Abstract:
A high density optical information recording medium comprises a substrate having rows of land and groove and a recording layer formed on the substrate. Each of the rows of land or groove includes at least two recording tracks separated by a denatured region produced by irradiating e.g. the center of each row in the recording layer with a denaturing light beam accompanied with one or two sub-beams for tracking control. Recording and reproduction of information can be performed likewise by using a recording or reproduction light beam accompanied with one or two tracking sub-beams.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical information recording/reproducing apparatus for recording information on or reproducing recorded information from an optical information recording medium. In particular, the invention relates to an optical information recording/reproducing apparatus for recording information on or reproducing recorded information from a recording medium having at least two rows of information tracks in one groove or between grooves.  
           [0003]    2. Related Background Art  
           [0004]    In general, reproduction of information recorded in an optical information recording medium is carried out in such way that a minute spot is formed on a signal surface by a condensing lens and then a reflected light is received by a photoelectric conversion device. The demand for high densification of this optical information recording is increasing year by year.  
           [0005]    As for a method of attaining this high densification, for example, there is known a method in which the high densification in a track direction is realized beyond restriction of optical resolution using means such as domain wall displacement detection (DWDD) means. For realizing the higher densification, it is necessary to narrow a track pitch to realize the densification in a radial direction. However, if a period of guide grooves is brought close to the resolution of an optical system, then a sufficient tracking signal can not be obtained. As well known, when a wavelength of a convergent light is assigned λ, and a numerical aperture of an objective lens is assigned NA, the resolution d of an optical system is determined on the basis of the following expression:  
             d=λ/ (2× NA )  
           [0006]    For example, since in an optical system as being used in DVD-RW, a wavelength λ of a light source is 635 nm, and a numerical aperture NA of an objective lens is 0.60, resolution d becomes 0.53 μm. If a track pitch of a medium is brought close to a value of 0.53 μm, a sufficient tracking signal can not be obtained. Thus, in DVD-RW, a track pitch is set to 0.74 μm and under this condition, a tracking signal is obtained.  
           [0007]    For the purpose of relaxing such a restriction due to the resolution of the optical system, as described in Japanese Patent Application Laid-Open No. 2000-331383, for example, there is proposed an optical recording medium in which tracks are formed so that widths of adjacent grooves become different from each other. FIG. 16 shows a recording medium described in Japanese Patent Application Laid-Open No. 2000-331383. In the recording medium described in Japanese Patent Laid-Open No. 2000-331383, since a period structure is provided in which two tracks apparently correspond to one period, a tracking signal can be obtained even if a track pitch is narrowed down to a track pitch beyond a resolution limit.  
           [0008]    However, in case of the recording medium described in Japanese Patent Laid-Open No. 2000-331383, it is necessary to form grooves having different widths on both sides of each track. Thus, in order that a tracking signal has sufficient modulation, it is necessary to increase a difference in width between the grooves. This is a cause for restricting the high densification.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an optical information recording medium with which even higher densification can be realized, and an optical information recording/reproducing apparatus.  
           [0010]    Therefore, according to the present invention, there is provided an optical information recording medium, comprising:  
           [0011]    a substrate having lands and grooves;  
           [0012]    a recording layer formed on the substrate;  
           [0013]    at least two rows of information tracks adapted to record therein or reproduce therefrom information and formed in the recording layer on the lands or grooves; and  
           [0014]    denatured regions formed on the lands or grooves and between the information tracks.  
           [0015]    Further, according to the present invention, there is provided an optical information recording/reproducing apparatus comprising:  
           [0016]    means for applying two sub-beams and a main beam onto the recording layer, the main beam being located between the sub-beams;  
           [0017]    a light receiving element for detecting reflected lights of the sub-beams from the medium; and  
           [0018]    means for generating a tracking error signal on the basis of outputs from the light receiving element,  
           [0019]    wherein information is recorded or reproduced with a spot of the main beam while carrying out tracking control on the basis of the tracking error signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIGS. 1A and 1B are cross sectional views each showing a structure of a magneto-optical disc for use in a first embodiment of an optical information recording/reproducing apparatus of the present embodiment;  
         [0021]    [0021]FIG. 2 is a view showing a structural example of an optical system of an annealing apparatus for annealing the magneto-optical disc shown in FIGS. 1A and 1B;  
         [0022]    [0022]FIG. 3 is a view showing a light receiving surface of a light receiving element for use in the annealing apparatus shown in FIG. 2;  
         [0023]    [0023]FIG. 4 is a view showing beam spots on a recording surface and a tracking error signal when each groove center of the magneto-optical disc shown in FIGS. 1A and 1B is annealed;  
         [0024]    [0024]FIG. 5 is a view showing beam spots on a recording surface and a tracking error signal when each upper surface of lands of the magneto-optical disc shown in FIGS. 1A and 1B is annealed;  
         [0025]    [0025]FIG. 6 is a block diagram showing the first embodiment of the optical information recording/reproducing apparatus according to the present invention;  
         [0026]    [0026]FIG. 7 is a diagram showing a light receiving surface of a light receiving element for use in the optical information recording/reproducing apparatus shown in FIG. 6;  
         [0027]    [0027]FIG. 8 is a view showing beam spots on a recording surface of a medium and a tracking error signal during recording or reproduction in the first embodiment;  
         [0028]    [0028]FIG. 9 is a view showing beam spots on a recording surface of a medium and a tracking error signal during recording or reproduction in a second embodiment of the present invention;  
         [0029]    [0029]FIG. 10 is a view showing a light receiving surface of a light receiving element for use in the second embodiment;  
         [0030]    [0030]FIGS. 11A and 11B are cross sectional views each showing a magneto-optical disc for use in a third embodiment of the present invention;  
         [0031]    [0031]FIG. 12 is a view showing a diffraction grating of an optical system for use in an annealing processing for the magneto-optical disc shown in FIGS. 11A and 11B;  
         [0032]    [0032]FIG. 13 is a view showing a light receiving element of an optical system for use in the annealing processing for the magneto-optical disc shown in FIGS. 11A and 11B;  
         [0033]    [0033]FIG. 14 is a view showing beam spots on a medium and a tracking error signal when the magneto-optical disc shown in FIGS. 11A and 11B is annealed;  
         [0034]    [0034]FIG. 15 is a view showing beam spots on a medium and a tracking error signal when information is recorded/reproduced on/from the magneto-optical disc shown in FIGS. 11A and 11B; and  
         [0035]    [0035]FIG. 16 is a view showing a conventional medium structure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.  
         [0037]    (First Embodiment)  
         [0038]    [0038]FIGS. 1A and 1B are cross sectional views each showing a structure of a magneto-optical disc for use in a first embodiment of an optical information recording/reproducing apparatus of the present invention. FIG. 1A is a cross sectional view of a substrate of a magneto-optical disc, and FIG. 1B is a cross sectional view of the magneto-optical disc. First of all, as shown in FIG. 1A, a substrate  30  is formed by injection molding of a resin material such as polycarbonate. Lands L and grooves G are formed on a surface of the substrate. In addition, a magneto-optical disc  8  shown in FIG. 1B is a domain wall displacement type magneto-optical medium from which a domain wall of a recording mark is displaced to reproduce recorded information.  
         [0039]    Since the domain wall displacement type magneto-optical medium is described in Japanese Patent Application Laid-Open No. 6-290496, for example, a detailed description thereof is omitted here for the sake of simplicity. A magnetic film M in which a displacement layer, a switching layer, and a recording layer are laminated is formed on the substrate  30 . It goes without saying that a layer structure of a domain wall displacement type magneto-optical medium is not limited to the structure described above, and various layer structures described in Japanese Patent Laid-Open No. 6-290496 or the like may be adopted.  
         [0040]    In addition, with respect to an information track, as shown in FIG. 1B, two rows of tracks, i.e., track T 1  and track T 2  are provided at an equal width in each groove G. An area A being changed in magnetism through an annealing processing is provided at a center of each groove G. Likewise, an area B being changed in magnetism through the annealing processing is provided along a center of each land L. Then, the track T 1  and the track T 2  are magnetically separated from each other through the area A and the area B.  
         [0041]    More specifically, a land width W L  is 0.2 μm, a groove width W G  is 0.8 μm, and a land pitch P L  is 1.0 μm, and a groove depth D is 0.03 μm. The annealed area A along the center of each groove has a width of about 0.2 μ, and the annealed area B on each land takes up nearly the whole land width W L . Note that, the land width and the groove width may be reversed so that the land width W L  is 0.8 μm and the groove width W G  is 0.2 μm. In such case, the track T 1  and the track T 2  are provided in each land L in order to carry out the recording.  
         [0042]    Next, description will hereinbelow be given with respect to a method of changing magnetism of the areas along the center of each groove G and each land L through the annealing processing. FIG. 2 shows an example of an optical system for use in the annealing processing for the magneto-optical disc  8 . First of all, a beam with 410 nm wavelength emitted from a semiconductor laser  1  is optically divided into three beams by a diffraction grating  2 , and each of the resultant three beams is collimated by a coupling lens  3 . Thereafter, the collimated beams are transmitted through a polarization beam splitter  4 , transmitted through a quarter-wave plate  5 , and then condensed by an objective lens  7  with 0.85 NA the position of which is controlled by actuators  6  to be made incident to the magneto-optical disc  8  from the film surface side.  
         [0043]    A reflected light from an information recording surface of the magneto-optical disc  8  is transmitted through the objective lens  7  again, transmitted through the quarter-wave plate  5 , reflected by the polarization beam splitter  4 , and then transmitted through a sensor lens  9  to be made incident to a light receiving element  10 . A focus error signal is obtained by utilizing an astigmatism method using a main beam. In addition, the annealing processing is executed with a main beam converged by the objective lens  7 .  
         [0044]    [0044]FIG. 3 shows a light receiving surface on the light receiving element  10 . A reflected light of the main beam is received by four divided light receiving surfaces  10   a ,  10   b ,  10   c  and  10   d  on the light receiving element  10 . Assuming that output signals thereof are assigned A, B, C and D, respectively, a signal which is obtained on the basis of the arithmetic operation of (A+C)−(B+D) is used as a focus error signal.  
         [0045]    Next, description will hereinbelow be given with respect to a tracking method when the magneto-optical disc  8  is annealed. FIG. 4 shows beam spots on a medium surface, and a tracking error signal for a spot position on an information recording surface of a main beam  13  when the area A along the center of each groove G of the magneto-optical disc  8  is annealed.  
         [0046]    A spot size is shown substantially in half maximum full-width. A distance of sub-beam  11  and sub-beam  12  in the disc radial direction is roughly made equal to or larger than ¼ of a land pitch P L  on the information recording surface, but equal to or smaller than a groove width W G  SO that the main beam  13  is located between the two sub-beams  11  and  12  in the middle thereof. Reflected lights of the sub-beams  11  and  12  are received by light receiving surfaces  10   e  and  10   f  on the light receiving element  10 , respectively. Then, assuming that output signals thereof are assigned TE 1  and TE 2 , respectively, the tracking control is carried out using a signal TE 3  obtained on the basis of the following expression:  
           TE   3 = TE   1 − TE   2   
         [0047]    While thus carrying out the focus control and the tracking control, the area along the center of each groove G is scanned with the main beam  13  to thereby anneal the area A of each groove G.  
         [0048]    Note that, as the tracking method, a push-pull signal of the sub-beams  11  and  12  may be used. In this case, a distance of the sub-beams  11  and  12  in the disc radial direction is set so as to be equal to the land pitch P L  on the recording surface.  
         [0049]    [0049]FIG. 5 shows beam spots on a medium surface, and a tracking error signal for a spot position of the main beam  13  when the area B on each land L of the magneto-optical disc  8  is annealed. In case where the area along the center of the land L is tracked with the main beam  13  as shown in FIG. 5, a reflected light of the main beam  13  is received by the four divided-light receiving surfaces  10   a ,  10   b ,  10   c  and  10   d  on the light receiving element  10 . Then, assuming that output signals thereof are assigned A, B, C and D, respectively, the tracking control is carried out using a so-called push-pull signal obtained on the basis of the arithmetic operation of (A+D)−(B+C) as the tracking error signal.  
         [0050]    The focus error signal is obtained in the same manner as that when each groove is annealed. Thus, the land L is scanned with the main beam  13  while carrying out the focus control and the tracking control using the push-pull signal as described above to thereby anneal the area B on the land L.  
         [0051]    Thus, by changing the tracking method for the case where each groove is annealed and the case where each land is annealed, the annealing processing for the area A along the center of each groove and the area B on each land is executed.  
         [0052]    Next, description will hereinbelow be given with respect to an apparatus for recording information or reproducing recorded information on or from the above-mentioned magneto-optical disc. FIG. 6 shows a configuration of a first embodiment of an optical information recording/reproducing apparatus according to the present invention. First of all, a beam with 650 nm wavelength emitted from a semiconductor laser  14  is optically divided into three beams including one main beam and two sub-beams by a diffraction grating  15 . Each of the beams is collimated by a coupling lens  16 , transmitted through a polarization beam splitter  17 , and then condensed by an objective lens  19  with 0.6 NA the position of which is controlled by actuators  18  to be applied onto the magneto-optical disc  8  in the form of a minute spot.  
         [0053]    A magnetic head  20  for applying a modulated magnetic field is provided above the upper surface of the magneto-optical disc  8  so as to face the objective lens  19 . A reflected light from an information recording surface of the magneto-optical disc  8  is transmitted through the objective lens  19  again and reflected by the polarization beam splitter  17  to be made incident to a light receiving element  23  through a sensor lens  21  and a Wollaston prism  22 .  
         [0054]    In addition, in the figure, reference numeral  31  designates a laser drive circuit for driving the semiconductor laser  14 , reference numeral  32  designates a magnetic head drive circuit for driving the magnetic head  20 , reference numeral  33  designates a recording circuit for modulating recorded data to generate a recording signal, and reference numeral  34  designates a reproduction circuit for generating a reproduction signal on the basis of output signals from the light receiving element  23  and executing predetermined signal processings using the resultant reproduction signal to thereby generate reproduced data.  
         [0055]    Moreover, reference numeral  35  designates a focus control circuit for generating a focus error signal on the basis of the output signals from the light receiving element  23  to carry out the focus control on the basis of the resultant focus error signal, reference numeral  36  designates a tracking control circuit for generating a tracking error signal on the basis of the output signals from the light receiving element  23  to carry out the tracking control on the basis of the resultant tracking error signal, and reference numerals  37  and  38  designate actuator drivers for driving the actuators  18 .  
         [0056]    When the recorded information in the magneto-optical disc  8  is reproduced, a beam for reproduction which is the main beam  13  from the objective lens  19  is applied to the magneto-optical disc  8  to cause the domain wall displacement to thereby read out information. At this time, the reproduction circuit  34  generates the reproduction signal on the basis of the output signals from the light receiving element  23  to execute predetermined signal processings such as binarization and demodulation using the resultant reproduction signal to thereby obtain reproduced data. Note that, since the domain wall displacement reproduction is well known, the detailed description thereof is omitted here for the sake of simplicity.  
         [0057]    Meanwhile, when information is recorded on the magneto-optical disc  8 , a magnetic field which is modulated in accordance with a recording signal is applied from the magnetic head  20  to the magneto-optical disc  8  while applying a beam for recording having a fixed intensity which is the main beam  13  from the objective lens  19  to the magneto-optical disc  8 , to thereby record information on the magneto-optical disc  8 . In this case, the magnetic head  20  is driven by the magnetic head drive circuit  32 , and the semiconductor laser  14  is driven by the laser drive circuit  31  to apply the modulated magnetic field to the magneto-optical disc  8  while applying the light beam for recording on the magneto-optical disc  8  to thereby record information on the magneto-optical disc  8 . Incidentally, the focus error signal upon recording/reproducing information is generated by utilizing the astigmatism method using the reflected light of the main beam.  
         [0058]    [0058]FIG. 7 shows the light receiving surface on the light receiving element  23 . The reflected light of the main beam is received by the four divided light receiving surfaces  23   a ,  23   b ,  23   c  and  23   d  on the light receiving element  23 . Then, assuming that output signals thereof are assigned A, B, C and D, respectively, a signal which is obtained on the basis of the arithmetic operation of (A+C)−(B+D) is used as a focus error signal. The focus error signal is obtained by utilizing the above-mentioned astigmatism method. The focus control circuit  35  generates the focus error signal and carries out the focus control so that the main beam is focused on the information recording surface of the medium on the basis of the resultant focus error signal.  
         [0059]    Meanwhile, when a reproduction signal is generated, two beams polarized and separated by the Wollaston prism  22  into components orthogonal to each other are received by light receiving surfaces  23   i  and  23   j , respectively. Then, assuming that output signals thereof are assigned I and J, respectively, a reproduction signal is obtained by carrying out the arithmetic operation of (I−J). The reproduction circuit  34  thus generates the reproduction signal to generate reproduced data using the resultant reproduction signal.  
         [0060]    Next, description will hereinbelow be given with respect to a tracking method when information is recorded and reproduced on and from the magneto-optical disc  8 . FIG. 8 shows beam spots on a medium surface and a tracking error signal when information is recorded and reproduced on and from the magneto-optical disc  8 . The spot size is shown substantially in half maximum full-width.  
         [0061]    A distance between sub-beam  24  and sub-beam  25  in the disc radial direction is made equal to ½ of the land pitch P L  on the information recording surface of the medium, and a main beam  26  is made located between the sub-beams  24  and  25  in the middle thereof. Reflected lights of the sub-beams  24  and  25  are received by light receiving surfaces  23   e ,  23   f , and  23   g ,  23   h  of the magneto-optical disc  8 , respectively, which are obtained by dividing the light receiving elements into two parts in a radial direction of the magneto-optical disc  8 . Assuming that output signals thereof are assigned E, F, and G, H, respectively, so-called push-pull signals are respectively obtained by carrying out the following arithmetic operations:  
           TE   4 = E−F    
           TE 5= G−H    
         [0062]    At this time, each of the push-pull signals TE 4  and TE 5  becomes a periodic signal having the land pitch P L  as a period. Assuming that a distance between the sub-beams  24  and  25  is ½ of the land pitch P L , the push-pull signals TE 4  and TE 5  of the sub-beams  24  and  25  are out of phase with each other by a half period. Accordingly, by further carrying out the arithmetic operation of TE 4 −TE 5 , a tracking signal TE 6  can be obtained by utilizing the well-known differential push-pull method. The tracking control circuit  36  thus generates the tracking error signal, and carries out the tracking control so that he main beam  26  traces the information tracks T 1  and T 2  on the basis of the resultant tracking error signal TE 6 .  
         [0063]    In addition, by changing a polarity of the tracking error signal, a beam can be drawn into either track interchangeably from the track T 1  to the track T 2 , or from the track T 2  to the track T 1 . The operation for changing the polarity of the tracking error signal is carried out by a control circuit (not shown). FIG. 8 shows a state in which one of the two rows of information tracks of the groove G is traced with the sub-beams and the main beam.  
         [0064]    As described above, the light beam for recording/reproduction can be accurately tracking-controlled so that two rows of information tracks within one groove (or land) of the magneto-optical disc  8  are respectively traced with the beam.  
         [0065]    (Second Embodiment)  
         [0066]    Next, a second embodiment of the present invention will hereinbelow be described in detail. FIG. 9 shows beam spots on a medium surface and a tracking error signal during recording/reproduction according to a second embodiment of the present invention. The magneto-optical disc  8  in this embodiment is the same as that shown in FIGS. 1A and 1B. It should be noted that a configuration of an optical information recording/reproducing apparatus is different in light receiving element  23  from that of the optical information recording/reproducing apparatus shown in FIG. 6, and positions of beams spots on an information recording surface of the medium are different. Configurations of other constituent elements are the same as those of the constituent elements shown in FIG. 6. In the figure, reference numeral  27  designates a sub-beam, and reference numeral  28  designates a main beam. The sub-beam  27  is obtained by optically dividing a beam emitted from the semiconductor laser  14  of FIG. 6 into beams by the diffraction grating  15 , and only one of the beams is used.  
         [0067]    In this embodiment, a distance between the sub-beam  27  and the main beam  28  in the disc radial direction is made equal to a half of the land pitch P L  on the information recording surface of the medium. In this connection, information is recorded and reproduced on and from the magneto-optical disc  8  using the main beam  28  converged by the objective lens  19 , and the focus error signal is generated by utilizing the astigmatism method using the reflected light of the main beam  28 .  
         [0068]    [0068]FIG. 10 shows a light receiving surface on the light receiving element  23  used in this embodiment. The reflected light of the main beam  28  is received by four divided light receiving surfaces  23   a ,  23   b ,  23   c  and  23   d  on the light receiving element  23 , respectively. Then, assuming that output signals thereof are assigned A, B, C and D, respectively, a focus error signal is obtained on the basis of the arithmetic operation of (A+C)−(B+D). The focus control circuit  35  thus generates the focus error signal on the basis of the arithmetic operation, and carries out the focus control on the basis of the resultant focus error signal.  
         [0069]    In addition, in case where a reproduction signal is generated, similarly to the first embodiment, two beams polarized and separated by the Wollaston prism  22  into components orthogonal to each other are received by light receiving surfaces  23   i  and  23   j , respectively. Then, assuming that output signals thereof are assigned I and J, respectively, a reproduction signal is obtained by carrying out the arithmetic operation of I−J.  
         [0070]    With respect to a tracking error signal, reflected lights of the sub-beam  27  and the main beam  28  are used. Assuming that a signal which is obtained by receiving the reflected light of the sub-beam  27  by the light receiving surface  23   e  on the light receiving element  23  is assigned E, signals which are obtained by receiving the reflected light of the main beam  28  by the four divided light receiving surfaces  23   a ,  23   b ,  23   c  and  23   d  on the light receiving element  23  are assigned A, B, C and D, respectively, and quantities of incident lights of the sub-beam and the main beam to the magneto-optical disc  8  are assigned P 1  and P 2 , respectively, a tracking error signal TE 9  is obtained by carrying out the following arithmetic operations:  
           TE   7 = E    
           TE   8 =( A+B+C+D )  
           TE   9 = TE   8 −( TE   7 · P   2 / P   1 )  
         [0071]    The tracking control circuit  36  generates the tracking error signal on the basis of the arithmetic operations, and carries out the tracking control on the basis of the resultant tracking error signal.  
         [0072]    In addition, similarly to the first embodiment, by changing a polarity of the tracking error signal, a beam can be drawn into either track interchangeably from the information track T 1  to the information track T 2 , or from the information track T 2  to the information track T 1 . This changing operation is carried out by the control circuit.  
         [0073]    As described above, in this embodiment, the beam for recording/reproduction can be accurately tracking-controlled without using divided light receiving elements so that two rows of information tracks within one groove (or land) are traced with the beam. In addition, in the second embodiment, since the light receiving element is simplified as compared with the first embodiment, the structure of the optical information recording medium can be simplified all the more. In addition, as compared with the tracking method using a push-pull signal, this embodiment has a superior advantage in that no offset due to shift or tilt of the objective lens in the radial direction of the disc is caused.  
         [0074]    Note that, in the second embodiment, it has been described that the distance between the sub-beam  27  and the main beam  28  in the disc radial direction is made equal to a half of the land pitch on the information recording surface. However, the present invention is not intended to be limited thereto and the distance may be a distance of N/2 (N: positive odd number) of the land pitch.  
         [0075]    (Third Embodiment)  
         [0076]    [0076]FIGS. 11A and 11B show another embodiment of the magneto-optical disc. In this embodiment, an example is shown in which the magneto-optical disc has four information tracks within one groove. FIG. 11A is a cross sectional view of a substrate of the magneto-optical disc, and FIG. 11B is a cross sectional view of the magneto-optical disc. The magneto-optical disc  8  is a domain wall displacement type magneto-optical medium. A magnetic film M in which a displacement layer, a switching layer, and a recording layer are laminated is formed on the substrate  30 . It should be noted that a layer structure of the domain wall displacement type magneto-optical medium is not limited thereto.  
         [0077]    In addition, with respect to an information track, as shown in FIG. 11B, four rows of tracks, i.e., tracks T 1 , T 2 , T 3  and T 4  are provided at an equal width in each groove G. An area A being changed in magnetism through an annealing processing is provided along a center of each groove G. Areas C being changed in magnetism through the annealing processing are provided along both sides of the area A so as to be apart therefrom by ¼ of the land pitch P L . Likewise, an area B being changed in magnetism through the annealing processing is provided along a center of each land L. The tracks T 1 , T 2 , T 3  and T 4  are magnetically separated from each other through the area A, the areas B and the areas C.  
         [0078]    A land width W L  is 0.2 μm, a groove width W G  is 2.0 μm, and a land pitch P L  is 2.20 μm, and a groove depth D is 0.03 μm. The annealed areas A and C of each groove have a width of about 0.2 μm, and the annealed area B on each land takes up nearly the whole land width W L . Note that, the land width and the groove width may be reversed so that the land width W L  is 2.0 μm and the groove width W G  is 0.2 μm. In this case, the tracks T 1 , T 2 , T 3  and T 4  are provided in each land L in order to carry out the recording.  
         [0079]    Next, description will hereinbelow be given with respect to a method in which annealing processing is performed to change the magnetism of the following areas: an area along a center of each groove; areas along both sides of the above-mentioned area along the center of the groove which are apart therefrom by ¼ of the land pitch P L ; and an area along a center of each land L. While a construction of an apparatus for use in the annealing processing is nearly the same as that of the apparatus shown in FIG. 2, the diffraction grating  2  and the light receiving element  10  in this embodiment are different in structure from those shown in FIG. 2. FIG. 12 shows the diffraction grating used in this embodiment, and FIG. 13 shows the light receiving element for use therein.  
         [0080]    Subsequently, a method of the annealing processing will hereinbelow be described. Firstly, a method of annealing the area A along the center of each groove G and the area B of each land L is the same as that in the first embodiment. FIG. 14 shows a beam arrangement when both areas C, which are apart from the area along the center of each groove by ¼ of the land pitch, are tracked with beams for the annealing. A distance between sub-beams  40  and  41  in the disc radial direction is made equal to ½ of the land pitch P L  on the information recording surface of the medium, and a main beam  42  is located between the sub-beams  40  and  41  in the middle thereof.  
         [0081]    In addition, a diffraction grating portion  15   a  of the diffraction grating, as shown in FIG. 12, has a circular shape which is smaller than an incident beam diameter D M , a beam diameter D S  of a diffracted light is made smaller than the incident beam diameter D M  to decrease NA when the diffracted light is condensed on the magneto-optical disc  8 , and each of the spots of the sub-beams  40  and  41  on the information recording surface is made substantially equal to the land pitch P L . Reflected lights of the sub-beams  40  and  41 , as shown in FIG. 13, are received by light receiving surfaces  10   e ,  10   f  and  10   g ,  10   h  which are obtained by dividing the light receiving elements into two parts in a radial direction of the magneto-optical disc  8 . Then, assuming that output signals of the light receiving surfaces  10   e ,  10   f  and  10   g ,  10   h  are assigned E, F and G, H, respectively, so-called push-pull signals are respectively obtained by carrying out the following arithmetic operations:  
           TE   10 = E−F    
           TE   11 = G−H    
         [0082]    Moreover, the arithmetic operation of TE 10 −TE 11  is carried out to obtain a tracking error signal TE 12  by utilizing the well-known differential push-pull method. Then, the areas C are annealed by using the tracking error signal TE 12 . Thus, the areas A and C within each groove, and the area B on each land are annealed.  
         [0083]    Next, description will hereinbelow be given with respect to an optical information recording/reproducing apparatus for recording/reproducing information on/from the magneto-optical disc  8  shown in FIG. 11B. A configuration of the apparatus is the same as that of the apparatus shown in FIG. 6 except that similarly to the case of the annealing processing, the diffraction grating shown in FIG. 12 is used as the diffraction grating  15 . FIG. 15 shows a beam arrangement on the medium and a tracking error signal upon recording/reproduction. A distance between sub-beams  43  and  44  in the disc radial direction is made equal to ½ of the land pitch P L  on the information recording surface of the medium, and a main beam  45  is located between the sub-beams  43  and  44  in the middle thereof.  
         [0084]    In addition, the diffraction grating portion  15   a  of the diffraction grating, as shown in FIG. 12, has a circular shape which is smaller than the incident beam diameter D M , the beam diameter D S  of the diffracted light is made smaller than the incident beam diameter D M  to decrease NA when the diffracted light is condensed on the magneto-optical disc  8 , and each of the spots of the sub-beams  43  and  44  on the information recording surface is made substantially equal to the land pitch P L .  
         [0085]    When tracks T 1  to T 4  within each groove are tracked with the main beam, similarly to the annealing processing, the differential push-pull signal (TE 12 ) is used. First of all, when the tracks T 1  and T 4  are tracked with the main beam, as shown in FIG. 15, a track offset is added to the tracking error signal to shift a tracking position by P L / 8 . Note that, the control circuit shown in FIG. 6 carries out the addition of the track offset through the tracking control circuit  36 .  
         [0086]    In addition, when the tracks T 2  and T 3  are tracked with the main beam, a track offset of a polarity reverse to that when the tracks T 1  and T 4  are tracked is added to the tracking error signal, and under this condition, the tracking control is carried out. In such a manner, information is recorded/reproduced in/from the four information tracks within one groove or land. While only the tracking control has been described in this embodiment, it should be noted that the focus control and recording/reproduction of information are the same as those in case of FIG. 6.  
         [0087]    Moreover, in the above-mentioned embodiments, the description has been given with respect to an example in which two or four information tracks are formed within one groove or land of the magneto-optical disc, and under this condition, information is recorded/reproduced. However, the present invention is not intended to be limited thereto. That is to say, it may also be available that three or five or more tracks are formed within one groove or land, and under this condition, information is recorded/reproduced. In such case, an offset and polarity are changed corresponding to a tracking position are carried out to thereby perform the tracking control for a desired track.  
         [0088]    While in the above-mentioned embodiments, the magneto-optical recording medium is used, it should be noted that a material of a recording layer is not limited thereto, and hence a phase change material may also be used.  
         [0089]    As set forth hereinabove, according to the present invention, even if the track density is increased beyond the optical resolution, the excellent recording/reproducing performance can be obtained, and hence the high densification of information recording can be realized as compared with the prior art.