Abstract:
A method for reproducing information from an optical medium including a substrate with grooves and lands alternately formed on the substrate in a radial direction. The grooves and the lands both serve as tracks which are divided into units in the circumferential direction. Each unit has a prepit area in a non-groove portion of the substrate with a first prepit being represented as VFO information in the prepit area and a second prepit being represented as address information in the prepit area. The first prepit and second prepit are located on both sides of a center line of one track and are arranged with respect to one another. The method includes irradiating an optical spot on the optical medium, detecting address information, controlling the irradiation position of the optical spot on the basis of the address information and reproducing information.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to optical recording methods and more particularly to a technique based on land/groove recording and suitable for performing high-density optical recording in which the track width is smaller than the optical spot diameter.  
           [0002]    A conventional method is disclosed in, for example, JP-A-59-191156. In the prior art, a laser beam generated from a laser diode carried on an optical head is formed into a collimated beam by means of a collimating lens, and the collimated beam passing through a beam splitter is focused by an objective lens so as to be converged into an optical spot on a magneto-optical recording medium. The position of the optical spot on the magneto-optical recording medium is controlled by moving the lens or the optical head by means of an optical spot scanning control means. Reflected light from the magneto-optical recording medium is guided to a photodetector through a beam splitter. A readout signal from the photodetector is processed by a reproduction circuit so as to be converted into reproduced data. Control of overall reproduction is carried out by a controller.  
           [0003]    JP-A-6-176404 describes a technique for performing high-density (narrow track) recording.  
           [0004]    A recording medium disclosed in JP-A-6-176404 is illustrated, in plan view form, in FIG. 5. Grooves  501  and lands  502  are formed on a substrate, information recording areas are formed in association with both the groove and the land, and prepits  504  are disposed on an extension line  503  of the boundary line between a groove  501  and a land  502 . Prepits  504  are positioned each groove on only one side relative to the center line of each groove. With this construction, recording information is recorded on both the groove  501  and the land  502 , address information representative of the recording areas are recorded in the form of prepits  504 , and one prepit is used in common to a pair of adjacent groove  501  and land  502  to provide address information therefor.  
           [0005]    When the technique as above is applied to, for example, a phase change recording medium or a magneto-optical recording medium, interference of information (crosstalk) between adjacent grooves  501  or lands  502  due to the optical interference effect within an optical spot  505  can be prevented, thereby permitting narrowing of track. On the other hand, in the prepit area free from the optical interference effect, the address information can be common to the paired groove and land and the effective track pitch can be increased to reduce crosstalk.  
           [0006]    In the example of JP-A-6-176404, however, the disposition of the prepit area is offset on one side of the center line of the groove and an offset tracking error signal is delivered out of the prepit area, with the result that when an optical spot is caused to track a groove or a land, a tracking error (tracking offset) increases, making it difficult to perform high-density recording in which the track pitch is narrowed.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a technique capable of suppressing the tracking offset to a value which is sufficiently low for practical use and permitting efficient disposition of address information even when recording is effected on both the groove and the land.  
           [0008]    To accomplish the above object, solutions of the invention are adopted as below.  
           [0009]    More particularly, in an optical recording medium having substantially concentric grooves and lands formed on a circular substrate and information recording areas formed in association with both the groove and the land, prepits are disposed on a virtual extension line of the boundary between a groove and a land, the disposition of the prepits satisfying all of the following four conditions:  
           [0010]    (i) Prepits are located on both sides of an extension of the center line of one groove;  
           [0011]    (ii) Prepits are located on both sides of an extension of the center line of one land;  
           [0012]    (iii) Prepits are not located on both sides of any specific position of the center line of one groove; and  
           [0013]    (iv) Prepits are not located on both sides of any specific position of the center line of one land.  
           [0014]    With this construction, disposition of prepits is not offset on either one side of a virtual extension of the center line of the groove or the land, so that an offset tracking error signal is not delivered out of the prepit area, making the tracking offset hardly occur. Further, since prepits do not exist on both sides of or symmetrically to a position on an extension of the center line of the groove or the land, interference of prepit information between adjacent tracks do not take place within a reproduction spo. Accordingly, recording can be performed on both the groove and the land and addresses can be reproduced without crosstalk to permit high-density narrow track recording.  
           [0015]    Preferably, prepits are disposed alternately at a period which is even times a channel bit length on both sides of a virtual extension of the center line of the groove.  
           [0016]    Thus, the prepits are uniformly disposed on both sides of a virtual extension of the center line of the groove or the land, making the tracking offset more hardly occur.  
           [0017]    Further, the groove and the prepit have the same depth which is 70 nm or less. More preferably, the depth is 40 nm or more and 60 nm or less.  
           [0018]    Through this, crosstalk between the groove and the land can duly be canceled and an excellent tracking servo signal can be obtained, thus making injection and production of a medium easy. With the groove depth being in excess of 70 nm, injection of the groove is difficult to achieve. With the groove depth being about 50 nm, tracking servo is maximized and substantially the same effect can be obtained at a groove depth which is 10 nm around 50 nm.  
           [0019]    Preferably, the groove and the land have substantially the same width which is in the range of from 0.3 μm to 0.75 μm.  
           [0020]    Through this, excellent tracking can be compatible with high-density recording. If the groove and the land has a width which is not greater than 0.3 μm, two sets of groove and land are concurrently within a single optical spot and any excellent tracking signal cannot be obtained. With the width of the groove and the land being in excess of 0.75 μm, practical high-density recording cannot be permitted.  
           [0021]    The minimal diameter of a prepit is made to be smaller than the width of each of the groove and land. More preferably, the diameter falls within the range of from 0.25 μm to 0.55 μm.  
           [0022]    Through this, an excellent prepit signal can be obtained without crosstalk. If the diameter is not greater than 0.25 μm, power of the prepit signal decreases extremely and with the diameter being in excess of 0.55 μm, crosstalk takes place.  
           [0023]    When an optical recording medium is used in which grooves and lands are formed on a substrate, information recording areas are formed in association with both the groove and the land, any groove is not formed but flat address areas are discretely formed in the information recording area, and first and second address pits are disposed in the address area on an extension of the boundary between the groove and the land, the first and second address pits being disposed to satisfy such requirements that the first and second address pits are disposed alternately on both sides of an extension of the center line of one groove, that the first and second address pits are disposed alternately on both sides of an extension of the center line of one land, that address pits do not exist on both sides of a position on an extension of the center line of the groove and that address pits do not exist on both sides of a position on an extension of the center line of the land, an optical spot is irradiated on the optical recording medium, a reflected beam from the optical recording medium is detected, an address pit is detected from the detected reflected beam to form an address pit readout signal, an address is detected on the basis of the address pit readout signal, an amplitude of a first readout signal obtained from the first address pit of the address pit readout signal is sampled and held, an amplitude of a second readout signal obtained from the second address pit of the address pit readout signal is sampled and held, the amplitudes of the first and second readout signals are compared together, an offset signal is formed on the basis of a result of comparison, and the irradiation position of the optical spot is controlled on the basis of the offset signal.  
           [0024]    Through this, the tracking offset can be suppressed sufficiently for practical use and address information can be obtained.  
           [0025]    When tracking is carried out by sequentially obtaining tracking servo signals through the use of a diffracted beam obtained from a groove and correcting an offset of a tracking servo signal with an offset signal, stabler tracking can be ensured. More particularly, upon detection of a reflected beam from the optical recording medium, a tracking servo signal is formed by detecting a light beam diffracted by a groove by means of a plurality of photodetectors, comparing diffracted beams detected by the plurality of photodetectors and detecting the relative positional relation between the groove and the optical spot, the tracking servo signal is corrected with an offset signal, and the irradiation position of the optical spot is controlled on the basis of the corrected tracking servo signal.  
           [0026]    Further, an optical recording/reproducing apparatus may be constructed which uses a similar optical recording medium, comprises a light beam source, a beam focusing means for focusing and irradiating a light beam generated by the light beam source on the optical recording medium, photodetecter detecting a reflected beam of the light beam irradiated by the beam focusing means, a reproduction circuit reproducing information by using a signal from the photodetector, and a scanner moving the position of an optical spot irradiated by the beam focusing means to a desired position on the optical recording medium, and further comprises means for detecting an address on the basis of a readout signal from a prepit, a low-pass filter for detecting an amplitude of a low frequency component of the readout signal from the prepit, and a circuit performing lock-in detection of a passed signal of the low-pass filter, whereby the position of the optical spot is controlled on the basis of the detected signal.  
           [0027]    With this construction, the tracking offset can be suppressed to a smaller value.  
           [0028]    According to an aspect of the present invention, as shown in for example FIG. 1, prepits are disposed on both sides of a virtual extension of the center line of the groove or the land in staggered relation. Accordingly, offset can be decreased to make the tracking offset hardly occur and prepits do not exist on both sides of a position on the extension of the center line of the groove or the land, with the result that interference of prepit information between adjacent tracks can be prevented within a reproduction spot and high-density narrow track recording can be ensured.  
           [0029]    Further, even if a tracking offset takes place as shown in FIG. 3, signal amplitudes of prepits on both sides are compared to perform accurate detection of the tracking offset amount. Accordingly, by feedback-controlling the information to the scanner, the tracking offset can be suppressed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a plan view of an embodiment of an optical recording medium according to the present invention.  
         [0031]    [0031]FIG. 2 is a fragmentary plan view of the FIG. 1 optical recording medium.  
         [0032]    [0032]FIG. 3 is a waveform diagram showing readout waveforms in the present invention.  
         [0033]    [0033]FIG. 4 is a block diagram of an embodiment of an optical recording/reproducing apparatus according to the present invention.  
         [0034]    [0034]FIG. 5 is a fragmentary plan view of a prior-art optical recording medium.  
         [0035]    [0035]FIG. 6 is a fragmentary plan view of another embodiment of the optical recording medium according to the present invention.  
         [0036]    [0036]FIG. 7 is a waveform diagram showing readout waveforms obtained from the FIG. 6 optical recording medium.  
         [0037]    [0037]FIG. 8 is a block diagram of an embodiment of an optical recording apparatus according to the present invention.  
         [0038]    [0038]FIG. 9 is a waveform diagram useful to explain the principle of the optical recording apparatus of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    Referring now to FIG. 1, there is illustrated, in plan view form, an optical recording medium of the present invention. Grooves  84  each having a width of 0.6 μm and a depth of 50 nm and lands  85  each having a width of 0.6 μm are formed alternately in the radial direction of the medium and recording marks  81  are recorded on the two kinds of areas. More particularly, the land  85  and the groove  84  are both recording areas which form tracks. In a prepit area  83 , any groove is not formed but prepits  82  are disposed on a flat area serving as an address area. This type of optical recording medium can be produced by forming a recording film on a disk-like substrate having grooves  84  and prepits  82 , which substrate being prepared through mass producing using a stamper. The prepit area  83  is formed radially of the substrate, having indicia such as marks in the form of prepits  82  which are disposed concentrically across a plurality of tracks. The prepits are not always required to be arranged in the radial direction over the entire radius of the substrate and the prepit area may be divided into a plurality of zones which are arranged in the radial direction, forming a ZCAV (zoned constant angular velocity) or ZCLV (zoned constant linear velocity) type format which is preferable from the standpoint of high-density recording.  
         [0040]    Referring to FIG. 2, the neighborhood of the prepit area  83  is illustrated in an enlarged view form. Pits  82  are disposed on an extension line of the boundary between a land and a groove. A pit has a width of 0.35 μm and a depth of 50 nm. The prepit area is divided into a first prepit area  831  and a second prepit area  832 . In the first prepit area  831 , pits  82  are disposed on the upper side, in the drawing, of the center line of a land  85  but in the second prepit area  832 , pits  82  are disposed on the lower side, in the drawing, of the center line of the land  85 . Accordingly, when an optical spot  21  scans, for example, the land  85 , a signal is always produced from either one of the first and second prepit areas and consequently, there is no fear that crosstalk will occur between adjacent tracks. Therefore, address information recorded in the form of prepits can duly be reproduced without crosstalk.  
         [0041]    Since pits  82  are not adjoined in the radius direction, injection can be facilitated upon formation by the stamper.  
         [0042]    Also, pits  82  are uniformly disposed on both sides of a track (a land or a groove) and hence the influence of a tracking error signal, which is delivered out of the prepit area while being offset due to pits  82 , upon a tracking servo signal can be canceled. Accordingly, the tracking offset can be suppressed to a minimum.  
         [0043]    Further, when reproducing, for example, a land  85 , reproduction of address information at the second prepit area  832  is carried out continuously with reproduction of address information at the first prepit area  831 . Accordingly, when the two areas are united into one area in which information is arranged to provide address information for one track, an address (track number) of a land and that of a groove can be set independently of each other.  
         [0044]    In the present embodiment, a magneto-optical recording film (TbFeCo) is used as the recording film. Accordingly, the recording mark is prepared in the form of a recorded domain. A known phase change film may also be used as the recording film. In the example of FIG. 2, one set of first and second prepit areas is provided but a plurality of sets may be provided as necessary.  
         [0045]    Referring to FIG. 6, there is illustrated, in enlarged fragmentary plan view form, another example of the optical recording medium of the present invention. Grooves  84  each having a width of 0.5 μm and a depth of 40 nm and lands  85  each having a width of 0.5 μm are formed alternately and recording marks  81  are recorded on the two kinds of areas. Thus, the land  85  and the groove  84  are both recording areas. In a prepit area  83 , any groove is not formed but substantially circular pits  82  (each having a diameter of 0.3 μm and a depth of 40 nm) are disposed on an extension line of the boundary between a land and a groove. The prepit area is divided into a VFO (Variable Frequency Oscillator) area  833  and an address area  834 .  
         [0046]    Especially, in the VFO area, pits  82  are disposed alternately on the upper and lower sides of the center line of a land  85 . In the address area, pits  82  are disposed alternately at the same period as that in the VFO area. Accordingly, there are no pits which exist on both sides of (or symmetrically to) a position on the center line of the land and the groove. In addition, in the address area, data for a particular track is so encoded as to differ by one pit from data for an adjacent track. In other words, the data takes the form of a Gray code. With this construction, when an optical spot  21  scans, for example, a land  85 , pits on either one side are always reproduced and there is no fear that crosstalk will occur between adjacent tacks. Therefore, address information distributed to the prepits can duly be reproduced without crosstalk. Since pits  82  for adjacent tracks do not adjoin to each other, injection can be facilitated. Also, pits  82  are uniformly disposed on both sides of a track (a land or a groove) and hence the influence of a tracking error signal, which is delivered out of the prepit area while being offset due to pits  82 , upon a tracking servo signal can be canceled. Accordingly, the tracking offset can be suppressed to a minimum.  
         [0047]    Referring to FIG. 7, readout signals obtained from the prepit area  83  in the FIG. 6 embodiment are illustrated. When an optical spot scans the center of individual tracks, signal waveforms shown in the Figure are generated of which signals  11  are generated from tracks constructed of lands  85  and signals  12  are generated from tracks constructed of grooves  84 . As is clear from the Figure, the generated signals are different for the individual tracks, demonstrating that address information is recorded very efficiently. By virtue of the use of the Gray code, an address can be reproduced in the course of inter-track access, ensuring suitability to high-speed access. Further, the use of the Gray code makes an error hardly occur even in the presence of crosstalk, thus ensuring suitability to narrowing of tracks.  
         [0048]    Referring now to FIG. 4, there is illustrated an example of construction of an optical recording/ reproducing apparatus of the present invention.  
         [0049]    In the present embodiment, a laser diode  311  having a wavelength of 680 nm is used as a light source, a laser beam is formed into a collimated beam by means of a collimating lens  312 , and the laser beam is focused to an optical spot  21  on an optical disk  8  by means of an objective lens  321 . As necessary, a beam shaping means such as a prism and other lenses may be provided in the optical path. The optical disk shown in FIGS. 1 and 2 is used as the optical disk  8 .  
         [0050]    Power of the laser diode  311  is controlled by a light power controller  71  having the auto-power control function. Beam splitters  324  and  325  are adapted to guide a light beam  23  reflected from the optical disk  8  to photodetectors  333 ,  334 ,  340  and  341 . In the present embodiment, the aperture ratio of the objective lens  321  is set to 0.55. Consequently, the diameter of the optical spot  21  on the optical disk  8  is 1.1 μm.  
         [0051]    The optical disk  8  is rotated by a motor  62 . The optical spot  21  can be moved to a desired position on the optical disk  8  by means of a scanning mechanism. In the present embodiment, the scanning mechanism, as designated at  6 , has an automatic position controller  6  also designated at  6  and having functions of auto-focus control and automatic tracking, and a lens actuator  61  controlled by the automatic position controller  6 .  
         [0052]    The reflected beam  23  from the optical disk is guided to a signal detection system by means of the beam splitters  324  and  325 . Part of the reflected beam is split to two beams having different polarization planes through a half-wave plate  337 , a lens  331  and a polarized beam splitter  332  and the two beams are detected by the photodetectors  333  and  334 , respectively. Readout signals from the two detectors  333  and  334  are differentially amplified by a differential amplifier  944  so that information magnetically recorded on the optical disk may be detected magneto-optically. The readout signals of the two detectors  333  and  334  are also added-together by means of an adder  941  so that information  14  recorded in the form of prepits on the optical disk may be detected.  
         [0053]    The automatic position controller  6  utilizes the reflected beam  23  from the optical disk  8  to cause a tracking servo signal detector  34  to detect an optical spot position which is used for feedback control. For detection of the optical spot position, the detectors  340  and  341  detect power of a diffracted light ray from a groove in the optical disk  8  and signals delivered out of the detectors  340  and  341  are differentially amplified by a differential amplifier  342  to produced a difference signal.  
         [0054]    Illustrated in FIG. 3 are signals obtained from the optical disk  8  shown in FIGS. 1 and 2 in the present embodiment. When the optical spot  21  scans a land  85 , a signal wave including magneto-electrical reproduced signal  12  shown in under side of FIG. 3 is obtained. As shown in FIG. 3, when the optical spot  21  deviates from the track center (being offset), an amplitude difference  13  takes place between prepit signal portions from the first and second prepit areas  831  and  832 . This amplitude difference  13  corresponds to an amount of tracking offset.  
         [0055]    The prepit signal  14  shown in FIG. 3 is fed to an address detection means  43  so as to be decoded to address information by an address decoder  431 . At the same time, timings for signals of the first and second prepit areas are generated by a timing controller  432 .  
         [0056]    On the basis of the timing information, a first amplitude sample and hold circuit  411  stores an amplitude (average maximum amplitude) of the first prepit area and a second amplitude sample and hold circuit  412  stores an amplitude (average maximum amplitude) of the second prepit area. Alternatively, the first and second amplitude sample and hold circuits  411  and  412  may have a common amplitude sampler.  
         [0057]    The thus held amplitudes are compared together by means of an amplitude comparator  42  to produce an amplitude difference  13 . On the basis of the amplitude difference  13 , a tracking offset signal  44  is formed. The tracking offset signal  44  is added with a tracking error signal  15  from the servo signal detector  34  by means of an adder  942  to produce a sum signal which in turn is fed back to the position moving means (scanning means)  6 .  
         [0058]    In the apparatus of the present embodiment, the tracking offset signal is formed on the basis of the amplitude difference  13  and the tracking error signal is corrected with the tracking offset signal to produce a corrected signal which is fed back to the position moving means. Accordingly, even when various kinds of external disturbance such as aberration of the optical spot is taken into consideration, the tracking offset can be decreased to ±0.03 μm or less. Under the nominal state devoid of optical aberration, the tracking offset is ±0.015 μm or less.  
         [0059]    In carrying out recording with the apparatus of the present embodiment, a recording beam  22  whose power is controlled by the light power controller  71  is irradiated on the optical disk  8  to form an optical spot  21 . While applying a bias field to the neighborhood of the optical spot  21  by means of a bias field power controller  72 , a bias field application circuit  73  and a bias coil  74 , the temperature of the recording film is heated by the optical spot  21  to a value near the Curie temperature to form a recorded domain in a heated area. In this example, the size of the recorded domain is assumed to be of a width of about 0.5 μm.  
         [0060]    Referring to FIG. 8, there is illustrated another example of construction of the optical recording/reproducing apparatus of the present invention. This example differs from the FIG. 4 embodiment in that the prepit signal is passed through a low-pass filter  45  and lock-in amplified by a lock-in amplifier (detector)  44 , thereby forming tracking error information. In this example, the optical recording medium of FIG. 6 is used and therefore, pits  82  are disposed alternately on both sides of the center of a land  85  or a groove  84  in the VFO area  833 .  
         [0061]    Illustrated in FIG. 9 are waveforms of a VFO signal (output of the amplifier  941 ) and a low-pass filter signal (output of the LPF  45 ). In the absence of an offset, a low-pass filter component  16  of a VFO signal  15  has no amplitude but in the presence of a tracking offset, a low-pass filter component  18  of a VFO signal  17  has an amplitude. This amplitude is lock-in amplified by the lock-in amplifier  44  to detect a tracking offset. Accordingly, by feedback-controlling the offset amount to the scanning means  6 , the tracking offset can be decreased. In the present embodiment, the tracking offset can be suppressed to ±0.025 μm or less.  
         [0062]    The present invention is in no way limited to the foregoing embodiments. For example, an optical head capable of generating a plurality of optical spots at a time may also be used. In addition to the magneto-optical recording medium, a phase change recording medium may be used. Further, in addition to the method using the diffracted beam, a three-spot detection method in which power levels of reflected beams from a plurality of optical spots are compared together or a pre-wobbling method may be used as the servo signal detection method.  
         [0063]    By using the optical recording medium of the present invention, the tracking offset can be suppressed to a level which is sufficiently small for practical use (0.03 μm or less) and address information can be obtained easily even during high-density narrow track recording.  
         [0064]    By using the optical recording/reproducing apparatus of the present invention, the tracking offset can be decreased easily through feedback control.