Source: http://www.google.com/patents/US6700842?ie=ISO-8859-1&dq=6462713
Timestamp: 2014-11-28 05:53:18
Document Index: 747060337

Matched Legal Cases: ['art 192', 'art 192', 'art 192', 'arts 92', 'arts 92', 'art 92', 'arts 92', 'art 92', 'art 92', 'arts 92', 'arts 92', 'art 92', 'art 92', 'arts 92']

Patent US6700842 - Optical head, photodetector, optical information recording and reproducing ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsDisclosed are an optical head, a photodetector, an optical recording and reproducing apparatus and a focus error detecting method, which can be adapted to a plurality of kinds of recording media and a land/groove recording system. A photoreceiving part for the main spot in a photodetector in an optical...http://www.google.com/patents/US6700842?utm_source=gb-gplus-sharePatent US6700842 - Optical head, photodetector, optical information recording and reproducing apparatus and focus error detecting methodAdvanced Patent SearchPublication numberUS6700842 B1Publication typeGrantApplication numberUS 09/671,103Publication dateMar 2, 2004Filing dateSep 27, 2000Priority dateSep 29, 1999Fee statusPaidAlso published asCN1178209C, CN1297225APublication number09671103, 671103, US 6700842 B1, US 6700842B1, US-B1-6700842, US6700842 B1, US6700842B1InventorsNoriaki NishiOriginal AssigneeSony CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (8), Referenced by (8), Classifications (27), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetOptical head, photodetector, optical information recording and reproducing apparatus and focus error detecting methodUS 6700842 B1Abstract Disclosed are an optical head, a photodetector, an optical recording and reproducing apparatus and a focus error detecting method, which can be adapted to a plurality of kinds of recording media and a land/groove recording system. A photoreceiving part for the main spot in a photodetector in an optical head is divided into a plurality of parts. An intermediate photoreceiving part consisting of four small photoreceiving parts is provided in a central area surrounded by four peripheral photoreceiving parts. Without using an output signal from the intermediate photoreceiving part positioning in an area where the intensity distribution is unstable in a converged light spot, a focus error signal is obtained only from output signals from the peripheral photoreceiving parts and a focusing control is performed by using the focus error signal. While maintaining compatibility by constructing the other parts of the optical head except for the photodetector in a manner similar to the case of the conventional astigmatism method, an excellent focusing control which does not cause much track crossing noise can be performed also on a land/groove recording medium.
focus pull-in signal FPI=reproduction signal RF=a+b+c+d (1) focus error signal FE=(a+c)−(b+d) (2) tracking error signal TE=phase difference between (a+c) and (b+d) (3) or
tracking error signal TE=e−f (4) The focus error signal FE expressed by the equation (2) is used for detecting a focus error by the astigmatism method. As described above, at the time of reproducing information from a DVD, in the photodetector 19 shown in FIG. 1, the shape of the beam spot 196 on the photoreceiving part 192 for the main spot becomes a circle or various ovals whose major axes are oriented differently in accordance with the degree of focusing. The focus error signal FE obtained by the equation (2) varies accordingly. More specifically, in the focusing state, output signals from the photoreceiving regions 192A, 192B, 192C and 192D of the photoreceiving part 192 for the main spot are almost equal to each other. Consequently, the focus error signal FE is almost zero. When the system is out of focus, the beam spot 196 has an oval shape. A difference therefore occurs between the sum (a+c) of the output signals from the photoreceiving regions 192A and 192C in one of diagonal line directions in the photoreceiving part 192 for the main spot and the sum (b+d) of output signals from the photoreceiving regions 192B and 192D in the other diagonal line direction. In this case, the sign of the difference between them depends on the direction of defocusing and the absolute value of the difference depends on the amount of defocusing. By moving the objective lens so that the focus error signal FE becomes zero, the best focusing state is maintained.
SUMMARY OF THE INVENTION The invention has been achieved in consideration of the above problems and its object is to provide an optical head, a photodetector, an optical recording and reproducing apparatus and a focus error detecting method which can be adapted to a plurality of kinds of recording media and also to a land/groove recording system in which both lands and grooves are used as information recording regions.
FCS={(a+c)−(b+d)}−K 1�{(mw+my)−(mx+mz)}−K 2�{(mw+my)+(mx+mz)} (5) where, a, b, c and d denote output signals from the peripheral photoreceiving parts 92A to 92D, respectively, mw, my, mx and mz denote output signals from the small photoreceiving parts 92Mw, 92Mx, 92My and 92Mz of the intermediate photoreceiving part 92M, respectively, and K1 and K2 are coefficients which can be positive or negative values or zero. As will be described hereinlater, the coefficient K1 is a correction coefficient for canceling the influence by the astigmatism of the optical system itself of the optical head 12. The coefficient K2 is a correction coefficient for canceling the influence when the gain of the focus servo control in the case where a spot is on a land and that in the case where a spot is on a groove are different from each other.
FCS1=(a+c)−(b+d) (6) A focus error signal FCS2 when it is assumed that K1≠0 and K2=0 is given by the following equation (7).
FCS2={(a+c)−(b=d)}−K 1�{(mw+my)−(mx+mz)} (7) A focus error signal FCS3 when it is assumed that K1=0 and K2≠0 is given by the following equation (8).
FCS3={(a+c)−(b+d)}−K 2�{(mw+my)+(mx+mz)} (8) A focus error signal FCS4 when it is assumed that K1≠0 and K2≠0 is given by the following equation (9) which is the same equation (5).
TRK1=phase difference signal between (a+c) and (b+d) (10) TRK2=phase difference signal between (a+mw+c+my) and (b+mx+d+mz) (11) TRK3={(a+d)−(b+c)}−K 3{(e−f)+(g−h)} (12)
TRK4={(a+d+mw+mz)−(b+c+mx+my)}−K 3{(e−f)+(g−h)} (13) TRK5=(e+f)−(g+h) (14) TRK2 denotes the tracking error signal used for performing detection by the differential phase difference method which is the same as that of the conventional technique. TRK4 is a tracking error signal used in the case of performing detection by a differential push-pull method. TRK1 is obtained by eliminating the output signals mw, mx, my and mz of the small photoreceiving parts 92Mw, 92Mx, 92My and 92Mz from TRK2. TRK3 is obtained by eliminating the output signals mw, mx, my and mz from TRK4. TRK5 is a tracking error signal used in the case of performing detection by the three-beam method (three-spot method).
RF=a+b+c+d+mw+mx+my+mz (15) The focus pull-in signal FPI is obtained by removing high frequency components of the RF signal by a low pass filter.
Reproduction of DVD First, the case of reproducing information recorded on the DVD 30 a will be described. In this case, the DVD 30 a may be a reproduction-only DVD (such as DVD-ROM or DVD-video) in which only either lands or grooves are used as a recording area or a DVD-RAM in which both lands and grooves are used as a recording area.
Reproduction of CD The case of reproducing information recorded on the CD 30 b will now be described. The CD 30 b may be an ordinary CD or CD-ROM for audio or a CD-R.
The arithmetic circuit 40 further generates the RF signal by the arithmetic process shown by the equation (15). The RF signal passes through the low pass filter 57 and becomes the focus pull-in signal FPI. The subsequent focus error detection and the focusing control are similar to those in the case of the DVD 30 a. Principle of Focus Error Detection The principle of detecting a focus error used in the embodiment will be described, in comparison with the astigmatism method in the conventional technique.
First Cause of Track Crossing Noise The first cause will be described first. Generally, a light beam condensed by the objective lens falls on a land or groove in the optical disk and is diffracted by reflection. In the case of the land/groove recording system, information is recorded on both lands and grooves, the track pitch (distance between lands or distance between grooves) to the diameter of the light beam spot condensed by the objective lens becomes relatively large. As a result, as shown in FIGS. 10A and 10B and FIGS. 11A and 11B, an overlapping manner of diffracted light from the optical disk on the pupil of the objective lens largely differs from that of the land recording system or the groove recording system (hereinbelow, referred to as a groove recording system for simplicity of explanation).
W L, G=�0.046 [λrms] (16) ΔL, G=W�λ�4�6�/NA2=0.83 [μm] (17) On the other hand, when it is assumed that the pull-in range of the astigmatism method is SPP and the magnification of a return system is β, in the return system, the astigmatic difference which is caused by the multilens is expressed by the following equation (18). The NA (referred to as NAback) of the return system is expressed by the following equation (19).
Δback=SPP�2�β2 (18) NA back =NA/β (19) The quantity of the astigmatism caused by the multilens of the return system is expressed by the following equation (20).
W back=Δback�NA back 2/λ/4/6� =SPP�NA 2/λ/2/6�[λrms] (20) For example, when it is assumed that the pull-in range SPP=6 [μm], Wback=0.67 [λrms].
MAS=A�ρ 2�Sin 2θ (21) LAS=−B�ρ 2�Cos 2θ (22) GAS=+B�ρ 2�Cos 2θ (23) From the equations (21) to (23), synthetic astigmatism TAS in the case where the spot is on a land is expressed by the following equation (24). Synthetic astigmatism TAS in the case where the spot is on a groove is expressed by the following equation (25).
TAS=(A 2 +B 2)��ρ2�Sin 2(θ−α/2) (24) TAS=(A 2 +B 2)��ρ2�Sin 2(θ+α/2) (25) The angle α is a value which satisfies the following equations (26) and (27).
Sin α=B/(A 2 +B 2)� (27) Since the diffraction pattern is inverted with respect to an area near the direction axis D4 in FIG. 17 as a center on the photoreceiving part, the arrangement direction of the diffraction pattern is turned from the original direction by 90 degrees. The direction of the inversion axis in the case where a spot is on a land is deviated from the direction axis D4 only by +α/2. The direction of the inversion axis in the case where a spot is on a groove is deviated from the direction axis D4 only by −α/2. As a result, as shown in FIGS. 18A and 18B, the interference region 51 in which the three diffraction rays are overlapped in the case where a spot is on a land and that in the case where a spot is on a groove are deviated from the dividing line DX by +α and −α, respectively.
Measure against the First Cause On the basis of the consideration, in the embodiment, on the precondition that the optical head includes little aberration, the photoreceiving part 92 for the main spot in the photodetector 9 is divided into the patterns shown in FIGS. 8 and FIGS. 18A and 18B. For example, the focus error signal FCS1 calculated by the equation (6) which does not use an output signal from the intermediate photoreceiving part 92M including the interference region 51 where the three diffraction rays are overlapped but uses only output signals from the peripheral photoreceiving parts 92A to 92D is used. That is, in FIG. 9, by setting that K1=0 and K2=0 by the setting signal SET, the focus error signal FCS1 is obtained.
Second Cause of Track Crossing Noise The second cause of the �track crossing noise� will now be explained. A case in which an outgoing beam from the optical head includes astigmatism will be considered. In this case, due to asymmetry of the phase distribution caused by astigmatism, the synthetic phase distribution of the diffraction rays becomes asymmetric, so that asymmetry occurs in the intensity distribution.
Measure Against the Second Cause On the basis of the consideration, in the embodiment, attention is paid to the fact that intensity change information of the interference region 51 caused by track crossing is obtained from output signals from the small photoreceiving parts 92Mw, 92Mx, 92My and 92Mz obtained by dividing the intermediate photoreceiving part 92M in the photoreceiving part 92 for the main spot into four parts. The �track crossing noise� caused by the astigmatism of the optical head included in the focus error signal FCS1 is cancelled by the output signals from the small photoreceiving parts 92Mw, 92Mx, 92My and 92Mz. More specifically, the focusing control is performed by using the focus error signal FCS2 calculated by the equation (7). In this case, it is sufficient to use the signal FCS2 as the focus error signal FCS by setting K2=0 by the setting signal SET in FIG. 9.
Problems Caused by Gain Difference in Focus Servo and Measure Against the Problems Also in the case of using the focus error signal FCS1 and in the case of adjusting the constant K1 so that the �track crossing noise� becomes the minimum at the time of focusing in the calculation of FCS2, for example, as shown in FIG. 23, when the gain of the focus error signal in the case where the light spot is on a land and that in the case where the light spot is on a groove are different from each other, a case such that the focus control is hindered may occur. In FIG. 23, although the objective lens position so that the level of the focus error signal becomes zero is almost unconditionally determined, a large difference in inclination occurs between the focus error signal FEL in the case where the spot is on a land and the focus error signal FEG in the case where the spot is on a groove. In such a state, therefore, the optimum value of the control gain in the control of moving the objective lens varies. It becomes necessary to switch the gain.
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