JP2013016242A - Optical drive device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical drive device capable of achieving a novel countermeasure against stray light.SOLUTION: An optical drive device 1 includes: a lens system that focuses a light beam generated by a laser light source 2 on an access target layer of a multi-layer optical disk 11 among its plurality of recording layers, and focuses a reflected light beam which is the light beam reflected on the multi-layer optical disk 11 on a photo-detector 5; an astigmatism optical element 25 for applying astigmatism to the reflected light beam having passed through the lens system; and a polarization conversion optical element 31 arranged between the astigmatism optical element 25 and the focusing point of the astigmatism optical element 25. The reflected light beam includes a signal light component reflected by the access target layer and a stray light component reflected by the recording layers among the plurality of recording layers other than the access target layer. The polarization conversion optical element 31 is configured such that the polarization direction of the signal light component and the polarization direction of the stray light component are different from each other at least in a partial region of a spot formed in a light reception plane by the signal light component.

Description

  The present invention relates to an optical drive device, and more particularly to an optical drive device that performs tracking servo using a three-beam method.

  When the three-beam method is used for the tracking servo, the light beam is irradiated onto the disk surface through the diffraction grating. Hereinafter, the 0th-order diffracted light generated by the diffraction grating is referred to as “main beam MB”, the + 1st-order diffracted light is referred to as “subbeam SB1”, and the −1st-order diffracted light is referred to as “subbeam SB2”. The main beam MB is a light beam having an intensity about 10 times that of the sub beams SB1 and SB2.

  The photodetector has three light receiving surfaces for receiving these three types of light beams. These light receiving surfaces are provided for the purpose of receiving the reflected light beam (signal light) reflected by the recording layer of the optical disc, but actually reflected not only by the signal light but also by other than the desired recording layer. So-called “stray light” is also received. When recording / reproducing of a multilayered optical disk (multilayer optical disk), the reflected light beam reflected by the recording layer other than the access target layer also becomes stray light.

  Each of the three types of light beams generates stray light, which spreads greatly and irradiates the three light receiving surfaces. In particular, the intensity of the main beam MB is about 10 times stronger than the intensity of the sub-beams SB1 and SB2, and therefore, the influence of stray light of the main beam MB on the light-receiving surfaces of the sub-beams SB1 and SB2 is large, resulting in a track king error. This can cause a large offset in the signal.

  Various methods have been tried in the past to remove this offset. For example, Patent Document 1 discloses a technique for blocking stray light by arranging a slit between a cylindrical lens used in the astigmatism method and a front focal line position thereof.

Japanese Patent No. 4434300

  By the way, according to the BDXL standard determined in 2010, highly multilayered optical discs such as three layers and four layers will be used in the future. In such an advanced multilayer optical disc, the interlayer distance is small, so that the intensity ratio of stray light to signal light is larger than the intensity ratio in the conventional two-layer optical disc.

  In addition to the amount of received signal light and the amount of received stray light, the interference between the signal light and stray light is added to the output signal of the light receiving surface, and this interference amount includes the intensity ratio of stray light to the signal light. There is a feature that the larger the larger, the larger. For this reason, the advanced multi-layer optical disc has a larger amount of interference than the two-layer optical disc, but particularly the sub-beams SB1 and SB2 have a high intensity ratio of signal light and stray light (especially stray light of the main beam MB). In a multilayer optical disc, the amount of interference becomes a very large value. Therefore, the offset disclosed in Patent Document 1 can no longer sufficiently remove the offset generated in the tracking error signal, and a further countermeasure against stray light is required.

  Accordingly, one of the objects of the present invention is to provide an optical drive device capable of realizing a new countermeasure against stray light.

  To achieve the above object, an optical drive device according to the present invention provides a laser light source, a photodetector having a light receiving surface, and a light beam generated by the laser light source to be accessed among a plurality of recording layers of a multilayer optical disc. A lens system for condensing the reflected light beam, which is the light beam reflected by the multilayer optical disc, and condensing the reflected light beam that has passed through the lens system; An astigmatism optical element; and a polarization conversion optical element disposed between the astigmatism optical element and a focal point of the astigmatism optical element, and the reflected light beam is reflected by the access target layer A signal light component and a stray light component reflected by a recording layer other than the access target layer among the plurality of recording layers, and the polarization conversion optical element has the signal light component formed in the light receiving surface. That for at least part of the area of the spot, the polarization direction of the polarization direction and the stray light component of the signal light component, characterized in that it is set differently.

  According to the present invention, it is possible to suppress interference between signal light and stray light with respect to at least a part of a spot formed by a signal light component in a light receiving surface. Therefore, it is possible to reduce the offset generated in the tracking error signal.

  In the optical drive device, the polarization conversion optical element may be disposed at a front focal line position close to the astigmatism optical element among two focal line positions of the astigmatism optical element.

  Further, in the optical drive device, the polarization conversion optical element is arranged such that half of the cross-sectional shape of the signal light component at the front focal line position overlaps the polarization conversion optical element. Also good.

  In the optical drive device, the polarization conversion optical element further sandwiches a straight line corresponding to a center line in a short direction in a cross-sectional shape of the signal light component at the front focal line position on the light receiving surface. It is good also as arrange | positioning so that the polarization direction of the said stray light component may differ in the area | region of one side, and the area | region of the other side.

  In this optical drive device, the polarization conversion optical element may be a right-angled isosceles triangle, and the base thereof may be arranged so as to coincide with the center line.

  In the optical drive device, the polarization conversion optical element may be arranged so that all of the cross-sectional shape of the signal light component at the front focal line position overlaps the polarization conversion optical element.

  Each of the optical drive devices may further include a shielding plate that shields both outer sides of the cross-sectional shape of the signal light component at the front focal line position in the short direction.

  In each of the above optical drive devices, the polarization conversion optical element may be configured integrally with the photodetector.

  In each of the above optical drive devices, the polarization conversion optical element may have at least one of a function of converting P-polarized light to S-polarized light or a function of converting S-polarized light to P-polarized light. The polarization conversion optical element may be a half-wave plate.

  According to the present invention, it is possible to suppress interference between signal light and stray light with respect to at least a part of a spot formed by a signal light component in a light receiving surface. Therefore, it is possible to reduce the offset generated in the tracking error signal.

1 is a schematic diagram of an optical drive device according to a first embodiment of the present invention. It is explanatory drawing of the astigmatism provided by a sensor lens. It is a top view of the photodetector by the 1st Embodiment of this invention. (A) is the figure which expanded and drawn the vicinity of the photodetector by the 1st Embodiment of this invention. (B) is the figure which drew the vicinity of the photodetector of the optical drive device by the background art of this invention similarly to (a). (A) and (b) are plan views of the front focal line position and the focal point, respectively, in the optical drive device according to the first embodiment of the present invention. (A) and (b) are the top views of the front focal line position and the focal point, respectively, when a positional deviation has occurred, according to the first embodiment of the present invention. (A) and (b) are respectively plan views of the front focal line position and the focal point when a lens shift occurs according to the first embodiment of the present invention. It is the figure which expanded the light-receiving surface by the 1st Embodiment of this invention. (A) and (b) are plan views of the front focal line position and the focal point, respectively, in the optical drive device according to the second embodiment of the present invention. (A) and (b) are the top views of the front focal line position and the focal point when the position shift has occurred according to the second embodiment of the present invention. (A) and (b) are respectively plan views of the front focal line position and the focal point in the optical drive device according to the third embodiment of the present invention.

  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 a first embodiment of the present invention.

  The optical drive device 1 performs at least one of reproduction and recording of the optical disk 11. Various optical recording media such as CD, DVD, and BD can be used as the optical disc 11, but in this embodiment, a disk-shaped optical disc (multilayer optical disc) having a recording surface that is 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.

  As shown in FIG. 1, a polarization conversion optical element 31 is fixed to the light receiving surface of the photodetector 5 with a glass 30 interposed therebetween. Accordingly, the polarization conversion optical element 31 is disposed between the sensor lens 25 and its focal point (described later). Although these are the features that characterize the present invention, in the following, other configurations will be described first, and the glass 30 and the polarization conversion optical element 31 will be separately described later.

  The optical system 3 includes a diffraction grating 21, a beam splitter 22, a collimator lens 23, a ¼ wavelength plate 24, and a sensor lens (astigmatism optical element, cylindrical lens) 25. The optical system 3 functions as an outward optical system that guides the light beam generated by the laser light source 2 to the optical disc 11, and also serves as a return optical system that guides the reflected light beam, which is a light beam reflected by the optical disc 11, to the photodetector 5. Function.

  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 makes it incident on the beam splitter 22 as S-polarized light. The beam splitter 22 reflects the incident S-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, together with the collimator lens 23, condenses the light beam generated by the laser light source 2 on the access target layer among the plurality of recording layers of the optical disc 11 and detects the reflected light beam reflected by the optical disc 11. A lens system for condensing light onto the device 5 is constructed. Specifically, the objective lens 4 condenses the light beam incident from the optical system 3 (the light beam that has passed through the collimator lens 23 and becomes parallel light) on the optical disk 11, and is recorded on the recording surface of the optical disk 11. The reflected light beam reflected is returned to parallel light. The reflected light beam returned to the parallel light passes through the collimator lens 23 and is condensed on the photodetector 5.

  Here, the reflected light beam is diffracted by the land group of the recording surface, and is decomposed into 0th order diffracted light and ± 1st order diffracted light. 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. In this specification, the 0th-order diffracted light, the + 1st-order diffracted light, and the −1st-order diffracted light decomposed by the diffraction grating 21 are referred to as a main beam MB, a subbeam SB1, and a subbeam SB2, respectively. In this case, it refers to the diffracted light generated by the diffraction at the land group of the recording surface. The main beam MB, the sub beam SB1, and the sub beam SB2 each independently generate a reflected light beam.

  In the return path optical system, the reflected light beam that has passed through the objective lens 4 and has become P-polarized light by reciprocating the quarter-wave plate 24 is incident on the collimator lens 23. The reflected 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 reflected light beam and makes it incident on the sensor lens 25 (cylindrical lens). The sensor lens 25 gives astigmatism to the reflected light beam incident from the beam splitter 22. The reflected 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 and the sensor lens 25 shown in FIG. 1 differs between the MY axis direction and the MX axis direction (bus line direction) that is perpendicular to the MY axis direction. ing.

  The position where the focus in the MY axis direction is focused is referred to as “front focal line position” as shown in FIG. The cross-sectional shape of the reflected light beam at this position is a rectangular shape elongated in the MX axis direction. Similarly, the position where the focal point in the MX-axis direction is focused is referred to as a “rear focal line position”. The cross-sectional shape of the reflected light beam at this position is a rectangular shape elongated in the MY axis direction. The point where the lengths of the reflected light beams in the MY-axis direction and the MX-axis direction are equal is called “focusing point”.

  In the optical drive device 1, an objective lens for causing the focal point of the reflected light beam (signal light component) reflected by the layer to be focused (access target layer) to be located on the photodetector 5. 4 is controlled (focus servo). In other words, the focal point of the light beam (stray light component) reflected by the recording layer other than the access target layer is not located on the photodetector 5, and the spot (stray light) formed on the photodetector 5 by the stray light component. The spot) has a shape that is greatly expanded as compared with a spot (signal light spot) formed by the signal light component on the photodetector 5.

  Returning to FIG. The photodetector 5 is installed on a plane that intersects the optical path of the reflected light beam emitted from the optical system 3. The photodetector 5 includes three light receiving surfaces, and each light receiving surface is divided into a predetermined number of light receiving regions. The photodetector 5 is configured to output a signal having an amplitude of a value (amount of received light) obtained by dividing the intensity of the light beam by the area of the light receiving surface for each light receiving region.

  FIG. 3 is a top view of the photodetector 5. In the figure, signal light spots of the respective reflected light beams are also shown. As shown in the figure, the photodetector 5 has light receiving surfaces 5a to 5c that are all square. The light receiving surfaces 5a to 5c are provided for the purpose of receiving the main beam MB, the sub beam SB1, and the sub beam SB2, respectively, and are arranged at positions where signal light spots of the corresponding reflected light beams are formed.

  As shown in FIG. 3, each of the light receiving surfaces 5a to 5c is divided into four light receiving regions each having a square shape. Specifically, counterclockwise from the upper left, the light receiving surface 5a is in four light receiving regions A to D, the light receiving surface 5b is in four light receiving regions E1 to E4, and the light receiving surface 5c is four light receiving regions F1 to F4. It is divided into four. The optical drive device 1 can generate various signals such as a focus error signal FE, a full addition signal (pull-in signal PI, an RF signal RF), and a tracking error signal TE by using an appropriate combination of these light receiving regions. It has become.

  Returning to FIG. 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, A focus error signal FE, a full addition signal (pull-in signal PI, RF signal RF), and a tracking error signal TE are generated.

A specific method for generating each signal by the processing unit 6 will be briefly described. In the following description, I _X represents the amount of light received in the light receiving region X.

  The focus error signal FE is preferably generated by the following equation (1) or equation (2). However, k is a constant. Equation (1) represents the astigmatism method by the one-beam method, and equation (2) represents the astigmatism method (differential astigmatism method) by the three-beam method.

  The tracking error signal TE is preferably generated by the following equations (3) to (5). However, k is a constant. Expressions (3) to (5) represent the differential push-pull method. Alternatively, the tracking error signal TE may be generated using a phase difference detection method.

  Here, the interference between the signal light and the stray light affects the amount of light received in each light receiving region. Therefore, when the interference between the signal light and the stray light occurs, each term on the right side of the equations (4) and (5) changes, which causes an offset in the tracking error signal. In the present invention, the occurrence of interference between signal light and stray light is suppressed, thereby reducing the offset generated in the tracking error signal.

  Since the pull-in signal PI and the RF signal RF are full addition signals, they can be generated by adding the received light amounts of all the light receiving regions of the light receiving surface 5a as in the following equation (6).

  Returning to FIG. 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). The processing unit 6 that has received this instruction signal performs focus servo and tracking servo. The focus servo is a process of focusing on the surface of the access target layer by moving the objective lens 4 perpendicularly to the surface of the optical disc 11 until the focus error signal FE becomes zero. The tracking servo is a process for realizing a track-on state by moving the objective lens 4 in parallel with the surface of the optical disc 11 to a position where the tracking error signal TE becomes zero (this movement is called “lens shift”). The tracking servo is executed in a state in which the focus servo is on (a state where the focal point of the signal light component is on the light receiving surface of the photodetector 5). When the track is turned on, the CPU 7 acquires the RF signal RF generated by the processing unit 6 as a data signal.

  Now, the configuration and function of the glass 30 and the polarization conversion optical element 31 will be described.

  FIG. 4A is an enlarged view of the vicinity of the photodetector 5. The figure also shows a substrate 40 that was not explicitly shown in FIG.

  As shown in FIG. 4A, the polarization conversion optical element 31 is fixed to the front surface of the photodetector 5 (the surface on which the light receiving surfaces 5a to 5c are provided) with the glass 30 interposed therebetween. These are integrally formed. The thickness D of the glass 30 takes into consideration the refractive index of the glass 30, and the focal point of the signal light component and the front side when the focus servo is on (when the focal point of the signal light component is on the light receiving surface of the photodetector 5). It is set to be equal to the distance between the focal line positions. Hereinafter, the front focal line position simply refers to the front focal line position of the signal light component when the focus servo is on. By setting the thickness D of the glass 30 as described above, the polarization conversion optical element 31 is disposed at the front focal line position.

  In addition, although it is preferable to arrange | position the polarization conversion optical element 31 in a front side focal line position as mentioned above, it does not necessarily have to correspond with a front side focal line position completely. The specific arrangement of the polarization conversion optical element 31 may be arbitrarily determined within a range in which the object of the present invention of suppressing interference between signal light and stray light is achieved.

  Each of the light receiving regions of the light receiving surfaces 5a to 5c has a wiring pattern formed on the surface of the substrate 40 via a through electrode TH penetrating the photodetector 5 and a bump BP provided on the back surface of the photodetector 5. (Not shown). A signal indicating the amount of light received in each light receiving region (the output signal of the photodetector 5) is transmitted to the substrate 40 through this route.

  As a reference, an example of an optical drive device according to the background art will be given. FIG. 4B is a diagram depicting the vicinity of the photodetector 5 of the optical drive device according to the background art in the same manner as FIG. 4A. As shown in the figure, in the background art, the output signal of the photodetector 5 is transmitted to the substrate 40 by using the bonding wire WB and the bonding pad P provided on the substrate 40. However, in the present embodiment, since the glass 30 is present and the bonding wire WB cannot be used, the output signal of the photodetector 5 is transmitted to the substrate 40 using the through electrode TH as described above. Communicating.

  The polarization conversion optical element 31 is an element that converts the polarization direction of a light beam that passes through. Specifically, at least one of a function of converting P-polarized light into S-polarized light and a function of converting S-polarized light into P-polarized light. Have For example, a half-wave plate can be suitably used as the polarization conversion optical element 31. In the present embodiment, the reflected light beam emitted from the optical system 3 toward the photodetector 5 is P-polarized light for both the signal light component and the stray light component. When the reflected light beam passes through the polarization conversion optical element 31, the polarization direction of the portion that has passed through changes to S-polarized light.

FIGS. 5A and 5B are plan views of the front focal line position and the focal point, respectively, in the optical drive device according to the present embodiment. FIG. 5A shows the cross-sectional shapes at the front focal line position for the signal light components of the main beam MB, the sub beams SB1 and SB2, and the stray light component (denoted as M _MB ) of the main beam MB. . FIG. 5B shows a spot (signal light spot) formed on the photodetector 5 by the signal light components of the main beam MB and the sub beams SB1 and SB2, and the stray light component M _MB of the main beam _MB is light. The spots (stray light spots) formed on the detector 5 are described. In both figures, these cross-sectional shapes and spots are hatched, and the polarization direction of each component is represented by this hatching. The specific correspondence between hatching and polarization direction is as shown in the figure.

  As shown in FIG. 5A, a polarization conversion optical element 31 is disposed at the front focal line position. The specific position and shape of the polarization conversion optical element 31 are set so that the polarization direction of the signal light component and the polarization direction of the stray light component are different with respect to at least a part of the signal light spot, preferably the whole region. . By doing so, the interference between the signal light and the stray light is suppressed, and the offset generated in the tracking error signal can be reduced.

  FIG. 5A discloses a specific example of the shape of the polarization conversion optical element 31 in which the polarization direction of the signal light component is different from the polarization direction of the stray light component in the entire region of the signal light spot. doing. As shown in FIG. 5A, the polarization conversion optical element 31 according to the present embodiment includes half-wave plates 31a, 31b, and 31c corresponding to the main beam MB and the sub beams SB1 and SB2, respectively. The half-wave plates 31a, 31b, and 31c are different only in the corresponding reflected light beams, and the arrangement relationship with respect to the corresponding reflected light beams is the same. Therefore, the half-wave plate 31a is described below as an example. To do.

  As shown in FIG. 5A, the cross-sectional shape of the signal light component of the main beam MB at the front focal line position is rectangular. When the center line C in the short direction is projected onto the light receiving surface 5a, a straight line D (FIG. 5B) that coincides with one diagonal line of the light receiving surface 5a that is square. When the region on one side of the straight line D of the light receiving surface 5a is projected on the front focal line position, a right isosceles triangular region is obtained at the front focal line position. The half-wave plate 31a is a right-angled isosceles triangle half-wave plate whose shape and position are both equal to this right-angled isosceles triangle region.

  When the half-wave plate 31a as described above is used, half of the cross-sectional shape of the signal light component of the main beam MB at the front focal line position is a half in the short direction (one side region with the center line C in between). As shown in FIG. 5A, the light passes through the half-wave plate 31a.

  The signal light component has a property that the area is switched between the front focal line position and the light receiving surface with the center line C or the straight line D interposed therebetween. In other words, the signal light component that passes through the region on the one side of the center line C (region on the upper right side of the drawing) at the front focal line position is transferred to the region on the other side of the straight line D (region on the lower left side of the drawing). Irradiated. On the other hand, the signal light component that passes through the other side of the center line C (region on the lower left side in the drawing) at the front focal line position is irradiated to the region on one side of the straight line D (region on the upper right side in the drawing) on the light receiving surface 5a. The Therefore, by arranging the half-wave plate 31a as described above, as shown in FIG. 5 (b), the region on the other side of the straight line D (region on the lower left side in the drawing) of the signal light spot is S-polarized light. The region on one side of the straight line D (the region on the upper right side of the drawing) is P-polarized light.

On the other hand, in the stray light component M _MB that is a reflected light beam that is out of focus, the replacement of the region like the signal light component does not occur. Further, since the stray light component M _MB may be considered to be substantially parallel light, the cross-sectional shape of the stray light component M _MB at the front focal line position and the shape of the stray light spot formed on the photodetector 5 are approximately Be the same. Accordingly, the S-polarized light in the stray light spot is a portion that is irradiated to the region on one side of the straight line D (region on the upper right side of the drawing) of the light receiving surface 5a.

  In this way, the signal light spot becomes P-polarized light in the region on one side of the light receiving surface 5a across the straight line D, and becomes S-polarized light in the region on the other side. On the other hand, the stray light spot becomes S-polarized light in the region on one side of the light receiving surface 5a across the straight line D, and becomes P-polarized light in the region on the other side. Therefore, in this embodiment, the polarization direction of the signal light component and the polarization direction of the stray light component are different in the entire region of the signal light spot, and interference between the signal light and the stray light is suppressed.

  Here, as described above, the shapes of the half-wave plates 31a, 31b, and 31c are right-angled isosceles triangles. However, when there is a positional shift or lens shift between the optical beam and the photodetector 5. Any shape other than a right-angled isosceles triangle can be adopted as long as it can cover the range in which the signal light shifts with respect to the light receiving surface.

  As described above, according to the optical drive device 1 according to the present embodiment, interference between signal light and stray light is suppressed. Therefore, it is possible to reduce the offset generated in the tracking error signal.

  For example, if the numerical aperture of the objective lens 4 is 0.85 and the return optical magnification is 15 times, the width of the cross-sectional shape of the signal light component of each beam at the front focal line position (length in the short direction) Is about 5 μm. Therefore, when the polarization conversion optical element 31 is arranged on the glass 30, it is necessary to carry out with the same high accuracy as a semiconductor process. For example, a thin film half-wave plate is used as the polarization conversion optical element 31, and another semiconductor component is formed on the substrate 40 shown in FIG. It is preferable to form the element 31.

  Up to this point, an ideal case in which there is no positional deviation between the optical beam and the photodetector 5 has been described, but in reality, a positional deviation caused by an assembly accuracy error at the time of manufacture or a lens caused by a tracking servo. Due to the shift or the like, a positional deviation occurs between the optical beam and the photodetector 5. In the following, two examples of cases where such misalignment occurs will be described, and it will be described that the effects of the present invention can be obtained even in such cases.

  FIGS. 6A and 6B are plan views of the front focal line position and the focal point, respectively, in the case where a positional shift due to an assembly accuracy error has occurred. This figure shows an example in which the signal light component does not completely pass through the polarization conversion optical element 31. In this example, as shown in the figure, the position of the signal light spot formed on the photodetector 5 is also shifted from the center of the corresponding light receiving surface. Further, the polarization direction of the signal light spot remains P-polarized in all regions.

  Since there is no positional deviation between the photodetector 5 and the polarization conversion optical element 31, the stray light spot is a straight line on the light receiving surface 5a as described in FIGS. 5 (a) and 5 (b). The part irradiated on one side of D (the upper right side of the drawing) is S-polarized light and the other part is P-polarized light.

  Also in this example, as shown in FIG. 6B, there is a slight overlap between the P-polarized signal light spot and the stray light spot that is S-polarized light. And stray light interference can be suppressed.

  As described above, according to the optical drive device 1, even when the positional deviation due to the assembly accuracy error occurs, interference between the signal light and the stray light is caused at least in a part of the signal light spot. Can be suppressed. Therefore, although it is not large, it is possible to obtain the effect of reducing the offset generated in the track king error signal.

  Next, FIGS. 7A and 7B are plan views of the front focal line position and the focal point when the lens shift occurs due to the tracking servo. When the lens shift occurs, the cross-sectional shape of the signal light component at the front focal line position moves in the longitudinal direction (direction A shown in the drawing). Since the amount of movement does not become so large, the state that the region on one side across the center line C passes through the polarization conversion optical element 31 is maintained even when a lens shift occurs.

  On the photodetector 5, the movement direction of the signal light spot when the lens shift occurs is the vertical direction (direction B shown in the drawing). Although the stray light spot also moves, the moving direction is different from the signal light spot.

  FIG. 8 is an enlarged view of the light receiving surface 5a. As shown in the figure, in this example, as a result of the signal light spot moving in the B direction, a part of the signal light spot that is S-polarized bites into the region where the stray light spot is S-polarized. (Part X shown in the figure). For this reason, the effect of suppressing the interference between the signal light and the stray light does not work in this biting portion, but the effect of suppressing the effect can be obtained as described above in the other portions.

  As described above, according to the optical drive device 1, even when a lens shift occurs due to tracking servo, the interference between the signal light and the stray light can be suppressed in most regions in the signal light spot. . Therefore, although not perfect, it is possible to obtain the effect of reducing the offset generated in the track king error signal.

  As is clear from FIG. 7B, the interference between the signal light and the stray light accompanying the lens shift occurs in each of the main beam MB and the sub beams SB1 and SB2, but from the viewpoint of the effect on the tracking error signal TE. Of these, the interference components in the sub-beams SB1 and SB2 are dominant. Therefore, in order to reduce the offset generated in the tracking error signal due to the lens shift, it is particularly important to reduce the interference component in the sub beams SB1 and SB2. In this regard, according to the optical drive device 1, the sub-beams SB1 and SB2 can also suppress the interference between the signal light and the stray light in almost the entire region of the signal light spot. It can be effectively reduced. This will be described in detail below.

  First, the interference components in the sub-beams SB1 and SB2 have a k-fold influence on the tracking error signal TE compared to the interference component in the main beam MB regardless of the presence or absence of lens shift. This is because the amount of received light of the sub-beams SB1 and SB2 is multiplied by k (k> 1) when calculating the tracking error signal TE as shown in the equation (3).

  When the lens shift occurs, interference components in the sub beams SB1 and SB2 increase. This is because not only the signal light spot but also the stray light spot moves with the lens shift, and as a result, the interference component becomes asymmetric between the light receiving surface 5b and the light receiving surface 5c. Therefore, when the lens shift occurs, the influence of the interference component in the sub beams SB1 and SB2 on the tracking error signal TE becomes more dominant. According to the optical drive device 1, since the interference components in the sub beams SB1 and SB2 having such a dominant influence can be suppressed, it is possible to effectively reduce the offset generated in the track king error signal.

  FIGS. 9A and 9B are plan views of the front focal line position and the focal point in the optical drive device 1 according to the second embodiment of the present invention. The meanings of the constituent elements and hatching appearing in the drawings are the same as those in FIGS. 5A and 5B, and a detailed description thereof will be omitted.

  The optical drive device 1 according to the present embodiment is optical in accordance with the first embodiment in that the shielding plate 32 is provided at the front focal line position and the shape of the polarization conversion optical element 31 is changed accordingly. Different from the drive device 1. Hereinafter, the differences will be mainly described in detail.

  The shielding plates 32 are provided so as to shield both outer sides of the cross-sectional shape of the signal light component at the front focal line position in the short direction. Specifically, as shown in FIG. 9A, the shield plate has an outer periphery sufficiently wider than a region corresponding to the light receiving surface and has openings 32a to 32c through which signal light components pass. That's fine. The openings 32a to 32c are preferably slightly larger than the cross-sectional shape of the signal light component.

  In FIG. 9A, the width of the opening 32a in the short direction is larger than that of the openings 32b and 32c. This is a configuration corresponding to the astigmatism method using the one-beam method. . That is, in order to perform focus servo by the astigmatism method, it is necessary to correctly receive the signal light component not only at the focal point but also at a position slightly deviated from the focal point. Therefore, the width in the short direction of the opening 32a is secured slightly larger so that all the signal light components can pass through at a position slightly shifted from the focal point. When performing the astigmatism method using the three-beam method, the widths in the short direction of the openings 32b and 32c may be secured slightly larger as well.

  In FIG. 9A, the width in the longitudinal direction of the openings 32a to 32c is made larger than the cross-sectional shape of the signal light component. This corresponds to the movement of the cross-sectional shape of the signal light component due to the lens shift described above.

  As in the first embodiment, the polarization conversion optical element 31 includes half-wave plates 31a, 31b, and 31c corresponding to the main beam MB and the sub beams SB1 and SB2, respectively. Also in this embodiment, the half-wave plates 31a, 31b, and 31c are different in the corresponding reflected light beams, and the arrangement relationship with respect to the corresponding reflected light beams is the same. A description will be given by taking the plate 31a as an example.

  Similarly to the first embodiment, the half-wave plate 31a is half of the short-side direction in the cross-sectional shape of the signal light component of the main beam MB at the front focal line position (on one side across the center line C). Is disposed so as to pass through the polarization converting optical element 31. Specifically, the half-wave plate 31a is constituted by a half-wave plate that occupies one region when the opening 32a is divided into two regions by the center line C.

  Therefore, for the signal light spot, as in the first embodiment, as shown in FIG. 9B, the other side of the straight line D (lower left side in the drawing) is one of the S-polarized light and the straight line D. The side region (the upper right region in the drawing) is P-polarized light.

  On the other hand, the stray light spot is the same as in the first embodiment in that the region on one side of the straight line D of the light receiving surface 5a (the region on the upper right side in the drawing) is S-polarized light and the other portion is P-polarized light. However, since it is largely shielded by the shielding plate 32, as shown in FIG. 9B, there are many areas where stray light spots are not formed. For this reason, as in the first embodiment, the interference between the signal light and the stray light does not occur, and the received light amount of the stray light itself is reduced. Therefore, in the present embodiment, further reduction of the offset generated in the track king error signal is realized. Similarly to the first embodiment, the sub-beams SB1 and SB2 can also suppress the interference between the signal light and the stray light in almost the entire region of the signal light spot. The offset that occurs in the track king error signal is effectively reduced.

  The present embodiment has a greater effect than the first embodiment particularly when the above-described “positional displacement due to assembly accuracy error” has occurred. Hereinafter, this point will be described.

  FIGS. 10A and 10B are plan views of the front focal line position and the focal point when a positional deviation due to the assembly accuracy error occurs. This figure shows an example in which the signal light component does not completely pass through the polarization conversion optical element 31. In this example, as in the case of the first embodiment shown in FIG. 6B, the position of the signal light spot formed on the photodetector 5 is also shifted from the center of the corresponding light receiving surface. Further, the polarization direction of the signal light spot remains P-polarized in all regions.

  The stray light spot is the same as the example shown in FIG. 6B in terms of the polarization direction. On the other hand, however, the provision of the shielding plate 32 prevents stray light spots from being formed in many portions of the light receiving surface as shown in FIG. For this reason, compared with the example shown in FIG. 6B, the area where the signal light and the stray light interfere clearly decreases. That is, the area where the stray light becomes P-polarized light, which is an interference area, is a half area of the opening, and thus is greatly reduced. From this, it can be said that according to the present embodiment, the offset generated in the track king error signal can be reduced as compared with the first embodiment.

  FIGS. 11A and 11B are plan views of the front focal line position and the focal point, respectively, in the optical drive device 1 according to the third embodiment of the present invention. The meanings of the constituent elements and hatching appearing in the drawings are the same as those in FIGS. 5A and 5B, and a detailed description thereof will be omitted.

  The optical drive device 1 according to the present embodiment is different from the optical drive device 1 according to the first embodiment in that the shape and arrangement of the polarization conversion optical element 31 are changed. Hereinafter, the differences will be mainly described in detail.

  As shown in FIG. 11A, the polarization conversion optical element 31 according to the present embodiment is arranged so that the entire cross-sectional shape of the signal light component at the front focal line position overlaps with the polarization conversion optical element 31. . In this case, the specific positions of the half-wave plates 31a to 31c constituting the polarization conversion optical element 31 may be the same as the openings 32a to 32c described in the second embodiment. Is preferred. On the other hand, as for the specific sizes of the half-wave plates 31a to 31c, as shown in FIG. 11 (a), the width in the shorter direction than the openings 32a to 32c shown in FIG. Can be small. That is, in the second embodiment using the shielding plate 32, it is necessary to make the width in the short direction of the openings 32a to 32c somewhat larger than the width of the signal light in order not to lose the signal light. On the other hand, in the present embodiment, no loss of signal light occurs. Therefore, even if the positional deviation between the optical system and the photodetector 5 is taken into consideration, at least in comparison with the openings 32a to 32c of the shielding plate 32, The width in the short direction of the polarization conversion optical element 31 can be greatly reduced. In addition, since it is not necessary to consider the signal light loss of the main beam MB according to the astigmatism method, the width in the short direction of the half-wave plate 31a applied to the main beam MB is set to 1 / of the sub beams SB1 and SB2. It becomes possible to make it as small as the two-wave plates 31b and 31c. Thus, by reducing the width in the short direction of the half-wave plates 31a to 31c, the region where the signal light and the stray light overlap can be reduced on the light receiving surface.

  By arranging the polarization conversion optical element 31 as described above, all the signal light spots become S-polarized light as shown in FIG. On the other hand, as shown in FIG. 11B, the stray light spot is S-polarized only in the region where the polarization conversion optical element 31 is projected on the photodetector 5, and the other is P-polarized.

  Therefore, as shown in FIG. 11B, in the vicinity of the center of the signal light spot, the signal light spot and the stray light spot are both S-polarized light on any of the light receiving surfaces 5a to 5c, and the signal light and the stray light interfere with each other. . On the other hand, according to the present embodiment, the area of the interference region does not change even during lens shift. That is, as compared with the first and second embodiments, the present embodiment can reduce the area of the interference region when there is a lens shift, and thus the effect of reducing stray light is increased.

  Here, as described above, the widths of the half-wave plates 31a to 31c constituting the polarization conversion optical element 31 need not be so large. Specifically, 5 to 6 μm is sufficient. The width in the short direction of the S-polarized light region of the stray light spot is approximately this level.

On the other hand, the diameter of the signal light spot is about 50 μm. Assuming that the diameter of the signal light spot is 50 μm and the width of the S polarization region of the stray light spot in the short direction is 5 μm, the area of the interference region (the area of the overlapping region of the signal light spot and the S polarization region of the stray light spot) ) Is approximately 250 μm ² . In view of the fact that the area of the signal light spot is π × 25 ² ≈1963 μm ² , in the present embodiment, the area of the interference region is (250/1963) × compared with the background art that does not use the polarization conversion optical element 31. That is, it is reduced to 100≈13%. This means that the interference between the signal light and the stray light is reduced to about 13%.

  According to the present embodiment, it is possible to reduce interference between signal light and stray light as compared with the first and second embodiments. This will be described in detail below.

  The magnitude of the interference can be compared using a parameter corresponding to X shown in FIG. 8 (width of a region where interference between signal light and stray light occurs. Hereinafter, referred to as “interference width”). The case where the shift amount of the signal light during the lens shift is about 10 μm will be described as an example. First, the interference width (= X) in the first embodiment (FIG. 8) is the signal light spot in the B direction. Is 10 μm, so that 10 × (1 / √2) = 7 μm. Next, the interference width in the second embodiment (FIG. 9) is that the width of the corresponding opening is 14 μm or less (the width of the corresponding half-wave plate is 7 μm or less). Equal to half the width, otherwise 7 μm. Explaining with specific numerical values, since the width of the opening 32a is about 50 μm and the width of the openings 32b and 32c is about 15 μm, respectively, the interference width is the main beam MB and the sub beams SB1 and SB2. It becomes about 7 μm.

  On the other hand, the interference width in the present embodiment (FIG. 11) is the above-mentioned “width in the short direction of the S-polarized region of the stray light spot”, which is about 5 to 6 μm. This value of about 5 to 6 μm is a value smaller than the value in any of the first and second embodiments described above. This means that the interference between the signal light and the stray light is reduced in the present embodiment as compared with the first and second embodiments.

  Regarding the second embodiment, for example, it is conceivable to make the widths of the openings 32b and 32c smaller (14 μm or less). In this way, the interference width can be reduced. However, since loss of signal light is likely to occur, it is necessary to make appropriate adjustments in the actual usage environment so as not to cause a problem in the operation of the optical drive device 1. become. In the present embodiment, such a loss of signal light does not occur in the first place. Therefore, it is possible to easily reduce the interference width as compared with the second embodiment.

  Since the interference between the signal light and the stray light is reduced as described above, the offset generated in the tracking error signal can be further reduced in the present embodiment as compared with the first and second embodiments. It has become. Similarly to the first and second embodiments, the sub-beams SB1 and SB2 can also suppress the interference between the signal light and the stray light in almost all areas in the signal light spot. Accordingly, the offset generated in the track king error signal is effectively 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.

DESCRIPTION OF SYMBOLS 1 Optical drive device 2 Laser light source 3 Optical system 4 Objective lens 5 Photodetector 5a-5c Light-receiving surface 6 Processing part 11 Optical disk 21 Diffraction grating 22 Beam splitter 23 Collimator lens 24 1/4 wavelength plate 25 Sensor lens 30 Glass 31 Polarization conversion Optical elements 31a to 31c Half-wave plate 32 Shield plates 32a to 32c Opening 40 Substrate A to D, E1 to E4, F1 to F4 Light receiving region BP Bump MB main beam M _MB main beam stray light component P Bonding pads SB1 to SB1 SB2 Sub beam TH Through electrode WB Bonding wire

Claims (10)

A laser light source;
A photodetector having a light receiving surface;
The light beam generated by the laser light source is focused on the access target layer of the plurality of recording layers of the multilayer optical disc, and the reflected light beam reflected by the multilayer optical disc is collected on the photodetector. A lens system to light,
An astigmatism optical element that gives astigmatism to the reflected light beam that has passed through the lens system;
A polarization conversion optical element disposed between the astigmatism optical element and a focal point of the astigmatism optical element;
The reflected light beam includes a signal light component reflected by the access target layer, and a stray light component reflected by a recording layer other than the access target layer among the plurality of recording layers,
The polarization conversion optical element is set such that the polarization direction of the signal light component and the polarization direction of the stray light component are different with respect to at least a partial region of the spot formed by the signal light component in the light receiving surface. An optical drive device. 2. The optical drive according to claim 1, wherein the polarization conversion optical element is disposed at a front focal line position close to the astigmatism optical element among two focal line positions of the astigmatism optical element. apparatus. The said polarization conversion optical element is arrange | positioned so that the half of a transversal direction among the cross-sectional shapes of the said signal light component in the said front side focal line position may overlap with the said polarization conversion optical element. Optical drive device. The polarization converting optical element includes a region on one side and a region on the other side across a straight line corresponding to a center line in a short direction in a cross-sectional shape of the signal light component at the front focal line position on the light receiving surface. The optical drive device according to claim 3, wherein the stray light component is arranged so that a polarization direction thereof differs from region to region. The optical drive device according to claim 4, wherein the polarization conversion optical element is a right-angled isosceles triangle, and is arranged so that a base thereof coincides with the center line. The optical drive device according to claim 2, wherein the polarization conversion optical element is arranged so that all of the cross-sectional shape of the signal light component at the front focal line position overlaps the polarization conversion optical element. The optical drive device according to any one of claims 1 to 6, further comprising shielding plates that shield both outer sides of the cross-sectional shape of the signal light component at the front focal line position in a short direction. . The optical drive device according to any one of claims 1 to 7, wherein the polarization conversion optical element is configured integrally with the photodetector. The said polarization conversion optical element has at least one of either the function which converts P polarized light into S polarized light, or the function which converts S polarized light into P polarized light. Optical drive device. The optical drive device according to claim 9, wherein the polarization conversion optical element is a half-wave plate.

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