JPS60113333A - Light pick up device - Google Patents

Abstract

PURPOSE:To eliminate cross talk between error signals of both sides due to a combination of an astigmatism method and a heterodyne method by dividing a light detector for detecting reflected light at least into eight. CONSTITUTION:The direction y of cylinder axis of a cylindrical lens is set to 0 deg. or 90 deg. though it is usually inclined by 45 deg. equivalently to the row of pits. Consequently, a change in shape of focus erros by a astigmatism method on a photodetector becomes a vertical ellipse 10a or a horizontal ellipse 10b. Accordingly, a focus error signl can be obtained by taking the difference between the sum of photodetector elements 9a, 9b, 9e, 9f and the sum of 9c, 9d, 9g, 9h, that is, (9a+ 9b+9e+9f)-(9c+9d+9g+9h). On the other hand, in the diffraction pattern due to pit, a change of distribution of strength is caused in the direction inclined by 45 deg. to the case of astigmatism. Therefore, track error detection by the heterodyne method can be obtained basically by operating (9a+9h+9d+9e)-(9b+9c+ 9f+9g).

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure and detection method of a photodetector for detecting defocus and track deviation in an optical pickup device.

Conventionally, various methods have been proposed for detecting defocus and track deviation of optical pickups used in optical disk devices. Typical methods are shown in Table 1.

Table 1 The circles in Table 1 indicate methods that have already been put into practical use and prototypes. Among these methods, the three-beam method and the heterodyne method are known to be stable as they are almost unaffected by the movement of the spot position of the return beam, and are also used as defocus detectors. It is known that the astigmatism method is stable against movement of the spot position of the force-returning light and can provide high sensitivity. Here, the three-beam method requires a diffraction grating for splitting the beam, and requires many adjustments, which makes it difficult to reduce costs. Therefore, the combination of the astigmatism method and the heterodyne method is a method that has both a simple structure and stable performance. However, when using this combination, the conventional 4-split photodetector shown in Figure 1 has the disadvantage that crosstalk occurs between the focus error signal and the tracking error signal, making it impossible to detect the error signal. It had not been put into practical use at all.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical pickup device which combines the astigmatism method and the heterodyne method by improving the photodetector.

First, the principles of the astigmatism method and the heterodyne method will be briefly described.

The astigmatism method is a method that uses the entire cylindrical lens 2. As shown in Fig. 2A, when focused light focused on Fl is passed through the cylindrical lens 2, the thickness remains constant in the axial direction y of the cylinder, so the focal point remains Flj, but in the direction perpendicular to the axial direction. Regarding X, since it has a lens effect, the focal point is F2. and two foci Fl and F'
2, the cross section of the beam changes from a horizontal ellipse 4α to a circle 4b to a vertical ellipse 4G, as shown in FIG. 2A. When the 4-segment photodetector 8 (not shown in Fig. 2A) is placed at a point where it becomes exactly circular at the focused point, the reflected light becomes more divergent than at the focused point when the disk is close, so % Fl 1 Fm becomes farther away, and the photodetector equivalently approaches, and the beam shape on the photodetector becomes a vertical ellipse as shown in FIG. 2B. If the circular button disk is far away, it will become a horizontal ellipse as shown in Figure 2BK. Therefore, among the outputs of the individual elements 8α, 8b, 8c, and 8d of the 4-split photodetector 80, the sum of the outputs of the diagonal elements 8α and 8b and 8c and 3d is taken, and the focus error signal is obtained by taking the difference between them. I know.

The heterodyne method is a method that focuses on changes in a dilute pattern, which is a diffraction pattern caused by bits, generated on a photodetector, and generates an error component through signal processing. FIG. 8 shows a light amount pattern 7t on the photodetector when the spot 5 passes over the bit 6. Figure 8A
8B shows the case where the spot is on the left side, FIG. 8B shows the case where the spot is in the center, and FIG. 3C shows the case where the spot is on the right side.

Then, this diffraction pattern is detected by a 4-split photodetector 8.
The light is received by the sensor, and the signal is processed by a signal detection circuit as shown in FIG.

In other words, the difference (
1+8)-(2+4)t-, total signal (1+2+
The difference between the diagonal sum signals is sampled and held at the rising edge of 8+4) at 9 o'clock and the falling edge at 9 o'clock.

The idea is to take this difference.

Now, as is clear from the above explanation, both the astigmatism method and the heterodyne method are methods for obtaining an error signal by taking the difference between the diagonal sum signals of the 4-split photodetector. Therefore, as described above, when a four-division photodetector is used, crosstalk between error signals for both focusing and tracking cannot be avoided.

FIG. 5 shows an optical pickup device of the present invention.

That is, the cylindrical lens 2 and the 8-segment photodetector 9 are used to detect focus and tracking errors. The dotted line in FIG. 5 shows the longitudinal cross-section and light ray locus along the cylindrical axis direction of the cylindrical lens, and the solid line shows the cross-sectional view and the light ray locus along the orthogonal direction.

Here 1. The cylindrical axis direction y of the cylindrical lens is normally tilted at an equivalent angle of 45 degrees with respect to the pit row.

In the present invention, the angle is set to 00° or 90°.

By doing so, the change in JF4 due to the astigmatism method on the photodetector becomes α for the vertical ellipse and 1 for the horizontal ellipse (lb) as shown in Figure 6, so the photodetector element 9cL, 9b, 9
The difference between the sum of g, 9f and the sum of 9c, 9d, 9g, and 9h, that is, (9a+9b+9g+9f)-(9c+9d+9
If g+9h)'Th is taken, a focus error signal is obtained, while the diffraction pattern due to the bit causes a change in intensity distribution in a direction tilted by 45 degrees from that in the case of astigmatism, as shown in an example in FIG. Therefore, track error detection using the heterodyne method is basically (9α+9 +9d+9g)
It can be obtained by calculating -(96+9c+9f+9g).

By doing so, crosstalk between the focus error signal and the tracking error signal is almost eliminated, and a stable optical pickup device can be realized.

By adopting the present invention as described above, it is possible to eliminate the crosstalk between the error signals of both the astigmatism method and the heterodyne method, which was the original purpose of Kyoru, and to achieve extremely stable performance. It is possible to realize an optical pickup device with In addition, although the heterodyne method has been mentioned as a tracking error detection method in the explanation so far, it goes without saying that it can be applied in exactly the same way to methods such as the time difference method that are common in principle.

[Brief explanation of drawings]

Figure 1 shows the four
Plan views showing the divided photodetector, FIGS. 2A and 2B, are as follows:
An explanatory diagram showing the principle of the astigmatism method, Fig. 8 A, B, and C are explanatory diagrams showing the diffraction pattern due to pits, Fig. 4 is an explanatory diagram showing the principle of the heterodyne method, and Fig. 5 A is an explanatory diagram showing the principle of the heterodyne method. FIG. 5B is a plan view showing an 8-segment photodetector used in the optical pickup device of the present invention; FIG. 6A is a plan view showing changes in spot shape on the photodetector; FIG. B is a plan view showing a diffraction pattern of pits on a photodetector. 2.e Cylindrical lens 9... 8-segment photodetector 9α, 9b, 9c, 9d, 9g, 9/, 9g. 9h...Photodetector element 10α, 10b...Light spot shape 1】-# diffraction pattern or more Applicant Seiko Electronic Industries Co., Ltd. Agent Patent attorney Mogami

Claims (1)

[Claims] In an optical pickup device that irradiates a light beam onto a recording medium and simultaneously reads a main signal, a focus error signal, and a tracking error signal from the reflected light, the photodetector for detecting the reflected light is divided into at least 8 sections. An optical pickup device characterized by: