording disc (15) to a photo detector (17), and a magnetic head (18) for impressing a reproducing magnetic field (H read ), wherein a magnetic domain forming a recording pit (RP) in said reproducing layer (61) and falling within said laser light spot (SP) and having opposite direction of magnetisation than the direction of said reproducing magnetic field (H read ) is read by means of a magneto-optical effect, characterized in that the pit arraying pitch (P) of recording signals (RP) along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam (LB), wherein upon reproduction a region (ER) of raised temperature is temporarily formed in said reproducing layer by the radiation of said read-out laser beam, said region (ER) being lightly shifted against the scanning direction of the read-out laser beam towards the rear side of the light spot (SP), said raised temperature being higher than said ambient temperature, wherein recording information within said shifted region (ER) is erased by being aligned with the direction of the reproducing magnetic field (H read ) so that said shifted recording-erased region (ER) is masking an adjacent pit. A method of reproducing information signals by means of a read-out laser beam from a magneto-optical recording disc having a structure in accordance with claim 1, wherein following steps are carried out: radiating the read-out laser beam (LB) on the reproducing layer (61) of said magneto-optical recording disc, thereby heating the recording disc to a temperature higher than said ambient temperature (T op ) within a region (ER) of the read-out laser beam (LB) which is slightly shifted against the scanning direction of the read-out laser beam towards the rear side of the light spot (SP) produced by the read-out laser beam (LB), aligning the magnetic domains in the reproducing layer (61) within said shifted region (ER) with the direction of a reproducing magnetic field (H read ) and thereby erasing recording information of pits in said reproducing layer within said shifted region (ER) of higher temperature, and reading the recording information from said optical recording medium within the light spot (SP) of the read-out laser beam (LB) by converting said magnetic recording signals within said light spot (SP) under photo-magnetic effects into optical signals while using said recording-erased region (ER) in said reproducing layer (61) as a mask for masking adjacent pits (Fig. 4). A reproducing device having a read-out laser beam and a magneto-optical recording disc (15) comprising a multi-layer film (71-74) as a recording layer, said multi-layer film having at least a reproducing layer (71) and a recording holding layer (74) magnetically coupled to each other, said recording holding layer (74) having signals magnetically recorded therein previously, said reproducing layer (71) having magnetic domains, said recording layer being a relief type recording layer and further comprises a reproduction assistant layer (72) and an intermediate layer (73) being stacked sequentially between said reproducing layer (71) and said recording holding layer (74) on a transparent substrate, said intermediate layer (73) having a Curie temperature lower than the Curie temperature of the reproducing layer (71) and approximately equal to the Curie temperature of said recording holding layer (74), and wherein the magnetic domains of said reproducing layer (71) and the reproduction assistant layer (72) are aligned before reproduction in one direction by applying an initialising magnetic field, said reproducing device further comprising means (12, 13, 14) for radiating the read-out laser beam to a reproducing surface of said magneto-optical disc (15) and forming a light spot (SP) thereon means (13, 14, 16) for directing laser light reflected from said surface of said magneto-optical disc (15) to a photo detector (17), and a magnetic head (18) for applying said initialising magnetic field in said one direction and impressing a reproducing magnetic field (H read ) opposite to said one direction, wherein only a magnetic domain forming a recording pit (RP) in said reproducing layer (71) by having a magnetisation in said one direction and falling within said laser light spot(SP) upon being transcribed from said recording holding laser (74) to said producing layer (71) is read by means of a magneto-optical effect when said reproducing magnetic field (H read ) is applied, characterized in that the pit arraying pitch (P) of recording signals (RP) along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam (LB), wherein upon reproduction by scanning with the read-out laser beam a region (DT) whose temperature exceeds a predetermined reproducing temperature (T RP ) is shifted slightly against the scanning direction from the read-out laser light spot (SP) towards the rear side thereof so that upon application of the reproducing magnetic field (H read ) the magnetic walls of the reproduction assistant layer are caused to disappear and the information of the recording holding layer (74) is transcribed to the reproducing layer and so that a further region within said light spot (SP) which does not yet reach said reproducing temperature (T RP ) is masked and only said region (DT) with the temperature above said reproducing temperature (T RP ) becomes the signal detecting region. A method of reproducing information signals by means of a read-out laser beam from a magneto-optical recording disc having a structure in accordance with claim 3, wherein following steps are carried out: radiating the read-out laser beam (LB) on the reproducing layer (71) of said magneto-optical recording disc, thereby heating the recording disc to a temperature higher than a predetermined reproducing temperature (T RP ) within a region (DT) which is slightly shifted against the scanning direction of the read-out laser beam (LB) towards the rear side of the light spot (SP) produced by the read-out laser beam (LB) and thereby lowering coercitivity of the reproduction assistant layer (72) within said region (DT) applying a reproducing magnetic field (H read ) and thereby effecting disappearance of the magnetic walls of the reproduction assistant layer (72) and transcribing the information of the recording holding layer (74) to the reproducing layer (71) within said region (DT), and reading the recording information thus transcribed within said region (DT) while using a region of said reproducing layer (71) within the light spot (SP) which does not yet reach the reproducing temperature (T RP ) as a mask for masking adjacent pits (Fig. 5). A reproducing device having a read-out laser beam and a magneto-optical recording disc (15) comprising a recording layer, an exchanged-coupled magnetic multi-layer film including a reproducing layer and a recording holding layer, wherein signals are magnetically recorded in the recording holding layer, and magnetic domains in the reproducing layer are aligned, said reproducing device further comprising means (12, 13, 14) for radiating the read-out laser beam to a reproducing surface of said magneto-optical recording disc (15) and forming a light spot (SP) thereon, means (13, 14, 16) for directing laser light reflected from said surface of said magneto-optical recording disc (15) to a photo detector and a magnetic head (18) for impressing a reproducing magnetic field (H read ), wherein magnetic domains forming recording pits (RP) in said reproducing layer (61) and falling within said laser light spot (SP) and having opposite magnetisation as the direction of said reproducing magnetic field (H read ) are read by means of a magneto-optical effect, characterized in that the pit arraying pitch (P) of recording signals (RP) along a direction at right angles to the read-out laser beam's scanning direction is selected to be not more than one half the spot diameter of the read-out laser beam (LB), said magneto-optical recording disc being a mixed type of the erasure and relief type, wherein the read-out laser beam is radiated to said magneto-optical recording disc in an initialised state thereof in which the recording pits are extinct at room temperature for heating the recording medium and thereby forming a recording relieved region (FL) at a position slightly deviated rearward from the laser beam spot (SP) against the scanning direction of the laser beam and at the same time forming a recording erased region (ER) at a higher. temperature within the recording relieved region (FL) so that only a recording pit (RP) within an area of the laser beam spot (SP) not masked by the recording erased region (ER) within the recording relieved region (FL) becomes a signal detection region (DT).

Inventors: FUKUMOTO ATSUSHI, OHTA MASUMI, ONO MASUMI, UDAGAWA TOSHIKI, YASUDA KOUICHI, YOSHIMURA SHUNJI, FUKUMOTO, ATSUSHI, OHTA, MASUMI, ONO, MASUMI, UDAGAWA, TOSHIKI, YASUDA, KOUICHI, YOSHIMURA, SHUNJI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G11B11
IPC codes: G11B7, G11B11