An optical pickup device

Abstract: In an optical pickup device in which a main beam and a pair of sub-beams are used and a diffraction device (3,13) is disposed between a recording medium (6) and a light-receiving device (7,27) such as a photodetector, the diffraction device (3) comprises first, second and third diffraction regions (3a, 3b, 3c). The second and third regions (3b, 3c) receive light beams from the recording medium (6) which are substantially identical in amount to each other. The light-receiving device (7) comprises first, second, third and fourth light-receiving regions (7a,7b,7e,7f). The first and second light-receiving regions (7a,7b) are juxtaposed, and separated by a line (A o ). The main beam which has been diffracted by the first diffraction region (3a) is focused (P₁) onto said line (A o ). The main beam which has been diffracted by the second diffraction region (3b) is focused (P₂) onto the first light-­ receiving region (7a). The main beam which has been diffracted by the third diffraction region (3c) is focused (P₃) onto the second light-receiving region (7b). The sub-beams which have been diffracted by the diffraction device (3) are focused onto the third and fourth light-receiving regions (7e, 7f) respectively. Alternatively, the diffraction device (13) comprises an area (13e) where one or more diffraction regions (13a,13b) are formed. The farthest point of the area (13e) at the side thereof towards the light-­ receiving device (27) is spaced from the optical axis (C) by a predetermined distance (L), to prevent the first-order diffracted beam from entering the optical system (4,5) disposed between the diffraction device (13) and the recording medium (6).

Description: This invention relates to an optical pickup device which is useful in an information recording and/or reproducing apparatus such as a compact disc reproducing apparatus, a video disc reproducing apparatus and the like. An optical pickup device according to the preamble of claim 1 is disclosed in e.g. EP-A-0.273.356. Fig.6 of the accompanying drawings shows a conventional optical pickup device. The device of Fig.6 comprises a semiconductor laser device 1, a first diffraction device 2, a second diffraction device 23, a collimating lens 4, an object lens 5, and a photodetector 17. A light beam from the semiconductor laser device 1 is diffracted by the first diffraction device 2 to produce three separate light beams, one of which is the zero-order diffracted beam (hereinafter, referred to as "the main beam"), and the others of which are the first-order diffracted beams (hereinafter, referred to as "the sub-beams") in the positive and negative directions which are substantially orthogonal to the sheet of the drawing of Fig. 6. These three separate beams are further diffracted by the second diffraction device 23. The resulting zero-order diffracted beam of each of the above-mentioned separate beams enters the object lens 5 through the collimating lens 4, and is focused on a recording medium 6. The main beam is focused on a pit of the recording medium 6. The two sub-beams, which are positioned symmetrically with respect to the above-mentioned main beam, are focused on the recording medium 6 in such a manner that they shift to a large extent in the tracking direction of the recording medium 6 and to a small extent in the radial direction. The beams reflected from the recording medium 6 pass through the object lens 5 and the collimating lens 4, and are diffracted by the second diffraction device 23. The resulting first-order diffracted beams are introduced into the photodetector 17. As shown in Fig. 7A, the diffraction device 23 is divided into two diffraction regions 23a and 23b by a division line 23c, when viewed from the side of the recording medium 6. The regions 23a and 23b have a number of grating lines which are inclined with respect to the division line 23c and which are symmetrical about the division line 23c. The photodetector 17 is divided into six regions 17a to 17f, as shown in Fig. 10B. The division line 23c elongates in the radial direction of the recording medium 6. When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6 or set at the correct focus, the resulting main beam which has been diffracted by the region 23a of the diffraction device 23 is focused on the division line A₁ of the photodetector 17 to form a spot Q₁, and the resulting main beam which has been diffracted by the region 23b of the diffraction device 23 is focused on the division line B₁ to form a spot Q₂. The resulting sub-beams are focused on the regions 17e and 17f of the photodetector 17. When output signals of the photodetecting regions 17a to 17f are represented respectively as S 1a to S 1f , a focus error signal is obtained by calculating (S 1a +S 1d )-(S 1b +S 1c ), a tracking error signal is obtained by calculating (S 1e -S 1f ), and a pit signal (i.e., an information signal) is obtained by calculating (S 1a +S 1b +S 1c +S 1d ). Figure 8 shows another conventional optical pickup device. The optical pickup device of Fig. 8 is different from the above-mentioned conventional device in the configurations of a second diffraction device 33 and a photodetector 27. The configuration of the grating lines of the diffraction device 33 and that of the photodetector 27, which are viewed from the recording medium 6, are shown in Figs. 9A and 9B, respectively. The diffraction device 33 is divided into two regions 33a and 33b by a division line 33c which elongates in the radial direction. The regions 33a and 33b have a number of grating lines which are at right angles to the division line 33c, and the grating period of the region 33a is different from that of the other region 33b. The photodetector 27 is divided into six regions 27a to 27f. When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6 or set at the correct focus, the resulting main beam which has been diffracted by the region 33a is focused on the division line A₂ to form a spot R₁ thereon, and the resulting main beam which has been diffracted by the region 33b is focused on the division line B₂ to form a spot R₂ thereon. The resulting sub-beams are focused on the photodetecting regions 27e and 27f. When output signals of the photodetecting regions 27a to 27f are represented respectively as S 2a to S 2f , a focus error signal is obtained by calculating (S 2a +S 2d )-(S 2b +S 2c ), a tracking error signal is obtained by calculating (S 2e -S 2f ), and a pit signal is obtained by calculating (S 2a +S 2b +S 2c +S 2d ). In the conventional optical pickup devices with the above-mentioned structures, the spots Q₁ and Q₂ (R₁ and R₂) based on the beams reflected from the recording medium 6 must be very precisely formed on the division lines of the photodetector 17(27). To achieve this, a delicate adjustment must be carried out so that the diffraction device 23(33) and the photodetector 17(27) respectively can be disposed at a given position. However, in order that the diffraction device 23(33) and the photodetector 17(27) are constructed to be moved separately or independently from the diffraction device 23(33), there must be a supporting structure by which the photodetector 17(27) can be freely moved. This makes the entire structure of the pickup device complicated, causing difficulties in obtaining a light-weight, miniaturized pickup device. Moreover, a number of positioning parts are needed, which makes the production process of the pickup device complicated and makes the production cost expensive. To solve these problems, the inventors of this invention designed to incorporate both the semiconductor laser device 1 and the photodetector 17(27) into the same package so that the positioning of the spots Q₁ and Q₂ (R₁ and R₂) on the division lines of the photodetector 17(27) can be carried out by the positional adjustment of the diffraction device 23(33) alone. However, in an optical pickup device with such a structure, the slight shifting of the positions of the semiconductor laser device 1 and the photodetector 17(27) from those of the initial plan makes it impossible to form the beam spots at the correct positions of the photodetector 17(27), resulting in a focus offset. To remove this focus offset, the position of the diffraction device 23(33) must be moved linearly and/or rotationally with respect to other components such as the semiconductor laser device 1 to shift the spots on the photodetector 17(27), so that the focus error signal becomes zero when the beam from the semiconductor laser device 1 is at the correct focus on the recording medium 6. However, the two spots on the photodetector 17(27) which are formed based on the main beams shift at the same time, resulting in that the position of each beam spot cannot be independently adjusted without the simultaneous shifting of these beam spots on the photodetector 17(27). Moreover, there is a possibility that the shifting of the two spots are countervailed on the focus error signals corresponding thereto. To avoid this, the diffraction device 23(33) must be moved to a great extent in the y-direction (Figs. 7B and 9B). Especially, in the optical pickup device shown in Fig. 8, the length of each of the divided regions of the photodetector 27 in the y-direction is short. Therefore, when a great focus offset occurs and the diffraction device 33 is moved to a great extent in the y-direction to compensate the said focus offset, the beam spots R₁ and R₂ on the photodetector 27 shift to a great extent in the y-direction and slip out of the photodetecting regions on which these spots must be formed. Moreover, because the diffraction device 23 (33) must be moved linearly to compensate for the focus offset phenomenon, the photodetector 17(27) is required to have a large enough size to receive the beam spots thereon, which makes the production cost thereof expensive. Fig.10 shows an optical pickup device which is described in U.S. patent application No.07/282,109, European Patent Appln.No. 88311665.9 (EP-A-0320276), a prior art under Article 54(3)EPC, and Canadian Patent Appln. No. 585,356. In the pickup device of Fig. 10, the diffraction device 43 consists of two diffraction regions 43a and 43b which are formed by dividing the whole area of the diffraction device by a division line 43c. The diffraction regions 43a and 43b have a number of grating lines which elongate in the track direction of the recording medium 6 (X-X′ direction in Fig. 10). The division line 43c elongates in the direction (Y-Y′ direction in Fig. 10) which is perpendicular to the track direction. The photodetector 37 is divided into five regions 37a to 37e. The division line 37f extends in the Y-Y′ direction to separate the regions 37a and 37b. The main beam which has been diffracted by the diffraction region 43a is focused on the division line 37f to form a spot S₁, and the main beam which has been diffracted by the diffraction region 43b is focused on the region 37c to form a spot S₂. The sub-beams are focused on the regions 37d and 37e to form spots S₃ to S₆. When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6, the spots S₁ to S₆ are formed as tiny spots as shown in Fig. 11B. In contrast, when the distance between the recording medium 6 and the object lens 5 becomes small (or large), the spots S₁ to S₆ are formed so as to be extended in semicircular shapes as shown in Figs. 11A (or in Fig. 11C). The regions 37a to 37e on which spots S₁ to S₆ are formed produce signals S a to S e , respectively. A focus error signal is obtained by calculating (S a -S b ), a tracking error signal by calculating (S d -S e ), and an information signal by calculating (S a +S b +S c ). The optical pickup device of Fig. 10 has a drawback in that a spurious signal is generated in the focus signal, tracking error signal and information signal. This will be described with reference to Figs. 12 and 13. A light beam emitted from the semiconductor laser device 1 is divided into a main beam and two sub-beams by the diffraction device 2, and enters in the diffraction device 43. For example, the resulting first-order diffracted beam diffracted by dividing the diffraction region 43b propagates as indicated by the phantom lines A, as if it is a light beam emitted from the spot S₂ toward the diffraction device 43. Therefore, this first-order diffracted beam is focused by the lenses 4 and 5 on the position of the recording medium 6, a position which corresponds to the spot S₂ (i.e., the position is the image point of the spot S₂). As shown by the phantom lines B, this first-order diffracted beam is reflected by the recording medium 6 to enter into the diffraction device 43 through the lenses 5 and 4, and the resulting zero-order diffracted beam is focused on the photodetector 37 as the spot S₂. Similarly, the resulting first-order diffracted beam diffracted by dividing the diffraction region 43a is reflected by the recording medium 6, and then focused on the photodetector 37 as the spot S₁. The first-order diffracted beams which are diffracted by the diffraction device 43 are not the light beams to be used for detecting signals. When these first-order diffracted beams are once received by the photodetector 37, therefore, a spurious signal is generated in the focus error signal and information signal. This causes the focus control to be incorrectly conducted, and an incorrect information signal to be produced. Also, the diffraction device 43 produces first-order diffracted beams based on the sub-beams which have been diffracted by the diffraction device 2, so that a spurious signal is also generated in the tracking error signal, thereby impeding the tracking control. In this way, the first-order diffracted beams produced by the diffraction device 43 cause a spurious signal in detection signals so that the optical pickup device cannot function properly. The optical pickup device according to the invention comprises a light source for emitting light; an optical system for focusing light emitted from said light source on to a recording medium, said optical system allowing said light reflected from said recording medium to pass therethrough, a light receiving means having a plurality of light receiving regions which receives said light reflected from said recording medium; diffraction means, disposed between said light source and said optical system across an optical axis of said light emitted from said light source, for allowing said light emitted from said light source to pass therethrough and for introducing said light reflected from said recording medium into said light receiving means, characterised in that said diffraction means has a diffraction area where one or more diffraction regions are formed, said optical axis of said light emitted from said light source passing through said diffraction area, and in that a distance from said optical axis to an end portion of said diffraction area is set such that first-order diffracted beams generated by said diffraction means when said light emitted from said light source passes therethrough are not incident on said optical system. In a preferred embodiment, the diffraction means comprises a portion where no diffraction grating is formed. Thus, the invention described herein makes possible the objectives of 1. providing an optical pickup device in which the focus offset can be adjusted simply by rotating a diffraction device; 2. providing an optical pickup device in which the focus offset can be easily adjusted without linearly moving a diffraction device; 3. providing an optical pickup device in which the size of a light receiving device can be reduced; 4. providing an optical pickup device in which a spurious signal is not produced; and 5. providing an optical pickup device which can function properly. The invention will be further described, by way of example, with reference to the accompanying drawings, wherein:- Fig. 1 is a perspective view illustrating an optical pickup device according to the invention. Fig. 2 is a section view of the optical pickup device of Fig. 1. Fig. 3 is a plan view of a diffraction device used in the optical pickup device of Fig. 2. Figs. 4A to 4D illustrate various modifications of the diffraction device useful in the optical pickup device of Fig. 2. Fig. 5 illustrates positional relations of spots formed on the diffraction device of Fig. 3. Fig. 6 illustrates diagrammatically a conventional optical pickup device. Fig. 7A is a plan view of a diffraction device used in the optical pickup device of Fig. 6. Fig. 7B is a plan view of a photodetector used in the optical pickup device of Fig. 6. Fig. 8 illustrates diagrammatically another conventional optical pickup device. Fig. 9A is a plan view of a diffraction device used in the optical pickup device of Fig. 8. Fig. 9B is a plan view of a photodetector used in the optical pickup device of Fig. 8. Fig. 10 is a perspective view illustrating an improved optical pickup device. Figs. 11A to 11C illustrate positional relations of spots formed on the diffraction device shown in Fig. 10. Fig. 12 is a section view illustrating the optical pickup device of Fig. 10. Fig. 13 is a plan view of the diffraction device used in the optical pickup device of Fig. 10. Fig. 1 shows an optical pickup device according to the invention. The pickup device of Fig.1 comprises a semiconductor laser device 1, a first diffraction device 2, a second diffraction device 3, a collimating lens 4, an object lens 5, and a photodetector 27. The pickup device of Fig. 1 is constructed in the same manner as the pickup device of Fig. 10 except for the second diffraction device 13. In this embodiment, the second diffraction device 13 has a square shape when viewed from the recording medium 6, and is divided into two rectangular areas 13e and 13f which are separated by a division line 13d. The division line 13d, which is not an actually formed one but an imaginary one for describing the area 13e, runs in the track direction (X-X′ direction) of the recording medium 6, and is separated from the optical axis C of a light beam emitted from the semiconductor laser device 1, by a predetermined distance L toward the photodetector 27. The distance between the optical axis and the farthest point of the area 13e at the opposite side of the photodetector 27 is greater than the distance L. The area 13e is divided by a division line 13c which is perpendicular to the division line 13d (i.e., the division line 13c runs in the Y-Y′ direction), into two one or more rectangular diffraction regions 13a and 13b where grating lines are formed in the track direction. The grating period of the first diffraction region 13a is slightly smaller than that of the second diffraction region 13b. The value of the distance L is selected so that the resulting first-order diffracted beams diffracted by the diffraction regions 13a and 13b do not enter the collimating lens 4. In the pickup device of Fig. 1 the diffracted beams diffracted by the first diffraction device 2 are further diffracted by the second diffraction device 13. Because the second diffraction device 13 is provided with gratings only in the area 13e, the diffraction direction of each of the first-order diffracted beams is restricted as shown by the phantom lines A in Fig. 2 so that the first-order diffracted beams are prevented from entering the collimating lens 4. That is, the first-order diffracted beams diffracted by the diffraction device 13 are not focused, and therefore the photodetector 27 is substantially prevented from receiving the first-order diffracted beams reflected from the recording medium 6, thereby effectively suppressing the level of a spurious signal generated in the focus error signal, tracking error signal and information signal. In the above-mentioned embodiment, gratings are formed in the whole of the area 13e of the diffraction device 13. The structure of the diffraction device 13 is not restricted to the above, but can be adequately formed in another manner. For example, the gratings are formed in a rectangular shape which is positioned at the center of the diffraction device 13 and elongates along the X-X′ direction (Fig. 4A), in a rectangular shape which is shorter than the diffraction device 13 in the X-X′ direction (Fig. 4B), in an oval shape (Fig. 4C), or in a shape which is a partly removed circle (Fig. 4D). The range of forming the diffraction ranges 13a and 13b may be restricted in either of the X-X′ direction or the Y-Y′ direction. When the diffraction device 13 is used in the three-beam method as in the case of this embodiment, the main beam (solid line) and sub-beams (two-dot dash lines) reflected from the recording medium 6 are slightly shifted from each other, as shown in Fig. 5. Hence, it is preferable that the diffraction ranges 13a and 13b are formed wider in the X-X′ direction. Moreover, the area 13f of the diffraction device 13 may be omitted as required.

Claims: An optical pickup device comprising a light source (1) for emitting light; an optical system (4,5) for focusing light emitted from said light source (1) on to a recording medium (6), said optical system (4,5) allowing said light reflected from said recording medium (6) to pass therethrough, a light receiving means (27) having a plurality of light receiving regions which receives said light reflected from said recording medium (6); diffraction means (13), disposed between said light source (1) and said optical system (4,5) across an optical axis (c) of said light emitted from said light source (1), for allowing said light emitted from said light source (1) to pass therethrough and for introducing said light reflected from said recording medium (6) into said light receiving means (27), characterised in that said diffraction means (13) has a diffraction area (13e) where one or more diffraction regions (13a,13b) are formed, said optical axis (c) of said light emitted from said light source (1) passing through said diffraction area (13e), and in that a distance (L) from said optical axis (c) to an end portion (13d) of said diffraction area (13e) is set such that first-order diffracted beams generated by said diffraction means (13) when said light emitted form said light source passes therethrough are not incident on said optical system (4,5). An optical pickup device according to claim 1, characterised in that the distance from said optical axis (c) to the end portion of the diffraction area (13e) opposite the said end portion (13d) is greater than the distance (L) from said optical axis (c) to the said end portion (13d). An optical pickup device according to claim 1, wherein said end portion (13d) of the diffraction area (13e) is closer to the light receiving means (27) than any other end portion of the diffraction area (13e). An optical pickup device according to any preceding claim, wherein tracks are formed on said recording medium (6), and said diffraction area (13e) is formed so that a width thereof in a direction parallel to said tracks is larger than a width of said diffraction area (13e) in a direction perpendicular to said tracks. An optical pick-up device, as claimed in any preceding claim, wherein said diffraction means (13) has a portion (13f) where no diffraction grating is formed. An optical pick-up device as claimed in claim 5, wherein said portion (13f) is closer to said light receiving means (27) than the diffraction area (13e). An optical pick-up device as claimed in any preceding claim, wherein a further diffraction means (2) is disposed between said light source (1) and said diffraction means (13) and wherein the light diffracted by the further diffraction means (2) is further diffracted by said diffraction means (13).

Inventors: KURATA YUKIO, NAGAHAMA TOSHIYA SAN-CORPO, NAKATA YASUO, YOSHIDA YOSHIO, KURATA, YUKIO, NAGAHAMA, TOSHIYA 407 SAN-CORPO, NAKATA, YASUO, YOSHIDA, YOSHIO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B7
IPC codes: G11B7