JP2010080038A - Optical drive device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce quantity of offset caused in a tracking error signal TE. <P>SOLUTION: In an optical drive device, a first push-pull signal and a first sum signal are generated based on respective light receiving quantity of light receiving areas 1Aa, 1Ba in which each width of a signal light tangent direction is shorter than a diameter pf a spot of signal light, a first normarized push-pull signal is generated by normalizing the first push-pull signal by using the first sum signal, then the tracking error signal is generated based on the first normalized push-pull signal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical drive device including an optical pickup, and more particularly to an optical drive device capable of reducing an offset generated in a tracking error signal.

  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. I do. The tracking servo will be briefly described below.

  FIG. 26A 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 regions E1 and E2. A region where the 0th-order diffracted light and the ± 1st-order diffracted light interfere with each other like the regions E1 and E2 is referred to as a “push-pull region”.

  Next, FIG. 27 is a diagram showing the light receiving surface 101 of the photodetector 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 light receiving surface 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 circular spot is drawn.

  As shown in FIG. 27, the light receiving surface 101 has a square shape and is vertically divided. As a result of such division, the upper light receiving area 101A receives the push-pull area E1, and the lower light receiving area 101B receives the push-pull area E2.

The photodetector that has received the light beam outputs, for each light receiving region, 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. 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 push-pull areas E1 and E2 is a value corresponding to the phase difference between the 0th-order diffracted light and the ± 1st-order diffracted light and the intensity of these diffracted lights. . 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. 26A) (hereinafter, this movement is referred to as track jump), the 0th-order diffracted light accompanies the movement. And the ± first-order diffracted light change in phase difference and intensity, and the light intensity in the push-pull regions E1 and E2 changes. As a result, each output signal also changes.

FIG. 26B 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 subtraction signal I _101A -I _101B (hereinafter referred to as push-pull signal PP) 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 push-pull signal PP, and the optical drive device outputs the push-pull signal PP as the tracking error signal TE, and the objective is set so that the value of the tracking error signal TE becomes zero. By controlling the position of the lens 100, the deviation of the focal position in the radial direction of the optical disk is adjusted.

  Incidentally, various offsets occur in the tracking error signal TE. Specifically, the boundary due to the offset due to the position movement (lens shift) of the objective lens by tracking servo, and the difference in reflectance between the recorded area (recording area) and the unrecorded area (unrecorded area). An offset occurring at (recording boundary) can be mentioned. In addition, in a multi-layered disc, an offset also occurs when a light beam (stray light) reflected by a layer other than the access target layer interferes with a light beam (signal light) reflected by the access target layer. Since such an offset causes an error in the tracking servo, it is required to reduce the offset from the tracking error signal TE.

  In Patent Document 1, the light flux of the signal light is divided into two, a photodetector is provided for each of the divided lights, and the difference of the push-pull signal PP obtained for each photodetector is used as the tracking error signal TE, thereby tracking error. A configuration is disclosed in which an offset due to an optical shift including the lens shift is reduced from the signal TE.

Patent Documents 2 and 3 disclose a configuration in which the offset due to the lens shift is reduced from the tracking error signal TE by a technique called a differential push-pull method. Similarly to the technique disclosed in Patent Document 1, the differential push-pull method also divides a light beam of signal light, provides a photodetector for each divided light, and provides a difference between push-pull signals PP obtained for each photodetector. Is used as the tracking error signal TE.
Japanese Patent Laid-Open No. 6-176181 (paragraph [0044] to [0050]) Japanese Patent Laying-Open No. 2005-346882 (Abstract) JP 2007-287232 A (Abstract)

  However, even with the configuration using a plurality of photodetectors as disclosed in Patent Documents 1 to 3, the amount of offset generated in the tracking error signal TE is still insufficient, and a new technique for reducing the offset is available. It has been demanded.

  Accordingly, an object of the present invention is to provide an optical drive device that can reduce the amount of offset generated in the tracking error signal TE.

  In order to achieve the above object, an optical drive device according to the present invention is symmetric with respect to a spot center of signal light, which is reflected light on an access target layer of a multilayered optical disc, and passes through the spot center and passes through the signal light tangent line. A photodetector having a first signal light receiving surface formed so as to be line symmetric with respect to a straight line parallel to the direction, and further divided into first A and first B signal light receiving regions by the straight line; A first push-pull signal and a first sum signal are generated based on the received light amounts of the 1A and 1B signal light receiving areas, and the first push-pull signal is normalized using the first sum signal. Tracking error signal generating means for generating a first normalized push-pull signal by generating and generating a tracking error signal based on the first normalized push-pull signal, And each width of the signal light tangent direction of the signal light receiving region of the 1B is characterized in that less than a diameter of the spot.

  According to the present invention, the amplitude of the first normalized push-pull signal is increased as compared with the case where the width in the signal light tangent direction of the 1A and 1B signal light receiving regions is longer than the diameter (spot diameter). This is because the push-pull area ratio becomes relatively large. That is, when the width of the signal light receiving area in the tangential direction of the signal light is narrowed, the area of the 0th order light is reduced, and the ratio of the 0th order light and the ± 1st order light generated in the land groove of the optical disc is occupied by the optical interference area. Therefore, the ratio of the 0th-order light component of the sum signal of the light receiving region which is the denominator is reduced. Accordingly, the amplitude of the first normalized push-pull signal is increased.

In general, the offset amount S _OFFSET of the signal S is expressed by the following equation (1) using the amplitude S _AMPLITUDE and the displacement amount S _DISPLACEMENT , so that the amplitude of the first normalized push-pull signal increases. Therefore, the offset amount of the first normalized push-pull signal is reduced. Therefore, the offset amount of the tracking error signal generated based on the first normalized push-pull signal is also reduced.

  In addition, it is preferable that each width | variety of the signal light tangent direction of the said 1A and 1B signal light light-receiving area | region is less than 20% of the said diameter. If it is less than 20%, the amplitude of the first normalized push-pull signal is stabilized, and at the same time, it is caused by the positional deviation of the spot in the signal light tangential direction to a level almost equal to the case of normalization with the signal of the entire beam diameter. Since an offset can be made, there is no problem in manufacturing.

  The optical drive device further includes a diffraction grating that divides a light beam applied to the multilayered optical disk into 0th-order diffracted light and ± 1st-order diffracted light, and the signal light is reflected light of the 0th-order diffracted light, and the light detection The device is formed so as to be point symmetric with respect to the spot center of the reflected light of the + 1st order diffracted light and to be symmetric with respect to a straight line passing through the spot center and parallel to the tangential direction of the signal light. The second signal light receiving surface divided into the second A and 2B signal light receiving regions and the point light symmetric with respect to the spot center of the reflected light of the −1st order diffracted light and passing through the spot center, the signal light A third signal light receiving surface formed so as to be line symmetric with respect to a straight line parallel to the tangential direction, and further divided into 3A and 3B signal light receiving regions by the straight line; 2 And the 3A signal light receiving area correspond to the area on the same side as the 1A signal light receiving area with the corresponding straight lines as boundaries, respectively, and the 2B and 3B signal light receiving areas correspond to each other. The tracking error signal generation means corresponds to the area on the same side as the 1B signal light receiving area with the respective straight lines as boundaries, and the tracking error signal generating means receives the 2A, 2B, 3A, and 3B signal light receiving areas. A second push-pull signal and a second sum signal are generated based on each received light amount of the region, and the second push-pull signal is normalized using the second sum signal, thereby generating a second normal signal. Of the second normalized push-pull signal, the tracking error signal based on the second normalized push-pull signal, the second A, second B, third A, and third B signal light in the light receiving region. Tangential Each width may be shorter than the diameter of the spot. According to this, when the differential push-pull method is used, the offset amount can be reduced for both the first and second normalized push-pull signals.

  Alternatively, in the optical drive device, the optical drive device further includes a diffraction grating that divides a light beam applied to the multilayered optical disc into 0th order diffracted light and ± 1st order diffracted light, and the signal light is reflected light of the 0th order diffracted light, The photodetector is formed so as to be point symmetric with respect to the spot center of the reflected light of the + 1st order diffracted light, and to be line symmetric with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. The second signal light receiving surface divided into the second A and second B signal light receiving regions by a straight line is point-symmetric with respect to the spot center of the reflected light of the −1st order diffracted light, and passes through the spot center. A third signal light receiving surface formed so as to be line symmetric with respect to a straight line parallel to the signal light tangential direction and further divided into third and third B signal light receiving regions by the straight line; The signal light receiving areas 2A and 3A correspond to areas on the same side as the signal light receiving areas 1A with the corresponding straight lines as boundaries, and the signal light receiving areas 2B and 3B. Corresponds to a region on the same side as the 1B signal light receiving region with each corresponding straight line as a boundary, and the tracking error signal generating means receives each light of the 2A and 2B signal light receiving regions. Generating a third push-pull signal and a third sum signal based on the quantity, and normalizing the third push-pull signal with the third sum signal to generate a third normalized push-pull signal; And a fourth push-pull signal and a fourth sum signal are generated based on the amounts of light received in the third and third signal light receiving areas, and the fourth push-pull signal is converted into the fourth push-pull signal. Normalize using sum signal Generating a fourth normalized push-pull signal, generating the tracking error signal based on the third and fourth normalized push-pull signals, and the second A, second B, third A, and Each width in the signal light tangent direction of the 3B signal light receiving region may be shorter than the diameter of the spot. Even in this case, when the differential push-pull method is used, the offset amount can be reduced for both the first and second normalized push-pull signals.

  In addition, it is preferable that each width | variety of the signal light tangent direction of the said 2A, 2B, 3A, and 3B signal light receiving area is less than 20% of the said diameter. If it is less than 20%, the amplitude of the second normalized push-pull signal is stabilized, and at the same time, compared with the case of normalization with the signal of the entire beam diameter, the position of the spot light in the tangential direction of the signal light is shifted. Since the resulting offset can be made, there is no problem in manufacturing.

  In the optical drive device described above, the widths of the first A, first B, second A, second B, third A, and third B signal light receiving regions in the signal light tangential direction may be the same. According to this, the offset amount can be reduced under the same conditions for each normalized push-pull signal.

  In each of the above optical drive devices, the photodetector has one or a plurality of stray light receiving regions arranged so as to be able to receive stray light that is reflected light from layers other than the access target layer, and the tracking error signal generating means Includes a first correction unit configured to correct at least one of the first push-pull signal or the first sum signal based on each received light amount of at least a part of the one or more stray light receiving regions. And the first normalized push-pull signal may be generated using each signal after correction by the first correction unit. According to this, the amount of offset generated in the tracking error signal due to stray light can be reduced.

  Further, in the optical drive device, the one or more stray light receiving regions include first A and second A stray light receiving regions provided on both sides of the first signal light receiving surface in the signal light radial direction. And the first light and the first stray light receiving area have the same width and position in the signal light tangent direction, and the first correction means includes the first push-pull signal. Alternatively, at least one of the first sum signals may be corrected based on the amount of received light in the first A and second A stray light receiving areas. According to this, since the influence of the recording boundary appearing in the stray light appears almost equally in the 1A and 2A signal light receiving areas and the 1A and 2A stray light receiving areas, there is a recording boundary in the stray light. Even if it appears, the amount of offset generated in the tracking error signal can be reduced.

  In the above optical drive device, the photodetector has one or a plurality of stray light receiving regions arranged to receive stray light that is reflected light from layers other than the access target layer, and the tracking error signal generating means includes: First correction means for correcting at least one of the second push-pull signal or the second sum signal based on the amount of light received in at least a part of the one or more stray light receiving regions. And the second normalized push-pull signal may be generated using each signal after correction by the first correction unit. According to this, even when the differential push-pull method is used, the amount of offset generated in the tracking error signal due to stray light can be reduced.

  Further, in the optical drive device, the one or more stray light receiving regions may include third and fourth A stray light receiving regions provided on both sides of the second signal light receiving surface in the signal light radial direction, and the third stray light receiving regions. 5A and 6A stray light receiving areas provided on both sides of the signal light receiving surface in the signal light radial direction, and the second A and 2B signal light receiving areas and the 3A and 4A stray light receiving areas. Each width and position in the signal light tangential direction are the same, and each width and position in the signal light tangent direction of the third light beam receiving area of 3A and 3B and the stray light light receiving area of 5A and 6A are the same. And the first correction means applies at least one of the second push-pull signal or the second sum signal to each of the received light amounts of the third A, fourth A, fifth A, and sixth A stray light receiving regions. As correcting based on Good. According to this, the 2A and 2B signal light receiving areas, the 3A and 4A stray light receiving areas, the 3A and 3B signal light receiving areas, and the 5A and 6A stray light receiving areas, Since the influence of the recording boundary appearing in the stray light appears almost equally, the amount of offset generated in the tracking error signal can be reduced even when the recording boundary appears in the stray light.

  Alternatively, in the optical drive device, the one or more stray light receiving areas may be a seventh A stray light receiving area provided between the first signal light receiving surface and the second signal light receiving surface, An 8A stray light receiving region provided between the first signal light receiving surface and the third signal light receiving surface, opposite to the seventh stray light receiving region across the second signal light receiving surface. A 3A stray light receiving area provided on the side, and a 6A stray light receiving area provided on the opposite side of the 8th stray light receiving area across the third signal light receiving surface. And the width and position of the signal light tangent direction of the 7A and 3A stray light receiving areas are the same, and the 3A and 3B signal light receiving areas and the 8A And the width and position in the signal light tangent direction with the 6A stray light receiving area are the same. And the tracking error signal generating means receives at least one of the second push-pull signal or the second sum signal as the amount of received light in each of the seventh A, eighth A, third A, and sixth A stray light receiving regions. The second normalization push-pull signal may be generated using each signal corrected by the first correction unit. According to this, the 2A, 2B, 3A, and 3B signal light receiving areas and the 7A, 8A, 3A, and 6A stray light receiving areas are affected by the recording boundary that appears in stray light. Appear substantially equally, it is possible to reduce the amount of offset generated in the tracking error signal even when a recording boundary appears in stray light.

  In each of the optical drive devices described above, the first signal light receiving surface includes first C and 1D signal light receiving areas provided on both sides of the first A signal light receiving area in the signal light tangential direction, 1B and 1F signal light receiving regions provided on both sides of the signal light receiving region in the signal light tangential direction, respectively, and the tracking error signal generating means includes the first normalized push-pull signal. Is corrected based on the amount of light received by each of the first C and first D signal light receiving areas and the first E and first F signal light receiving areas, and the second correcting means The tracking error signal may be generated using the corrected first normalized push-pull signal. According to this, even if the spot of the signal light is shifted in the signal light tangent direction, a suitable first normalized push-pull signal can be obtained.

  In each of the optical drive devices described above, an addition signal of the received light amounts of the first and first signal light receiving regions may be used as a pull-in signal used when focus servo is executed. According to this, when a multilayer optical disk is used, the interlayer separation of the pull-in signal becomes easy. Therefore, the focus servo is stabilized and the signal on the in-focus surface can be easily detected.

  In each of the optical drive devices described above, the first signal light receiving surfaces receive 1C and 1D signal light received at predetermined distances on both sides in the signal light tangential direction of the 1A signal light receiving area. The tracking error signal generating means further includes a first E and a first F signal light receiving areas provided at a predetermined distance from each other on both sides in the signal light tangential direction of the 1B signal light receiving area. The first push-pull signal and the first sum signal are generated based on the received light amounts of the first A and the first B signal light receiving areas, and the first C to F first signal light receiving areas are received. Generate a fifth push-pull signal and a fifth sum signal based on the quantity, and normalize the first and fifth push-pull signals by at least one of the first and fifth sum signals, respectively. It may generate the tracking error signal by Rukoto. This makes it possible to take advantage of the advantages of the 1A and 1B signal light receiving regions that receive light near the center of the spot and the advantages of the 1C to 1F signal light receiving regions that receive light near the outer edge of the spot, thereby reducing noise and offset. It is possible to generate a tracking error signal with less.

  An optical drive device according to another aspect of the present invention includes a diffraction grating that divides a light beam applied to a multilayered optical disk into 0th-order diffracted light and ± 1st-order diffracted light, and a spot center of reflected light of the + 1st-order diffracted light. It is symmetrical with respect to a straight line that passes through the center of the spot and is parallel to the signal light tangential direction, and is further divided into the second and second signal light receiving regions by the straight line. It is point-symmetric with respect to the second signal light receiving surface and the spot center of the reflected light of the minus first-order diffracted light, and is line-symmetric with respect to a straight line passing through the spot center and parallel to the signal light tangential direction And a photodetector having a third signal light receiving surface formed by the straight line and divided into third A and 3B signal light receiving regions by the straight line, and the second signal light receiving region and the third A light receiving region. Signal light receiving area Corresponds to a region on the same side with each corresponding straight line as a boundary, and the 2B signal light receiving region and the 3B signal light receiving region are on the same side with each corresponding straight line as a boundary. The second push-pull signal is generated based on the received light amounts of the 2A, 2B, 3A, and 3B signal light receiving areas, and is based on the second push-pull signal. Tracking error signal generating means for generating a tracking error signal, and the width of each of the 2A, 2B, 3A, and 3B signal light receiving regions in the signal light tangential direction is shorter than the diameter of each spot. It is characterized by. According to this, the offset of the tracking error signal TE caused by the interference between the stray light of the main beam MB and the sub-beams SB1 and SB2 in the 2A, 2B, 3A, and 3B signal light receiving areas can be reduced.

  Further, in the optical drive device, the tracking error signal generating means generates a second sum signal based on the amount of light received in the second A, second B, third A, and third B signal light receiving regions, A second normalized push-pull signal is generated by normalizing the second push-pull signal using the second sum signal, and a tracking error signal is generated based on the second normalized push-pull signal. It is good to do. According to this, when the differential push-pull method is used, the offset for the second normalized push-pull signal can be reduced.

  In the optical drive device, the tracking error signal generating means generates a third sum signal based on the received light amounts of the second and second signal light receiving areas, and the second push-pull signal. The third normalized push-pull signal is generated by normalizing the components based on the received light amounts of the second A and second B signal light receiving areas using the third sum signal, and generating the third A , A fourth sum signal is generated based on the respective received light amounts of the 3B signal light receiving region, and based on the received light amounts of the third A and 3B signal light receiving regions of the second push-pull signal. A component is normalized using the fourth sum signal to generate a fourth normalized push-pull signal, and the tracking error signal is generated based on the third and fourth normalized push-pull signals. And It may be. According to this, when the differential push-pull method is used, the offset for the third and fourth normalized push-pull signals can be reduced.

  According to the present invention, the amount of offset generated in the tracking error signal TE can be reduced in an optical recording medium including a multilayer optical recording medium.

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

  FIG. 1 is 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. In this embodiment, a disc-shaped optical disc having a recording surface multilayered 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 diffraction grating 21, 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.

  First, in the forward optical system, the diffraction grating 21 decomposes the light beam emitted from the laser light source 2 into three beams (0th-order diffracted light and ± 1st-order diffracted light) and 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 converts the light beam incident from the beam splitter 22 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. This return light beam is diffracted on the recording surface and is decomposed into 0th order diffracted light and ± 1st order diffracted light as described with reference to FIG. The 0th order diffracted light and the ± 1st order diffracted light are different from the 0th order diffracted light and ± 1st order diffracted light generated by the diffraction grating 21 and are confusing. The −1st order diffracted light is referred to as main beam MB, subbeam SB1, and subbeam SB2, respectively. In the case of 0th order diffracted light and ± 1st order diffracted light, it refers to diffracted light generated by diffraction on the recording surface. The main beam MB, sub beam SB1, and sub beam SB2 each independently generate reflected light having a push-pull region as described with reference to FIGS.

  Next, in the return path optical system, 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 objective lens 4 is arranged so that the joint point of the light beam (signal light) reflected by the layer to be focused (access target layer) is positioned on the photodetector 5. Position control is performed (focus servo). 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 is installed on a plane that intersects the optical path of the return light beam emitted from the optical system 3. The photodetector 5 includes a large number of light receiving surfaces, and each light receiving surface is divided into a large number of light receiving regions. In the optical drive device 1, the tracking error signal TE can be generated by various generation processes by appropriately combining these light receiving regions. The specific contents will be described later.

  The processing unit 6 is configured by a DSP (Digital Signal Processor) having an A / D conversion function that converts analog signals for multiple channels into digital data as an example, and receives an output signal from the photodetector 5, Various signals such as a tracking error signal TE, a focus error signal FE, and a pull-in signal PI are generated. Details of the processing of the processing unit 6 will also be described later.

  The CPU 7 is a processing device incorporated in a computer, a DVD recorder, or the like, and transmits an instruction signal for specifying an access position on the optical disc 11 to the processing unit 6 via an interface (not shown). Receiving this instruction signal, the processing unit 6 controls the objective lens 4 and moves it parallel to the surface of the optical disk 11 (this movement is called “lens shift”), thereby realizing a track-on state (tracking servo). . In the track-on state, the CPU 7 acquires an RF signal generated by the processing unit 6 (a total signal of received light amounts of light receiving areas in a main beam light receiving surface S1a described later) as a data signal.

  Here, lens shift and stray light will be described in detail using a photodetector (FIG. 27) according to the background art. In this description, attention is focused only on the main beam MB.

  First, FIGS. 3A and 3B show spots of the main beam MB irradiated on the light receiving surface 101 of the photodetector according to the background art. In FIG. 3 and FIGS. 5 and 6 described later, contour lines of light intensity are shown in the spots. Further, the light beam spot has directions corresponding respectively to the tangential direction and the radial direction of the optical disc 11, and in the following, the direction corresponding to the tangential direction (signal light tangent direction) is defined as the X axis with respect to the spot of the main beam MB. A direction corresponding to the radial direction (signal light radial direction) is referred to as a Y-axis.

  The light receiving surface 101 has a square shape and is symmetric with respect to the spot center of the main beam MB and is symmetrical with respect to a straight line passing through the spot center and parallel to the signal light tangential direction (X axis). ing. Further, the light receiving areas 101A and 101B are divided by this straight line. Each diagonal line of the light receiving surface 101 coincides with the MX axis direction and the MY axis direction described above.

  FIG. 3A shows an example of a signal light spot when there is no lens shift, and FIG. 3B shows an example of a signal light spot that has moved to the maximum by the lens shift. As shown in the figure, the spot of the main beam MB moves in the Y-axis direction with lens shift. The size of the light receiving surface 101 is determined so that the entire spot of the main beam MB can be received even if this movement occurs. 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 spot of the main beam MB are parallel to the Y axis and the X axis, respectively.

  Next, FIG. 4 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. And the lens shift direction line and the lens shift reference line of the spot of the stray light x-y are represented as LDx-y and LBx-y, respectively.

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

  FIG. 5 shows spots formed on the light receiving surface 101 for each main beam MB and stray light when there is no lens shift (shift amount of the objective lens 4 = 0). As shown in each drawing of FIG. 5, the spot formed by each stray light is larger than the size of the light receiving surface 101 and protrudes greatly from the light receiving surface 101.

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

  As shown in FIG. 5, the lens shift direction line and the lens shift reference line of each stray light spot are oblique compared to the lens shift direction line LDS and the lens shift reference line LBS of the spot of the main beam MB due to the influence of the sensor lens 25. Leaning on.

  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. 5A and 5B). 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. 5D and 5E).

  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. 6 shows each spot moved in a certain direction by lens shift in correspondence with each figure in FIG. As shown in the figure, the stray light spot is also moved by lens shift similarly to the spot of the main beam MB, and the center of intensity of the stray light spot (black point in the figure) is located on the lens shift direction line.

  FIG. 7A is a diagram illustrating the spot of the main beam MB, the stray light spot, and the respective lens shift direction lines and lens shift reference lines shown in each drawing of FIG. Similarly, FIG. 7B illustrates the spot of the main beam MB, 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. 7 (a) and 7 (b). As shown in FIGS. 7A and 7B, each spot is irradiated on the photodetector 5 in an overlapping manner.

  Now, details of the configuration of the photodetector 5 and details of the processing of the processing unit 6 will be described.

  First, FIG. 8 is a top view of the photodetector 5 according to the present embodiment, and shows a light receiving surface and a light receiving region. In the figure, a spot of signal light is also drawn. As shown in the figure, the photodetector 5 has a total of nine light receiving surfaces including a main beam light receiving surface S1a, sub-beam light receiving surfaces S2a and S3a, and stray light receiving surfaces S1b to S6b.

  The main beam light receiving surface S1a is a square whose side is x (≧ spot diameter r = 50 μm), is point-symmetric with respect to the spot center of the main beam MB, passes through the spot center, and is in the tangential direction of the signal light It is formed so as to be line symmetric with respect to a straight line B1 parallel to. In general, the value of x is about twice the spot diameter. This is because the area necessary for receiving the signal of the entire spot when performing the focus control using the astigmatism method is about twice the spot diameter.

  Further, the main beam light receiving surface S1a is divided in the vertical direction of the drawing by a straight line B1, a light receiving region 1Aa is provided at the lower center of the drawing, and light receiving regions 1Ca and 1Da are provided on both sides in the signal light tangent direction of the light receiving region 1Aa. Is provided. In addition, a light receiving region 1Ba is provided at the upper center of the drawing, and light receiving regions 1Ea and 1Fa are provided on both sides of the light receiving region 1Ba in the signal light tangent direction.

  The widths of the light receiving area 1Aa and the light receiving area 1Ba in the signal light tangent direction are both w (<spot diameter), and the positions in the signal light tangential direction are the same. The widths of the light receiving areas 1Ca, 1Da, 1Ea, and 1Fa in the signal light tangent direction are the same.

  In addition, the light receiving areas 1Ca and 1Da are separated from the light receiving area 1Aa by a predetermined distance g1 (≧ 0). Similarly, the light receiving areas 1Ea and 1Fa are also separated from the light receiving area 1Ba by a predetermined distance g1.

  The sub beam receiving surface S2a is a square having the same size as the main beam receiving surface S1a, is symmetric with respect to the spot center of the reflected light of the sub beam SB1, passes through the spot center, and is a straight line B2 parallel to the signal light tangential direction. It is formed so as to be line symmetric with respect to. In addition, the sub-beam light receiving surface S2a is divided into upper and lower portions with the straight line B2 as a boundary, as with the main beam light receiving surface S1a, and each region divided vertically is divided in the same manner as the main beam light receiving surface S1a. Light receiving areas 2Aa to 2Fa are provided corresponding to the light receiving areas 1Aa to 1Fa, respectively.

  The sub-beam light receiving surface S2a is disposed at a position shifted by d1 in the signal light tangential direction with respect to the main beam light receiving surface S1a. This is because the spot positions of the main beam MB and the sub beam SB1 are shifted by d1 in the signal light tangent direction in the present embodiment, but the magnitude of the beam shift varies depending on how the optical system 3 is configured. .

  The sub beam receiving surface S3a is disposed on the opposite side of the sub beam receiving surface S2a with the main beam receiving surface S1a interposed therebetween. Specifically, the sub-beam light receiving surface S3a is also a square having the same size as the main beam light-receiving surface S1a, is symmetric with respect to the spot center of the reflected light of the sub beam SB2, passes through the spot center, and is tangential to the signal light. It is formed so as to be line symmetric with respect to a straight line B3 parallel to. Further, the sub-beam light receiving surface S3a is also divided into upper and lower parts like the main beam light receiving surface S1a with the straight line B3 as a boundary, and the respective areas divided up and down are also divided in the same manner as the main beam light receiving surface S1a. Light receiving areas 3Aa to 3Fa are provided corresponding to the light receiving areas 1Aa to 1Fa, respectively.

  Similarly to the sub beam light receiving surface S2a, the sub beam light receiving surface S3a is also arranged at a position shifted by d1 in the signal light tangential direction with respect to the main beam light receiving surface S1a. However, the direction of deviation is opposite to that of the sub-beam light receiving surface S2a.

  Here, the main beam receiving surface S1a and the sub beam receiving surfaces S2a and S3a are square, but the shape of each light receiving surface is not limited to a square.

  The stray light receiving surface S1b is a rectangle having the same width and position in the signal light tangential direction as that of the main beam receiving surface S1a. The stray light receiving surface S1b has a predetermined width on one side (upper side in the drawing) of the main beam receiving surface S1a in the signal light radial direction. They are separated by a distance g2 (≧ 0). The light receiving area 1Ab has the same width and position in the signal light tangent direction as the light receiving area 1Aa and the like, and the light receiving areas 1Bb and 1Cb provided on both sides of the light receiving area 1Ab in the signal light tangential direction, respectively. The width and position of the light receiving areas 1Bb and 1Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving regions 1Bb and 1Cb are provided apart from the light receiving region 1Ab by a predetermined distance g1.

  The stray light receiving surface S2b is a rectangle having the same width and position in the signal light tangential direction as that of the main beam receiving surface S1a, and on the other side (lower side of the drawing) of the main beam receiving surface S1a in the signal light radial direction. They are separated by a predetermined distance g2. The light receiving area 2Ab has the same width and position in the signal light tangent direction as the light receiving area 1Aa and the like, and light receiving areas 2Bb and 2Cb provided on both sides of the light receiving area 2Ab in the signal light tangential direction, respectively. The width and position of the light receiving areas 2Bb and 2Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving regions 2Bb and 2Cb are provided apart from the light receiving region 2Ab by a predetermined distance g1.

  The stray light receiving surface S3b is a rectangle having the same width and position in the signal light tangential direction as that of the sub beam receiving surface S2a, and a predetermined distance g2 on one side of the sub beam receiving surface S2a in the signal light radial direction (upper side in the drawing). Are provided only apart. The light receiving region 3Ab has the same width and position in the signal light tangent direction as the light receiving region 2Aa and the like, and light receiving regions 3Bb and 3Cb provided on both sides of the light receiving region 3Ab in the signal light tangent direction, respectively. The width and position of the light receiving areas 3Bb and 3Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving areas 3Bb and 3Cb are provided apart from the light receiving area 3Ab by a predetermined distance g1.

  The stray light receiving surface S4b is a rectangle having the same width and position in the signal light tangential direction as that of the sub beam receiving surface S2a. The stray light receiving surface S4b has a predetermined distance on the other side in the signal light radial direction of the sub beam receiving surface S2a. They are separated by g2. The light receiving region 4Ab has the same width and position in the signal light tangential direction as the light receiving region 2Aa and the like, and light receiving regions 4Bb and 4Cb provided on both sides of the light receiving region 4Ab in the signal light tangential direction, respectively. The width and position of the light receiving areas 4Bb and 4Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving areas 4Bb and 4Cb are provided apart from the light receiving area 4Ab by a predetermined distance g1.

  The stray light receiving surface S5b is a rectangle having the same width and position in the signal light tangential direction as that of the sub-beam light receiving surface S3a. Are provided only apart. The light receiving area 5Ab has the same width and position in the signal light tangent direction as the light receiving area 3Aa and the like, and light receiving areas 5Bb and 5Cb provided on both sides of the light receiving area 5Ab in the signal light tangential direction, respectively. The width and position of the light receiving areas 5Bb and 5Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving areas 5Bb and 5Cb are provided apart from the light receiving area 5Ab by a predetermined distance g1.

  The stray light receiving surface S6b is a rectangle having the same width and position in the signal light tangential direction as that of the sub beam receiving surface S3a. The stray light receiving surface S6b has a predetermined distance on the other side in the signal light radial direction of the sub beam receiving surface S3a. They are separated by g2. The light receiving area 6Ab has the same width and position in the signal light tangent direction as the light receiving area 3Aa and the like, and light receiving areas 6Bb and 6Cb provided on both sides of the light receiving area 6Ab in the signal light tangential direction, respectively. The width and position of the light receiving areas 6Bb and 6Cb in the signal light tangent direction are the same as the width and position of the light receiving areas 1Ca and 1Da, respectively. The light receiving areas 6Bb and 6Cb are provided apart from the light receiving area 6Ab by a predetermined distance g1.

The photodetector 5 outputs, for each light receiving region, a signal having an amplitude of a value (light receiving amount) obtained by dividing the intensity of the light beam by the area of the 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 configuration of the photodetector 5 has been described in detail above. Next, FIG. 9 is a diagram showing functional blocks of the processing unit 6. As shown in the figure, the processing unit 6 includes a tracking error signal generation unit 61 (tracking error signal generation unit), a focus error signal generation unit 62 (focus error signal generation unit), and a pull-in signal generation unit 63 (pull-in signal generation unit). And the objective lens control unit 64, and the tracking error signal generation unit 61 includes a first correction unit 65 (first correction unit) and a second correction unit 66 (second correction unit).

  The tracking error signal generator 61 generates a tracking error signal TE based on each output signal of the photodetector 5. The specific generation process is diverse, and will be described later.

  The focus error signal generation unit 62 generates a focus error signal FE based on each output signal of the photodetector 5. Details of this generation processing will also be described later.

  The pull-in signal generation unit 63 (pull-in signal generation means) generates a pull-in signal PI based on each output signal of the photodetector 5. This pull-in signal PI is generally a total signal of the total amount of light received by the main beam light receiving surface S1a, and is used by the objective lens control unit 64 during focus servo. It can also be used as a signal for detecting a recording surface signal. Details of this generation processing will also be described later.

  The objective lens control unit 64 generates a control signal for the objective lens 4 based on the tracking error signal TE generated by the tracking error signal generation unit 61 and outputs the control signal to an actuator (not shown) for position control of the objective lens 4. (Tracking servo). Further, the pull-in signal generated by the pull-in signal generation unit 63 is monitored, and when the value of the pull-in signal exceeds a predetermined value, the objective lens 4 is controlled based on the focus error signal FE generated by the focus error signal generation unit 62. A control signal is generated and output to an actuator (not shown) for position control of the objective lens 4 (focus servo).

  Hereinafter, signal generation by each signal generation unit will be described. In the following description, first, the tracking error signal generation processing of the tracking error signal generation unit 61 will be described with reference to first to sixth generation processing. Thereafter, signal generation processing of the focus error signal generation unit 62 and the pull-in signal generation unit 63 will be described.

[First generation process]
In the first generation process, only the light receiving areas 1Aa and 1Ba are used. The tracking error signal generation unit 61 generates a main push-pull signal MPP (first push-pull signal) and a main sum signal SUMm (first sum signal) based on the received light amounts of the light receiving regions 1Aa and 1Ba. The normalized main push-pull signal MPPN (first normalized push-pull signal) is generated by normalizing the pull signal MPP using the main sum signal SUMm, and the tracking error signal is generated based on the normalized main push-pull signal MPPN. Generate TE.

  Specifically, the tracking error signal generation unit 61 generates the tracking error signal TE by performing calculations of the following formulas (2) to (5).

  The amplitude of the normalized main push-pull signal MPPN generated by this generation process increases as the width w of the light receiving regions 1Aa and 1Ba decreases. This is because the push-pull area ratio is relatively increased as the width w is decreased.

  FIG. 10 is a diagram in which the normalized main push-pull signal MPPN generated by the generation process is acquired for each width w by simulation and plotted with respect to the disk position (μm). This figure shows the case of widths w = 10 μm and 20 μm and the case of w ≧ 50 μm (= spot diameter) as a comparative example. The optical magnification of the optical system 3 was 15 times, the optical disk 11 was a single-layer optical disk with a track pitch of 0.32 μm and a groove depth of 0.02 μm, and the length x of one side of the main beam light receiving surface S1a was 100 μm. Further, the main push-pull signal MPP and the main sum signal SUMm were obtained by dividing the intensity of the main beam MB in the light receiving areas 1Aa and 1Ba, respectively.

  10 that the amplitude of the normalized main push-pull signal MPPN increases as the width w decreases.

  As described above, since the signal offset amount is generally expressed by the above-described equation (1), the normalized main push-pull signal can be obtained by reducing the width w and increasing the amplitude of the normalized main push-pull signal MPPN. The offset amount of the signal MPPN is reduced. Therefore, by reducing the width w, the offset amount of the tracking error signal TE generated by the equation (5) is also reduced.

  When the optical disk 11 is a multilayer optical disk, the offset displacement amount of the normalized main push-pull signal MPPN decreases as the width w decreases. That is, when the optical disk 11 is a multilayer optical disk, stray light is received in each light receiving region in addition to signal light. The normalized main push-pull signal MPPN is displaced by this amount of stray light. However, the smaller the width w, the smaller the ratio of the received light amount to the received light amount. This is because the received light amount of stray light may be considered to be substantially uniform over the entire light receiving region S1a, but the received light amount of the signal light increases as it is closer to the center of the light receiving region S1a. Therefore, the smaller the width w, the smaller the offset displacement amount of the normalized main push-pull signal MPPN. As a result, the offset amount of the normalized main push-pull signal MPPN expressed by the above equation (1) is further reduced. Is done.

  Here, FIG. 11 is a diagram in which the main push-pull signal MPP is plotted under the same conditions as in FIG. As shown in the figure, the amplitude of the main push-pull signal MPP decreases as the width w decreases. However, if normalization is not performed, it is difficult to apply in practice because it is subject to reflectance fluctuations. This is because, in order to reduce the offset amount of the tracking error signal TE by reducing the width w, the normalized main push-pull signal MPPN must be used as the tracking error signal TE instead of the main push-pull signal MPP. It shows that. That is, when the main push-pull signal MPP which is not normalized is used, when a signal is amplified, a noise component newly generated in an amplifier circuit or a transmission path is also amplified, and this noise component appears as an offset. This is because, depending on the noise component, the offset amount of the tracking error signal TE expressed by the equation (1) eventually increases.

  FIG. 12 is a diagram in which the amplitudes of the main push-pull signal MPP and the normalized main push-pull signal MPPN are plotted with the width w being further varied. The simulation conditions are the same as in FIG. However, the main push-pull signal MPP that has not been normalized is displayed with the amplitude of the main push-pull signal MPP set to 35% in consideration of fluctuations in amplitude due to recording boundaries and other reflectance fluctuations. It can also be seen from the figure that the normalized main push-pull signal MPPN increases and the main push-pull signal MPP decreases as the width w is reduced.

  In addition, according to FIG. 12, when the width w is less than 10 μm (20% of the spot diameter of 50 μm), the amplitude of the normalized main push-pull signal MPPN is almost saturated and stabilized. Therefore, in order to realize a stable tracking servo, the width w is preferably less than 20% of the spot diameter. However, since manufacturing becomes difficult if the width w is too small, it is preferable that the width w be made manufacturable.

  Further, by making the width w as small as possible, it is possible to reduce the offset caused by the positional deviation of the spot of the main beam MB in the signal light tangential direction. This is because the change in the amount of received light when the spot is shifted is small.

[Second generation process]
In the second generation process, the light receiving areas 1Aa, 1Ba, 2Aa, 2Ba, 3Aa, and 3Ba are used. This generation process uses a so-called differential push-pull method. The tracking error signal generation unit 61 generates the normalized main push-pull signal MPPN in the same manner as in the first generation process, and also normalizes the sub push-pull signal SPPN (second normalized push-pull signal) as follows. ) Is generated.

  That is, the tracking error signal generator 61 generates a sub push-pull signal SPP (second push-pull signal) and a sub sum signal SUMs (second sum signal) based on the amounts of light received in the light receiving regions 2Aa, 2Ba, 3Aa, and 3Ba. ) And normalize the sub push-pull signal SPP using the sub sum signal SUMs to generate a normalized sub push pull signal SPPN (second normalized push pull signal).

  Specifically, the tracking error signal generation unit 61 performs calculations of the following formulas (6) to (8) to generate a normalized sub push-pull signal SPPN.

  Next, the tracking error signal generation unit 61 generates a tracking error signal TE based on the normalized main push-pull signal MPPN and the normalized sub push-pull signal SPPN. Specifically, as shown in the following equation (9), the normalized sub push-pull signal SPPN is multiplied by a constant k and subtracted from the normalized main push-pull signal MPPN. The resulting signal is referred to as a tracking error signal TE. When the main and sub are normalized separately, normally k = 1, but this k is determined so that the offset at the time of shifting the main and sub lenses is cancelled.

The spot shifts in the radial direction of the signal light (up and down direction in FIG. 8) by the lens shift, but the movement direction is the same for the main beam MB and the sub beams SB1 and SB2. Is moved, the light receiving amount of the light receiving areas 1Ba, 2Ba, 3Ba is increased, and the light receiving amount of the light receiving areas 1Aa, 2Aa, 3Aa is decreased. As a result, the output signal I _1Ba increases, while the output signal I _1Aa decreases, and an offset in the increasing direction occurs in the main push-pull signal MPP. Similarly, while the output signals I _2Ba and I _3Ba increase, the output signals I _2Aa and I _3Aa decrease and an offset in the increasing direction also occurs in the sub push-pull signal SPP. Therefore, it is possible to cancel these changes by using Expression (9). Note that changes due to lens shift hardly appear in the main sum signal SUMm and the sub sum signal SUMs.

  On the other hand, in the main beam MB and the sub beams SB1 and SB2, since the vertical relationship of the push-pull region is inverted, the phases of the main push-pull signal MPP and the sub push-pull signal SPP are different from each other by 180 °. Therefore, the main push-pull signal MPP and the sub push-pull signal SPP do not cancel each other according to the equation (9), and the constant k is set so that the offset between the main push-pull signal MPP and the sub push-pull signal SPP is canceled when the lens is shifted. By determining in advance, it becomes possible to perform tracking servo according to equation (9).

  As in the normalized main push-pull signal MPPN, the amplitude of the normalized sub push-pull signal SPPN increases as the width w of each light receiving region decreases. Therefore, by reducing the width w, the offset of the normalized sub push-pull signal SPPN is reduced, and as a result, the offset amount of the tracking error signal TE generated by Expression (9) is also reduced.

  Here, the effect of reducing the offset amount of the tracking error signal TE is conspicuous when the sub beams SB1 and SB2 interfere with stray light of the main beam MB during lens shift. Hereinafter, this point will be described.

  FIG. 13 is a diagram illustrating a result of simulating the offset amount of the tracking error signal TE generated by Expression (9). Although details will be described later, in calculating the tracking error signal TE, it is assumed that the main push-pull signal MPPN on the right side of Equation (9) has no influence of stray light. On the other hand, for the sub push-pull signal SPPN on the right side of Equation (9), two types of simulations are performed, assuming that there is interference between the stray light of the main beam MB and the sub beams SB1 and SB2, and assuming no interference. went. Thereby, in FIG. 13, it is possible to confirm the difference in the offset amount of the tracking error signal TE depending on the presence or absence of interference between the stray light of the main beam MB and the sub beams SB1 and SB2. In FIG. 13, the horizontal axis represents the width w and the vertical axis represents the percentage.

  In this simulation, the amount of lens shift was changed from 0 mm to 0.3 mm in increments of 0.05 mm, and for each lens shift amount, the magnitude of the offset generated in the tracking error signal TE was obtained by equation (1). In FIG. 13, the maximum value of the offset amount when offset in the plus direction and the maximum value of the offset amount when offset in the minus direction are plotted. In addition, the simulation results when there is interference for each and the simulation results when there is no interference are shown. In addition, the optical magnification of the optical system 3 was 15 times, the optical disk 11 was a two-layer optical disk with an interlayer distance of 10 μm, and the length x of one side of the main beam light receiving surface S1a was 100 μm.

  Each signal was obtained as follows. That is, first, the main push-pull signal MPP and the main sum signal SUMm were obtained by dividing the intensity of the main beam MB by the light receiving regions 1Aa and 1Ba, respectively. Note that the stray light component and the interference component are not taken into account when the main push-pull signal MPP and the main sum signal SUMm are obtained. That is, the main push-pull signal MPP and the main sum signal SUMm are ideal signals that are not affected by stray light.

Next, a signal in which the amplitudes of the main push-pull signal MPP and the main sum signal SUMm are respectively reduced to 1/10 is obtained, and the push-pull signal PP _STRAY and the sum signal SUM _STRAY generated by stray light of the main beam MB are added, respectively. The main push-pull signal MPP ′ and the main sum signal SUMm ′ when there is no interference are obtained and used as the sub push-pull signal SPP and the sub sum signal SUMs when there is no interference. Specifically, it calculated | required by Formula (10) and Formula (11). Note that the push-pull signal PP _STRAY and the sum signal SUM _STRAY were determined by dividing the stray light intensity of the main beam MB by the light receiving areas 1Aa and 1Ba, respectively.

On the other hand, if there is interference, a simulation of interference between the main push-pull signal MPP and the main sum signal SUMm having a 1/10 amplitude and stray light is performed, and the main push-pull signal MPP ″ and the main sum signal SUMm ″. And used as the sub push-pull signal SPP and the sub sum signal SUMs when there is interference. Approximately, it can be written in a form in which interference components are added to Expressions (10) and (11), and is expressed as Expressions (12) and (13). Here, φ ₁ and φ ₂ are phase differences between the interference lights.

  If there is no interference, as shown in FIG. 13, the offset amount remains at a low level regardless of the width w, and there is almost no change. On the other hand, if there is interference, an extremely large offset amount occurs when w ≧ 50 μm, but the offset amount decreases as the width w decreases. The reason why the offset amount behaves in this way is as follows.

The interference component of Expression (12) is ΔMPP = ((1/10) · I _1Ba · I _{1Ba_STRAY} ) ^0.5 · cos φ ₁ − ((1/10) · I _1Aa · I _{1Aa_STRA y} ) ^0.5 · cos φ ₂ is there. Reducing the width w, component _I 1Ba of the main beam MB of _DerutaMPP, to _more than _{I 1Aa} is reduced, the stray light component _{I 1Ba_STRAY,} is _{I 1Aa_STRAY} decreases. This is a cause of reducing the offset amount caused by interference by reducing the width w. For simplicity, for example, when the stray light component of the sum signal is corrected by using the stray light receiving surface and the sum signal does not have the stray light component, that is, SUMm ″ ≈SUMm / 10, The signal is only a factor that reduces the intensity of the main beam MB. If the stray light components I _{1Ba_STRAY} and I _{1Aa_STRAY of ΔMPP} are also reduced by the same ratio as that of the main beam MB, since the square root of multiplication is obtained, the reduced ratio becomes equal between ΔMPP and the sum signal, and the influence of interference is width w. It does not change even if it is made smaller. However, since the stray light component becomes smaller as described above, the offset displacement amount caused by the influence of the interference decreases when the width w is decreased, and the offset amount decreases. This is because the stray light intensities I _{1Ba_STRAY} and I _{1Aa_STRAY} can be considered to have a constant intensity distribution locally, and when the width w is reduced, the ratio is substantially reduced by that ratio, whereas the intensities I _1Ba and I _{1Aa of the} main beam MB are _reduced. This is because, from the Gaussian distribution, the intensity of the central portion of the spot light size is increased, and therefore the ratio does not decrease with the width w reduced.

Further, in DerutaMPP, when decreasing the width w, the _local, since regarded _{I 1Ba_STRAY} ≒ _{I 1Aa_STRAY,} and φ ₁ ≒ φ _2, may be referred to variable factors during lens shift is small.

  Further, since the interference component of Expression (12) has a shape including the square root of the product of the sub beam intensity and the stray light intensity of the main beam, the intensity ratio of the main beam and the sub beam is k (here, k = 10). As a result, in Equation (9), the interference component between the sub-beam signal component and the main beam stray light component can be written in a form multiplied by the square root of k as compared with the interference component between the main beam signal component and the main beam stray light component. Therefore, it can be seen that when the intensity ratio k between the main beam and the sub beam is large, the influence of the interference is greater in the sub beam receiving region than the main beam receiving region, being multiplied by the square root of k. In the above description, by reducing the width w, the push-pull amplitude of the tracking error signal can be increased, and as a result, the offset amount can be reduced. Therefore, the width w is reduced in the patterns of both the main beam receiving area and the sub beam receiving area. It was more effective to do. By the way, when considering the offset of the tracking error signal generated at the time of lens shift, the stray light of the main beam loses symmetry between the two sub-beam light-receiving areas when the lens shift occurs, so the interference component with the sub-beam signal light is symmetric. It is considered that the loss of the value causes an offset. In other words, from the viewpoint that the main beam stray light and the interference of the sub beam signal are dominant, especially when the intensity ratio k between the main beam and the sub beam is large, the influence of the interference is greater in the sub beam receiving region. Since the light receiving area is multiplied and multiplied by the square root of k, the offset amount can be sufficiently reduced even if the width w is reduced only by the sub-pattern. However, interference between the main beam and stray light of the main beam may be caused by a decrease in the intensity ratio of the main beam and the sub beam, or the stray light intensity due to the main beam MB increases due to multilayering of the optical disc even in the main pattern. If the offset displacement amount due to is increased, it is better to reduce the interference offset displacement amount by reducing the width w of the main pattern as well as the sub-pattern.

  The reduction of the interference offset displacement amount by reducing the width w can also be explained from the results of FIGS. FIG. 12 shows the dependency of the push-pull amplitude on the width w, but the interference offset amount in FIG. 13 is smaller than the ratio at which the push-pull amplitude is increased by reducing the width w. This also shows that by reducing the width w, the interference offset displacement amount is reduced and the interference offset amount is reduced in addition to the factor that increases the push-pull amplitude.

  As described above, the influence of the interference is verified by setting the signal intensity to 1/10 using the main pattern to be the same as the sub beam intensity. From this, when the intensity ratio of the main beam and the sub beam is large, the tracking error signal TE It can be said that the effect of reducing the offset amount appears significantly when the sub beams SB1 and SB2 interfere with the stray light of the main beam MB. Although the case of normalization has been described here, the same can be said for the case of no normalization.

  By the way, since the above simulation is merely for the purpose of seeing the influence of interference, interference with stray light in the main pattern has been observed. However, in practice, it is necessary to simulate sub beams SB1 and SB2 and stray light in the sub pattern. FIG. 14 is a diagram illustrating a result of obtaining the offset amount of the tracking error signal TE by the simulation for obtaining each signal as described above. In this case, the simulation considering the interference component between the sub beams SB1 and SB2 and the stray light cannot be performed.

  As can be understood from FIG. 14, in this simulation, the offset amount remains at a low level regardless of the width w and hardly changes. From this result, it was confirmed that reducing the width w does not adversely affect the offset.

  The tracking error signal generator 61 may normalize the push-pull signal for each light receiving surface. In this case, the tracking error signal generation unit 61 generates a sub push-pull signal SPP2 (third push-pull signal) and a sub sum signal SUMs2 (third sum signal) based on the amounts of light received by the light receiving regions 2Aa and 2Ba. Then, the sub push-pull signal SPP2 is normalized using the sub sum signal SUMs2, thereby generating a normalized sub push-pull signal SPPN2 (third normalized push-pull signal). Further, the sub push-pull signal SPP3 (fourth push-pull signal) and the sub-sum signal SUMs3 (fourth sum signal) are generated based on the received light amounts of the light-receiving regions 3Aa and 3Ba, and the sub-push-pull signal SPP3 is used as the sub-sum signal. Normalized sub push-pull signal SPPN3 (fourth normalized push-pull signal) is generated by normalization using SUMs3. Then, the tracking error signal TE is generated based on the normalized sub push-pull signal SPPN2 and the normalized sub push-pull signal SPPN3.

  Specifically, the tracking error signal generator 61 generates the normalized sub push-pull signals SPPN2 and SPPN3 by performing calculations of the following equations (14) to (20). In equation (20), k is usually 1/2, but this k is determined so as to cancel the offset at the time of shifting the main lens and the sub lens.

  Even when the tracking error signal TE is generated as described above, the offset generated in the tracking error signal TE can be reduced as described above.

  Further, even when the tracking error signal TE is generated by the equation (21) instead of the equations (9) and (20), the offset generated in the tracking error signal TE can be reduced as described above. Here, the constant k is usually a value determined by the intensity ratio of the main signal and the sub signal, but as described above, it is determined so as to cancel the offset at the time of shifting the main and sub lenses.

[Third generation process]
In the third generation process, the light receiving areas 1Aa, 1Ba, 2Aa, 2Ba, 3Aa, 3Ba, 1Ab, 2Ab, 3Ab, 4Ab, 5Ab, and 6Ab are used. In this generation process, the offset of the main sum signal SUMm and the subsum signal SUMs caused by stray light is to be canceled by the amount of light received in the light receiving region that receives only stray light.

The first correction unit 65 corrects each sum signal obtained by the equations (3) and (7) based on the received light amounts of the light receiving regions 1Ab, 2Ab, 3Ab, 4Ab, 5Ab, and 6Ab. Specifically, the correction process is performed according to Expression (22) and Expression (23). However, SUMmA is a corrected main sum signal, and SUMsA is a corrected subsum signal. The constants k _{1 to} k ₃ are determined in advance so that the offset (stray light component) of each sum signal is minimized.

  In each sum signal corrected in this way, the stray light component is smaller than before correction. Therefore, by calculating the equations (4) and (8) using each corrected sum signal, the amount of offset generated in the tracking error signal TE can be reduced.

  That is, since the stray light included in the sum signal is reflected light from other than the focusing layer, it is independent of the variation factor of the sum signal from the focusing layer, so the effect of normalization cannot be sufficiently obtained, This is the cause of the offset. In particular, in the sub sum signal, since the sub beam intensity is weak, the ratio of stray light becomes high and the constant k needs to be increased accordingly. Therefore, the fluctuation of the stray light component included in the sum signal is emphasized. Will be. Therefore, it is preferable to correct the stray light component of the thumb signal.

  Here, an example in which both the main sum signal and the sub sum signal are corrected has been described. However, the offset amount reduction effect of the tracking error signal TE can be obtained to some extent even if only one of the corrections is performed. Therefore, it is good also as correcting only either one. As described above, since the influence of stray light appears particularly in the subsum signal, it is more preferable to correct the subsum signal.

  Further, when the subsum signal is generated not by the formula (7) but by the formula (15) and the formula (18), the first correction unit 65 uses the subsum signal SUMs2 based on the received light amounts of the light receiving regions 3Ab and 4Ab. The subsum signal SUMs3 may be corrected based on the received light amounts of the light receiving regions 5Ab and 6Ab. Specifically, the correction process is performed according to Expression (24) and Expression (25). By doing this, the same effect as described above can be obtained even when the subsum signal is generated by the equations (15) and (18).

[Fourth generation process]
In the fourth generation process, as in the third generation process, the light receiving areas 1Aa, 1Ba, 2Aa, 2Ba, 3Aa, 3Ba, 1Ab, 2Ab, 3Ab, 4Ab, 5Ab, and 6Ab are used. In addition to the offsets of the main sum signal SUMm and the sub sum signal SUMs caused by stray light, the offsets similarly generated in the main push pull signal MPP and the sub push pull signal SPP can be canceled.

The point which the 1st correction | amendment part 65 correct | amends each sum signal by Formula (22) and Formula (23) mentioned above is the same as the above. Here, the 1st correction | amendment part 65 also correct | amends each push pull signal calculated | required by Formula (2) and Formula (6) based on each light reception amount of light reception area | region 1Ab, 2Ab, 3Ab, 4Ab, 5Ab, and 6Ab. To do. Specifically, the correction process is performed according to Expression (26) and Expression (27). However, MPPA is the corrected main push-pull signal, and SPPA is the corrected sub push-pull signal. The constants k _{4 to} k ₆ are optimized by experiment so that the offset of each push-pull signal is minimized.

  In each push-pull signal corrected in this way, the offset caused by stray light is smaller than before correction. Therefore, the amount of offset generated in the tracking error signal TE can be further reduced by calculating the equations (4) and (8) using the push-pull signals and the sum signals after correction. .

  In addition, although the example which correct | amends all of each push-pull signal and each sum signal was demonstrated here, it is not necessarily necessary to correct | amend all, and which signal is correct | amended is based on the result of an experiment suitably. It is a problem to be determined. Therefore, depending on the result of the experiment, there may be a case where only the sub push-pull signal SPP and the sub sum signal SUMs are corrected.

  Further, when the sub push-pull signal is generated not by the formula (6) but by the formula (14) and the formula (17), the first correction unit 65 receives the sub push-pull signal SPP2 in the light receiving regions 3Ab and 4Ab. The sub push-pull signal SPP3 may be corrected based on the received light amounts of the light receiving regions 5Ab and 6Ab. Specifically, the correction process is performed according to Expression (28) and Expression (29). In this way, the same effect as described above can be obtained even when the sub push-pull signal is generated by Expression (14) and Expression (17).

[Fifth generation process]
In the fifth generation process, all the light receiving regions shown in FIG. 8 are used. This generation processing is intended to correct the influence of the lateral shift in case the spot is laterally shifted in the signal light tangent direction.

In a typical example, the second correction unit 66 corrects the normalized main push-pull signal MPPN obtained by Expression (4) based on the amounts of received light in the light receiving regions 1Ca, 1Da, 1Ea, and 1Fa. Specifically, the correction process is performed according to Expression (30). However, MPPNA is the normalized main push-pull signal MPPN after correction. The constant k ₇ is optimized by experiment so that the offset generated in the normalized main push-pull signal due to the lateral shift is minimized.

  Since the amount of the lateral deviation is reflected in the second term parenthesis of the equation (30), the offset caused by the lateral displacement can be suitably removed from the normalized main push-pull signal by the equation (30).

  Here, FIG. 15 is a diagram illustrating a simulation result when correction processing is performed using Expression (30). In the figure, the amount of offset generated in the tracking error signal TE before and after correction by the equation (30) is shown for each amount of lateral deviation. In this simulation, w = 20 μm and stray light and lens shift are not considered. In addition, the normalized main push-pull signal at the position of the track center was simulated, and the displacement from zero was used as an offset. From the result of FIG. 15, it is understood that the offset amount is reduced by the correction process.

  FIG. 16 shows the offset amount for each width w when the lateral shift amount is 5 μm and the focal point of the light beam is at the track center in Equation (9). From the result of FIG. 6, it is understood that the offset amount is smaller as the width w is smaller in the range of width w ≦ 25 μm. In this case, in the range of width w ≦ 10 μm, since the offset amount is equal to or less than that in the case of width w = 50 μm (beam diameter), the problem of misalignment occurs when the width is less than 20% of the beam diameter. Is hard to get up.

Furthermore, in order to remove the offset generated in the normalized main push-pull signal MPPN due to the lateral shift, the second correction unit 66 further receives the light receiving regions 1Bb, 1Cb, and 1Fa in addition to the light receiving amounts of the light receiving regions 1Ca, 1Da, 1Ea, and 1Fa. The normalized main push-pull signal MPPN is corrected based on the received light amounts of 2Bb and 2Cb. Specifically, the normalized main push-pull signal MPPN is corrected by the following equation (31). Note that the constants k ₈ , k ₉ , and k ₁₀ are optimized by experiment so that the offset generated in the normalized main push-pull signal MPPN due to the lateral shift is minimized. Moreover, you may correct | amend individually with respect to each light reception area | region.

In addition, various patterns can be considered for the correction processing by the second correction unit 66. In the first pattern, the second correction unit 66 removes the offset generated in the normalized sub push-pull signal SPPN due to the lateral shift, so that the light receiving regions 2Ca, 2Da, 3Ca, 3Da, 2Ea, 2Fa, 3Ea, and 3Fa are removed. The normalized sub push-pull signal SPPN is corrected based on each received light amount. Specifically, the normalized sub push-pull signal SPPN is corrected by the equation (32). However, SPPNA is the normalized main push-pull signal SPPN after correction. Incidentally, the constant k ₁₁ is, as an offset generated in the normalized sub push-pull signal SPPN by lateral displacement is minimized and optimized by experiments.

In the second pattern, the second correction unit 66 removes the offset generated in the normalized sub push-pull signal SPPN due to the lateral shift, and receives the light receiving areas 2Ca, 2Da, 3Ca, 3Da, 2Ea, 2Fa, 3Ea, and 3Fa. The normalized sub push-pull signal SPPN is corrected based on the received light amounts of the light receiving regions 3Bb, 3Cb, 4Bb, 4Cb, 5Bb, 5Cb, 6Bb, and 6Cb. Specifically, the normalized sub push-pull signal SPPN is corrected by the equation (33). The constants k ₁₂ , k ₁₃ , and k ₁₄ are optimized by experiment so that the offset generated in the normalized sub push-pull signal SPPN due to the lateral shift is minimized. Moreover, you may correct | amend individually with respect to each light reception area | region.

The third pattern is applied when the normalized sub push-pull signals SPPN2 and SPPN3 are generated by performing the operations of the equations (14) to (19). The second corrector 66 removes the offset generated in the normalized sub push-pull signal SPPN2 due to the lateral shift, and outputs the normalized sub push-pull signal SPPN2 based on the received light amounts of the light receiving regions 2Ca, 2Da, 2Ea, and 2Fa. to correct. Specifically, the normalized sub push-pull signal SPPN2 is corrected by the equation (34). However, SPPN2A is the normalized main push-pull signal SPPN2 after correction. Incidentally, the constant k ₁₅ is, as an offset generated in the normalized sub push-pull signal SPPN2 by lateral displacement is minimized and optimized by experiments.

Further, the second correction unit 66 removes the offset generated in the normalized sub push-pull signal SPPN3 due to the lateral shift, and the normalized sub push-pull signal based on the respective received light amounts of the light receiving regions 3Ca, 3Da, 3Ea, and 3Fa. Correct SPPN3. Specifically, the normalized sub push-pull signal SPPN3 is corrected by the equation (35). However, SPPN3A is the normalized main push-pull signal SPPN3 after correction. Incidentally, the constant k ₁₆ is, as an offset generated in the normalized sub push-pull signal SPPN3 by lateral displacement is minimized and optimized by experiments.

In correcting the normalized sub push-pull signal SPPN2 or SPPN3 as described above, the second correction unit 66 further receives each of the light receiving regions 3Bb, 3Cb, 4Bb, 4Cb, 5Bb, 5Cb, 6Bb, and 6Cb. The normalized sub push-pull signal SPPN2 or SPPN3 may be corrected based on the amount. As an example, the normalized sub push-pull signal SPPN2 or SPPN3 may be corrected by Expression (36) or Expression (37). The constants k _{15 to} k ₂₀ are optimized by experiments so that the offsets generated in the normalized sub push-pull signals SPPN2 and SPPN3 due to the lateral shift are minimized. Moreover, you may correct | amend individually with respect to each light reception area | region.

[Sixth generation process]
In the sixth generation process, each light receiving region in the main beam light receiving surface S1a is used. This generation process uses the fact that a gap of a predetermined distance g1 is provided between the light receiving area 1Aa and the light receiving areas 1Ca and 1Da.

  The tracking error signal generation unit 61 generates a push-pull signal XPP (first push-pull signal) and a sum signal SUMx (first sum signal) based on the received light amounts of the light receiving regions 1Aa and 1Ba, and push-pull signal A normalized push-pull signal XPPN is generated by normalizing XPP using the sum signal SUMx. The tracking error signal generator 61 generates a push-pull signal YPP (fifth push-pull signal) and a sum signal SUMy (fifth sum signal) based on the received light amounts of the light receiving regions 1Ca to 1Fa, and pushes them. A normalized push-pull signal YPPN is generated by normalizing the pull signal YPP using the sum signal SUMy. Then, a tracking error signal TE is generated using these signals.

  Specifically, the tracking error signal generation unit 61 performs the calculations of the following equations (38) to (44) to generate the tracking error signal TE.

  Here, FIG. 17 shows the amount of offset caused by the lens shift, each normalized push-pull signal XPPN when w = 10 μm, w = 20 μm, w ≧ 50 μm, and each normalized when w + 2g1 = 10 μm, 20 μm. It is the figure shown with the amount of lens shifts (mm) about push pull signal YPPN. As shown in the figure, in the normalized push-pull signal YPPN, the amount of offset caused by lens shift is larger than that in the normalized push-pull signal XPPN.

  FIG. 18 is a diagram in which each normalized push-pull signal XPPN and each normalized push-pull signal YPPN similar to FIG. 17 are plotted with respect to the disk position (μm). As shown in the figure, the normalized push-pull signal XPPN has a larger amplitude than the normalized push-pull signal YPPN.

As can be understood from FIGS. 17 and 18, in the normalized push-pull signal XPPN and the normalized push-pull signal YPPN, the magnitude relationship between the amounts of offset caused by lens shift and the magnitude relationship in amplitude are reversed. Yes. Therefore, by reducing the constant k ₂₁ and generating the tracking error signal TE as in Expression (44), compared with the case of generating the tracking error signal TE according to Expression (5), the normalized push-pull signal YPPN is used. The amount of offset generated can be reduced.

  The tracking error signal TE may be generated by the following equation (45) instead of the equation (44). Even in this case, the same effect as described above can be obtained.

  Heretofore, the generation process of the tracking error signal TE by the tracking error signal generation unit 61 has been described with reference to the first to sixth generation processes.

  Next, focus error signal generation processing of the focus error signal generation unit 62 will be described.

  The focus error signal generation unit 62 generates the focus error signal FE using only each light receiving area constituting the main beam light receiving surface S1a. Specifically, the calculation of the following equation (46) is performed to generate the focus error signal FE.

  Here, FIG. 19 is a view in which only the main beam light receiving surface S1a is extracted from the top view of the photodetector 5. FIG. In the example shown in the figure, dividing lines are provided along the signal light tangential center line C1 of the light receiving regions 1Aa and 1Ba, and the light receiving regions 1AaL, 1AaR, 1BaL, and 1BaR are provided. In this case, it is preferable that the focus error signal generation unit 62 generates the focus error signal FE by performing the calculation of the following equation (47) instead of the equation (46). However, as shown in FIG. 20, even if the equation (46) is used, no particular problem occurs. Even if the spot is shifted left and right, there is almost no influence on the focus error signal FE calculated by the equation (46). FIG. 20 is a diagram showing the results of simulation of Expressions (47) and (46) assuming that the optical magnification is 15 times and the size of the main beam light receiving surface S1a is 100 μm square. In this simulation, w = 10 μm and g1 = 0 μm.

  Also, the focus error signal FE may be normalized in the same manner as the tracking error signal.

  Next, the pull-in signal generation process of the pull-in signal generation unit 63 will be described.

  The pull-in signal generation unit 63 generates the pull-in signal PI using only the light receiving regions 1Aa and 1Ba in the central portion among the light receiving regions constituting the main beam light receiving surface S1a. Specifically, the pull-in signal PI is generated by performing the calculation of the following equation (48).

  In this way, by generating the pull-in signal PI using only the light receiving regions 1Aa and 1Ba, the objective lens control unit 64 has an effect of facilitating interlayer separation of the pull-in signal PI. This will be described in detail below.

  FIG. 21 is a plot of the focus error signal FE generated by the equation (47) and the simulation result of the pull-in signal PI generated by the equation (48) plotted against the focal length (μm). As for the pull-in signal PI, two cases of width w = 10 μm and 100 μm are shown. However, the pull-in signal PI when the width w = 10 μm is multiplied by 5. The focus error signal FE shows only when the width is w = 100 μm. In addition, the simulation was performed with the optical magnification of the optical system 3 being 15 times, the optical disk 11 being a two-layer optical disk with an interlayer distance of 10 μm, and the length x of one side of the main beam light receiving surface S1a being 100 μm.

  In FIG. 21, the focal length (two locations) at which the focus error signal FE zero-crosses from the plus side to the minus side is the focal length at which the recording surface is focused. The pull-in signal PI has peaks near these two focal lengths, but the pull-in signal PI having a width w = 10 μm is smaller than the pull-in signal PI having a width w = 100 μm between the two focal lengths. The drop in the signal is getting bigger. Therefore, when the objective lens control unit 64 performs the focus servo, it can be said that the interlayer separation by the pull-in signal PI is facilitated. Therefore, the focus servo is stabilized and the signal on the in-focus surface can be easily detected.

  The signal generation processing of the focus error signal generation unit 62 and the pull-in signal generation unit 63 has been described above.

  As described above, by using the optical drive device 1 of the present embodiment, the amount of offset caused by various factors in the tracking error signal TE can be reduced. Also, interlayer separation during focus servo is facilitated.

  Hereinafter, other effects obtained by using the optical drive device 1 will be described.

  Further, depending on the optical disc, the reflectance differs between an area where data is recorded (recording area) and an unrecorded area (unrecorded area). For example, in BD, the reflectance on the recording area side is 60% or less of the reflectance on the unrecorded area side. Therefore, an offset may occur in the tracking error signal TE at the boundary (recording boundary) between the recording area and the unrecorded area. According to the optical drive device 1, such an offset can also be reduced. This will be described in detail below.

  First, the diffracted light due to the recording boundary appearing in the signal light spot will be described.

  In general, it is known that diffracted light due to a recording boundary appears at a period (for example, twice) longer than the track pitch at the time of track jump. FIG. 22 shows an end surface of a recording surface of an optical disk 11 composed of a plurality of tracks, a schematic diagram of a recording boundary, an objective lens 4, a light beam (incident light, reflected light (0th order diffracted light, ± on a track). 1st order diffracted light, ± 1st order diffracted light at the recording boundary)).

  As shown in FIG. 22, the diffraction angle of ± first-order diffracted light at the recording boundary is smaller than the diffraction angle of ± first-order diffracted light at the track. This is because, as described above, the period diffracted at the recording boundary during track jump is longer than the track pitch. Since each diffracted light has such a diffraction angle, the ± 1st order diffracted light at the recording boundary interferes with the 0th order diffracted light and the ± 1st order diffracted light at the track.

  FIG. 23 shows spots formed on the main beam receiving surface S1a by the main beam of the light beam shown in FIG. As shown in the figure, interference areas F1 to F3 appear in this spot.

  The interference area F1 is an area where the three lights of the 0th order diffracted light and the ± 1st order diffracted light at the recording boundary interfere with each other. The interference region F2 is an interference region between the 0th order diffracted light and the ± 1st order diffracted light at the recording boundary. The interference region F3 is an interference region between the above-described push-pull regions E1 and E2 and ± first-order diffracted light at the recording boundary.

  FIG. 24 shows, for each interference region, the amplitude of the differential signal in the region above and below the vertical dividing line (line B1 in the figure) of the main beam light receiving surface S1a in terms of disk position (μm). In the figure, a recording boundary exists at the center of the horizontal axis. As shown in the figure, the differential signals in the interference areas F2 and F3 are positive near the recording boundary. On the other hand, the differential signal in the interference area F1 is negative near the recording boundary. These variations in the differential signal are offset due to the presence of recording boundaries.

  When the main push-pull signal MPP is generated using the entire main beam light receiving surface S1a, the sum of the differential signals in the interference regions F1 to F3 substantially coincides with the main push-pull signal MPP, as shown in FIG. Thus, an offset occurs in the differential signal at the recording boundary, and the absolute value of the offset in the interference area F1 is smaller than the absolute value of the offset in the interference areas F2 and F3, so the offset is not canceled out, and the main push-pull signal MPP In addition, the offset of the recording boundary remains.

  On the other hand, for example, when the main push-pull signal MPP is generated using Expression (2), the proportion of the interference region F1 in the entire light receiving region used for generating the main push-pull signal MPP increases as the width w decreases. Therefore, the negative contribution of the differential signal in the interference area F1 increases, and the offset generated in the main push-pull signal MPP due to the presence of the recording boundary decreases. Therefore, as the width w is smaller, the offset of the tracking error signal TE at the recording boundary is also reduced.

  Next, the recording boundary that appears in the stray light spot will be described.

  Usually, since the recording boundary on the optical disk is parallel to the tangential direction of the optical disk, the recording boundary appears in parallel to the lens shift reference line in the spot formed by the reflected light on the photodetector 5.

  As shown in FIG. 6, the lens shift reference line of the stray light spot is substantially parallel to the Y axis (signal light radial direction). Therefore, for the main push-pull signal and the sub push-pull signal, the influence of the recording boundary in the stray light spot is almost canceled in the calculation process. On the other hand, the main sum signal and the sub sum signal are offset by the recording boundary of the stray light spot. With respect to the main sum signal, the intensity of the main beam is sufficiently higher than that of stray light, so that there is almost no problem. However, the sub sum signal largely fluctuates due to this offset and affects the accuracy of the tracking error signal TE.

  By the way, since the lens shift reference line of the stray light spot is substantially parallel to the Y axis, the influences of the recording boundaries appearing in the light receiving areas 2Aa, 2Ba, 3Ab, and 4Ab are almost equal to each other. Similarly, the influence of the recording boundary appearing in the light receiving areas 3Aa, 3Ba, 5Ab, 6Ab is almost equal to each other. Therefore, by making the width and position of each stray light receiving surface in the signal light tangent direction the same as that of the corresponding sub beam receiving surface by the above-described equation (23), the sub push-pull signal SUMs is generated by the recording boundary of the stray light spot. It is possible to cancel the resulting offset. As a result, the offset of the tracking error signal TE at the recording boundary is also reduced.

  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.

  For example, depending on the configuration of the optical system, a photodetector as shown in FIG. 25 can be used. This photodetector is used when there is not enough space between the main pattern and the sub pattern, and the width and position of the sub pattern and the signal light tangential direction are the same with priority given to sub signal correction. Are arranged as follows. That is, the stray light component of the sum signal may be corrected using the stray light receiving surface only for the sub signal.

  Even when this photodetector is used, it is possible to reduce the offset of the tracking error signal TE by calculating various signals as in the above embodiment. Hereinafter, specific generation processing of various signals will be described.

  For example, Expression (23) and Expression (27) are used in place of the following Expression (49) and Expression (50), respectively. This replacement means that, in the equations (23) and (26), the output signal of the light receiving region 7Ab is used instead of the output signal of the light receiving region 4Ab, and the output signal of the light receiving region 8Ab is used instead of the output signal of the light receiving region 5Ab. It is what I did. By generating various signals in this way, the offset of the tracking error signal TE can be reduced.

  In the above embodiment, the width of the light receiving area located at the center of each light receiving surface such as the light receiving area 1Aa is the same. This is to reduce the offset amount under the same condition for each normalized push-pull signal, but it may not necessarily be the same. As examples of such cases, the main beam MB and the sub beams SB1 and SB2 have different spot diameters, or the main beam light receiving surface S1a places importance on the interlayer separation of the pull-in signal PI, and the sub beam receiving surfaces S2a and S3a. For example, the width may be different. Therefore, the width of each light receiving region is preferably adjusted as appropriate based on other configurations of the optical drive device 1.

1 is a schematic diagram of an optical drive device according to an embodiment of the present invention. It is explanatory drawing of the astigmatism provided by the sensor lens by embodiment of this invention. FIG. 4A is a diagram showing a spot of a main beam MB irradiated on a light receiving surface of a photodetector according to the background art of the present invention. FIG. 4A is an example of a signal light spot when there is no lens shift, and FIG. It is an example of the signal light spot moved to the maximum by. It is a figure which shows an example of the laminated constitution of the optical disk by embodiment of this invention. It is the figure which showed the spot formed on the light-receiving surface by embodiment of this invention when there is no lens shift for every main beam and stray light. It is the figure which drew each spot which moved to the fixed direction by lens shift corresponding to each figure of FIG. (A) is the figure which drew the spot of the main beam shown in each figure of FIG. 5, a stray light spot, and each lens shift direction line, and a lens shift reference line in one figure. FIG. 7B is a diagram illustrating the spot of the main beam, the stray light spot, and the lens shift direction lines and the lens shift reference lines shown in FIGS. It is a top view of the photodetector by embodiment of this invention. It is a figure which shows the functional block of the process part by embodiment of this invention. It is the figure which acquired the normalized main push pull signal by embodiment of this invention for every width w by simulation, and plotted with respect to disk position (micrometer). It is the figure plotted on the same conditions as FIG. 10 about the main push pull signal MPP by embodiment of this invention. It is the figure which plotted the amplitude of the main push-pull signal by the embodiment of this invention, and the normalization main push-pull signal by changing the width w to various values. It is a figure which shows the result of having simulated the offset amount of the tracking error signal TE by embodiment of this invention. It is a figure which shows the result of having calculated | required the offset amount of the tracking error signal by embodiment of this invention by the simulation which calculates | requires a sub push pull signal and a subsum signal by dividing the intensity | strength of a sub beam in each light reception area | region. It is a figure which shows the simulation result at the time of performing the correction process by embodiment of this invention. The offset amount when the lateral deviation amount is 5 μm and the focal point of the light beam is on the track is shown for each width w. The amount of offset caused by the lens shift is determined for each normalized push-pull signal when w = 10 μm, w = 20 μm, w ≧ 50 μm, and for each normalized push-pull signal when w + 2g1 = 10 μm, 20 μm. It is the figure shown with the shift amount. FIG. 18 is a diagram in which normalized push-pull signals similar to those in FIG. 17 are plotted with respect to disk position (μm). It is the figure which extracted only the main beam light-receiving surface part from the top view of the photodetector by embodiment of this invention. FIG. 6 is a plot of the focus error signal simulation results generated according to the embodiment of the present invention in terms of focal length (μm). FIG. FIG. 7 is a plot of simulation results of a focus error signal and a pull-in signal generated by an embodiment of the present invention with respect to a focal length (μm). FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention, an end face of a cross section of an optical disk recording surface, a schematic diagram of a recording boundary, an objective lens, a light beam (incident light, reflected light (0th order diffracted light, ± 1st order diffracted light at a track, recording boundary) Is a diagram showing ± 1st order diffracted light)). It is a figure which shows the spot which the main beam of the light beam shown in FIG. 22 forms in the main beam light-receiving surface by embodiment of this invention. It is a figure which shows the amplitude of the differential signal of the area | region above and below the dividing line of the main beam light receiving surface by embodiment of this invention with respect to a disk position for every interference area | region. It is a figure which shows the modification of the photodetector by 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.

Explanation of symbols

E1, E2 Push-pull area F1 to F3 Interference area S1a Main beam light receiving surface S2a, S3a Sub beam light receiving surface S1b to S8b Stray light receiving surface 1Aa to 1Fa, 2Aa to 2Fa, 3Aa to 3Fa, 1Ab to 1Cb, 2Ab to 2Cb, 3Ab to 3Cb, 4Ab to 4Cb, 5Ab to 5Cb, 6Ab to 6Cb, 7Ab to 7Cb, 8Ab to 8Cb Light receiving area 1 Optical drive device 2 Laser light source 3 Optical system 4 Objective lens 5 Optical detector 6 Processing unit 7 CPU
11 Optical disk 21 Diffraction grating 22 Beam splitter 23 Collimator lens 24 1/4 wavelength plate 25 Sensor lens 61 Tracking error signal generation unit 62 Focus error signal generation unit 63 Pull-in signal generation unit 64 Objective lens control unit 65 First correction unit 66 Second Correction unit

Claims (17)

It is formed so as to be point symmetric with respect to the spot center of the signal light that is reflected light on the access target layer of the multilayer optical disc, and to be symmetric with respect to a straight line passing through the spot center and parallel to the tangential direction of the signal light. And a photodetector having a first signal light receiving surface divided into first A and 1B signal light receiving regions by the straight line;
A first push-pull signal and a first sum signal are generated based on the received light amounts of the first and first signal light receiving areas, and the first push-pull signal is used as the first sum signal. Tracking error signal generating means for generating a first normalized push-pull signal by normalizing and generating a tracking error signal based on the first normalized push-pull signal,
2. An optical drive device according to claim 1, wherein each of the first and first signal light receiving areas has a width in the signal light tangent direction shorter than the diameter of the spot.   2. The optical drive device according to claim 1, wherein each of the first light A and the first light signal receiving regions has a width in a signal light tangent direction of less than 20% of the diameter. A diffraction grating that further divides the light beam applied to the multilayered optical disc into 0th order diffracted light and ± 1st order diffracted light;
The signal light is reflected light of the 0th-order diffracted light,
The photodetector is
It is formed so as to be point symmetric with respect to the spot center of the reflected light of the + 1st order diffracted light, and to be symmetric with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. A second signal light receiving surface divided into second B signal light receiving regions;
The first-order diffracted light is formed so as to be point-symmetric with respect to the spot center of the reflected light and to be symmetrical with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. And a third signal light receiving surface divided into 3B signal light receiving regions,
The 2A and 3B signal light receiving areas correspond to areas on the same side as the 1A signal light receiving area with the corresponding straight lines as boundaries, and the 2B and 3B signal light receiving areas. Corresponds to the area on the same side as the signal light receiving area of the 1B with each corresponding straight line as a boundary,
The tracking error signal generation means generates a second push-pull signal and a second sum signal based on the received light amounts of the second A, second B, third A, and third B signal light receiving regions, A second normalized push-pull signal is generated by normalizing a second push-pull signal using the second sum signal, and the tracking error signal is also generated based on the second normalized push-pull signal. Produces
3. The optical drive device according to claim 1, wherein each of the widths of the second A, second B, third A, and third B signal light receiving regions in the signal light tangent direction is shorter than the diameter of the spot. A diffraction grating that further divides the light beam applied to the multilayered optical disc into 0th order diffracted light and ± 1st order diffracted light;
The signal light is reflected light of the 0th-order diffracted light,
The photodetector is
It is formed so as to be point symmetric with respect to the spot center of the reflected light of the + 1st order diffracted light, and to be symmetric with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. A second signal light receiving surface divided into second B signal light receiving regions;
The first-order diffracted light is formed so as to be point-symmetric with respect to the spot center of the reflected light and to be symmetrical with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. And a third signal light receiving surface divided into 3B signal light receiving regions,
The 2A and 3B signal light receiving areas correspond to areas on the same side as the 1A signal light receiving area with the corresponding straight lines as boundaries, and the 2B and 3B signal light receiving areas. Corresponds to the area on the same side as the signal light receiving area of the 1B with each corresponding straight line as a boundary,
The tracking error signal generating means generates a third push-pull signal and a third sum signal based on the amounts of light received in the second and second signal light receiving areas, and outputs the third push-pull signal. A third normalized push-pull signal is generated by normalization using the third thumb signal, and a fourth push-pull signal is generated based on the received light amounts of the third and third signal light receiving regions. And a fourth sum signal, and the fourth push-pull signal is normalized using the fourth sum signal to generate a fourth normalized push-pull signal, and the third and fourth Generating the tracking error signal based on the normalized push-pull signal of
3. The optical drive device according to claim 1, wherein each of the widths of the second A, second B, third A, and third B signal light receiving regions in the signal light tangent direction is shorter than the diameter of the spot.   5. The optical drive according to claim 3, wherein a width of each of the 2A, 2B, 3A, and 3B signal light receiving regions in a signal light tangential direction is less than 20% of the diameter. apparatus.   6. The signal light tangent width of each of the first A, first B, second A, second B, third A, and third B signal light receiving regions is the same as any one of claims 3 to 5. The optical drive device according to one item. The photodetector has one or a plurality of stray light receiving regions arranged so as to be capable of receiving stray light that is reflected light in layers other than the access target layer,
The tracking error signal generation unit corrects at least one of the first push-pull signal or the first sum signal based on each received light amount of at least a part of the one or more stray light receiving regions. 7. The first normalized push-pull signal is generated using each signal corrected by the first correcting unit, and the first normalized push-pull signal is generated. The optical drive device according to Item. The one or more stray light receiving regions include 1A and 2A stray light receiving regions provided on both sides in the signal light radial direction of the first signal light receiving surface,
The widths and positions of the first A and 1B signal light receiving areas and the first A and second A stray light receiving areas in the signal light tangential direction are the same,
The first correction unit corrects at least one of the first push-pull signal or the first sum signal based on the amount of light received in each of the first A and second A stray light receiving regions. The optical drive device according to claim 7. The photodetector has one or a plurality of stray light receiving regions arranged so as to be capable of receiving stray light that is reflected light in layers other than the access target layer,
The tracking error signal generation unit corrects at least one of the second and second push-pull signals or the second sum signal based on each received light amount of at least a part of the one or more stray light receiving regions. 4. The optical drive according to claim 3, further comprising: a first correction unit configured to generate the second normalized push-pull signal using each signal corrected by the first correction unit. apparatus. The one or more stray light receiving areas are the 3A and 4A stray light receiving areas provided on both sides of the second signal light receiving surface in the signal light radial direction, and the signal light of the third signal light receiving surface. Including 5A and 6A stray light receiving areas provided on both sides in the radial direction,
Each width and position in the signal light tangential direction of the 2A and 2B signal light receiving areas and the 3A and 4A stray light receiving areas are the same,
The widths and positions of the 3A and 3B signal light receiving areas and the 5A and 6A stray light receiving areas in the signal light tangential direction are the same,
The first correction means determines at least one of the second push-pull signal or the second sum signal based on the received light amounts of the third A, fourth A, fifth A, and sixth A stray light receiving regions. The optical drive device according to claim 9, wherein the optical drive device is corrected. The one or more stray light receiving areas include a seventh A stray light receiving area provided between the first signal light receiving surface and the second signal light receiving surface, and the first signal light receiving surface. An 8A stray light receiving region provided between the third signal light receiving surface and a 3A provided on the opposite side of the seventh stray light receiving region across the second signal light receiving surface. A stray light receiving region, and a 6A stray light receiving region provided on the opposite side of the eighth stray light receiving region across the third signal light receiving surface,
The widths and positions of the second A and 2B signal light receiving regions and the seventh A and 3A stray light receiving regions in the signal light tangential direction are the same,
The widths and positions of the signal light tangent directions of the third A and 3B signal light receiving areas and the 8A and 6A stray light receiving areas are the same,
The tracking error signal generating means generates at least one of the second push-pull signal or the second sum signal based on the received light amounts of the seventh A, eighth A, third A, and sixth A stray light receiving regions. 10. The optical system according to claim 9, further comprising a first correcting unit that corrects, wherein the second normalized push-pull signal is generated using each signal corrected by the first correcting unit. Drive device. The first signal light receiving surface includes first C and first D signal light receiving areas respectively provided on both sides of the first A signal light receiving area in the signal light tangential direction, and the first B signal light receiving area. 1E and 1F signal light receiving regions provided respectively on both sides of the signal light tangential direction,
The tracking error signal generation unit corrects the first normalized push-pull signal based on the received light amounts of the first C and first D signal light receiving regions and the first E and first F signal light receiving regions. The tracking error signal is generated using the first normalized push-pull signal corrected by the second correction unit. The optical drive device according to any one of the above.   The optical drive according to any one of claims 1 to 12, wherein an addition signal of each received light amount of the first and first signal light receiving areas is used as a pull-in signal used when focus servo is executed. apparatus. The first signal light receiving surface includes first C and first D signal light receiving regions provided at a predetermined distance from both sides of the first A signal light receiving region in the signal light tangential direction, and the first B light receiving surface. 1E and 1F signal light receiving areas provided at a predetermined distance from each other on both sides of the signal light receiving area in the signal light tangential direction,
The tracking error signal generator generates a first push-pull signal and a first sum signal based on the received light amounts of the first and first signal light receiving areas, and the first to first F signals. A fifth push-pull signal and a fifth sum signal are generated based on the amount of light received in the signal light receiving area, and the first and fifth push-pull signals are respectively selected from the first and fifth sum signals. The optical drive apparatus according to claim 1, wherein the tracking error signal is generated by normalization by at least one of the two. A diffraction grating that divides a light beam applied to the multilayer optical disk into a zero-order diffracted light and a ± first-order diffracted light;
It is formed so as to be point symmetric with respect to the spot center of the reflected light of the + 1st order diffracted light, and to be symmetric with respect to a straight line passing through the spot center and parallel to the signal light tangential direction. The second signal light receiving surface divided into the 2B signal light receiving area and the point symmetric with respect to the spot center of the reflected light of the −1st order diffracted light, passing through the spot center, and in the signal light tangential direction A photodetector having a third signal light receiving surface formed so as to be line symmetric with respect to a parallel straight line and further divided into 3A and 3B signal light receiving regions by the straight line;
The second signal light receiving region and the 3A signal light receiving region correspond to regions on the same side with the corresponding straight lines as boundaries, respectively, and the 2B signal light receiving region and the 3B signal light. The light receiving area corresponds to the area on the same side with each corresponding straight line as a boundary,
A second push-pull signal is generated based on the received light amounts of the second A, second B, third A, and 3B signal light receiving regions, and a tracking error signal is generated based on the second push-pull signal. Tracking error signal generating means for
An optical drive device characterized in that the widths of the signal light receiving regions of the second A, second B, third A, and third B in the signal light tangent direction are shorter than the diameter of each spot.   The tracking error signal generating means generates a second sum signal based on the amount of light received in the second A, second B, third A, and third B signal light receiving areas, and the second push-pull signal is generated as the second push-pull signal. The second normalized push-pull signal is generated by normalizing using the second sum signal, and a tracking error signal is generated based on the second normalized push-pull signal. 15. The optical drive device according to 15. The tracking error signal generating means includes
A third sum signal is generated based on the received light amounts of the second A and second B signal light receiving areas, and each of the second push-pull signals in the second A and second B signal light receiving areas is received. Generating a third normalized push-pull signal by normalizing a quantity-based component with the third sum signal;
A fourth sum signal is generated based on the amount of light received in each of the 3A and 3B signal light receiving areas, and each light received in the 3A and 3B signal light receiving areas of the second push-pull signal. Generating a fourth normalized push-pull signal by normalizing a component based on a quantity with the fourth sum signal;
16. The optical drive device according to claim 15, wherein the tracking error signal is generated based on the third and fourth normalized push-pull signals.

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