1. Field of the Invention
The present invention relates to an optical head apparatus and an optical information recording and reproducing apparatus that record and reproduce information to and from an optical recording medium, in particular, to those that allow the deviation of the thickness of the substrate of an optical recording medium and the radial tilt thereof to be detected without offsets of a focusing error signal and a tracking error signal.
2. Description of the Related Art
On a write-once type optical recording medium and a rewritable optical recording medium on which an RF (Radio Frequency) signal has not been recorded, grooves have been formed as tracks. Generally, viewed from the incident light side of such an optical recording medium, a concave portion and a convex portion of a groove are referred to as land portion and groove portion, respectively. When a focusing error signal of a write-once type optical recording medium or a rewritable optical recording medium is detected, the signal level of the focusing error signal does not strictly accord with the defocus amount. In other words, at the position where the defocus amount is zero, the signal level of the focusing error signal is not strictly zero. Since grooves have been formed, theoretically, the sign of an offset of the focusing error signal detected at the land portion is reverse of that detected at the groove portion. The offset is referred to as groove traverse noise offset. When a tracking error signal is detected from a write-once type optical recording medium or a rewritable optical recording medium, push-pull method is normally used. However, in the push-pull method, when an objective lens of an optical head apparatus is shifted in the radial direction of an optical recording medium, the tracking error signal has an offset. This offset is referred to as lens shift offset. To prevent recording and reproduction characteristics from deteriorating due to such offsets, it is necessary for an optical head apparatus and an optical information recording and reproducing apparatus to provide a mechanism that allows the focusing error signal and the tracking error signal to have no offsets.
FIG. 1 shows the structure of a conventional optical head apparatus that allows the focusing error signal and the tracking error signal to have no offsets. The optical head apparatus has been disclosed as Japanese Patent Laid-Open Publication No. 2000-82226. Light emitted from a semiconductor laser 1 is separated into three pieces of 0-th order light and ±1st order diffracted light by a diffractive optical element 3p. Around 50% of these three pieces of light transmits a beam splitter 24. A collimator lens 2 collimates those pieces of light. An objective lens 6 focuses those pieces of light on a disk 7. Three pieces of light that are reflected from the disk 7 transmit the objective lens 6 and the collimator lens 2 in the reverse direction. The beam splitter 24 reflects around 50% of those piece of light. Those pieces of light transmit a cylindrical lens 8. A photo detector 10b receives those pieces of light. The photo detector 10b is disposed at the middle position of two focal lines of the collimator lens 2 and the cylindrical lens 8.
FIG. 2 shows the positions of focused spots on the disk 7. Focused spots 13a, 13t, and 13u correspond to 0-th order light, +1st order diffracted light, and −1st order diffracted light that are separated by the diffractive optical element 3p. The focused spot 13a is placed on a track 12 (land portion or groove portion). The focused spot 13t is placed on a right adjacent track (groove portion or land portion) of the track 12. The focused spot 13u is placed on a left adjacent track (groove portion or land portion) of the track 12.
FIG. 3 shows the pattern of a light receiving portion of the photo detector 10b and the positions of light spots on the photo detector 10b. A light spot 15a corresponds to 0-th order light separated by the diffractive optical element 3p. The 0-th order light is received by four light receiving portions 25a to 25d divided by a division line that passes through the optical axis and that parallels the tangential direction of the disk 7 and a division line that parallels the radial direction thereof. A light spot 15j corresponds to +1st order diffracted light separated by the diffractive optical element 3p. The light spot 15j is received by four light receiving portions 25e to 25h divided by a division line that passes through the optical axis and that parallels the tangential direction of the disk 7 and a division line that parallels the radial direction thereof. A light spot 15k corresponds to −1st order diffracted light separated by the diffractive optical element 3p. The light spot 15k is received by four light receiving portions 25i to 25l divided by a division line that passes through the optical axis and that parallels the tangential direction of the disk 7 and a division line that parallels the radial direction thereof. The focused spots 13a, 13t, and 13u are trained nearly in the tangential direction of the disk 7. In contrast, the light spots 15a, 15j, and 15k are trained nearly in the radial direction due to the operations of the collimator lens 2 and the cylindrical lens 8.
When outputs of the light receiving portions 25a to 25l are denoted by V25a to V25l, respectively, the focusing error signal can be obtained by differential astigmatism method as (V25a+V25d)−(V25b+V25c)+K{(V25e+V25h+v25i+V25l)−(V25f+V25g+V25j+V25k)} (where K is constant). On the other hand, the tracking error signal can be obtained by differential push-pull method as (V25a+V25b)−(V25c+V25d)−K{(V25e+V25f+V25i+V25j)−(V25g+V25h+V25k+V25l)}. In addition, an RF signal of the focused spot 13a can be obtained as V25a+V25b+V25c+V25d . 
FIGS. 4A, 4B, and 4C show various types of focusing error signals. In FIGS. 4A to 4C, the horizontal axis represents the defocus amount of the disk 7, whereas the vertical axis represents each focusing error signal. A focusing error signal 26a shown in FIG. 4A is a focusing error signal of the focused spot 13a that is placed on a land portion. A focusing error signal 26b shown in FIG. 4A is a focusing error signal of the focused spot 13a placed on a groove portion. A focusing error signal 26c shown in FIG. 4B is a focusing error signal of the focused spots 13t and 13u in the case that the focused spot 13a is placed on a land portion. A focusing error signal 26d shown in FIG. 4B is a focusing error signal of the focused spots 13t and 13u in the case that the focused spot 13a is placed on a groove portion. The signal level of a focusing error signal is not strictly zero at the position where the defocus amount is zero. In FIG. 4A, a focusing error signal has a positive offset at a land portion, whereas it has a negative offset at a groove portion. In FIG. 4B, a focusing error signal has a negative offset at a land portion, whereas it has a positive offset at a groove portion. In contrast, a focusing error signal 26e shown in FIG. 4C is a final focusing error signal that is the sum of the focusing error signal of the focused spot 13a and the focusing error signals of the focused spots 13t and 13u in the case that the focused spot 13a is placed at a land portion and a groove portion. In FIG. 4C, the offsets of the focusing error signals shown in FIGS. 4A and 4B are cancelled each other. Thus, the focusing error signal 26e does not have an offset.
FIGS. 5A, 5B, and 5C show various types of tracking error signals. In FIGS. 5A, 5B, and 5C, the horizontal axis represents the off track amount of the disk 7, whereas the vertical axis represents each tracking error signal. A tracking error signal 27a shown in FIG. 5A is a tracking error signal of the focused spot 13a in the case that the objective lens 6 is outward shifted in the radial direction of the disk 7. A tracking error signal 27b shown in FIG. 5A is a tracking error signal of each of the focused spots 13t and 13u in the case that the objective lens 6 is outward shifted in the radial direction of the disk 7. A tracking error signal 27c shown in FIG. 5B is a tracking error signal of the focused spot 13a in the case that the objective lens 6 is inward shifted in the radial direction of the disk 7. A tracking error signal 27d shown in FIG. 5B is a tracking error signal of each of the focused spots 13t and 13u in the case that the objective lens 6 is inward shifted in the radial direction of the disk 7. The polarity of the tracking error signal of the focused spot 13a is the reverse of the polarity of the tracking error signal of each of the focused spots 13t and 13u. However, when the objective lens 6 is shifted in the radial direction of the disk 7, the sign of the offset of the tracking error signal of the focused spot 13a is the same as the sign of the offset of the tracking error signal of each of the focused spots 13t and 13u. In FIG. 5A, the offsets of the tracking error signal 27a and the tracking error signal 27b are positive. In FIG. 5B, the offsets of the tracking error signals 27c and 27d are negative. In contrast, a tracking error signal 27e shown in FIG. 5C is a final tracking error signal that is the difference between a tracking error signal of the focused spot 13a and a tracking error signal of each of the focused spots 13t and 13u in the case that the objective lens 6 is outward and inward shifted in the radial directions of the disk 7. In FIG. 5C, the offsets of the tracking error signals shown in FIGS. 5A and 5B are cancelled each other. Thus, the tracking error signal 27e shown in FIG. 5C does not have an offset.
The recording density of an optical information recording and reproducing apparatus is reversely proportional to the second power of the diameter of a focused spot formed on a record medium by an optical head apparatus. In other words, as the diameter of a focused spot is smaller, the recording density becomes higher. The diameter of a focused spot is reversely proportional to the numerical aperture of an objective lens of the optical head apparatus. In other words, as the numerical aperture of the objective lens is higher, the diameter of a focused spot becomes smaller. On the other hand, when the thickness of the substrate of the optical recording medium deviates from a designed value, the shape of a focused spot deforms due to the spherical aberration caused by the deviation of the thickness of the substrate. As a result, the recording and reproduction characteristics deteriorate. Since the spherical aberration is proportional to the fourth power of the numerical aperture, as the numerical aperture of the objective lens is higher, the margin of the deviation of the thickness of the substrate of the optical recording medium becomes narrower. When the optical recording medium tilts in the radial direction against the objective lens, the shape of a focused spot deforms due to the comatic aberration caused by the radial tilt. As a result, the recording and reproduction characteristics deteriorate. Since the comatic aberration is proportional to the third power of the numerical aperture of the objective lens, as the numerical aperture of the objective lens is higher, the margin of the radial tilt of the optical recording medium against the recording and reproduction characteristics becomes narrower. Thus, in the optical head apparatus and the optical information recording and reproducing apparatus that use an objective lens having a higher numerical aperture for a higher recording density, it is necessary to detect and compensate the deviation of the thickness of the substrate of the optical recording medium and the radial tilt thereof so as to prevent the recording and reproduction characteristics from deteriorating.
An example of a conventional optical head apparatus that can detect the radial tilt of an optical recording medium has been described in SPIE Proceedings, Vol. 4090, pp. 309–318. The optical head apparatus features a diffractive optical element 3q with which the diffractive optical element 3p of the optical head apparatus shown in FIG. 1 is substituted.
FIG. 6 is a plan view showing the diffractive optical element 3q. The diffractive optical element 3q has a structure in which a diffraction grating is formed in only the inside of a circular area 28 of which diameter is smaller than the effective diameter (denoted by a dotted line in FIG. 6) of the objective lens 6. The direction of grating members of the diffraction grating parallels the radial direction of the disk 7. The grating members are linearly formed and equally spaced. Light that enters the inside of the circular area 28 partly transmits it as 0-th order light and is diffracted as +1st order diffracted light. In contrast, light that enters the outside of the circular area 28 fully transmits it. In other words, 0-th order light separated by the diffractive optical element 3q contains both light that transmits the inside of the circular area 28 and light that transmits the outside of the circular area 28. Thus, the numerical aperture of 0-th order light depends on the effective diameter of the objective lens 6. On the other hand, +1st order diffracted light separated by the diffractive optical element 3q contains only light diffracted on the inside of the circular area 28. Thus, the numerical aperture of ±1st order diffracted light depends on the diameter of the circular area 28. As a result, the distribution of the intensity of 0-th order light separated by the diffractive optical element 3q is different from that of each of +1st order diffracted light separated thereby. Thus, the intensity of the peripheral portion of each of +1st order diffracted light is lower than that of 0-th order light.
FIG. 7 shows the positions of focused spots on the disk 7. Focused spots 13a, 13v, and 13w correspond to 0-th order light, +1st order diffracted light, and −1st order diffracted light separated by the diffractive optical element 3q, respectively. The focused spot 13a, 13v, and 13w are placed on the same track 12 (land portion or groove portion). Since the intensity of the peripheral portion of each of ±1st order diffracted light is lower than that of 0-th order light, the diameter of each of the focused spots 13v and 13w that are +1st order diffracted light is larger than the diameter of the focused spot 13a that is 0-th order light.
The pattern of the light receiving portion of the photo detector of the optical head apparatus and the positions of the light spots on the photo detector are the same as those shown in FIG. 3.
FIGS. 8A, 8B, and 8C show various types of tracking error signals for detecting a radial tilt. In FIGS. 8A, 8B, and 8C, the horizontal axis represents the off track amount of the disk 7, whereas the vertical axis represents each tracking error signal. A tracking error signal 29a shown in FIG. 8A is a tracking error signal of each of the focused spots 13a, 13v, and 13w in the case that the disk 7 does not have a radial tilt. In contrast, a tracking error signal 29b shown in FIG. 8B is a tracking error signal of the focused spot 13a in the case that the disk 7 has a positive radial tilt. A tracking error signal 29c shown in FIG. 8B is a tracking error signal of each of the focused spots 13v and 13w in the case that the disk 7 has a positive radial tilt. A tracking error signal 29d shown in FIG. 8C is a tracking error signal of the focused spot 13a in the case that the disk 7 has a negative radial tilt. A tracking error signal 29e shown in FIG. 8C is a tracking error signal of each of the focused spots 13v and 13w in the case that the disk 7 has a negative radial tilt. The position at which the tracking error signal of the focused spot 13a traverses zero from the − side to the + side as the curve on the graph moves rightward corresponds to a land portion, whereas the position at which the tracking error signal of the focused spot 13a traverses zero from the + side to the − side as the curve on the graph moves rightward corresponds to a groove portion. When the disk 7 does not have a radial tilt, the zero crossing point of the tracking error signal of each of the focused spots 13v and 13w matches the zero crossing point of the tracking error signal of the focused spot 13a. Thus, the signal level of each of the tracking error signals becomes zero regardless of whether each of the focused spots 13v and 13w is placed on a land portion or a groove portion. When the disk 7 has a positive radial tilt, the zero crossing point of the tracking error signal of each of the focused spots 13v and 13w shifts leftward against the tracking error signal of the focused spot 13a. Thus, when each of the focused spots 13v and 13w is placed at a land portion, the signal level of the tracking error signal becomes positive. In contrast, when each of the focused spots 13v and 13w is placed at a groove portion, the signal level of the tracking error signal becomes negative. When the disk 7 has a negative radial tilt, the zero crossing point of the tracking error signal of each of the focused spots 13v and 13w shifts rightward against the tracking error signal of the focused spot 13a. Thus, when each of the focused spots 13v and 13w is placed at a land portion, the signal level of the tracking error signal becomes negative. In contrast, when each of the focused spots 13v and 13w is placed at a groove portion, the signal level of the tracking error signal becomes positive. Thus, the tracking error signal of each of the focused spots 13v and 13w when performing a tracking servo using the tracking error signal of the focused spot 13a can be used as a radial tilt signal.
In the conventional optical head apparatus that allows the focusing error signal and the tracking error signal to have no offsets, the sum of a focusing error signal of 0-th order light separated by a diffractive optical element and a focusing error signal of each of ±1st order diffracted light separated by the diffractive optical element is a final focusing error signal. In addition, the difference between a tracking error signal of 0-th order light and a tracking error signal of each of ±1st order diffracted light is a final tracking error signal. To cancel an offset of a focusing error signal of 0-th order light separated by a diffractive optical element and that of each of ±1st order diffracted light separated thereby and to cancel an offset of a tracking error signal of 0-th order light separated by the diffractive optical element and that of each of ±1st order diffracted light separated thereby, it is necessary to cause the distribution of the intensity of 0-th order light separated by the diffractive optical element to be the same as that of each of ±1st order diffracted light separated thereby.
On the other hand, in the conventional optical head apparatus that can detect a radial tilt of an optical recording medium, a radial tilt thereof is detected corresponding to the deviation between the zero crossing point of a tracking error signal of 0-th order light separated by the diffractive optical element and the zero crossing point of the tracking error signal of each of ±1st order diffracted light separated thereby, so it is necessary to cause the distribution of the intensity of 0-th order light separated by the diffractive optical element to be different from that of each of ±1st order diffracted light separated thereby.
Thus, in the conventional optical head apparatus, both the structure that allows a focusing error signal and a tracking error signal to have no offsets and the structure that allows a radial tilt of an optical recording medium to be detected cannot be satisfied at a time.