OPTICAL DISK

Abstract: An optical disc adapted so that concentrical or spiral guide grooves are formed at a base and emboss shaped pits are molded between these respective guide grooves is disclosed. In this optical disc, depth of pit is set so that the push-pull signal by the pits is caused to have reverse polarity with respect to the push-pull signal by the guide grooves. For example, when wavelength of reproduction laser beams is λ, refractive index of the base is n, and track pitch is t, the depth Dp of the pit is set so as to fall within the range expressed as 0.32 λ/n ≤ Dp ≤ 0.51 λ/n , the width Wp of the pit is set so as to fall within the range expressed as 0.27 t ≤ Wp ≤ 0.36 t , and the depth Dg and the width Wg of the guide groove are respectively set so as to fall within the ranges expressed as 0.0875 λ/n ≤ Dg ≤ 0.1625 λ/n and expressed as 0.27 t ≤ Wg ≤ 0.36 t .

Description: Technical Field This invention relates to an optical disc including guide grooves and pits, and more particularly to improvements for obtaining reasonable push-pull signals. Background Art For example, optical discs represented by the so-called ISO standard have spiral guide grooves, wherein emboss shaped pits are molded (formed) in advance between respective grooves at the inner circumferential portion or the header portion. In such optical discs, the optical pick-up is subjected to seek operation in the radial direction of the disc to move it to the target track on the basis of tracking error signal (so called push-pull signal) from the groove. Namely, threshold is set in the vicinity of center value of the push-pull signal to detect how many times the push-pull signal crosses (traverses) this threshold to thereby count the number of tracks to detect the target track. At the same time, the target address is detected on the basis of an address signal from the emboss shaped pit to carry out recording or reproduction of information. Meanwhile, in the optical disc where pits in emboss form are molded in advance between respective grooves as described above, the phenomenon appears that (the signal amplitude of) the tracking error signal (so called push-pull signal) in the area including pits is reduced to one half (1/2) - one third (1/3) as compared to the signal amplitude in the area comprised of only grooves. This is because an approach has been conventionally preferentially employed to take the information signal amplitude from the area comprised of only grooves as a large value, so the tracking error signal from the area including pits is sacrificed. Accordingly, when the tracking error signal is greatly reduced in the area including pits by phenomenon as previously described above, there take place inconveniences such that tracking cannot be precisely taken and/or any error takes place in the tracking count operation. EP-A-0 546 525 discloses an optical disc including at least one tracking groove and a plurality of pits which are arranged in-between the spaces devided by the tracking groove as is the case with the present optical disc. A depth of the tracking groove from a disc surface is selected to lie in a range from about λ/16 to 3λ/16 for the wave length λ of a reproducing laser beam. A depth of each of the pits from the disc surface is selected to lie in a range from 5λ/16 to λ/2. This document does not deal with the width of the pits and the width of grooves in combination with their respective depths. JP-A-63 050 933 and the corresponding abstract discloses an optical disc which propose for stabilizing a tracking servo loop and to obtain a recording and reproducing signal with high quality setting the guide track depth between λ/4 and λ/2. The depth of address pits coexisting between continuous guide tracks is proposed to lie in a range of λ/16 to λ/2. That is, the value of the depth of the track proposed in this document is outside of the range of the depth of the guide groove in the present application. Further, this document does not explicitly deal with a width of the pit and the width of the guide grooves as well in combination with their respective depth. Disclosure of the invention An object of this invention is to provide an optical disc capable of suppressing decrease of (the signal amplitude of) the pushpull signal in the area where grooves and pits are mixed. To attain the above-described object, an optical disc of this invention is directed to an optical disc in which concentrical or spiral guide grooves are formed at a base (substrate), and pits in emboss form are molded between these respective guide grooves. When wavelength of reproduction laser beam is λ and refractive index of the base is n, the depth Dp of the pit is caused to fall within the range expressed as 0.32 λ/n ≤ Dp ≤ 0.51 λ/n. In the optical disc, when the track pitch is t, the width Wp of the pit is expressed as 0.27 t ≤ W p ≤ 0.36 t. Further, the depth Dg and the width Wg of the guide groove are respectively expressed as 0.0875 λ/n ≤ D g ≤ 0.1625 λ/n and 0.27 t ≤ W g ≤ 0.36 t. In this invention, since the depth of the pit is set so that a push-pull signal caused by the pit has the same polarity as that of the push-pull signal caused by the grooves at the time the reproduction laser beam crosses the grooves, there is no possibility that bad influence is exerted on the push-pull signal. As a result, e. g., the possibility that any erroneous counting may take place at the time of tracking count operation is lowered. In addition, the information signal amplitude from the area including pits is ensured. Namely, in accordance with this invention, in the area comprised of only grooves and the area where pits exist between respective grooves, an approach can be made such that the polarities and the magnitudes of push-pull signals (from the respective areas), which are the most important for the purpose of taking tracking of laser beam spot, are not caused to be changed. Since the previously described effect can be obtained also in the case where the laser beam spot crosses (traverses) the header area (area from which address signal is obtained) of the optical disc, erroneous count can be reduced in the case of the system in which the push-pull signal is used for the track counting operation. Brief Description of the Drawings FIG. 1 is a model view showing surface configuration of optical disc base where pits and grooves are formed. FIG. 2 is a model view for explaining the principle of the push-pull method. FIG. 3 is a characteristic diagram showing, in a comparative manner, push-pull signal by the grooves and push-pull signal by the pits. FIG. 4 is a characteristic diagram conceptually, along with TCS signal, the state of change of push-pull signal by depth of groove. FIG. 5 is a characteristic diagram showing a typical example of push-pull signal including noises. FIG. 6 is a characteristic diagram showing push-pull signal in which the pit depth is caused to be reasonable so that noise is reduced. FIG. 7 is a characteristic diagram showing change of push-pull signal by pit depth Dp. FIG. 8 is a characteristic diagram showing the state of push-pull signal in the case where track where pits exist and track where no pit exists are adjacent to each other. FIG. 9 is a characteristic diagram showing change of TCS signal by pit depth Dp. FIG. 10 is a characteristic diagram showing, in a model form, groove width and groove depth dependency of TCS signal and push-pull signal. Best Mode for Carrying Out the Invention FIG. 1 represents, in a model form, the surface configuration of an optical disc base (substrate) where grooves G and emboss shaped pits P are formed. In this case, the area where the pits P are formed corresponds to the header portion. In this example, reflection film, recording film, and dielectric film and/or protective film, etc. are formed by a configuration corresponding to use (purpose) on the base so that the optical disc is provided. When laser beams are irradiated onto the optical disc in the in-focus state to scan (seek) the laser (beam) spot in the radial direction of the disc, push-pull signal is observed. In the figure, arrow A indicates the state (direction) where the portion comprised of only grooves (groove portion or area) is scanned, and arrow B indicates the state (direction) where the portion including pits (the pit portion or area) is scanned. The push-pull method is a method of taking out light reflected and diffracted at track on the disc as an output difference at two light receiving portions (photo-detecting portions) on the bi-sected detector PD symmetrically disposed with respect to the track center to thereby detect tracking error. The explanatory view of the principle is shown in FIG. 2. In the case where the center of the laser spot and the center of the guide groove G are in correspondence with each other, reflected and diffracted light symmetrical in left and right directions can be obtained. In contrast, in the case where those centers are not in correspondence with each other, there results reflected and diffracted light asymmetrical in left and right directions. As a result, difference in the light intensity obtained at (respective detecting portions of) the bi-sected photodetector PD takes place. A difference signal between output signals from these two photo-detecting portions is obtained as a push-pull signal having polarity, and is used for tracking. Moreover, when a difference between output signals obtained by the two photo-detecting portions when the laser spot crosses (traverses) the track at the time of track access is taken, the difference signal thus obtained represents S-shaped curve. For example, the push-pull signal (S-shaped curve) obtained from the grooves and the push-pull signal (S-shaped curve) obtained from the pits are indicated as shown in FIG. 3. A signal obtained by combining (synthesizing) these both signals results in an actual push-pull signal obtained at the header portion. In this case, in accordance with the study of the inventor of this application, it has been found that the push-pull signal changes in dependency upon depth of the groove or the pit and its polarity is inverted with λ/4 (λ is wavelength of laser beams) being as the boundary. Let now consider the case where the push-pull signal from the grooves and the push-pull signal from the pits are the same. It is to be noted that since the push-pull signal from the grooves is the primary push-pull signal, the depth of the groove is caused to be value (λ/8) where (the amplitude of) the push-pull signal PP becomes maximum in FIG. 4. As shown in FIG. 3A, the push-pull signal by grooves crosses (traverses) the base line (line of the center value) at the centers of respective grooves. At this time, the crossing (traversing) direction is the upper direction (the polarity of the push-pull signal at this time is assumed to be positive). On the other hand, the pits exist at the intermediate portions between the respective grooves, and the push-pull signal by pits is as shown in FIG. 3B. In the case where setting is made with respect to the depth of pit such that (polarity of) the push-pull signal at the time the laser beam crosses the pit is positive, the push-pull signal is as indicated by line s in FIG. 3B. At the centers of respective pits, similarly to the previously described push-pull signal derived from grooves (Fig. 3A), the direction in which the push-pull signal S crosses (transverses) the base line is the upper direction. On the other hand, at the respective groove centers, the direction of the push-pull signals is the lower direction. As a result, at the groove centers, there results the state where the push-pull signal caused by the grooves is canceled by the push-pull signal caused by the pits. Thus, (the signal amplitude of) the resultant push-pull signal is greatly lowered. As a result, large noise is observed. The signal actually observed is shown in FIG. 5. This FIG. 5 shows the state where the push-pull signal is observed at the pit portion (the groove portion with header which corresponds to (the area scanned in the direction indicated by) arrow B). It is found from Fig. 5 that signals (signal components) from the pits P are superposed on the (resultant) push-pull signal PP as respective noises (noise components) N, and there thus results high possibility that erroneous counting may take place, e.g., at the time of tracking count operation by the push-pull signal. On the contrary, in the case where setting is made with respect to the depth of the pit such that the push-pull signal caused by the pits becomes negative at the time the laser beam crosses the pit P, the push-pull signal by pits is as indicated by line m in FIG. 3B. At the respective groove centers, when the light spot moves (shifts) from the left to the right in the Fig. 3B , the direction in which the push-pull signal in crosses (traverses) the base line is the upper direction. This direction is the same direction as that of the push-pull signal caused by grooves. Accordingly, there is no possibility that there results great lowering in the push-pull signal by grooves. FIG. 6 shows the state where the push-pull signal is observed at the pit portion (the groove portion with header which corresponds to (the area scanned in the direction indicated by) the arrow B) when such a setting is made. Noise N is hardly observed. How the push-pull signal (P-P) and the TCS (Track-cross signal) change when the depth of the pit is actually changed was studied on the basis of the above-mentioned finding. In the above-mentioned study, parameters employed are as indicated in the Table 1. In addition, scalar analytic method of diffraction was used in the analysis. FIG. 7 is a diagrammatical view showing difference of the push-pull signal by the depth Dp of the pit. In this figure, the right end and the left end correspond to the track center. Accordingly, the push-pull signal when the light spot crosses (traverses) a single track is indicated with the signal intensity being taken on the ordinate in FIG. 7. Moreover, in the figure, the solid line a indicates the push-pull signal in the groove area (the pit depth Dp=0 which corresponds to (the area scanned in the direction indicated by) the arrow A in the FIG. 1 mentioned above), and other lines b, c, d, e indicate push-pull signals in the areas including pits different in depth (hereinafter each simply referred to as the pit area as occasion may demand). In the figure, the line b indicates the push-pull signal at the pit depth of λ/4n, the line c indicates the push-pull signal at the pit depth of 7λ/24n, the line d indicates the push-pull signal at the pit depth of λ/3n, and the line e indicates the push-pull signal at the pit depth of 3λ/8n. It has been found from this figure that the push-pull signal in the pit area has substantially the same intensity as that of the push-pull signal in the groove area when the pit depth Dp is equal to 3λ/8n (line e). Further, FIG. 8 is a diagram showing push-pull signals similar to the above in connection with the case where the track where pits exist and the track where no pit exists are adjacent to each other. In FIG. 8, the line f indicates the push-pull signal in the state where only grooves exist, and the line g indicates the push-pull signal in the state where the track where pits exist and the track where no pit exist are adjacent to each other. In this case, the depth Dp of pit is 3λ/8n. Also in this case, the influence that the pit exerts on the push-pull signal can be hardly observed. Even if a track having pit at one side and a track having no pit at the other side are adjacent to each other, the deviation quantity of push-pull is less than 0.01 µm. This value is very small as compared to the track pitch. On the other hand, a TCS signal when the depth Dp of pit is changed is shown in FIG. 9. From this FIG. 9, change of the pit modulation factor (degree) based on difference of pit depth Dp can be recognized. When viewed from this figure, the pit modulation factor becomes maximum when the pit depth Dp is a value in the vicinity of λ/4n (I SM /I OL = 0.89: When pits exist at respective tracks, I SM /I OL becomes equal to 0.84), and the pit modulation factor is decreased by about 25% when the pit depth Dp is equal to 3λ/8n (I SM /I OL = 0.54: When pits exist at respective tracks, I SM /I OL is equal to 0.51). While, in consideration of the data accuracy, etc., in the prior art, the pit depth Dp was set to λ/4n so that the information signal amplitude from the pit area becomes maximum, setting is assumed to be made in this invention such that the pit depth Dp is a value in the vicinity of 3λ/8n in consideration of the push-pull signal for tracking. In this case, also as previously indicated, the pit modulation factor is somewhat decreased, but decrease of the pit modulation factor to such a degree can be tolerable (allowed). In this connection, the tolerance (allowable) limit (limitation) in the ISO standard (e.g., ISO-IEC-DIS 14517: magneto-optical disc cartridge having diameter of 130 mm and disc memory capacity of 2.6 GB) is expressed as 0.45 ≤ I SM /I OL ≤ 0.95. FIG. 10 shows, by the contour (value) line, the state of change of the push-pull signal and the TCS signal when the depth and the width of the groove are used as parameter. (The condition is recited as below.) <Condition> Disc: Track pitch 1.1 µm Groove shape Parabola Polycarbonate base (substrate) (refractive index 1.58) Optical system: Wavelength λ 680 nm NA 0.55 It can be seen from this FIG. 10 that when the groove becomes deep, the polarity of the push-pull signal is inverted. The gist of the invention of this application resides in that such inverting phenomenon of the push-pull signal is applied to the optical disc including the area where grooves and pits are mixed to thereby suppress lowering of the push-pull signal at the pit portion. The fact that the modulation factor of the pit is caused to be maximum corresponds to the fact that (the amplitude of) the TCS signal is caused to be large. When the area of the groove width of 0.3 ∼ 0.4 µm is assumed, the area where (the amplitude of) the TCS signal is large within the inverting push-pull area is the area where the groove depth is 140 ∼ 220 nm. It is thus sufficient to employ pits of this depth. Accordingly, when it is assumed that the pit width is taken on the ordinate and the pit depth is taken on the abscissa in FIG. 10, it is sufficient to make a setting such that the pit depth becomes equal to the depth corresponding to the slanting line area in the figure. In more practical sense, it can be seen from this FIG. 10 that, in order to ensure the required push-pull signal intensity while ensuring the TCS signal intensity, it is sufficient to set the pit depth Dp so as to fall within the range of 0.32 λ/n - 0.51 λ/n.

Claims: An optical disc adapted so that concentrical or spiral guide grooves (G) are formed at a base, and emboss shaped pits are molded between these respective guide grooves, wherein when wavelength of reproduction laser beams is λ, refractive index of the base is n, and track pitch is t, the depth D p of the pit (P) is set so as to fall within the range expressed as 0.32 λ/n ≤ D p ≤ 0.51 λ/n, the width W p of the pit (P) is set so as to fall within the range expressed as 0.27 t ≤ W p ≤ 0.36 t, and depth D g and width W g of the guide grooves (G) are respectively set so as to fall within the ranges expressed as 0.0875 λ/n ≤ Dg ≤ 0.1625 λ/n and expressed as 0.27 t ≤ W g ≤ 0.36 t, wherein a push-pull signal (PP) caused by the pit (P) has the same polarity as that of the push-pull signal (PP) caused by grooves (G) at the time the reproduction laserbeam crosses the grooves (G).

Inventors: KAGAWA MASAKI, NAKAYAMA HIROSHI, KAGAWA, MASAKI, NAKAYAMA, HIROSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1996
Main classification: G11B7
IPC codes: G11B7