JPS63244416A - Optical information detector - Google Patents

Abstract

PURPOSE:To prevent the deterioration in quality of a focusing error caused by the aberration of an optical system by interposing a turnable parallel plate tilted from an optical axis to a divergent or converging part of light in an optical path. CONSTITUTION:The astigmatism in a direction in parallel or not orthogonal to an information track remaining in the optical system is removed. That is, the direction of the astigmatism is controlled by turning a parallel plate 12 around the optical axis 13 and the quantity of astigmatism is controlled by varying a tilt angle theta of the parallel plate or using the parallel plate different in plate thickness (t) and refractive index (n). Thus, it is prevented that the aberration of the optical system influences the diffraction pattern by a guide track projected onto a photodetector, resulting in deteriorating the quality of a focusing error signal.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an optical information detection device, particularly an optical information detection device that records and reproduces information using an information recording medium provided with a circular or spiral guide track. The present invention relates to a device that prevents deterioration in the quality of a focusing error signal.

<Prior Art> Conventionally, there has been an optical information detection device as shown in FIG. In the figure, 1 is a semiconductor laser, 2 is a collimator lens, 3/i beam splitter, 4 is an objective lens, 5 is a substrate, 6 is an information track provided on the substrate 5, and 6'
are guide tracks existing on both sides of the information track 6,
7 is an information recording medium such as an amorphous alloy thin film of rare earth/transition metal formed on the substrate 5, 8 is a converging lens, 9 is a cylindrical lens, and 10 is a four-part photodetector divided into a square shape. It is.

Next, the operation of the optical information detection device having the above configuration will be explained.

The light beam emitted from the semiconductor laser 1 is made into parallel light by the collimator lens 2, passes through the beam splitter 3, enters the objective lens 4, and is focused by the objective lens 4 to form a spot on the information recording medium 7. . The reflected light from the information recording medium 7 passes through the objective lens 4 again, is reflected by the beam splitter 3, and then passes through the converging lens 8 and the cylindrical lens 9 to become a light beam having astigmatism.The generating line of the cylindrical lens 9 The light is incident on a four-split photodetector 10 arranged at an angle of 45° with respect to the direction.

Here, a substrate 5 made of a light-transmitting material such as glass, etc.
In order to perform high-density recording and reproduction, groove-like tracks 6 and 6' are formed in advance in a circular or spiral shape. Furthermore, on the track 6.6', an information recording medium 7 made of, for example, an amorphous alloy thin film of rare earth and transition metals is placed.
is coated by vapor deposition, sputtering, etc.

Next, the principle of focus detection in the optical information detection device will be explained.

FIG. 3 is a diagram showing the principle of focus detection in a conventional optical information detection device. four light receiving sections 10a of the four-split photodetector 10,
The amount of light received at IOb, IOc, and IOd is Sa and S, respectively.
b, Sc, and Sd. When in focus, the beam spot on the photodetector 10 becomes circular as shown in Figures (a) and (b), so the amount of received light Sa, S detected by the four light receiving sections is
b, Sc, and Sd are equal, so (Sa+5C)−(
Sb+Sd) is 0. Next, when the objective lens 4 relatively approaches the information recording medium 7, the beam spot on the photodetector 10 moves in a direction parallel to the generatrix of the cylindrical lens 9, as shown in FIG. . It becomes an ellipse with the major axis being , and (Sa+Sc) - (Sb+Sd) is negative. On the other hand, when the objective lens 4 moves relatively away from the information recording medium 7, the beam spot on the photodetector 10 moves in the direction a perpendicular to the generatrix of the cylindrical lens 9, as shown in FIGS. It becomes an ellipse with its long axis at (
Sa+Sb) -(Sc+Sd) becomes positive. As the focusing error signal used as the signal for correcting the relative distance between the information recording medium 7 and the objective lens 4 as described above, the result of calculating (Sa+5b)-(Sc+Sd) may be used. Problems to be Solved by the Invention Next, at the time of focusing shown in FIGS. 3(a) and 3(b), the image is projected onto the photodetector 10 due to the diffraction by the guide track 6' due to the track deviation. Focusing error signal (hereinafter referred to as crosstalk) that is erroneously generated due to changes in the diffraction pattern that occur in the beam spot.
In this study, we examine the influence of

First, consider an ideal case where the optical system from the semiconductor laser 1, which is the light source, to the information recording medium 7 has no aberration.

As shown in FIG. 4, the photodetector 10 is arranged so that the illustrated X direction is the track direction and the illustrated Y direction is perpendicular to the track.
is irradiated and the reflected light is received by the photodetector 10. At this time, the positional relationship between the information track 6 and the beam spot 11 shown in FIG. 4 (FIGS. 4(a) and 4(b) shows the state in which the beam spot 11 is shifted downward, and FIG. 4(C) shows the positional relationship between the information track 6 and the beam spot 11, respectively.
, (d) No deviation, Figure 4 (e), (f
) respectively indicate a state in which the beam spot 11 is shifted upward. ) The diffraction pattern projected onto the photodetector 10 changes depending on the angle.

However, since the diffraction patterns in FIG. 4 are all symmetrical about the Y axis, crosstalk will not occur if the above calculation is performed.

Next, consider the case where the optical system has aberrations. When the optical system from the semiconductor laser 1, which is the light source, to the information recording medium 7 has aberration (particularly astigmatism), and its direction is parallel to the information track 6 (or deviated from the orthogonal direction), crosstalk occurs. occurs. Figure 5 shows this situation.
The arrangement direction of the photodetector 10 and the positional relationship between the information track 6 and the beam spot 11 are set in the same manner as in FIG. 4.

However, in this case, the aberration of the optical system disturbs the diffraction pattern projected onto the photodetector 10, causing crosstalk. Therefore, even though it is in focus, the focusing error signal is as shown in Fig. 5(a).
(b), Figure 5(C), (d), Figure 5(e)
)(f) will take positive, 0, and negative values, respectively,
Focusing control becomes unstable because an incorrect signal is generated.

<Object of the Invention> The present invention provides a means for preventing aberrations of an optical system from affecting a diffraction pattern caused by a guide track projected onto a photodetector, resulting in a decrease in the quality of a focusing error signal. An object of the present invention is to provide an optical information detection device equipped with the following.

<Example> Hereinafter, an example of the optical information detection device according to the present invention will be described with reference to the drawings.

FIG. 1 shows a configuration diagram of an embodiment of an optical information detection device according to the present invention. In the configuration shown in FIG. 1, a parallel flat plate 12 made of glass is arranged between a semiconductor laser 1 serving as a light source and a collimator lens 2, and the parallel flat plate 12 is arranged along an optical axis 13.
It is possible to rotate around the axis. Here, astigmatism is generated by arranging a parallel plate having a thickness t and a refractive index n and tilting it by an angle θ into a light beam having a divergence angle α. The present invention utilizes this astigmatism to remove the astigmatism remaining in the optical system in a direction parallel or not perpendicular to the information track 6, thereby making it possible to obtain a good focusing error signal. That is, the direction of astigmatism can be determined by rotating the parallel plate 12 around the optical axis 13, and the amount of astigmatism can be determined by changing the inclination θ of the parallel plate or by changing the parallel plate with a different plate thickness [, refractive index n]. It is possible to control by using . With the above configuration, it is possible to solve the problem of deterioration of the focusing error signal due to astigmatism of the optical system.

FIG. 6 is a diagram showing a focusing error signal when the information track 6 is traversed. FIG. 6(a) shows a signal obtained by a conventional optical information detection device, and the information track 6
Crosstalk occurs when the beam spot 11 traverses the beam spot 11, and especially crosstalk near the in-focus state causes malfunctions. FIG. 6(b) shows a signal obtained by the optical information detection device according to the present invention, and near the in-focus state,
There is almost no crosstalk, and stable focusing control is possible. Here, in the above embodiment, the parallel plate 12 is interposed in the part where the laser beam diverges, but the effects of the present invention can also be obtained even if this parallel plate 12 is interposed in the part where the laser beam is converged. can.

<Effects of the Invention> According to the present invention described above, it is possible to prevent the quality of the focusing error signal from deteriorating due to aberrations of the optical system.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of an optical information detection device according to the present invention, FIG. 2 is a configuration diagram of a conventional optical information detection device, and FIG. 3 is a diagram explaining the principle of focus detection of the optical information detection device. 4 and 5 are explanatory diagrams of changes in the position of the beam spot and the information track and the resulting changes in the state of the detection light, and FIG. 6 is a characteristic diagram of the focusing error signal. In the figure 1: Semiconductor laser 2: Collimator lens 3; Beam splitter 4: Objective lens 5: Substrate
6. Information track 7: Information recording medium 8: Convergent lens 9 Nijilintorical lens

Claims (1)

[Claims] 1. A parallel plate that is tilted from the optical axis and rotatable about the optical axis is interposed in the diverging or converging part of the light in the optical path from the light source to the information recording medium, and the changes in the parallel plate change the optical system. An optical information detection device characterized by adjusting to reduce the influence caused by aberrations.