OPTICAL DISK APPARATUS AND TRACKING METHOD

Abstract

A tracking offset is reduced. The effect of an SPP signal fluctuation is reduced by using a variable mixing ratio DPP method, and a residual tracking offset is minimized.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing experiment results providing a design value of a mixing ratio α.

FIGS. 2A and 2B are diagrams showing results of measuring push-pull signals during reproduction from a dual-layer disk.

FIG. 3 is a schematic diagram showing the theory of interlayer crosstalk generation.

FIG. 4 is a diagram showing a DPP signal in an OPC area.

FIG. 5 gives an enumeration of causes for a tracking offset.

FIG. 6A is a diagram for explaining how to measure SPP signal fluctuation.

FIG. 6B is a diagram showing a distribution of SPP signal fluctuation.

FIG. 7A is a diagram showing an MPP signal over a disk revolution period.

FIG. 7B is a block diagram showing an example configuration of an LSI used for measuring decentering of a disk.

FIG. 7C is a diagram showing a configuration of a zero-cross detector.

FIG. 8 is a diagram showing a relationship between a lens shift caused by disk decentering and an MPP signal.

FIG. 9 is a diagram showing how to adjust a tracking control gain.

FIG. 10 is a diagram showing an example configuration of a tracking error signal generator.

FIG. 11A is a diagram showing results of DPP signal simulation performed with a variable mixing ratio α in an OPC area.

FIG. 11B is a diagram showing singular point definition.

FIG. 11C is a diagram for explaining designing of a mixing ratio α for reproduction operation.

FIG. 11D is a diagram for explaining designing of a mixing ratio α for recording operation.

FIG. 12 is a flowchart of processing during reproduction from a dual-layer disk.

FIG. 13 is a flowchart of processing in an OPC area.

FIG. 14 is a block diagram of servo control.

FIG. 15 is a diagram showing an example configuration of a sequencer for changing the mixing ratio α.

FIG. 16 is a block diagram showing an example configuration of an optical disk apparatus.

FIGS. 17A and 17B show results of optical constant identification performed for an optical simulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in detail with reference to a preferred embodiment.

FIG. 16 is a block diagram showing an example configuration of an optical disk apparatus according to the present invention. An optical disk medium 100 is rotated by a spindle motor 160. For a read operation, an electric current controlled by a laser power/pulse controller 120 is applied to a laser diode 112 included in an optical head 110, and a laser beam is generated. At this time, the laser power/pulse controller 120 controls the electric current such that the laser beam has a light intensity prescribed by a CPU 140. The laser beam is split into a main beam and two sub-beams by a diffraction grating 116. The split beams are then converged by an objective lens 111 to form an optical spot 101 on the optical disk medium 100. The optical spot 101 includes a main spot formed by converging the main beam and two sub-spots formed by converting the two sub-beams. The two sub-spots are positioned, being shifted by half a track pitch each, on both sides of the main spot. The light reflected from the optical spot 101 is detected, via the objective lens 111, by a photo-detector 113. The photo-detector 113 includes, as shown in FIG. 10, plural photo-detecting elements.

A signal processor 130, while reproducing information recorded on the optical disk medium 100 using the signal detected by the optical head 110, generates an MPP signal and an SPP signal, from which it then generates a VDPP signal. A servo-controller 150 performs focusing and tracking. For tracking, the servo-controller 150 uses the VDPP signal. Focusing is performed based on a focus error signal generated from a main spot detection signal. A reproduction signal is generated from the main spot detection signal. For recording, the laser power/pulse controller 120 converts prescribed recording data into a prescribed recording pulse current, and performs control so that pulsed light is emitted from the laser diode 112. The circuit required for using the tracking method according to the present invention is incorporated in the signal processor 130. Processing for learning the mixing ratio α referred to in the foregoing is executed as a program stored in the CPU 140. The optical disk apparatus of the present invention can be provided based on the configuration described above.

First, the tracking method according to the present invention will be described. FIG. 12 is a flowchart of the tracking method according to the present invention used in reading a dual-layer disk. The initial value of the mixing ratio α is 0.5. The MPP and SPP signals are measured in a focused state, and the fluctuation of the SPP signal is calculated. Shifting of the lens is then measured. When the SPP signal fluctuation is smaller than a threshold value, the mixing ratio α is left unchanged at 0.5, and tracking servo control is performed. When the SPP signal fluctuation is larger than the threshold value, an optimum value of the mixing ratio α is determined based on the amount of the offset due to disk decentering and the amount of the fluctuation, and the residual tracking offset is estimated. When the residual tracking offset becomes smaller than a threshold value, the tracking servo control is performed using the mixing ratio α thus determined. When the residual tracking offset is larger than the threshold value, the condition is regarded as an error.

FIG. 13 is a flowchart of the tracking method according to the present invention performed in a single-layer OPC area. The process up to the step where the SPP signal fluctuation is calculated is the same as shown in FIG. 12. Subsequently, whether or not the current operation is performed in an OPC area is determined. When the current operation is being performed in other than an OPC area, the mixing ratio α is set to 0.5. When the current operation is being performed in an OPC area, the mixing ratio α is set to a value lower than 0.5, for example, 0.2 for reproduction or a value higher than 0.5, for example, 0.7 for recording. For the mixing ratio α, plural appropriate values may be stored in advance so that one of them may be selected for use according to the condition of operation.

FIG. 14 is a block diagram of preferred servo control for the optical disk apparatus according to the present invention. Referring to FIG. 14, control of the objective lens actuator by the VDPP signal is as described in the foregoing. FIG. 14 also shows a servo control loop for a thread motor. The objective lens is driven by the objective lens actuator and the optical head is driven by the thread motor both in a direction transversal to the track, namely, in a radial direction of the optical disk. If the thread motor is controlled so that the offset of the VDPP signal approaches zero, the signal offset relative to the lens shift will become smaller than appropriate, for example, in a case where the mixing ratio α is set to 0.5. Controlling the thread motor in such a way is not appropriate. To realize stable position control for the thread motor without depending on the value of the mixing ratio α, it is appropriate to control the thread motor so that the offset of the MPP signal approaches zero. Namely, the thread servo control is to be performed using the MPP signal that is sensitive (showing a large offset) to disk decentering.

FIG. 15 is a diagram showing an example configuration of a sequencer for changing the mixing ratio α in the optical disk apparatus according to the present invention. According to the present invention, it is necessary to change the mixing ratio α between when accessing an OPC area and when recording data. Instantly changing the mixing ratio α makes tracking control unstable. Referring to FIG. 15, gain settings stored in a data array 142 by a sequencer 141 are automatically transferred to a tracking error signal generator 30 as serial data 54 at appropriate intervals. Changing the mixing ratio α in steps as in this case secures tracking control stability.

The present invention is applied to an optical disk apparatus in which tracking control is performed using three beams.

Claims

1. An optical disk apparatus, comprising: an optical head including a light source, a beam splitter which splits a beam from the light source into a plurality of beams, an objective lens which converges the plurality of beams split by the beam splitter into a plurality of optical spots on an optical disk, a photo-detector which detects the plurality of optical spots reflected from the optical disk, and an actuator which drives the objective lens in a radial direction of the optical disk;a thread motor which drives the optical head in the radial direction of the optical disk;a unit which generates a plurality of tracking spot signals from detection signals generated by detecting the plurality of optical spots;a unit which generates a tracking control signal for driving the actuator by mixing the plurality of tracking spot signals; anda mixing ratio changing unit which changes a mixing ratio of the plurality of tracking spot signals.

2. The optical disk apparatus according to claim 1; wherein the mixing ratio changing unit determines the mixing ratio based on a measured amount of fluctuation of each of the tracking spot signals and a measured amount of decentering of the optical disk.

3. The optical disk apparatus according to claim 1; wherein a plurality of values of the mixing ratio are stored in advance, and the mixing ratio changing unit uses a value selected from the plurality of values stored in advance.

4. The optical disk apparatus according to claim 1; wherein the mixing ratio changing unit changes the mixing ratio in steps.

5. The optical disk apparatus according to claim 1; wherein the plurality of optical spots include a main spot and two sub-spots positioned, being shifted by half a track pitch each, on both sides of the main spot, and the thread motor is controlled using a tracking spot signal which is generated by a detection signal generated by detecting the main spot.

6. The optical disk apparatus according to claim 1: wherein the plurality of optical spots include a main spot and two sub-spots positioned, being shifted by half a track pitch each, on both sides of the main spot;wherein the unit that generates a plurality of tracking spot signals generates a main push-pull signal from a detection signal generated by detecting the main spot and a sub-push-pull signal from detection signals generated by detecting the sub-spots; andwherein the mixing ratio changing unit changes the mixing ratio based on an initial state where gain correction has been made to equalize the main push-pull signal and the sub-push-pull signal in amplitude at a time of crossing a track of the optical disk.

7. A tracking method of an optical disk apparatus which irradiates an optical disk with a plurality of optical spots and generates a tracking signal from detection signals generated by detecting light reflected from the plurality of optical spots, the tracking method comprising the steps of: generating a tracking signal with a variable mixing ratio by adjusting outputs of a plurality of tracking error signals, which are generated from detection signals generated by detecting the light reflected from the plurality of optical spots, and by performing addition or subtraction using the plurality of adjusted tracking error signals;changing the mixing ratio according to an operating condition of the optical disk apparatus, andcontrolling positions in a disk radial direction of the plurality of optical spots by using the tracking signal with a variable mixing ratio.

8. The tracking method according to claim 7: wherein the plurality of optical spots include a main spot and two sub-spots positioned, being shifted by half a track pitch each, on both sides of the main spot; andwherein the step of changing the mixing ratio according to an operating condition of the optical disk apparatus includesobtaining an amount of fluctuation of a second tracking error signal which is generated from detection signals generated by detecting the two sub-spots and an amount of shifting of an objective lens which converges the plurality of optical spots on the optical disk,obtaining, when the amount of fluctuation is greater than a predetermined threshold value, an amount of offset of the second tracking error signal resulting from the shifting of the objective lens, andobtaining the mixing ratio based on the amount of offset and the amount of fluctuation.

9. The tracking method according to claim 7: wherein a plurality of values of the mixing ratio are stored in advance, and a value selected, according to an operating condition of the optical disk apparatus, from the plurality of values stored in advance is used.

10. The tracking method according to claim 7: wherein the plurality of optical spots include a main spot and two sub-spots positioned, being shifted by half a track pitch each, on both sides of the main spot; andwherein the mixing ratio is set such that, when the optical spots are engaged in reproduction operation in an OPC area, a ratio of a tracking error signal which is generated from detection signals generated by detecting the two sub-spots increases, and such that, when the optical spots are engaged in recording operation in the OPC area, s ratio of a tracking error signal which is generated from a detection signal generated by detecting the main spot increases.

11. The tracking method according to claim 7: wherein the mixing ratio is changed in steps.

12. The tracking method according to claim 7: wherein the plurality of optical spots include a main spot and two sub-spots positioned, being shifted by half a track pitch each, on both sides of the main spot; andwherein the step of generating a tracking signal with a variable mixing ratio includes generating a main push-pull signal from a detection signal generated by detecting light reflected from the main spot and a sub-push-pull signal from detection signals generated by detecting light reflected from the sub-spots; andperforming gain adjustment to generate an initial state where the main push-pull signal and the sub-push-pull signal have an equal amplitude at a time of crossing a track of the optical disk.