Optical pickup device

Abstract: An optical pickup device for supplying a correct focus error signal includes a light source (1) for generating a light beam; a recording medium (4) for receiving and reflecting the light beam; and a photo-detector (5) for receiving the reflected light beam from the recording medium and for outputting a focus error signal of the light beam. The photo-detector is divided into at least two regions (D₁-D₆) and includes a reference portion within at least two regions. When the light beam received by the photo-detector is not within the reference portion of the photo-detector, a portion of the light beam is not detected by at least one of the at least two regions, thereby supplying a correct focus error signal.

Description: The present invention relates to an optical pickup device suitable for use with optical disk devices, compact disk players, optical video disk players and the like. Fig. 1 shows the arrangement of a conventional optical pickup device which includes a light source (e.g. a semiconductor laser) 1, a parallel planar plate 2, an objective lens 3, a recording medium (e.g. an optical disk) 4, and a photodetector 5. As shown by the arrows of Fig. 1, a divergent light beam is emitted from the light source 1 to the surface of the parallel planar plate 2 where the light beam is reflected to the objective lens 3. The objective lens 3 serves to focus the reflected light beam onto the optical disk 4. The focused light beam on disk 4 is then reflected to the photo-detector 5 through the objective lens 3 and the parallel planar plate 2. Since the parallel planar plate 2 consists of a glass plate or the like, and since plate 2 is disposed in the light beam converging path between the objective lens 3 and the photo-detector 5, the light beam received by the photo-detector 5 necessarily suffers from astigmatism. Figs. 2(a)-(e) show the photo-detector 5 of Fig. 1, wherein the photo-detector is divided into four regions D₁-D₄ by a first straight line 51, and a second straight line 52 which is perpendicular to the first straight line 51. When the light beam from the objective lens is correctly focused onto the disk 4, a so-called "focus state" is achieved. More particularly, in the focus state, a substantially circular light spot is formed on the photo-detector 5 as shown in Figure 2(a). However, when the disk 4 is moved towards or away from the lens 3, a substantially elliptical light spot is formed on the photo-detector as shown in Figs. 2(b) and 2(c). Therefore, according to the principle of astigmatism, a focus error signal having a characteristic as indicated by the curve a in Fig. 3 can be obtained. The focus error (FE) signal associated with the light spots of the conventional pickup device can be detected from the output of the photo-detector 5. More particularly, the focus error signal is equal to the difference between the sum of the outputs of regions D₁ and D₃ and the sum of the outputs of regions D₂ and D₄. Curve a of Fig. 3 shows the focus error signal of the light spots of Figs. 2(a), 2(b), and 2(c), wherein the minimum point of curve a is the focus error of Fig. 2(b), the origin is the focus error of Fig. 2(a) (i.e. no focus error), and the maximum point of signal a is the focus error of Fig. 2(c). In addition to detecting the focus error signal, the photo-detector 5 can detect variations in the level of the reflected light beam from disk 4 (which is modulated with signals (pits) recorded on the disk) from the sum of the outputs of regions D₁-D₄ of photo-detector 5. A problem occurs with the detection of the focus error signal of the pickup device of Fig. 1 when the optical components of the pickup device are not properly positioned, or when the objective lens 3 is moved in a tracking direction. More particularly, in either of these situations, the light spot reflected onto the photo-detector 5 from the disk 4 will be shifted from the center of the photo-detector 5 (Fig. 2(a)) to, for example, the position shown in Figure 2(d). As a result, the focus error signal becomes offset (from the origin) as shown by curve b of Fig. 3. In addition, the situation may arise where the objective lens is moved such that the light spot has the shape and position as shown in Fig. 2(e). Although the light spot of Fig. 2(e) has a zero focus error signal, it is not in the focus state. As a result, the degree of modulation of the light beam by the signals (pits) recorded on the disk 4 is lowered, thereby causing the pickup device to be sensitive to noise. Thus, there is a continuing need in the art for a pickup device which can provide a correct focus error signal even when the light beam applied to the photo-detector is somewhat shifted from a predetermined reference position on the photo-detector. In accordance with the invention, an optical pickup device of the type having a light source for generating a light beam, an objective lens for focusing the light beam on a recording medium which, in use, receives and then reflects the focused light beam, and photo-detector means for receiving the reflected light beam from the recording medium such that a light spot falls on the photo-detector means, the photo-detector means comprising a plurality of separate regions, a logical combination of the outputs from which providing an indication of an in focus or out of focus condition of the light spot falling on the photo-detector means; is characterised by the individual photo-detectors having a shape and orientation such that when the light spot in the in focus condition is moved a short distance in a predetermined direction relatively to the photo-detectors at least one of the regions does not detect a part of the reflected light beam so that the part of the light which is not detected compensates for the movement in the predetermined direction so that the logical combination of outputs continues to indicate the in focus condition. In the accompanying drawings:- Fig. 1 is a plan view showing the arrangement of a conventional optical pickup device. Figs. 2(a)-(e) each show a plan view of a photo-detector in the conventional optical pickup device of Fig. 1. Fig. 3 is a graphical representation of the two focus error signals according to the conventional optical pickup device of Figure 1. Figs. 4(a)-(b) are plan views showing a photo-detector according to an embodiment of the present invention. Fig. 5 is a graphical representation of two focus error signals according to the present invention. Fig. 6 is an plan view showing an embodiment of the optical pickup device of the present invention. Figs.7(a)-(b) are plan views of a photo-detector according to an embodiment of the present invention. The optical pickup device according to the present invention is similar to the arrangement of the conventional optical pickup device of Fig. 1; however, the present invention is quite different from the prior art device with regard to how the photo-detector 5 is divided. Fig. 4 shows a photo-detector 5 according to the optical pickup device of the present invention. In Fig. 4, the photo-detector 5 has six regions, labelled D₁ - D₆. Regions D₁ through D₄ are defined by first and second division lines 51 and 52, respectively. Each of the regions D₁ through D₄ has a drawdown point 53 on its periphery which serves to delimit the respective regions of the photo-detector. As shown in Fig. 4, each of the drawdown points is located on an imaginary line (not shown) forming a 45° angle with respect to the division lines 51 and 52. Further, each of the drawdown points is shown to be touching to the circumference of a focused light spot which is positioned at the center of the photo-detector 5 as indicated by broken lines in Fig. 4. Referring to Fig. 4(b), the light spot is shown to have moved a relatively short distance towards the drawdown point 53 of region D₄ in a tracking direction T. More particularly, a portion of the light spot has moved outside of region D₄ into region D₅. As a result, the portion of the light spot which is within region D₅ will not be detected by region D₄. Thus, although the light spot in Fig. 4(b) has moved from its initial position in Fig. 4(a), the sum of the outputs of regions D₄ and D₂ will still be equal to to the sum of the outputs of regions D₁ and D₃. Referring now to Fig. 5, wherein curve a represents the focus error signal of the light spot centered as in Fig. 4(a), and curve b represents the focus error signal of the light spot off center as in Fig. 4(b). As shown in the curve b of Fig. 5, even when the light spot is shifted in a tracking direction T, the focus error signal does not become offset. The invention provides an accurate focus error signal even when the light spot on the photo-detector is moved in a selected tracking direction. In the embodiment of Fig. 4, it was assumed that the light spot was circular; however, those skilled in the art will understand that, in practice, the configuration of the drawdown points 53 will be determined according to the aberration and the distribution of intensity of the light beam. Although the photo-detector of Fig. 4 provides a correct focus error signal, the sum of the outputs of regions D₁ through D₄ of Fig. 4(b) is less than that the sum of the outputs of the regions D₁ through D₄ of Fig. 4(a). Thus, although a correct focus error signal is detected when the light is moved as shown in Fig. 4(b), a correct reproduction signal (RF signal) of the light spot will not be obtained. For the purposes of obtaining a correct reproduction signal, the present invention provides two additional regions. More particularly, photo-detector 5 includes region D₅ which is formed outside of regions D₁ and D₄, and region D₆ which is formed outside of regions D₂ and D₃. As a result, a correct reproduction signal can be detected by simply calculating the sum of the outputs of regions D₁ through D₆. Referring now to Fig. 6, where there is shown another embodiment of the present invention. In this embodiment, the photo-detector 5 is divided by division lines 51 and 52 as in the embodiment of Fig. 4; however, instead of drawdown points, a light shielding member 6 for intercepting a part of the light beam is included between the parallel planar plate 2 and the photo-detector 5. The light shielding member 6 performs the same function as the drawdown points in Fig. 4. Fig. 7 shows another embodiment of the present invention. In this embodiment, photo-detector 5 is divided into two regions, namely, an inner circular region D₇, and an outer annular region D₈ having two drawdown points 53 in a tracking direction T. In Fig. 7, the focus error signal is equal to the difference between the outputs of regions D₇ and D₈. As shown in Fig. 7(a), when the light beam is focused onto the photo-detector 5, the difference between the outputs of regions D₇ and D₈ is zero. When the light spot is moved from its correct position shown in Fig. 7(a) towards one of the drawdown points 53 as shown in Fig. 7(b), the amount of light detected by region D₈ is decreased in proportion to the amount in which the light spot has moved outside of region D₈. In addition, the output of region D₇ is decreased because the energy of the light spot gradually decreases as it moves from the center of the photo-detector 5 (Fig. 7(a)) towards the outer periphery of region D₈ (Fig. 7(b)). Therefore, in this embodiment the drawdown points 53 are designed such that the changes in the outputs of regions D₇ and D₈ are cancelled out, thereby preventing the focus error signal from becoming offset. The photo-detector of Fig. 7 may also include data detecting regions, located outside of region D₈, for detecting a reproduction signal as in the embodiment of Fig. 4. Further, instead of drawdown points 53, the light shielding member 6 of Fig. 6 may be used.

Claims: An optical pickup device of the type having a light source (1) for generating a light beam, an objective lens (3) for focusing the light beam on a recording medium (4) which, in use, receives and then reflects the focused light beam, and photo-detector means (5) for receiving the reflected light beam from the recording medium such that a light spot falls on the photo-detector means, the photo-detector means comprising a plurality of separate regions (D₁-D₄; D₇,D₈) a logical combination of the outputs from which providing an indication of an in focus or out of focus condition of the light spot falling on the photo-detector means; characterised by the individual photo-detectors having a shape and orientation such that when the light spot in the in focus condition is moved a short distance in a predetermined direction (T) relatively to the photo-detectors at least one of the regions does not detect a part of the reflected light beam so that the part of the light which is not detected compensates for the movement in the predetermined direction so that the logical combination of outputs continues to indicate the in focus condition. A device according to claim 1, further comprising a light shielding member (6) positioned between the recording medium (4) and the photo-detector means (5) for intercepting a portion of the light beam. A device according to claim 1, wherein the photo-detector means (5) is divided into four separate photo-detector regions (D₁-D₄) by a pair of perpendicular division lines (51,52) intersecting at a common point, and wherein a radius from the common point to the surface of each of the four separate photodetector regions varies, the radius being a minimum when it forms a 45° angle to the division lines, and being a maximum when it is perpendicular to one of the division lines. A device according to claim 3, further comprising a first data detecting region (D₅) located outside of the first and fourth photo-detector regions (D₁,D₄), and a second data-detecting region (D₆) located outside of the second and third photo-detector regions (D₂,D₃). A device according to claim 3 or claim 4, wherein the focus error signal is calculated by the difference between the sum of the outputs of the first and the third photo-detector regions (D₁,D₃) and the sum of the outputs of the second and fourth photo-detector regions (D₂,D₄). A device according at least to claim 4, wherein a reproduction signal of the light beam is calculated by the sum of the outputs of the first, second, third, and fourth photo-detector regions (D₁-D₄) and the first and second data-detecting regions (D₅,D₆). A device according to claim 1 or claim 2, wherein the photo-detector means (5) is divided into an inner circular region (D₇), and an outer annular region (D₈), the outer annular region having two drawdown points (53) on its circumference such that a distance between each of the drawdown points and a centre point of the inner circular region is less than a distance between any point on the circumference of the outer annular region which is not at the drawdown points and the centre point of the inner circular region. The device according to claim 7, wherein the light focus error signal is detected by calculating the difference between the outputs of the inner circular region (D₇)and the outer annular region (D₈). The device according to claim 8, further comprising a data-detecting region located outside of the annular region. The device according to claim 9, wherein a reproduction signal of the light beam is detected by calculating the sum of the outputs of the inner circular region (D₇), the outer annular region (D₈), and the data detecting region.

Inventors: MAEDA TAKANORI PIONEER ELECTRO, MAEDA, TAKANORI PIONEER ELECTRONIC CORPORATION
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1988
Main classification: G11B7
IPC codes: G02B7, G11B7