JP2012133852A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup in which a diffraction optical element is disposed in an optical path through which return light passes, with a technique capable of obtaining a proper TE signal stably.SOLUTION: An optical pickup includes a diffraction optical element 17 for guiding reflection light (return light) from an optical disk to a light detection unit 19 in an optical path. The diffraction optical element 17 includes a substantially band-like diffraction region 171 with a direction substantially orthogonal to an arrangement direction of two push-pull signal component areas W as a longitudinal direction and through which a center part of the return light is transmitted. In the light detection unit 19, a main light receiving unit 191 for receiving light transmitted through the diffraction optical element 17 without being diffracted by the diffraction optical element 17, and sub light receiving units 192, 193 for receiving diffraction light diffracted by the substantially band-like diffraction region 171 are formed. A TE signal is generated by using signals output from the main light receiving unit 191 and the sub light receiving units 192, 193.

Description

  The present invention relates to an optical pickup used when reading information recorded on an optical disc or writing information on an optical disc.

  Conventionally, for the purpose of reading information and writing information on an optical disc such as a Blu-ray disc (hereinafter referred to as BD), a digital versatile disc (hereinafter referred to as DVD), and a compact disc (hereinafter referred to as CD). A pickup is used. In this optical pickup, an objective lens for condensing the light emitted from the light source on the information recording surface of the optical disc, a photodetector (PDIC) for receiving reflected light (return light) reflected by the optical disc, Is included.

  When reading information on an optical disk or writing information on an optical disk by an optical pickup, it is necessary to control the position of the light spot collected by the objective lens as follows. First, it is necessary to perform control (focusing control) so that the light spot condensed by the objective lens always exists on the information recording surface of the optical disc. Second, it is necessary to perform control (tracking control) so that the light spot focused on the information recording surface by the objective lens always follows the track formed on the optical disk.

  For this reason, in the optical pickup, based on the signal output from the photodetector, a focus error (FE) signal indicating the amount of deviation between the light spot and the information recording surface (target information recording surface), and the light spot And a tracking error (TE) signal representing the amount of deviation between the track and the target track (target track). Control for moving the position of the objective lens in the focus direction (focusing control) is performed based on the FE signal, and control for moving the position of the objective lens in the tracking direction (tracking control) is performed based on the TE signal.

  The focus direction is a direction substantially perpendicular to the information recording surface of the optical disc, and the tracking direction is a direction substantially perpendicular to the track of the optical disc (formed concentrically or spirally on the information recording surface).

  Conventionally known methods for calculating the TE signal include a PP (Push-Pull) method and a DPP (Differential Push-Pull) method. Since the DPP method can obtain the TE signal while suppressing the influence of lens shift (offset of the objective lens in the tracking direction), it has been widely used in optical pickups.

  However, for example, a BD is generally a multi-layer disc having a plurality of information recording surfaces, and recently, multi-layer discs having three or four layers are also commercially available. When such a multilayer disk is handled with an optical pickup, reflected light from an information recording surface different from the target information recording surface is generated as stray light. The DPP method that employs a configuration in which light emitted from a light source is irradiated onto an optical disk by being divided into three light beams, which are main light and two sub-lights, by a diffraction grating (grating) is easily affected by the stray light described above. Not suitable for multi-layer discs.

  Also, in the DPP method, since the light emitted from the light source is divided into three lights and irradiated onto the optical disk as described above, the light use efficiency is higher than in the case where only one light is irradiated onto the optical disk. It will decline. That is, it can be said that the DPP method is not preferable in terms of light utilization efficiency.

  For this reason, recently, as shown in Patent Document 1, for example, there is one light spot formed on an optical disk by light emitted from a light source, and a plurality of return lights from the optical disk are formed by a diffractive optical element. A configuration has been developed in which a TE signal is obtained by receiving light with a photodetector separately.

  FIG. 9 is a diagram showing a configuration of a diffractive optical element that splits return light provided in a conventional optical pickup (Patent Document 1). FIG. 9 also shows the return light incident on the diffractive optical element 101.

  The track formed on the optical disk acts as a diffraction grating for light incident on the optical disk. For this reason, the return light includes zero-order light and ± first-order light. As shown in FIG. 9, the return light has a push-pull signal component region W that is generated by overlapping zero-order light and ± first-order light. This push-pull signal component region W is a region including an AC signal component (push-pull signal component) generated when light incident on the optical disk crosses the track.

  The diffractive optical element 101 is divided into three regions 101a, 101b, and 101c. The three regions 101a, 101b, and 101c are arranged in a direction substantially orthogonal to the arrangement direction of two push-pull signal component regions W included in the return light. There is no diffraction groove in the central region 101a, and this portion is a transparent substrate. Diffraction grooves are provided in the regions 101b and 101c arranged so as to sandwich the central region 101a. The three regions 101a, 101b, and 101c are formed so that the push-pull signal component region W is within the central region 101a, and the push-pull signal component region W does not enter the regions 101b and 101c at both ends.

  FIG. 10 is a diagram showing a light receiving pattern formed on a photodetector provided in a conventional (PTL 1) optical pickup. FIG. 10 also shows a light spot formed on the light receiving surface of the photodetector 102.

  Three photodetectors 102a, 102b, and 102c are formed in the photodetector 102, and the sub-light-receiving unit 102b and the sub-light-receiving unit 102c are arranged symmetrically about the main light-receiving unit 102a. The arrangement direction of the three light receiving portions 102a, 102b, and 103c is the direction in which the light spot formed on the light receiving surface of the photodetector 102 moves when the objective lens included in the optical pickup is lens shifted (moved in the tracking direction). (Hereinafter referred to as the first direction) and a parallel direction. This first direction is a direction corresponding to the tracking direction.

  The main light receiving unit 102a is divided into four regions a, b, c, and d by a cross-shaped dividing line. The sub light receiving unit 102b is divided into two regions by a dividing line orthogonal to the first direction to form two regions e and f. Similarly, the sub light receiving unit 102c is divided into two by a dividing line orthogonal to the first direction to form two regions g and h.

  The light (0th order light) transmitted through the diffractive optical element 101 without being diffracted by the diffractive optical element 101 becomes a light spot SP1 and is received by the main light receiving unit 102a. The + 1st order light diffracted by the grating surface edge regions 101b and 101c of the diffractive optical element 101 is received by the sub light receiving unit 102b as substantially half-moon or crescent-like light spots SP2a and SP2b. On the other hand, the −1st-order light diffracted by the lattice plane edge regions 101b and 101c of the diffractive optical element 101 is received by the sub light receiving unit 102c as light spots SP3a and SP3b having a substantially half-moon shape or a substantially crescent shape. .

In this configuration, the TE signal is obtained by the following equation (A). In the formula (A), signals output from the respective regions of the photodetector 102 are indicated with “S” in front of the region name.
TE = (Sa + Sb) − (Sc + Sd) −k * ((Se−Sf) + (Sg−Sh)) (A)
Note that k in Equation (1) is a coefficient.

  In the formula (A), (Sa + Sb) − (Sc + Sd) in the previous term is a signal obtained from the light spot SP1 received by the main light receiving unit 102a. This signal is a signal obtained by adding a DC offset signal generated when the objective lens is shifted to the push-pull signal generated by the track structure of the optical disk.

  The first term (Se−Sf) in the equation (A) is a signal obtained from the light spots SP2a and SP2b received by the sub light receiving unit 102b. The light spots SP2a and SP2b received by the sub light receiving unit 102b are light spots of + 1st order light diffracted by the grating surface edge regions 101b and 101c of the diffractive optical element 101, and this light spot has a push-pull signal component. Is not (or almost) not included. Therefore, the (Se−Sf) signal does not include the push-pull signal, and includes only the DC offset signal resulting from the movement of the light spot accompanying the lens shift of the objective lens. In this respect, the same applies to the second term (Sg-Sh) in the second term in the formula (A). Therefore, according to the formula (A), a good TE signal from which the influence when the objective lens is shifted is removed can be obtained.

JP 2009-9628 A

  However, the optical pickup disclosed in Patent Document 1 has the following problems. First, the light spots received by the sub light receiving units 102b and 102c are light (diffracted light) from the grating surface edge regions 101b and 101c of the diffractive optical element 101, and are received by the sub light receiving units 102b and 102c. Compared to the area, the size tends to be quite small. As a result, signals output from the sub light receiving units 102b and 102c are easily affected by noise, and the TE signal may be deteriorated.

  Second, an appropriate TE signal cannot be obtained when the mounting accuracy of the photodetector 102 cannot be sufficiently secured in a direction parallel to the traveling direction of light reaching the photodetector 102 (hereinafter referred to as Z direction). There's a problem. This will be described with reference to FIG. FIG. 11 is a schematic diagram for explaining a problem in the case where the variation in mounting of the photodetector occurs in the Z direction.

  As shown in FIG. 11, when the position of the photodetector 102 is shifted in the Z direction and changed from the position P1 to the position P2, the position where the diffracted light diffracted by the diffractive optical element 101 falls on the photodetector 102 changes. I understand that In the conventional optical pickup, the diffraction direction of the diffracted light (± first order light) generated by the diffractive optical element 101 is parallel to the first direction (see FIG. 10). For this reason, when the mounting position of the photodetector 102 in the Z direction deviates from the target position, the positions of the light spots SP2a and SP2b formed on the sub light receiving unit 102b even though the objective lens is not shifted by the lens. However, it becomes a state biased to one of the two regions e and f. A similar bias also occurs in the sub light receiving unit 102c. As a result, there arises a problem that an appropriate TE signal cannot be obtained.

  In view of the above points, an object of the present invention is to provide a technique capable of stably obtaining an appropriate TE signal in an optical pickup in which a diffractive optical element is disposed in an optical path through which return light passes.

  In order to achieve the above object, an optical pickup according to the present invention has a diffractive optical element in an optical path for guiding reflected light from an optical disk to a light detection unit, and uses a signal output from the light detection unit to track a tracking error signal. When the two areas generated by overlapping the zero-order light and the ± first-order light generated by the track structure of the optical disc among the reflected light are used as the push-pull signal component area, The diffractive optical element includes a substantially band-shaped diffractive region in which a direction substantially orthogonal to an arrangement direction of the two push-pull signal component regions is a longitudinal direction and a central portion of the reflected light passes, and the light detection unit Includes a main light receiving portion that receives light that is transmitted through the diffractive optical element without being diffracted by the diffractive optical element, and receives diffracted light that is diffracted by the substantially band-shaped diffraction region. And a sub light receiving portion, is formed, the tracking error signal using the signal output from the main light receiving portion and said sub light receiving portion is generated.

  According to this configuration, the TE signal can be obtained using the diffractive optical element disposed in the optical path through which the return light passes. For this reason, in the optical pickup of this configuration, it is not necessary to divide the light emitted from the light source into three lights and irradiate the optical disk, and the light utilization efficiency is higher than that in the case where the TE signal is generated using the DPP method. Enhanced. Further, in this configuration, when dealing with a multilayer disk, the influence of stray light, which is a serious problem in the DPP method, can be reduced.

  Further, in this configuration, the central portion sandwiched between the push-pull signal component regions of the return light is received by the sub light receiving unit. For this reason, it is possible to make the area of the light spot formed in the sub light receiving portion relatively large with respect to the area of the sub light receiving portion. Therefore, according to this configuration, it is possible to reduce the influence of noise on the signal output from the sub light receiving unit, and it is possible to stably obtain an appropriate TE signal.

  In the optical pickup configured as described above, it is preferable that a dead zone that does not detect light is formed at the center of the sub light receiving unit.

  According to this configuration, even when the objective lens shifts the lens and the push-pull signal component region of the return light enters the band-shaped diffraction region, the push-pull signal component region of the return light can be prevented from entering the sub light receiving unit. . Further, according to this configuration, the light receiving area of the sub light receiving unit can be reduced, so that the influence of stray light can be reduced. That is, in this configuration, the signal output from the sub light receiving unit can be stabilized, and an appropriate TE signal can be easily obtained.

  In the optical pickup having the above-described configuration, the dead zone may be formed by the sub light receiving unit including two portions arranged at intervals. The dead zone can also be formed using a light shielding member (this configuration is also included in the scope of the present invention). However, as in this configuration, it is advantageous in terms of cost, workability at the time of manufacturing, and the like to form the dead zone by forming the sub light receiving portion with two portions.

  In the optical pickup having the above configuration, an objective lens that condenses light from a light source on an information recording surface of the optical disc, and an actuator that moves the objective lens in a tracking direction substantially orthogonal to the track of the optical disc, When the objective lens moves in the tracking direction and the direction in which the light spot formed on the light receiving surface of the light detecting unit moves is the first direction, the main light receiving unit and the sub light receiving unit are It is preferable that they are arranged in parallel in a direction substantially orthogonal to the first direction. According to this configuration, it is possible to obtain a good TE signal even if the mounting accuracy of the photodetector in the Z direction is somewhat low.

  In the optical pickup having the above-described configuration, two sub light receiving units may be provided, and the two sub light receiving units may be arranged symmetrically so as to sandwich the main light receiving unit. This configuration is a preferable mode when a blazed grating is not employed in the diffractive optical element.

  In the optical pickup having the above-described configuration, the diffractive optical element is provided with another diffractive region so as to sandwich the substantially band-shaped diffractive region, and the diffracted light diffracted by the other diffractive region is the main diffractive region. It is preferable that no light is received by the light receiving unit and the sub light receiving unit. If comprised in this way, it is hard to generate | occur | produce imbalance of light quantity in the light spot formed in a main light-receiving part, and it is preferable.

  According to the present invention, an appropriate TE signal can be stably obtained in an optical pickup in which a diffractive optical element is disposed in an optical path through which return light passes.

Schematic plan view showing the configuration of the optical pickup of the present embodiment Schematic plan view showing the optical configuration of the optical pickup of the present embodiment Schematic plan view showing the configuration of the diffractive optical element provided in the optical pickup of the present embodiment Schematic plan view showing a light receiving pattern formed on a light receiving surface of a photodetector provided in the optical pickup of the present embodiment Schematic diagram showing the relationship between the diffractive optical element and the return light when the objective lens is shifted. The figure for demonstrating the modification of the diffractive optical element with which the optical pick-up of this embodiment is provided. The figure for demonstrating the modification of the sub light-receiving part formed in the photodetector with which the optical pick-up of this embodiment is provided. The figure which shows the modification regarding the photodetector with which the optical pick-up of this embodiment is provided. The figure which shows the structure of the diffractive optical element which splits the return light with which the conventional optical pickup is equipped. The figure which shows the light reception pattern formed in the photodetector with which the conventional optical pick-up is equipped. Schematic diagram for explaining problems when variation in mounting of photodetectors occurs in the Z direction

  Hereinafter, embodiments of an optical pickup according to the present invention will be described in detail with reference to the drawings.

  1A and 1B are schematic plan views showing the configuration of the optical pickup according to the present embodiment. FIG. 1A is a top view of the optical pickup, and FIG. 1B is a side view of the optical pickup. In addition, FIG.1 (b) is the figure seen along the arrow L shown to Fig.1 (a). In FIG. 1B, the optical disk D is also shown for easy understanding.

  As shown in FIG. 1, the optical pickup 1 of the present embodiment includes a pickup base 10 and an objective lens actuator 30 that is fixedly disposed on the pickup base 10. The objective lens actuator 30 is an example of the actuator of the present invention.

  An optical member included in the optical pickup 1 is mounted on the pickup base 10. Further, bearing portions 10 a and 10 b are provided at the left and right ends of the pickup base 10. The pickup base 10 is slidable on a guide shaft 100 (shown by a broken line in FIG. 1A) provided in the optical disc device (device for reproducing and recording the optical disc D) by the bearing portions 10a and 10b. Will be supported. The guide shaft 100 provided in the optical disc apparatus is disposed so as to extend in the radial direction (radial direction; Rad direction) of the optical disc. The optical pickup 1 slidable with respect to the guide shaft 100 can read and write information by accessing a desired address of the rotating optical disk D.

  The objective lens actuator 30 is a device that enables the objective lens 16 provided in the optical system of the optical pickup 1 to move in the focus direction and the tracking direction (the same direction as the Rad direction).

  In the optical pickup 1, it is necessary to perform focusing control so that the focal position of the objective lens 16 always matches the information recording surface RS of the optical disk D when reading or writing information. Further, in the optical pickup 1, the position of the light spot condensed on the information recording surface RS of the optical disc D by the objective lens 16 always follows the track of the optical disc D when reading or writing information. Tracking control needs to be performed. The objective lens actuator 30 is driven, for example, when performing these focusing control and tracking control.

  The objective lens actuator 30 has a lens holder that holds the objective lens 16, and is configured to support the lens holder so as to be swingable with a wire. And a lens holder is moved with the force generated using a coil and a magnet. Since this type of objective lens actuator is known, detailed description thereof is omitted here.

  FIG. 2 is a schematic plan view showing the optical configuration of the optical pickup of the present embodiment. The type of the semiconductor laser 11 (an example of the light source of the present invention) is determined by the type of the optical disc D that is supported by the optical pickup 1. For example, if the optical pickup 1 is BD compatible, a semiconductor laser 11 that emits laser light having a wavelength of 405 nm band is used.

  Laser light emitted from the semiconductor laser 11 is sent to the polarization beam splitter 12. The polarization beam splitter 12 reflects S-polarized light (an example of linearly polarized light, which is not limited to this) emitted from the semiconductor laser 11. The laser beam reflected by the polarization beam splitter 12 is converted into circularly polarized light by the quarter wavelength plate 13. The laser light emitted from the quarter-wave plate 13 is reflected by the rising mirror 15 after passing through the collimating lens 14. The laser beam reflected by the raising mirror 15 reaches the objective lens 16 above the raising mirror 15. The objective lens 16 has a function of condensing incident laser light on the information recording surface RS of the optical disc D.

  After being focused on the information recording surface RS by the objective lens 16, the reflected light (return light) reflected by the information recording surface RS passes through the objective lens 16 and is reflected by the rising mirror 15. Then, the return light passes through the collimating lens 14, is converted into P-polarized light by the quarter wavelength plate 13, and passes through the polarization beam splitter 12. The return light that has passed through the polarizing beam splitter 12 passes through a sensor optical system including the diffractive optical element 17 and a sensor lens 18 including a cylindrical surface to a photodetector 19 (an example of the light detection unit of the present invention). Focused.

  The photodetector 19 functions as a photoelectric conversion unit that converts the received optical signal into an electrical signal. The electrical signal output from the photodetector 19 is sent to the signal processing unit 20. In the signal processing unit 20, a reproduction signal, an FE signal, a TE signal, and the like are generated.

  The collimating lens 14 can be moved in the optical axis direction M (left and right direction in FIG. 2), and the position thereof is appropriately moved according to a layer jump or the like. Thereby, in the optical pickup 1, the influence of spherical aberration is suppressed appropriately. The diffractive optical element 17 is provided so as to be able to generate a TE signal. Details of this point will be described later.

  The reason why the sensor lens 22 is provided with a cylindrical surface so as to provide astigmatism is to enable generation of an FE signal using the astigmatism method. Further, based on the FE signal and the TE signal generated by the signal processing unit 20, a control unit (not shown) of the optical pickup 1 controls driving of the objective lens actuator 30 so as to perform focusing control and tracking control. .

  FIG. 3 is a schematic plan view showing the configuration of the diffractive optical element provided in the optical pickup of the present embodiment. In FIG. 3, the return light incident on the diffractive optical element 17 is also shown. As shown in FIG. 3, the diffractive optical element 17 includes a first region 171 disposed in the middle, and a second region 172a and a third region 172b disposed so as to sandwich the first region 171. Are divided into three areas. The alignment direction of the three regions 171, 172a, 172b is substantially parallel to the alignment direction (vertical direction in FIG. 3) of the push-pull signal component regions W (two) in the return light incident on the diffractive optical element 17. is there.

  The first region 171 is a substantially band-like region whose longitudinal direction is a direction (right and left direction in FIG. 3) that is substantially orthogonal to the direction in which the push-pull signal component regions W of return light are arranged. The first region 171 is a region through which the central portion of the return light (portion sandwiched between the regions W) passes. In the present embodiment, the width (length in the short direction) of the first region 171 is determined so that the push-pull signal component region W does not enter when the objective lens 16 is not lens-shifted.

  Note that if the width of the first region 171 is too wide (in some cases, the configuration of the return light region W may be somewhat intruded), an appropriate TE signal can be obtained when a lens shift occurs. Since it may disappear, it is preferable to determine the optimum width by experiment or the like.

  In the first region 171, a diffraction grating composed of a striped uneven pattern is formed. The diffraction grating formed in the first region 171 is arranged so that the primary light generated by diffracting the incident return light in the first region 171 is incident on a sub light receiving portion (described later) of the photodetector 19. A pattern is formed. The first region 171 is an example of a substantially band-shaped diffraction region of the present invention.

  The second region 172a and the third region 172b are both provided in a substantially rectangular shape and are disposed adjacent to the first region 171. The second region 172a and the third region 172b are also formed with a diffraction grating having a striped uneven pattern. However, the diffraction grating formed in these regions 172a and 172b has a main light receiving unit and a sub light receiving unit in which diffracted light generated by diffracting incident return light in these regions 172a and 172b is formed in the photodetector 19. The pattern is formed so as not to enter (described later). The second region 172a and the third region 172b are examples of other diffraction regions in the present invention.

  The diffraction grating formed in each of the regions 171, 172 a, and 172 b is made of a rectangular diffraction groove in this embodiment, but is not limited to this, and may be a blazed grating made of a sawtooth diffraction groove, for example.

  FIG. 4 is a schematic plan view showing a light receiving pattern formed on the light receiving surface of the photodetector provided in the optical pickup of the present embodiment. FIG. 4 also shows a light spot formed on the light receiving surface of the photodetector 19. This light spot is shown assuming that the objective lens 16 is not lens-shifted.

  As shown in FIG. 4, the photodetector 19 is formed with a main light receiving portion 191 disposed in the middle and two sub light receiving portions 192 and 193 disposed so as to sandwich the main light receiving portion 191. ing. The two sub light receiving portions 192 and 193 are arranged symmetrically with the main light receiving portion 191 as the center.

  The arrangement direction of the three light receiving portions 191, 192, and 193 is the direction in which the light spot formed on the light receiving surface of the photodetector 19 moves when the objective lens 16 is shifted (hereinafter referred to as a first direction). In a direction substantially orthogonal to The first direction is a direction corresponding to the tracking direction.

  The main light receiver 191 is an example of the main light receiver of the present invention, and the sub light receiver 192 and the sub light receiver 193 are examples of the sub light receiver of the present invention. In the present embodiment, the number of sub light receiving units is two, but may be one in some cases.

  The main light receiving unit 191 is provided with four divided regions A, B, C, and D formed by dividing a rectangular light receiving region into four equal parts by the first dividing line DL1 and the second dividing line DL2. ing. The first dividing line DL1 is a dividing line that is substantially orthogonal to the first direction, and the first dividing line DL1 and the second dividing line DL2 are substantially orthogonal to each other. In a state where the objective lens 16 is not lens-shifted, the center of the 0th-order light from the diffractive optical element 17 (light transmitted through the diffractive optical element 17 without being diffracted by the diffractive optical element 17) and the first dividing line The main light receiving unit 191 is provided so that the intersection of DL1 and the second dividing line DL substantially matches.

  The sub light receiving unit 192 includes two substantially rectangular portions 192a and 192b that are arranged in parallel at an interval. That is, the sub light receiving unit 192 has a configuration in which a dead zone ND that does not detect light is provided at the center thereof. The two portions 192a and 192b have the same shape and the same size. The two portions 192a and 192b are both divided into two equal parts by a third dividing line DL3 that is substantially orthogonal to the first direction (substantially parallel to the first dividing line DL1). Region F is formed.

  The sub light receiving unit 192 is provided so that the center of the + 1st order light from the diffractive optical element 17 and the center M1 of the sub light receiving unit 192 substantially coincide with each other when the objective lens 16 is not shifted. The center M1 of the sub light receiving unit 192 corresponds to the midpoint position of a line (shown by a broken line) connecting the third dividing lines DL3 of the two portions 192a and 192b (see FIG. 4).

  Similar to the sub light receiving unit 192, the sub light receiving unit 193 that is in a symmetrical relationship with the sub light receiving unit 192 includes two substantially rectangular portions 193a and 193b arranged in parallel at an interval. Then, it is divided into two equal parts by the third dividing line DL3, and a region G and a region H are formed respectively. Similarly to the sub light receiving unit 192, the sub light receiving unit 193 has a configuration in which a dead zone ND that does not detect light is provided at the center thereof. The sub light receiving unit 193 is provided so that the center of the −1st order light from the diffractive optical element 17 and the center M2 of the sub light receiving unit 193 substantially coincide with each other in a state where the objective lens 16 is not lens-shifted.

  The reason why the dead zone ND is provided in the sub light receiving units 192 and 193 will be described later. Further, the shape of the light spot formed in the sub light receiving portions 192 and 193 shown in FIG. 4 does not correspond to the shape of the first region 171 of the diffractive optical element 19, but this is because the sensor lens 18. This is because the shape of the light is distorted by the action of the cylindrical surface provided on the surface.

  The signal processing unit 20 (see FIG. 2) uses a signal output from each of the light receiving units 191 to 193 (each region) of the photodetector 19 configured as described above, according to the following arithmetic expression. A reproduction signal (RF signal), an FE signal, and a TE signal are generated.

First, the RF signal is generated by the following equation (1).
RF = SA + SB + SC + SD (1)
In Expression (1), a signal output from each divided region of the main light receiving unit 191 is indicated by adding “S” to the name of each divided region. The same notation is used in the following formulas (2) and (3).

  The RF signal may be obtained by using signals output from the sub light receiving units 192 and 193 in addition to the signal obtained from the main light receiving unit 191.

The FE signal is generated by the following equation (2).
FE = (SA + SC)-(SB + SD) (2)
Note that this arithmetic expression is based on an astigmatism method which is a well-known method, and a detailed description thereof will be omitted. In the present embodiment, the FE signal is generated using the astigmatism method, but a configuration in which the FE signal is generated by another method may be adopted.

The TE signal is generated by the following equation (3).
TE = (SA + SB)-(SC + SD) -K * ((SE-SF) + (SG-SH)) (3)
In Equation (3), K is a coefficient. The coefficient K can be obtained by, for example, a method of experimentally obtaining a value for obtaining an appropriate TE signal while changing the coefficient to various values.

  In Expression (3), (SA + SB) − (SC + SD) in the previous term is a signal obtained from the light spot S1 received by the main light receiving unit 191. This signal is a signal obtained by adding a DC offset signal generated when the objective lens 16 is shifted to the push-pull signal generated by the track structure of the optical disc D.

  The first term (SE-SF) in equation (3) is a signal obtained from the light spot S2 received by the sub light receiving unit 192. The light spot S2 received by the sub light receiving unit 192 is a light spot of + 1st order light diffracted by the first region 171 of the diffractive optical element 17, and this light spot contains almost no push-pull signal component. . The sub light receiving portions 192 and 192 are provided with a dead zone ND at the center. For this reason, the push-pull signal is not included in the (SE-SF) signal, and only the DC offset signal resulting from the movement of the light spot accompanying the lens shift of the objective lens 16 is included. In this respect, the same applies to (SG-SH) in the second term of the latter term in the formula (3). Therefore, according to the expression (3), a good TE signal from which the influence when the objective lens is shifted is removed can be obtained.

  Here, the operation of the dead zone ND provided in the sub light receiving portions 192 and 193 will be described. FIG. 5 is a schematic diagram showing the relationship between the diffractive optical element and the return light when the objective lens is lens-shifted. In the present embodiment, a configuration is adopted in which the width (length in the short direction) of the first region 171 provided in the diffractive optical element 17 is as wide as possible. When the objective lens 16 is lens-shifted, the push-pull signal component area W of the return light may enter the first area 171 as shown in FIG.

  The push-pull signal component region W enters from the shape (shape of the region W) in the center of the first region 171 (see FIG. 5). For this reason, in the optical pickup 1, a dead zone ND is provided at the center of the sub light receiving portions 192 and 193 so that the light in the portion where the region W enters is not received by the sub light receiving portions 192 and 193. . As a result, the (SE-SF) signal and the (SG-SH) signal do not include the push-pull signal but include only the DC offset signal.

  The effect of the optical pickup 1 provided as described above will be described. The optical pickup 1 is configured such that light irradiated from the semiconductor laser 11 to the optical disc D becomes one light (only one light spot is formed on the optical disc D). For this reason, the light utilization efficiency can be improved as compared with an optical pickup that employs the DPP method to obtain a TE signal. Further, the optical pickup 1 can reduce the influence of stray light, which is a serious problem in the DPP method, when dealing with a multilayer disk (for example, often used in a BD).

  Further, in the optical pickup 1, the area of the light spot received by the sub light receiving units 192 and 193 can be made relatively large with respect to the area of the sub light receiving units 192 and 193, so It is possible to reduce the influence of noise on the signals output from the units 192 and 193. Further, since the light receiving area of the sub light receiving portions 192 and 193 can be reduced by forming the dead zone ND, the influence of stray light in the sub light receiving portions 192 and 193 can be reduced. In other words, the optical pickup 1 can stabilize the signals output from the sub light receiving units 192 and 193 and easily obtain an appropriate TE signal.

  Further, in the optical pickup 1, a good TE signal can be obtained even if the mounting accuracy of the photodetector 19 in the Z direction is somewhat low. In the optical pickup 1, the arrangement direction of the light receiving portions 191 to 193 in the photodetector 19 is a direction substantially orthogonal to the first direction (corresponding to the tracking direction) (see FIG. 4). For this reason, even if the mounting position of the photodetector 19 in the Z direction varies, the light spot is biased to one of the region E and the region F (the region G and the region H in the sub light receiving unit 193) in the sub light receiving unit 192. Is not formed. For this reason, as compared with the above-described conventional optical pickup, it is easy to ensure the performance with respect to the variation in the mounting of the photodetector in the Z direction.

  The embodiment described above is an example of the present invention, and the optical pickup of the present invention is not limited to the configuration described above.

  For example, the configuration of the diffractive optical element 17 described above is merely an example, and can be changed as appropriate. For example, the substantially band-shaped diffraction region (first region) 171 of the diffractive optical element 17 may be configured as shown in FIG. That is, the narrow portion 171a may be provided in the vicinity of the central portion of the substantially band-shaped diffraction region 171 (the range indicated by the arrow T) so that the push-pull signal component region W of the return light does not enter. The shape of the narrow portion 171a may be appropriately changed from the shape shown in FIG. 6 within a range that satisfies the purpose of avoiding the push-pull signal component region W from entering. Note that the shape of the second region 172a and the third region 172b is changed in accordance with the change of the substantially band-shaped diffraction region (first region) 171.

  Further, the second region 172a and the third region 172b of the diffractive optical element 17 may not be provided with a diffraction grating, and this portion may be configured to simply transmit light.

  In addition, the dead zone ND in the sub light receiving units 192 and 193 is not obtained by dividing the sub light receiving units 192 and 193 into two parts, but for example on the sub light receiving units 192 and 193 (or the photodetector 19 and the diffractive optical element 17). And a light shielding member may be disposed between them. Further, the area where the dead zone ND is formed may be provided so that the push-pull signal component area W can be prevented from entering, and as shown in FIG. 7, it is provided only in the central part of the sub light receiving parts 192 and 193. May be.

  Further, by shortening the width (length in the short direction) of the substantially band-like diffraction region (first region) 171 provided in the diffractive optical element 17, the return light is pushed even if the objective lens 16 shifts the lens. As long as the pull signal component region W does not enter the band-shaped diffraction region 171, depending on the case, the sub light receiving portions 192 and 193 may not be provided with dead zones.

  In addition, in the optical pickup 1, when the mounting accuracy of the photodetector 19 in the Z direction is high, as shown in FIG. ) And a parallel direction.

  In the embodiment described above, the optical pickup 1 is configured to correspond to one optical disc. However, the present invention is naturally applicable to, for example, an optical pickup that supports a plurality of types of optical pickups (for example, two types of BD and DVD, three types of BD, DVD, and CD).

  The present invention is suitable for an optical pickup corresponding to, for example, a multilayer disk.

1 Optical pickup 11 Semiconductor laser (light source)
16 Objective Lens 17 Diffractive Optical Element 19 Photodetector (Photodetection Unit)
30 Objective lens actuator 171 First region (substantially belt-like diffraction region)
172a Second region (other diffraction regions)
172b Third region (other diffraction regions)
191 Main light receiving part 192, 193 Sub light receiving part D Optical disc ND Dead band RS Information recording surface W Push-pull signal component area

Claims (6)

An optical pickup having a diffractive optical element in an optical path for guiding reflected light from an optical disc to a light detection unit, and generating a tracking error signal using a signal output from the light detection unit,
Of the reflected light, when two regions generated by overlapping zero-order light and ± first-order light generated by the track structure of the optical disc are set as push-pull signal component regions,
The diffractive optical element includes a substantially band-shaped diffractive region in which a direction substantially orthogonal to the direction in which the two push-pull signal component regions are arranged is a longitudinal direction, and a central portion of the reflected light passes through,
In the light detection unit,
A main light receiving portion that receives light that passes through the diffractive optical element without being diffracted by the diffractive optical element;
A sub light-receiving unit that receives diffracted light diffracted by the substantially band-shaped diffraction region;
Formed,
An optical pickup in which the tracking error signal is generated using signals output from the main light receiving unit and the sub light receiving unit.   The optical pickup according to claim 1, wherein a dead zone that does not detect light is formed at a central portion of the sub light receiving unit.   The optical pickup according to claim 2, wherein the dead zone is formed by the sub light receiving unit including two portions arranged at intervals. An objective lens for condensing the light from the light source on the information recording surface of the optical disc;
An actuator for moving the objective lens in a tracking direction substantially orthogonal to the track of the optical disc;
With
When the objective lens moves in the tracking direction and the direction in which the light spot formed on the light receiving surface of the light detection unit moves is the first direction,
The optical pickup according to claim 1, wherein the main light receiving unit and the sub light receiving unit are arranged in parallel in a direction substantially orthogonal to the first direction. Two sub light receiving portions are provided,
5. The optical pickup according to claim 1, wherein the two sub light receiving parts are arranged symmetrically so as to sandwich the main light receiving part. The diffractive optical element is provided with another diffraction region so as to sandwich the substantially band-shaped diffraction region,
6. The optical pickup according to claim 1, wherein the diffracted light diffracted by the other diffraction region is not received by the main light receiving unit and the sub light receiving unit.

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