JP2009230840A - Photodetector and optical drive device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To cancel an offset generated in a tracking error signal due to lens shift. <P>SOLUTION: In a photodetector, a light receiving surface is divided into a plurality of light receiving areas by a first division line T2 that is either a first straight-line between lens shift directional lines of the respective light beams reflected by the respective layers other than a layer to be accessed and passing through a spot center provided in an area including a lens shift reference line of a signal light beam in the absence of the lens shift of an objective lens or a second straight-line, as a straight-line passing through the spot center, in which the absolute value of the sums of differences between the light receiving amounts of the light beams reflected by the respective layers other than the layer to be accessed at both sides of the straight-line is a local maximum value or the difference with the local maximum value is within a prescribed amount. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a photodetector and an optical drive device, and more particularly to a photodetector and an optical drive device suitable for an optical disc having a multilayered recording layer.

  An optical drive device for reproducing and recording an optical disk such as a CD (Compact Disc), a DVD, or a BD (Blu-ray Disc (registered trademark)) includes an optical pickup. The optical pickup has a forward optical system that generates a light beam and converges it on the recording surface of the optical disk by an objective lens, and a return optical system that includes a photodetector that receives the light beam reflected by the recording surface of the optical disk. ing.

  Since the focal position of the light beam needs to be aligned with the center of the track formed on the recording surface of the optical disk, the optical drive device is a control called tracking servo for adjusting the deviation of the focal position in the radial direction of the optical disk. (For example, the control by the differential push-pull method described in Patent Document 1 and the one push-pull method described in Patent Document 2) is performed. The tracking servo will be briefly described below.

  FIG. 23A shows an end surface of the recording surface of the optical disk 11 composed of a plurality of lands L and grooves G, an objective lens 100, a light beam (incident light, reflected light (0th order diffracted light, ± 1st order). Origami)). As shown in the figure, the 0th-order diffracted light and the ± 1st-order diffracted light interfere with each other in the interference regions E1 and E2.

  Next, FIG. 24 is a diagram showing a light receiving surface of the photodetector 101 that receives the light beam reflected by the recording surface of the optical disc 11. As shown in the figure, the 0th-order diffracted light reflected by the recording surface of the optical disk 11 forms a spot at the center of the photodetector 101. This spot has various shapes such as a square and a circle depending on various lenses arranged in the optical path, but here, a square spot is drawn.

  As shown in FIG. 24, the light receiving surface of the photodetector 101 has a square shape and is divided into upper and lower portions. As a result of such division, the upper light receiving region 101A receives the interference region E1, and the lower light receiving region 101B receives the interference region E2.

The photodetector 101 that has received the light beam outputs a signal having an amplitude of a value (amount of received light) obtained by dividing the intensity of the light beam by the area of the light receiving surface for each light receiving region. In the following, output signals corresponding to the light receiving areas 101A and 101B are I _101A and I _101B , respectively.

  The light intensity of the interference regions E1 and E2 has a value corresponding to the phase difference between the 0th-order diffracted light and the ± 1st-order diffracted light, but this phase difference changes depending on the unevenness on the recording surface. Therefore, when the focal position of the incident light moves in the optical disk radial direction, that is, in the direction crossing the track (the horizontal direction in FIG. 23A) (hereinafter, this movement is referred to as track jump), the zero-order diffracted light accompanies the movement. And the ± first-order diffracted light change, and the light intensity of the interference regions E1 and E2 changes. As a result, each output signal also changes.

FIG. 23B shows such a change in each output signal. As shown in the figure, the output signals I _101A and I _101B exhibit opposite phase changes around a predetermined value a, and the addition signal I _101A + I _101B always has a constant value 2a.

On the other hand, the subtracted signal I _101A -I _101B (hereinafter referred to as tracking error signal TE) of the output signal becomes 0 when the focal position of the incident light is at the center of the land L or the groove G, and 0 otherwise. It becomes a value other than. The tracking servo uses such a property of the tracking error signal TE, and the optical drive device controls the position of the objective lens so that the value of the tracking error signal TE becomes 0, so that the focal point in the radial direction of the optical disk is controlled. Adjust the misalignment.
Japanese Laid-Open Patent Publication No. 4-168631 JP 2007-335047 A

  By the way, when the position of the objective lens is moved by the tracking servo (hereinafter, this movement is referred to as a lens shift), the range of the spot formed on the photodetector 101 and the intensity center are shown in FIG. Shift in direction. Therefore, there is a problem that an offset occurs in the tracking error signal TE and the accuracy of the tracking servo deteriorates.

  Here, the shift of the spot range and the intensity center due to the lens shift will be described.

  First, FIG. 25 is a schematic diagram of an optical drive device for performing reproduction and recording of the optical disc 11. As shown in the figure, the optical drive device includes a laser light source 102, a beam splitter 104, a collimator lens 105, a quarter wavelength plate 106, and a sensor lens 107 in addition to the objective lens 100 and the photodetector 101. The

  The light beam emitted from the laser light source 102 is reflected by the beam splitter 104 and travels in the direction of the optical disk 11, is converted into parallel light by the collimator lens 105, passes through the quarter-wave plate 106, and is reflected by the objective lens 100. 11 Condensed on the recording surface.

  The light beam reflected by the recording surface of the optical disk 11 becomes parallel light by the objective lens 100 and enters the collimator lens 104 by going backward through the quarter-wave plate 106. The collimator lens 104 condenses the incident light beam on the light receiving surface of the photodetector 101. The sensor lens 107 gives astigmatism to the light beam condensed on the light receiving surface of the photodetector 101.

FIG. 26A shows a contour line (light intensity I at the intensity center) of the light intensity of the spot formed on the light receiving surface when the light receiving surface of the photodetector 101 is located at the focal point of the sensor lens 107. FIG. it is a diagram showing a predetermined number times) _0. The intersection (origin) of the X axis and the Y axis is the center of the light receiving surface, and the collimator lens 105 is arranged so that this origin is the focal point. The range of the spot is determined by the diameter of the objective lens 100, and the diameter of the objective lens 100 is usually designed so that up to 77% of the light intensity I ₀ is within the range of the spot. The contour lines of light intensity are actually longer in the Y-axis direction than in the X-axis direction, but are drawn in a perfect circle in FIG. 26 and FIG. 28 described later for the sake of simplicity.

  FIG. 26B shows the light intensity corresponding to the Y axis in FIG. As shown in the figure, the light intensity of the spot can be approximated by a normal distribution centered on the intensity center.

  The example of FIG. 26A is an example in the case where there is no lens shift. In this case, as shown in FIG. 26, the focal point of the collimator lens 105, the center of the spot (the geometric center of the spot range), and the intensity center Are in the same position.

  When there is a lens shift, the optical axis of the objective lens 100 and the optical axis of the collimator lens 105 are shifted. FIG. 27 shows the state of the light beam in such a state. When there is a lens shift, the focal position of the objective lens 100 moves, so the spot formed on the recording surface of the optical disk 11 by the light beam that has passed through the objective lens 100 moves in the direction of movement of the objective lens 100 (the tracking servo is Use movement). Further, the spot formed on the light receiving surface of the photodetector 101 via the collimator lens 104 by the light beam reflected by the recording surface of the optical disk 11 also moves in the moving direction of the objective lens 100.

  FIG. 28A is a diagram showing the light intensity of the spots formed on the light receiving surface of the photodetector 101 in the example shown in FIG. 27 with the same contour lines as in FIG. As shown in the figure, when there is a lens shift, both the intensity center and the spot range move in the Y-axis direction. However, these movement amounts do not match, and the movement amount at the intensity center becomes larger than the movement amount in the spot range. As a result, the intensity center does not coincide with the spot center.

  FIG. 28 (b) shows the light intensity corresponding to the Y axis in FIG. 28 (a).

  As described above, when there is a lens shift, the spot range and the intensity center shift. This shift causes an offset in the value of the tracking error signal TE.

  FIG. 29 is a diagram showing the relationship between the shift amount of the objective lens 100 and the offset amount of the tracking error signal TE. As shown in the figure, the offset amount of the tracking error signal TE changes substantially in proportion to the shift amount. When the shift amount is 0, the offset amount = 0.

  Accordingly, an object of the present invention is to provide a photodetector and an optical drive device that can cancel an offset generated in a tracking error signal due to a lens shift.

  In order to solve the above problems, a photodetector according to the present invention is a light-receiving surface that receives a light beam reflected by a recording surface of a multilayered optical disc, collimated by an objective lens, and collected by a collimator lens. The light receiving surface is provided in a region including a lens shift reference line of signal light, which is sandwiched between lens shift direction lines of each light beam reflected by each layer other than the access target layer. The first straight line passing through the spot center when there is no lens shift of the objective lens, or the straight line passing through the spot center, and the straight line of each light beam reflected by each layer other than the access target layer The first dividing line, which is either the absolute value of the sum of the differences between the received light amounts on both sides, is a maximum value, or is a second straight line whose difference from the maximum value is within a predetermined amount. Characterized in that it is divided into the optical domain.

  According to the present invention, the amount of change that occurs in the stray light spot due to lens shift can be measured, and the offset that occurs in the tracking error signal can be canceled using the result.

  In the photodetector, the first dividing line is a straight line passing through the center of the spot, and a difference in received light amount on both sides of each light beam reflected by each layer other than the access target layer. The absolute value of the sum may be a maximal value, and an angle difference with each lens shift direction line may be a first peak line within a predetermined angle, and the first peak line may be Of the lens shift direction lines, the angle difference with the lens shift direction line of the adjacent layer having a smaller number of layers as viewed from the access target layer may be within a predetermined angle. According to this, it is possible to accurately measure the amount of change generated in the stray light spot by the lens shift.

  In the photodetector, the light receiving surface includes the first dividing line when the access target layer is the first layer and the first division when the access target layer is the second layer. The line may be divided into a plurality of light receiving areas. According to this, the amount of change that occurs in the stray light spot due to lens shift can be preferably measured for each layer.

  In the photodetector, the same straight line may be used as the first dividing line when the access target layer is the first layer and when the access target layer is the second layer. According to this, dividing lines can be reduced.

  Further, in the photodetector, the light receiving surface is provided in a region including a lens shift reference line of each light beam reflected by each layer other than the access target layer and including a lens shift direction line of the signal light. In addition, the third straight line passing through the spot center or the straight line passing through the spot center and the sum of the difference in received light amount on both sides of each straight line of each light beam reflected by each layer other than the access target layer The absolute value may be a maximum value or may be divided into a plurality of light receiving regions by a second dividing line which is one of the fourth straight lines whose difference from the maximum value is within a predetermined amount. According to this, it is possible to accurately measure the amount of change generated in the stray light spot by the lens shift.

  In the photodetector, the second dividing line is a straight line passing through the center of the spot, and a difference in received light amount on both sides of each light beam reflected by each layer other than the access target layer. The absolute value of the sum may be a local maximum value, and the second peak line may have a difference in angle with each lens shift reference line within a predetermined angle. Of the lens shift reference lines, the angle difference from the lens shift reference line of the adjacent layer on the side with the larger number of layers as viewed from the access target layer may be within a predetermined angle. According to this, the amount of change generated in the stray light spot by the lens shift can be measured with higher accuracy.

  In the photodetector, the light receiving surface includes the second dividing line when the access target layer is the first layer and the second division when the access target layer is the second layer. The line may be divided into a plurality of light receiving areas. According to this, the amount of change that occurs in the stray light spot due to lens shift can be preferably measured for each layer.

  In the photodetector, the same straight line may be used as the second dividing line when the access target layer is the first layer and when the access target layer is the second layer. According to this, dividing lines can be reduced.

  Further, in the photodetector, the light receiving surface includes the second dividing line when the access target layer is the first outermost layer and the second dividing line when the access target layer is the second outermost layer. It may be divided into a plurality of light receiving areas by dividing lines. Even in this way, the dividing line can be reduced.

  In the photodetector, the light receiving regions may be provided symmetrically with respect to the lens shift direction line of the signal light. The light receiving regions may be provided so as to overlap with the signal light receiving region. According to this, the value of the measured change amount can be increased.

  A photodetector according to another aspect of the present invention is a light receiving surface that receives a light beam reflected by a recording surface of a multilayered optical disc, converted into parallel light by an objective lens, and further collected by a collimator lens. The light receiving surface is provided in a region including a lens shift reference line for each light beam reflected by each layer other than the access target layer and including a lens shift direction line of the signal light. The absolute value of the sum of the difference in received light amount on both sides of each of the light beams reflected by each layer other than the access target layer, and the straight line passing through the spot center or the straight line passing through the spot center. Is divided into a plurality of light receiving regions by a dividing line that is either a maximum value or a straight line whose difference from the maximum value is within a predetermined amount.

  Also according to the present invention, the amount of change that occurs in the stray light spot due to the lens shift can be measured, and the offset that occurs in the tracking error signal can be canceled using the result.

  In addition, an optical drive device according to the present invention includes each of the above-described photodetectors, and a change caused by a lens shift of a spot formed on the photodetector by stray light included in the light beam based on the amount of light received by each of the light-receiving regions. Measuring means for measuring the quantity, and tracking error signal generating means for generating a tracking error signal using a measurement result of the measuring means.

  According to the present invention, the offset generated in the tracking error signal can be canceled using the measurement result of the amount of change generated in the stray light spot due to the lens shift.

  An optical drive device according to another aspect of the present invention includes the above-described photodetectors, and stray light included in the light beam is generated based on the amount of light received from a part of the plurality of light receiving regions. It is characterized by comprising measuring means for measuring the amount of change caused by lens shift of a spot formed on the detector, and tracking error signal generating means for generating a tracking error signal using the measurement result of the measuring means.

  According to the present invention, the area of the stray light receiving region can be reduced. Further, by reducing the area of the stray light receiving region, it is possible to reduce the influence of stray light reflected from other than the optical disk.

  According to the present invention, the offset generated in the tracking error signal TE can be canceled using the measurement result of the amount of change generated in the stray light spot due to the lens shift.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is an example of a schematic diagram of an optical drive device 1 according to the present embodiment.

  The optical drive device 1 performs reproduction and recording of the optical disk 11. Various optical recording media such as CD, DVD, and BD can be used as the optical disc 11, but in this embodiment, a disc-shaped optical disc having a recording surface that is multilayered into two or more layers by a multilayer film is used. .

  As shown in FIG. 1, the optical drive device 1 includes a laser light source 2, an optical system 3, an objective lens 4, a photodetector 5, and a processing unit 6. Among these, the laser light source 2, the optical system 3, the objective lens 4, and the photodetector 5 constitute an optical pickup.

  The optical system 3 includes a beam splitter 22, a collimator lens 23, a quarter wavelength plate 24, and a sensor lens (cylindrical lens) 25.

  The optical system 3 functions as an outward optical system that guides the light beam emitted from the laser light source 2 to the optical disk 11, and also functions as a backward optical system that guides the return beam from the optical disk 11 to the photodetector 5.

  In the forward optical system of the optical system 3, the light beam emitted from the laser light source 2 enters the beam splitter 22 as P-polarized light. The beam splitter 22 reflects the incident P-polarized light and bends its path in the direction of the optical disk 11. The collimator lens 23 is a biconvex lens, and the light beam incident from the beam splitter 22 is converted into parallel light. The quarter wavelength plate 24 converts the light beam that has passed through the collimator lens 23 into circularly polarized light. The light beam that has passed through the quarter-wave plate 24 enters the objective lens 4.

  The objective lens 4 condenses the light beam incident from the optical system 3 (a light beam in a parallel light state) on the optical disk 11 and returns the return light beam reflected by the recording surface of the optical disk 11 to parallel light.

  In the return path optical system of the optical system 3, a light beam that has passed through the objective lens 4 and has become S-polarized light by reciprocating the quarter-wave plate 24 is incident on the collimator lens 23. The light beam that has passed through the collimator lens 23 is incident on the beam splitter 22 while being condensed. The beam splitter 22 transmits the incident light beam 100% so as to enter the sensor lens 25. The sensor lens 25 gives astigmatism to the light beam incident from the beam splitter 22. The light beam provided with astigmatism enters the photodetector 5.

  FIG. 2 is an explanatory diagram of astigmatism imparted by the sensor lens 25. As shown in the figure, the sensor lens 25 has a lens effect only in one direction (MY axis direction = subordinate direction). Therefore, the focal position of the optical system constituted by the collimator lens 23 (FIG. 1) and the sensor lens 25 differs between the MY axis direction and the MX axis direction (bus line direction) which is a direction perpendicular to the MY axis direction. (MY axis focus and MX axis focus shown in FIG. 2). A point where the lengths of the light beams in the MY axis direction and the MX axis direction are equal is referred to as a focal point.

  In the optical drive device 1, the position control of the objective lens 4 is performed so that the joint point of the light beam (signal light) reflected by the layer to be focused (access target layer) is positioned just on the photodetector 5. (Focus servo described later). In other words, the joint point of the light beam (stray light) reflected by a layer other than the access target layer is not located on the photodetector 5, and the spot (stray light spot) that stray light forms on the photodetector 5 is As compared with a spot (signal light spot) formed on the photodetector 5 by the signal light, the signal light has a shape spreading in at least one of the MY axis direction and the MX axis direction.

  Returning to FIG. The photodetector 5 includes a signal light receiving area for receiving signal light and a stray light receiving area for receiving stray light.

  FIG. 3A shows the signal light receiving region 8 of the photodetector 5. As shown in the figure, the signal light receiving area 8 is a four-divided light receiving area in which four light receiving areas 8A, 8B, 8C and 8D having a square light receiving surface of a predetermined size are arranged counterclockwise from the upper left of the drawing. It is. Hereinafter, the vertical dividing line of the signal light receiving region 8 is referred to as a Y axis, and the horizontal dividing line is referred to as an X axis. The signal light receiving area 8 is arranged such that the MX axis direction and the MY axis direction described above coincide with the diagonal line of the signal light receiving area 8.

  FIG. 3A shows an example of a signal light spot when there is no lens shift. As shown in the figure, the center of the signal light spot in this case is the origin (the intersection of the X axis and the Y axis).

  FIG. 3B shows an example of the signal light spot that has moved to the maximum by the lens shift. As shown in the figure, the size of the signal light receiving area 8 is determined so that the entire signal light spot can be received even when the signal light spot is moved by lens shift. In the following, the spot movement direction due to lens shift is referred to as a lens shift direction line LDS, and a line that serves as a reference for spot movement due to lens shift is referred to as a lens shift reference line LBS. The lens shift direction line LDS and the lens shift reference line LBS of the signal light spot coincide with the Y axis and the X axis, respectively. Further, in the following, a plus direction and a minus direction are provided in the moving direction by the lens shift of the objective lens 4, and in the example of FIG. 3, the objective lens 4 that moves the signal light spot to the light receiving regions 8B and 8C side. The moving direction of the objective lens 4 that moves the signal light spot to the opposite side is referred to as the plus direction, and the minus direction.

The photodetector 5 that has received the light beam outputs a signal having an amplitude of a value (light reception amount) obtained by dividing the intensity of the light beam by the area of the light receiving surface for each light receiving region. Hereinafter, an output signal corresponding to the light receiving region X (X is a code of the light receiving region) is described as I _X.

  The stray light receiving area of the photodetector 5 will be described later.

Returning to FIG. As an example, the processing unit 6 is configured by a DSP (Digital Signal Processor) having an A / D conversion function for converting analog signals for multiple channels into digital data, and receives an input of an output signal of the photodetector 5. Thus, the tracking error signal TE and the focus error signal FE described above are generated. In the present embodiment, the tracking error signal TE and the focus error signal FE are expressed by Expression (1) and Expression (2), respectively. Note that PP in equation (1) is a push-pull signal and is represented by equation (3). TEa is a change amount of the stray light spot due to lens shift, which will be described in detail later.
TE = PP-TEa (1)
FE = (I _8A + I _8C ) − (I _8B + I _8D ) (2)
PP = (I _8A + I _8D ) − (I _8B + I _8C ) (3)

  The processing unit 6 also has a function of moving the objective lens 4 in a direction perpendicular to the recording surface of the optical disc 11 so that the position of the spot formed on the recording surface by the light beam moves toward and away from the recording surface. The objective lens 4 is moved so that the generated focus error signal FE becomes zero (focus servo). The processing unit 6 stores the vertical position of the objective lens 4 for each layer, and first moves the objective lens 4 to the vertical position stored for the access target layer. Thereafter, a process of moving the objective lens 4 so that the focus error signal FE becomes 0 is executed. Thereby, the processing unit 6 focuses the light beam on the recording surface.

  Further, the processing unit 6 has a function of moving the position of the spot formed on the recording surface by the light beam in the radial direction of the optical disk 11 by moving the objective lens 4 in the radial direction of the optical disk 11 (lens shift). The objective lens 4 is shifted so that the generated tracking error signal TE becomes 0 (tracking servo). Thereby, the processing unit 6 focuses the light beam on the track.

  As described above, the signal light spot formed on the photodetector 5 is shifted due to the lens shift, which causes an offset in the tracking error signal TE. In order to prevent the tracking servo accuracy from deteriorating due to the lens shift, it is preferable to be able to cancel the offset generated in the tracking error signal TE due to the lens shift. Therefore, in this embodiment, a stray light spot is used as one method for that purpose. That is, the stray light spot is also changed by the lens shift similarly to the signal light spot, but the processing unit 6 measures the amount of the change and cancels the offset generated in the tracking error signal TE using the measurement result.

  FIG. 4 is a diagram showing functional blocks of the processing unit 6 for realizing such functions. As shown in the figure, the processing unit 6 includes a push-pull signal generation unit 61, a change amount measurement unit 62, a tracking error signal generation unit 63, and an objective lens control unit 64.

  The push-pull signal generation unit 61 receives the output signal of the signal light receiving region 8 of the photodetector 5 and generates a push-pull signal PP by Expression (3). The change amount measuring unit 62 (measuring unit) receives an output signal of the stray light receiving region of the photodetector 5. Then, the amount of change due to the lens shift of the stray light spot is measured by the received output signal.

  The tracking error signal generation unit 63 (tracking error signal generation means) substitutes the push-pull signal PP generated by the push-pull signal generation unit 61 and the change amount measured by the change amount measurement unit 62 into Equation (1). Thus, the tracking error signal TE is calculated. The objective lens controller 64 generates a control signal for the objective lens 4 based on the value of the tracking error signal TE calculated by the tracking error signal generator 63, and an actuator (not shown) for controlling the position of the objective lens 4. Output to.

  Hereinafter, the measurement of the amount of change performed by the stray light receiving region of the photodetector 5 and the change amount measuring unit 62 will be described. Before that, the relationship between the stray light and the lens shift will be described in detail.

  First, FIG. 5 is a diagram illustrating an example of a layer configuration of the optical disc 11. As shown in the figure, the optical disc 11 according to this example has a five-layer configuration having layers L0 to L4 in order from the side far from the objective lens 4, and the layer spacing is 16 μm, 10 μm, in order from between the layers L0 and L1. 10 μm and 16 μm.

  In the following description, when the access target layer is Lx (here, x = 0 to 4), the stray light reflected by the layer Ly (here y = 0 to 4, y ≠ x) is reflected as stray light xy. It will be expressed as Further, the lens shift direction line and the lens shift reference line of the spot of the stray light xy will be expressed as LDx-y and LBx-y, respectively.

  Here, only the case where the access target layer is the layer L2 will be described.

  FIG. 6 shows spots formed on the photodetector 5 for each signal light and stray light when there is no lens shift (shift amount of the objective lens 4 = 0). A signal light receiving region 8 is shown at the center of each figure.

  As shown in each drawing of FIG. 6, the spot formed by each stray light is larger than the size of the signal light receiving area 8 and protrudes greatly from the signal light receiving area 8.

  Of each stray light, the stray light 2-3 and stray light 2-4 spots have a larger spread in the MX-axis direction than in the MY-axis direction. This is because the stray light 2-3 and the stray light 2-4 form a spot on the photodetector 5 at a position closer to the sensor lens 25 than the MY-axis focal point (FIG. 2). The spot of stray light 2-3 is smaller than that of stray light 2-4. This is because the layer L3 is closer to the access target layer L2 than the layer L4.

  On the other hand, the spot of the stray light 2-1 and the stray light 2-0 has a larger spread in the MY axis direction than the spread in the MX axis direction. This is because the stray light 2-1 and the stray light 2-0 form a spot on the photodetector 5 at a position farther from the sensor lens 25 than the MX axis focal point (FIG. 2). Further, the spot of the stray light 2-1 is smaller than that of the stray light 2-0. This is because the layer L1 is closer to the access target layer L2 than the layer L0.

  FIG. 6 also shows a lens shift direction line and a lens shift reference line for each spot.

  As shown in the figure, the lens shift direction line and the lens shift reference line of each stray light spot are inclined more obliquely than the lens shift direction line LDS and the lens shift reference line LBS of the signal light spot due to the influence of the sensor lens 25. ing.

  Specifically, both the lens shift direction line and the lens shift reference line have a spot formation position that approaches the sensor lens 25 from the focal point in FIG. 2 and approaches the MX axis, and coincides with the MX axis at the MY axis side focal point. When the sensor lens 25 is further approached from there, it further tilts beyond the MX axis (FIGS. 6A and 6B). On the other hand, in both the lens shift direction line and the lens shift reference line, the spot formation position moves away from the sensor lens 25 from the focal point in FIG. 2 and approaches the MY axis, and coincides with the MY axis at the MX axis side focal point. When the sensor lens 25 is further approached from there, it further tilts beyond the MY axis (FIGS. 6D and 6E).

  The lens shift direction line and the lens shift reference line are axisymmetric with respect to the bus line or the child line of the sensor lens 25.

  FIG. 7 shows each spot moved in the plus direction by the lens shift in correspondence with each diagram in FIG. As shown in the figure, the stray light spot is also moved by lens shift like the signal light spot, and the center of intensity of the stray light spot (black dot in the figure) is located on the lens shift direction line.

  FIG. 8A shows the signal light spot, the stray light spot, and the respective lens shift direction lines and the lens shift reference lines shown in FIG. Similarly, FIG. 8B illustrates the signal light spot, the stray light spot, and the respective lens shift direction lines and lens shift reference lines shown in the respective drawings of FIG. The contour lines are omitted in FIGS. 8 (a) and 8 (b). As shown in FIGS. 8A and 8B, each spot is irradiated on the light detector 5 in an overlapping manner.

  Next, FIG. 9 is a diagram illustrating another example of the layer configuration of the optical disc 11. As shown in the figure, the optical disc 11 according to this example has a four-layer configuration having layers L0 to L3 in order from the side far from the objective lens 4, and the layer spacing is 13 μm, 16 μm, in order from between the layers L0 and L1. 10 μm. The light intensity of the reflected light from each layer is almost the same.

  10 and 11 show spots formed on the photodetector 5 by the light beam every time the access target layer is each of the layers L0 to L3. In FIG. 10A, the access target layer is the layer L0, and the signal light is reflected light from the layer L0. Further, the stray light 0-1, 0-2, 0-3 forms a spot on the photodetector 5. Similarly, in FIG. 10B, the access target layer is the layer L1, and the signal light is reflected light from the layer L1. Further, the stray light 1-0, 1-2, 1-3 forms spots on the photodetector 5. In FIG. 11A, the access target layer is the layer L2, and the signal light is reflected light from the layer L2. Further, the stray lights 2-0, 2-1, and 2-3 form spots on the photodetector 5. In FIG. 11B, the access target layer is the layer L3, and the signal light is reflected light from the layer L3. Further, the stray light 3-0, 3-1, 3-2 forms a spot on the photodetector 5.

  10 and 11 also show a lens shift direction line and a lens shift reference line for each spot. Each spot shown in FIGS. 10 and 11 is obtained when the objective lens 4 is moved in the plus direction by lens shift.

  Here, the result of having performed the simulation of received light amount using the optical disk 11 by the example of FIG. 9 is shown.

  First, FIG. 12 is an explanatory diagram of simulation. As shown in the figure, in this simulation, a square stray light receiving area 9 (including the signal light receiving area 8) centered on the origin is provided. Note that the size of the stray light receiving area 9 is such that all stray light can be received (in FIG. 12 and the following drawings, it is not necessarily so because priority is given to ease of viewing).

  Here, a square stray light receiving area is used. However, since the intensity of the center of the laser beam is dominant and the intensity distribution in the tangential direction of the disk is particularly steeper, the stray light receiving area is a rectangle that is long in the X-axis direction. It is good. Moreover, even if it is the size and shape which do not contain all the stray light of an adjacent layer, what is necessary is just to have an effect.

  Further, a straight line L that passes through the origin and forms an angle of θ with the X axis is provided. The stray light receiving region 9 is divided into regions A and B by this straight line L, and stray light is changed while changing θ from −180 ° to 180 °. The amount of light received in each of the areas A and B was simulated for each time. In addition, the lens shift amount of the objective lens 4 is 0.3 mm, and the simulation was performed when the access target layers were the layer L1 and the layer L2.

13 and 14 show the simulation results when the access target layers are the layer L1 and the layer L2, respectively. 13 and FIG. 14, the “difference” on the vertical axis indicates the difference between the received light amounts on both sides of the straight line L, that is, the difference between the received light amount in the region A and the received light amount in the region B (the received light amount IA in the region _A). -Indicates the amount of received light I _B ) in region B. Further, stray light 1-3 and 2-0 is set to a value of 25% of the difference in the amount of received light, assuming that the stray light receiving area 9 is not so large as to include all of stray light 1-3 and 2-0.

  Tables 1 to 8 are numerical data of the simulation results shown in FIGS. Tables 1 to 4 correspond to FIG. 13, and Tables 5 to 8 correspond to FIG. In these tables, only numerical data of −90 ° to 90 ° is shown. However, since “difference” between two straight lines L different in θ by 180 ° differs only in sign, −180 ° to −90 °. The data of 90 ° to 180 ° can be easily obtained by changing the sign of the numerical data of −90 ° to 90 °. Further, the reference numerals beginning with P or S in the table correspond to the same reference numerals in FIGS. However, only the underlined reference numerals are described in the column corresponding to θ different from the same reference numerals in FIGS. 13 and 14 by 180 degrees.

  In the example illustrated in FIG. 13, the stray light includes three stray lights 1-0, 1-2, and 1-3 (see FIG. 10B). When the light detector 5 receives the light beam actually reflected by the optical disk 11, these stray lights are simultaneously received. Therefore, the “sum of the differences between the stray lights 1-0, 1-2, and 1-3” shown in FIG. 13 is the difference between the actual received light amount of the region A and the received light amount of the region B (however, here) Signal light is not taken into account.) As shown in FIG. 13, the “sum of the differences between the stray lights 1-0, 1-2, and 1-3” has an absolute value that is a maximum between θ = −90 ° and θ = 90 °. It has poles P1 and P2. The straight lines L respectively corresponding to these pole points P1 and P2 are hereinafter referred to as peak lines. Although there are pole points at -180 ° to -90 ° and 90 ° to 180 °, the corresponding straight line L is the same as that of the pole point at -90 ° to 90 °, so it will not be taken up here.

  Further, as shown in FIG. 13, each stray light has pole points P10, P12, and P13, respectively. A straight line L corresponding to each of these poles is approximately equal to the lens shift reference line of each stray light (since the simulation is performed in units of 5 °, it is not exactly equal). Moreover, as shown in FIG. 13, each stray light and the θ-axis intersect at intersections S10, S12, and S13. A straight line L corresponding to each of these intersections is equal to the lens shift direction line of each stray light.

  As shown in FIG. 13, the value of θ corresponding to the pole P1 is close to the value of θ corresponding to the poles P10, P12, P13. On the other hand, the value of θ corresponding to the pole P2 is close to the value of θ corresponding to the intersections S10, S12, S13. Accordingly, it can be said that the pole points P1 and P2 are located near the lens shift reference line and the lens shift direction line of each stray light, respectively, and the pole point P1 is determined by the positions (θ) of the pole points P10, P12, and P13. Similarly, the pole P2 is also determined by the positions (θ) of the intersections S10, S12, S13.

  More precisely, the pole P2 is located within the θ range (within the θ range including the lens shift reference line (θ = 0 °) of the signal light) sandwiched between the lens shift direction lines of each stray light. On the other hand, the pole P1 is located outside the θ range (in the θ range including the lens shift direction line (θ = −90 °) of the signal light) sandwiched between the lens shift reference lines of each stray light. . This is because the difference is gentle in the vicinity of the lens shift reference line. That is, while the vicinity of the poles P12 and P13 of the stray light 1-2 and 1-3 changes gently, the stray light 1-0 changes suddenly after passing the pole P10. A pole P1 is generated at a position slightly deviated from the poles P12 and P13.

  In the example illustrated in FIG. 14, the stray light includes three stray lights 2-0, 2-1, and 2-3 (see FIG. 11A). As shown in FIG. 14, the “sum of differences between stray light 2-0, 2-1, and 2-3” has two absolute values between the maximum value between θ = −90 ° and θ = 90 °. And have pole points P5 and P6. Straight lines L corresponding to these pole points P5 and P6 are also peak lines. Although there are extreme values at -180 ° to -90 ° and 90 ° to 180 °, they are not taken up here as in FIG.

  Further, as shown in FIG. 14, each stray light has pole points P20, P21, and P23, respectively. A straight line L corresponding to each of these poles is approximately equal to the lens shift reference line of each stray light (since the simulation is performed in units of 5 °, it is not exactly equal). Moreover, as shown in FIG. 14, each stray light and the θ axis intersect at intersections S20, S21, and S23. A straight line L corresponding to each of these intersections is equal to the lens shift direction line of each stray light.

  As shown in FIG. 14, the value of θ corresponding to the pole P5 is close to the value of θ corresponding to the intersections S20, S21, S23. On the other hand, the value of θ corresponding to the pole P6 is close to the value of θ corresponding to the poles P20, P21, P23. Therefore, it can be said that the poles P5 and P6 are located in the vicinity of the lens shift direction line and the lens shift reference line of each stray light, respectively, and the pole P6 is determined by the positions (θ) of the poles P20, P21, and P23. Similarly, the pole P5 is also determined by the positions (θ) of the intersections S20, S21, S23.

  More precisely, the pole P5 is located within the θ range (within the θ range including the lens shift reference line (θ = 0 °) of the signal light) sandwiched between the lens shift direction lines of each stray light. On the other hand, the pole P6 is located outside the θ range (within the θ range including the lens shift direction line (θ = 90 °) of the signal light) sandwiched between the lens shift reference lines of each stray light. This is because the difference is gentle in the vicinity of the lens shift reference line. That is, the vicinity of the poles P21 and P20 of the stray light 2-1 and 2-0 changes gently, whereas the stray light 2-3 changes suddenly after passing the pole P23. A pole P6 is generated at a position slightly deviated from the poles P21 and P20.

The example shown in FIGS. 13 and 14 is an example in which the lens shift amount of the objective lens 4 is 0.3 mm. However, when the lens shift amount changes, the sum of the differences of the respective stray light changes accordingly. Therefore, the sum of the differences between the stray light can be said to reflect the amount of change in lens shift, and thus the amount of change TEa can be obtained by the following equation (4).
TEa = (sum of differences of each stray light) × k (4)

  Note that k in Equation (4) is a constant, and the optimum value is obtained in advance by measurement, but this constant k is essentially the amplification factor of the sum of the differences of each stray light. Therefore, the amount of change TEa can be obtained with higher accuracy as the value of the sum of the differences of each stray light is larger. In the example of FIG. 13 (when the access target layer is the layer L1), the sum of the differences of each stray light is the maximum at the extreme value P2, and therefore, when the access target layer is the layer L1, By dividing the stray light receiving region 9 by the peak line corresponding to the extreme value P2, the change amount TEa can be obtained with the highest accuracy. Similarly, when the access target layer is the layer L2, the change amount TEa can be obtained with the highest accuracy by dividing the stray light receiving region 9 along the peak line corresponding to the extreme value P5.

  Hereinafter, the measurement of the amount of change performed by the stray light receiving region of the photodetector 5 and the change amount measuring unit 62 will be described in detail with reference to three embodiments. In the following description, the optical disk 11 according to the example of FIG. 9 is assumed, and the description will be continued with reference to FIGS.

[First Embodiment]
In the present embodiment, the stray light receiving area divided by using the peak lines corresponding to the extreme values P2 and P5 (peak lines near the lens shift direction line) is used. In the present embodiment, description will be given focusing only on the case where the access target layer is the layer L1 or the layer L2.

  FIG. 15 is a diagram showing the stray light receiving region 10 of the photodetector 5 according to the present embodiment. The stray light receiving area 10 does not include the signal light receiving area 8. Further, the stray light receiving region 10 is divided into four light receiving regions 10A to 10D by a peak line T2 and a peak line T5 corresponding to the pole points P2 and P5, respectively. In the figure, θ (x) indicates the value of θ corresponding to the pole point x.

The change amount measuring unit 62 measures the change amount TEa based on the received light amounts of the light receiving regions 10A to 10D. Specifically, when the access target layer is the layer L1, the change amount measuring unit 62 calculates the change amount TEa by the following equation (5). In the right parenthesis of Equation (5), there is a difference in the amount of received light in both side regions of the peak line T2. Also, k ₁ is a constant, the optimum value previously obtained in advance by measurement.
TEa = ((I _10C + I _10D ) − (I _10A + I _10B )) × k ₁ (5)

On the other hand, when the access target layer is the layer L2, the change amount measuring unit 62 obtains the change amount TEa by the following equation (6). In the right parenthesis of Equation (6), the difference in the amount of received light in the both side regions of the peak line T5 is shown. Also, k ₂ is a constant, the optimum value previously obtained in advance by measurement.
TEa = ((I _10A + I _10D ) − (I _10B + I _10C )) × k ₂ (6)

  In this way, when the access target layer is the layer L1 or the layer L2, the change amount measuring unit 62 can obtain the change amount TEa with high accuracy.

  It is also preferable to obtain the change amount TEa based on only a part of the received light amount instead of all of the light receiving regions 10A to 10D. As an example, an example in which the change amount TEa is obtained based on the amount of light received only in the light receiving regions 10A and 10C will be described.

In this case, when the access target layers are the layers L1 and L2, the change amount measuring unit 62 obtains the change amount TEa by the following equations (7) and (8), respectively. However, It should be noted, k ₄ is a constant, the optimum value previously obtained in advance by measurement for each access target layer.
TEa = (I _10C −I _10A ) × k ₃ (7)
TEa = (I _10A −I _10C ) × k ₄ (8)

Each of the above equations will be described in detail. The right side of equation (7) can be transformed as in equation (9).
_{(I 10C -I 10A) × k} 3 = {((I 10C + I 10D) - (I 10A + I 10B)) - ((I 10A + I 10D) - (I 10B + I 10C))} × k 3/2 · (9)

In formula (9), (I _10C + I _10D ) − (I _10A + I _10B ) is the difference in the amount of light received on both sides of the peak line T2, and (I _10A + I _10D ) − (I _10B + I _10C ) is the peak. This is the difference in the amount of light received at both sides of the line T5. The peak lines T2 and T5 correspond to the poles P2 and P5, respectively. From Tables 3 and 6, the poles P2 and P5 correspond to θ = 10 and −10. Therefore, for example, the access target layer is the layer L1. In Table 2 and Table 3, the right side of Equation (9) can be calculated as (0.65062−0.45686) × k ₃ /2=0.09958×k ₃ . Similarly, from Tables 2 and 3, the right side of the formula (X7) can be calculated as (0.71281-0.55285) × k ₄ /2=0.07998×k ₄ .

  As described above, the change amount TEa in the equations (7) and (8) has a non-zero value. Therefore, the change amount TEa can be obtained from the equations (7) and (8). In this way, the area of the stray light receiving region can be reduced. Further, by reducing the area of the stray light receiving region, it is possible to reduce the influence of stray light reflected from other than the optical disk 11.

  Here, the light receiving regions 10A and 10C are provided symmetrically with respect to the lens shift direction line (= Y axis) of the signal light. In this way, when the change amount TEa is obtained based only on the difference between the light receiving areas provided symmetrically with respect to the lens shift direction line of the signal light, each light receiving area is provided so as to overlap the signal light receiving area 8. Is preferred. FIG. 16 shows an example in which the light receiving areas 10 </ b> A and 10 </ b> C are provided so as to overlap the signal light receiving area 8. This superimposition is possible because each light receiving area is axisymmetric with respect to the lens shift direction line of the signal light, so that the amount of change due to the lens shift of the signal light is cancelled. In this way, the value of the change amount TEa can be increased.

  Moreover, since the stray light receiving region 10 is divided for various reasons, the peak lines T2 and T5 are not always available. In such a case, the stray light receiving region 10 may be divided by a straight line near the peak lines T2 and T5.

  Specifically, the first straight line passing through the origin, or the straight line passing through the origin, is provided in a region sandwiched between the lens shift direction lines of each stray light and including the lens shift reference line of the signal light. The first absolute value of the sum of the differences between the received light amounts of the respective stray light on both sides of the straight line is a maximum value or a second straight line whose difference from the maximum value is within a predetermined amount. What is necessary is just to divide | segment into a some light reception area | region with a dividing line.

  For example, the case where the access target layer is the layer L1 will be described. The first straight line is a straight line that passes through the origin and is provided in the hatched area (first straight line installation area) in FIG. The first straight line installation region is sandwiched between the lens shift direction lines LD1-0, LD1-2, and LD1-3 of the stray light 1-0, 1-2, and 1-3, and the lens shift reference line of the signal light. (= X-axis).

  On the other hand, the second straight line is a straight line passing through the origin, and the absolute value of the sum of the differences in the amounts of received light on both sides of each stray light is a maximum value, or a difference from the maximum value is a predetermined amount. It is a straight line that satisfies being within. For example, when the predetermined amount is 10%, the straight line in the hatched area shown in FIG. The predetermined amount is preferably determined so that one peak line is included in the installation area of the second straight line shown in FIG.

  As described above, the first dividing line is a straight line passing through the origin in the hatched area in FIG.

  The first dividing line is a lens shift direction line of an adjacent layer having a smaller number of layers as viewed from the access target layer (a lens shift direction line LD1-0 when the access target layer is the layer L1). The angle difference may be within a predetermined angle. The same applies to the case where a dividing line is provided near the peak line T5 (when the access target layer is the layer L2).

  In FIG. 15, the peak lines T2 and T5 are used to divide the stray light receiving area 10 and divided into four light receiving areas 10A to 10D. However, the access target layer is the layer L1 and the layer L2. The same dividing line may be used. In this case, although there are two divided areas, the change amount TEa obtained from the difference in the received light amount of the divided areas has a non-zero value regardless of whether the access target layer is the layer L1 or the layer L2. In this case, the dividing line can be reduced.

  When the access target layer is the L0 layer, the same operation as that for the L1 layer may be performed, and when the access target layer is the L3 layer, the same operation as that for the L2 layer may be performed.

[Second Embodiment]
In the present embodiment, stray light divided using peak lines (peak lines in the vicinity of the lens shift reference line) corresponding to extreme values P1 and P6 in which the difference in the amount of received light in both side regions increases after extreme values P2 and P5. A light receiving area is used. In the present embodiment, the description will be given focusing only on the case where the access target layer is the layer L1 or the layer L2.

  FIG. 19 is a diagram showing the stray light receiving region 10 of the photodetector 5 according to the present embodiment. As shown in the figure, the stray light receiving region 10 is divided into four light receiving regions 10E to 10H by a peak line T1 and a peak line T6 respectively corresponding to the pole points P1 and P6.

The change amount measuring unit 62 measures the change amount TEa based on the received light amounts of the light receiving regions 10E to 10H. Specifically, when the access target layer is the layer L1, the change amount measuring unit 62 calculates the change amount TEa by the following equation (10). In the right parenthesis of Equation (10), the difference in the amount of received light in the both side regions of the peak line T1 is shown. Also, k ₅ are constants, the optimum value previously obtained in advance by measurement.
TEa = ((I _10E + I _10F ) − (I _10G + I _10H )) × k ₅ (10)

On the other hand, when the access target layer is the layer L2, the change amount measuring unit 62 calculates the change amount TEa by the following equation (11). In the right parenthesis of Equation (11), the difference in the amount of received light in both side regions of the peak line T6 is shown. Also, k ₆ are constants, the optimum value previously obtained in advance by measurement.
TEa = ((I _10E + I _10H ) − (I _10F + I _10G )) × k ₆ (11)

  In this way, when the access target layer is the layer L1 or the layer L2, the change amount measuring unit 62 can obtain the change amount TEa with high accuracy.

It is also preferable to obtain the change amount TEa based on only a part of the received light amount instead of all of the light receiving regions 10E to 10H. As an example, when based on the received light amount of only the light receiving regions 10E and 10G, the change amount measuring unit 62 obtains the change amount TEa by the following equation (12) regardless of whether the access target layer is the layer L1 or the layer L2. . However, It should be noted, k ₇ are constants, the optimum value previously obtained in advance by measurement for each access target layer.
TEa = (I _10E −I _10G ) × k ₇ (12)

  The change amount TEa obtained from the equation (12) has a non-zero value as in the equations (7) and (8) described in the first embodiment. Therefore, the change amount TEa can be obtained from the equation (12). In this way, the area of the stray light receiving region can be reduced. Further, by reducing the area of the stray light receiving region, it is possible to reduce the influence of stray light reflected from other than the optical disk 11.

  Here, the light receiving regions 10E and 10G are provided symmetrically with respect to the lens shift direction line (= Y axis) of the signal light. Therefore, similarly to the light receiving regions 10A and 10C described above, the light receiving regions 10E and 10G can be provided so as to overlap the signal light receiving region 8. In this way, the value of the change amount TEa can be increased.

  Further, since the stray light receiving region 10 is divided for various reasons, the peak lines T1 and T6 are not always available. In such a case, the stray light receiving region 10 may be divided by a straight line near the peak lines T1 and T6.

  Specifically, it is a third straight line passing through the origin, or a straight line passing through the origin, provided in a region between the lens shift reference lines of each stray light and including the lens shift direction line of the signal light. The second absolute value of the sum of the differences between the received light amounts of the respective stray light on both sides of the straight line is a local maximum value or a fourth straight line whose difference from the local maximum value is within a predetermined amount. What is necessary is just to divide | segment into a some light reception area | region with a dividing line.

  For example, the case where the access target layer is the layer L1 will be described. The third straight line is a straight line that passes through the origin and is provided in the shaded area (installation area of the third straight line) in FIG. This third straight line installation region is sandwiched between the lens shift reference lines LB1-0, LB1-2, and LB1-3 of the stray light 1-0, 1-2, and 1-3, and the lens shift direction line of the signal light. (= Y axis).

  On the other hand, the fourth straight line is a straight line passing through the origin, and the absolute value of the sum of the differences in the amounts of received light on both sides of each stray light is a maximum value or a difference from the maximum value is a predetermined amount. It is a straight line that satisfies being within. For example, when the predetermined amount is 10%, the straight line in the hatched area shown in FIG. This is the same as in the first embodiment. The predetermined amount is preferably determined so that one peak line is included in the installation area of the fourth straight line shown in FIG.

  As described above, the second dividing line is a straight line passing through the origin in the hatched area in FIG.

  The second dividing line is a lens shift reference line of an adjacent layer having a larger number of layers as viewed from the access target layer (or a lens shift reference line LB1-2 when the access target layer is the layer L1). The angle difference may be within a predetermined angle. The same applies to the case where the dividing line is provided near the peak line T6 (when the access target layer is the layer L2).

  Further, in FIG. 19, the peak lines T1 and T6 are used to divide the stray light receiving region 10 and divided into four light receiving regions 10A to 10D, but the access target layer is the layer L1 and the layer L2. The same dividing line may be used. In this case, although there are two divided areas, the change amount TEa obtained from the difference in the received light amount of the divided areas has a non-zero value regardless of whether the access target layer is the layer L1 or the layer L2. In this case, the dividing line can be reduced.

  When the access target layer is the L0 layer, the same operation as that for the L1 layer may be performed, and when the access target layer is the L3 layer, the same operation as that for the L2 layer may be performed.

[Third Embodiment]
In this embodiment, a stray light receiving area divided by using two peak lines corresponding to two extreme values per access target layer is used. In the present embodiment, description will be given focusing only on the case where the access target layer is the layer L1.

  FIG. 21 is a diagram showing the stray light receiving region 10 of the photodetector 5 according to the present embodiment. The stray light receiving area 10 is divided into, for example, four light receiving areas 10I to 10L by a peak line T1 and a peak line T2 respectively corresponding to the pole points P1 and P2.

  In the present embodiment, two examples of the measurement of the change amount TEa by the change amount measuring unit 62 are given.

In the first example, the change amount measuring unit 62 calculates the change amount TEa by the following equation (13). Incidentally, k ₈ is a constant, the optimum value previously obtained in advance by measurement.
TEa = {− ((I _10J + I _10K ) − (I _10I + I _10L )) + ((I _10K + I _10L ) − (I _10I + I _10J ))} × k ₈ (13)

(I _10J + I _10K ) − (I _10I + I _10L ) in the right parenthesis of the equation (13) is the difference in the amount of received light in both side regions of the peak line T1. Similarly, (I _10K + I _10L ) − (I _10I + I _10J ) is a difference in the amount of received light in both side regions of the peak line T2. Therefore, taking the case where the lens shift amount is 0.3 mm as an example, the right side of Expression (13) is (−0.53534 + 0.65062) × k ₈ = 0.11528 × k ₈ from Tables 1 and 3. Can be calculated.

  Thus, the change amount TEa obtained from Expression (13) has a non-zero value. Therefore, the change amount TEa can be obtained from the equation (13).

Here, Expression (13) is rewritten as Expression (14).
TEa = (I _10L −I _10J ) × 2 × k ₈ (14)

  As is apparent from the equation (14), the light receiving areas required in this example are actually only the light receiving areas 10L and 10J, and the light receiving areas 10K and 10I are unnecessary. Therefore, by not providing these, the area of the stray light receiving region can be reduced. Further, by reducing the area of the stray light receiving region, it is possible to reduce the influence of stray light reflected from other than the optical disk 11.

In the second example, the change amount measuring unit 62 calculates the change amount TEa by the following equation (15). Incidentally, k ₉ are constants, the optimum value previously obtained in advance by measurement.
TEa = {((I _10J + I _10K ) − (I _10I + I _10L )) + ((I _10K + I _10L ) − (I _10I + I _10J ))} × k ₉ (15)

The difference between Expression (15) and Expression (13) is only the sign of (I _10J + I _10K ) − (I _10I + I _10L ) in the right parenthesis. In this case, taking the case where the lens shift amount is 0.3 mm as an example, the right side of Expression (15) is (0.53534 + 0.65062) × k ₉ = 1.185596 × k ₉ from Tables 1 and 3. Can be calculated.

  Thus, the change amount TEa obtained from the equation (15) also has a non-zero value. Therefore, the change amount TEa can be obtained from the equation (15).

Further, equation (15) can be rewritten as equation (16).
TEa = (I _10K −I _10I ) × 2 × k ₉ (16)

  As is clear from the equation (16), the light receiving areas required in this example are only the light receiving areas 10K and 10I, and the light receiving areas 10J and 10L are unnecessary. Therefore, by not providing these, the area of the stray light receiving region can be reduced. Further, by reducing the area of the stray light receiving region, it is possible to reduce the influence of stray light reflected from other than the optical disk 11.

  Although two examples are given above, it can be seen that the use of only the light receiving regions 10K and 10I is dominant as shown in the equation (16).

  Even when the access target layer is the layer L2, for example, when the access target layer is the layer L1, if the access target layer is divided into four light receiving regions by the peak line T5 and the peak line T6 respectively corresponding to the pole points P5 and P6 It can be considered as well. That is, the stray light receiving region is divided into eight by the peak lines T1, T2, T5, and T6. When accessing the layer L0 and the layer L3, calculations may be performed using the peak lines T1 and T6 as dividing lines, respectively. Further, as described above, the dividing lines corresponding to the peak lines T2 and T5 may be divided into six, or the dividing lines corresponding to the peak lines T1 and T6 may be divided into six. The dividing lines corresponding to the lines T2 and T5 and the dividing lines corresponding to the peak lines T1 and T6 may be divided into four at the same time.

[Fourth Embodiment]
In the present embodiment, in addition to the case where the access target layers are the layers L1 and L2, the case where the layers are the layers L0 and L3 will also be described.

  First, in the case where the access target layers are the layer L0 and the layer L3, as in the second embodiment, the peak line near the lens shift reference line or the straight line (the second dividing line described above) near the lens shift reference line. Use to divide the stray light receiving area. Since the layer L0 and the layer L3 are both outermost layers, the lens shift reference lines are gathered on one side of the Y axis as shown in FIGS. 10 (a) and 11 (b). A dividing line may be provided in the vicinity of these lens shift reference lines. FIG. 22 shows an example of the dividing lines T7 and T8 set in this way.

  Next, regarding the case where the access target layers are the layer L1 and the layer L2, as described in the third embodiment, the two peak lines corresponding to the two extreme values (the peak line for the layer L1). T1 and T2. The layer L2 is divided using the peak lines T5 and T6.) However, since the peak lines T1 and T6 are close to the dividing lines T7 and T8, respectively, these can be substituted.

  After all, in the present embodiment, as shown in FIG. 22, the stray light receiving region 10 divided into eight light receiving regions 10M to 10T using the peak lines T2 and T5 and the dividing lines T7 and T8 is used.

The change amount measuring unit 62 measures the change amount TEa based on the received light amounts of the light receiving regions 10M to 10T. Specific calculation formulas are Expressions (17) to (20) when the access target layer is each of the layers L0 to L3. In the right parentheses in the equations (17) and (20), the difference in the amount of received light in the both side regions of the dividing lines T7 and T8 is shown. Moreover, Formula (18) and Formula (19) apply Formula (16) shown in 3rd Embodiment. Further, k _{10 to} k ₁₃ are constants, and the optimum values are obtained in advance by measurement.
TEa = ((I _10N + I _10O + I _10P + I _10Q ) − (I _10M + I _10R + I _10S + I _10T )) × k ₁₀ (17)
TEa = ((I _10P + I _10Q ) − (I _10M + I _10T )) × k ₁₁ (18)
TEa = ((I _10S + I _10T ) − (I _10O + I _10P )) × k ₁₂ (19)
TEa = ((I _10M + I _10N + I _10S + I _10T ) − (I _10O + I _10P + I _10Q + I _10R )) × k ₁₃ (20)

  In this way, the change amount measuring unit 62 can obtain the change amount TEa regardless of which of the layers L0 to L3 is the access target layer. Moreover, the division lines can be reduced by substituting the division lines T7 and T8 for the peak lines T1 and T6.

  In the above example, the equation (16) shown in the third embodiment is applied to the equation for calculating the change amount TEa when the access target layers are the layer L1 and the layer L2. Of course, it may be applied. Needless to say, the peak lines T2 and T5 may be filled with the above-described first dividing line.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to such embodiment at all, and this invention can be implemented in various aspects in the range which does not deviate from the summary. Of course.

It is an example of the schematic diagram of the optical drive device by embodiment of this invention. It is explanatory drawing of the astigmatism provided by the sensor lens by embodiment of this invention. Both (a) and (b) show the signal light receiving area of the photodetector according to the embodiment of the present invention. (A) shows an example of the signal light spot when there is no lens shift, and (b) shows an example of the signal light spot moved to the maximum by the lens shift. It is a figure which shows the functional block of the process part by embodiment of this invention. It is a figure which shows an example of the laminated constitution of the optical disk by this invention. It is a figure which shows the spot formed on a photodetector in the case where there is no lens shift using the optical disk by the example of FIG. 6 for every signal light and stray light. FIG. 7 is a diagram depicting spots that are moved in the plus direction due to lens shift when the optical disc according to the example of FIG. 6 is used, corresponding to the diagrams of FIG. 6. (A) is the figure which drawn the signal light spot shown in each figure of FIG. 6, the stray light spot, each lens shift direction line, and a lens shift reference line in one figure. FIG. 8B is a diagram illustrating the signal light spot, the stray light spot, and the respective lens shift direction lines and lens shift reference lines shown in the respective drawings of FIG. It is a figure which shows another example of the laminated constitution of the optical disk by this invention. FIG. 10 is a diagram illustrating spots formed on a photodetector for each access target layer when the optical disc according to the example of FIG. 9 is used. FIG. 10 is a diagram illustrating spots formed on a photodetector for each access target layer when the optical disc according to the example of FIG. 9 is used. It is explanatory drawing of the simulation of the light reception amount performed using the optical disk by the example of FIG. It is a figure which shows the result of the said simulation in case an access object layer is the layer L1. It is a figure which shows the result of the said simulation in case an access object layer is the layer L2. It is a figure which shows the stray light light reception area | region of the photodetector by the 1st Embodiment of this invention. In the first embodiment of the present invention, an example is shown in which a divided light receiving region is provided so as to overlap the signal light receiving region. It is a figure which shows the installation area | region of the 1st straight line by the 1st Embodiment of this invention. It is a figure which shows the area | region containing the straight line in which the difference of the absolute value of the sum of the difference in a both-sides area | region and its maximum value is less than predetermined amount regarding the 1st, 2nd embodiment of this invention. It is a figure which shows the stray light light-receiving area | region of the photodetector by the 2nd Embodiment of this invention. It is a figure which shows the installation area | region of the 3rd straight line by the 2nd Embodiment of this invention. It is a figure which shows the stray light light-receiving area | region of the photodetector by the 3rd Embodiment of this invention. It is a figure which shows the stray light light-receiving area | region of the photodetector by the 4th Embodiment of this invention. (A) is a figure which shows the end surface of the cross section of the optical disk recording surface by the background art of this invention, an objective lens, and a light beam (incident light, reflected light (0th order diffracted light, +/- 1st order diffracted light)). (B) is a figure which shows the change of each output signal. It is a figure which shows the light-receiving surface of the photodetector by the background art of this invention. 1 is a schematic diagram of an optical drive device for reproducing and recording an optical disc according to the background art of the present invention. (A) shows a case where the light intensity of a spot formed on the light receiving surface is a contour line (a predetermined number times the light intensity I _{0 at} the intensity center) when the light receiving surface of the photodetector is located at the focal point of the sensor lens. ). (B) is a figure which shows the light intensity corresponding on the Y-axis of (a). FIG. 2 is a schematic diagram of an optical drive device for reproducing and recording an optical disc according to the background art of the present invention, and is a diagram showing a state of a light beam particularly when there is a lens shift. (A) is the figure which showed the light intensity of the spot formed on the light-receiving surface of a photodetector with the contour line in the example shown in FIG. (B) is a figure which shows the light intensity corresponding on the Y-axis of (a). It is a figure which shows the relationship between the shift amount of the objective lens by the background art of this invention, and the offset amount of a tracking error signal.

Explanation of symbols

L0, L1, L2, L3 Layer 1 Optical drive device 2 Laser light source 3 Optical system 4 Objective lens 5 Photo detector 6 Processing unit 8, 8A, 8B, 8C, 8D Signal light receiving areas 9, 10, 10A, 10B, 10C , 10D, 10E, 10F, 10G, 10H, 10I, 10J, 10K, 10L, 10M, 10N, 10O, 10P, 10Q, 10R, 10S, 10T Stray light receiving area 11 Optical disc 22 Beam splitter 23 Collimator lens 24 1/4 wavelength Plate 25 Sensor lens 61 Push-pull signal generation unit 62 Change amount measurement unit 63 Tracking error signal generation unit 64 Objective lens control unit

Claims (16)

A photodetector having a light receiving surface that receives a light beam reflected by a recording surface of a multilayered optical disc, converted into parallel light by an objective lens, and further collected by a collimator lens,
The light receiving surface is
Spot when there is no lens shift of the objective lens, which is sandwiched between the lens shift direction lines of each light beam reflected by each layer other than the access target layer and is included in the region including the lens shift reference line of the signal light The first straight line through the center, or
It is a straight line passing through the spot center, and the absolute value of the sum of the difference in the amount of received light on both sides of each light beam reflected by each layer other than the access target layer is a maximum value or the maximum value. The photodetector is divided into a plurality of light receiving regions by a first dividing line which is one of the second straight lines whose difference is within a predetermined amount.   The first dividing line is a straight line passing through the center of the spot, and the absolute value of the sum of the differences in the amount of light received on both sides of each light beam reflected by each layer other than the access target layer is a maximum value. 2. The photodetector according to claim 1, wherein the photodetector is a first peak line having an angle difference with each lens shift direction line within a predetermined angle.   The first peak line has an angle difference within a predetermined angle with respect to a lens shift direction line of an adjacent layer having a smaller number of layers as viewed from the access target layer among the lens shift direction lines. The photodetector according to claim 2.   The light receiving surface is divided into a plurality of light receiving regions by the first dividing line when the access target layer is the first layer and the first dividing line when the access target layer is the second layer. The photodetector according to any one of claims 1 to 3, wherein the photodetector is divided.   The same straight line is used as the first dividing line when the access target layer is the first layer and when the access target layer is the second layer. The described photodetector. The light receiving surface is
A third straight line passing through the center of the spot provided in a region between the lens shift reference lines of each light beam reflected by each layer other than the access target layer and including the lens shift direction line of the signal light, or ,
It is a straight line passing through the spot center, and the absolute value of the sum of the difference in the amount of received light on both sides of each light beam reflected by each layer other than the access target layer is a maximum value or the maximum value. 6. The light according to claim 1, wherein the light is divided into a plurality of light receiving regions by a second dividing line which is one of the fourth straight lines whose difference is within a predetermined amount. Detector.   The second dividing line is a straight line passing through the spot center, and the absolute value of the sum of the differences in the amounts of light received on both sides of each light beam reflected by each layer other than the access target layer is a maximum value. The photodetector according to claim 6, wherein the photodetector is a second peak line having an angle difference from each lens shift reference line within a predetermined angle.   The second peak line has an angle difference within a predetermined angle with respect to a lens shift reference line of an adjacent layer having a larger number of layers as viewed from the access target layer among the lens shift reference lines. The photodetector according to claim 7.   The light receiving surface is divided into a plurality of light receiving regions by the second dividing line when the access target layer is the first layer and the second dividing line when the access target layer is the second layer. The photodetector according to claim 6, wherein the photodetector is divided.   The same straight line is used as the second dividing line when the access target layer is the first layer and when the access target layer is the second layer. The described photodetector.   The light receiving surface is divided into a plurality of light receiving regions by the second dividing line when the access target layer is the first outermost layer and the second dividing line when the access target layer is the second outermost layer. The photodetector according to claim 6, wherein the photodetector is divided.   The photodetector according to claim 1, wherein each of the light receiving regions is provided symmetrically with respect to a lens shift direction line of the signal light.   The photodetector according to claim 12, wherein each of the light receiving regions is provided so as to overlap with the signal light receiving region. A photodetector having a light receiving surface that receives a light beam reflected by a recording surface of a multilayered optical disc, converted into parallel light by an objective lens, and further collected by a collimator lens,
The light receiving surface is
A straight line passing through the center of the spot, provided in the region including the lens shift direction line of the signal light, sandwiched between the lens shift reference lines of each light beam reflected by each layer other than the access target layer, or
It is a straight line passing through the spot center, and the absolute value of the sum of the difference in the amount of received light on both sides of each light beam reflected by each layer other than the access target layer is a maximum value or the maximum value. A photodetector that is divided into a plurality of light receiving regions by a dividing line that is one of straight lines whose difference is within a predetermined amount. A photodetector according to any one of claims 1 to 14, comprising:
Measuring means for measuring the amount of change due to lens shift of the spot formed on the photodetector by the stray light contained in the light beam, based on the amount of light received by each of the light receiving regions;
Tracking error signal generating means for generating a tracking error signal using the measurement result of the measuring means;
An optical drive device comprising: A photodetector according to any one of claims 4 or 6 to 11,
Measuring means for measuring the amount of change due to lens shift of a spot formed on the photodetector by the stray light included in the light beam based on the amount of received light of the plurality of light receiving regions;
Tracking error signal generating means for generating a tracking error signal using the measurement result of the measuring means;
An optical drive device comprising:

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