JPS6139240A - Focus detector - Google Patents

Abstract

PURPOSE:To decrease the optical length of a detection system while a beam diameter on a four-division detector is kept to a prescribed size by providing an astigmatic difference lens where one face has a spherical lens face and the other has a cylindrical lens face in an optical path of a focused luminous flux. CONSTITUTION:The one face of the astigmatic difference lens 30 is a concave spherical fase 30A and the other face has a convex cylindrical face 30B. Since the astigmatic difference lens 30 is arranged in the optical path, the focused angle of the focus luminous flux 32 is decreased by the concave spherical face 30A and the flux is irradiated on the four-division detector 31 as the focus luminous flux 33 having an astigmatic difference in combination with the convex cylindrical face 30B. The optical length of the detection system is reduced by providing the condenser lens 29 and the astigmatic difference lens 30 without reducing the beam diameter on the four-division detector 31 in the focusing state, that is, while the focusing angle is kept to a conventional angle.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus detection device in an optical information recording/reproducing device such as an optical disk file device.

Conventional Structure and Problems In an optical information recording/reproducing apparatus, focus control is performed to accurately focus a light beam emitted from a light source as a light spot on the information recording surface of an optical disk. Since this control is feedback control, it is necessary to detect focus errors with high precision, and various focus detection devices have been proposed for this purpose.

FIG. 1 shows a schematic diagram of an optical system of a conventional focus detection device.

FIG. 1 shows a focus detection device based on the principle called astigmatism. In the figure, a light beam 2 emitted from a semiconductor laser 1 is
The collimator lens 3 converts the light into parallel light, which passes through the polarizing beam splitter 4, the wavelength plate 5, and the aperture lens 6, and is focused as a light spot 8 on the information recording surface 7 of the optical disc. After the light spot 8 is reflected by the information recording surface 7, it passes through the diaphragm lens 6 again and becomes a parallel light, which is then passed through the K wavelength plate 5.
The light is then reflected and separated by the wandering beam splitter 4, and travels downward in the plane of the paper as a light beam 9. The light beam 9 is further made into a focused light beam by a condensing lens 10, and by arranging a cylindrical lens 11 in its optical path, it is made into a focused light beam 12 having two focal planes (astigmatism difference) due to astigmatism. . When the information recording surface 7 of the optical disc is in the focused position, a four-split detector 13 is placed at a position where the beam shape of the focused light beam 12 becomes a perfect circle. When the information recording surface 7 changes before and after the in-focus position, the beam shape of the focused beam 12 at the 4-split photodetector 13 becomes a vertically long or horizontally long ellipse, and the difference in the sum of the diagonal outputs of the 4-split photodetector 13 is By taking this, a focus error signal can be obtained.

In the conventional focus detection device configured as described above,
Reducing the optical path length L (see FIG. 1) of the detection system is an important issue in downsizing the optical head. In addition, the focus detection performance is improved by reducing the effects of optical axis misalignment and optical axis tilt between the 4-split detector 13 and the light beam 9 due to thermal deformation of the optical component support member due to thermal expansion or elastic deformation due to external loads such as vibration. It is important to shorten the optical path length also for stabilization. Conventionally, in order to shorten the optical path length, measures have been taken such as shortening the distance between the condenser lens 1o and the polarizing beam splitter 4, and shortening the focal length of the condenser lens 1o. However, no great improvement can be expected with the former, and when the focal length of the latter condenser lens 10 is shortened, the convergence angle α increases, and the perfect circular beam diameter of the converged light beam 12 on the 4-split detector 13 decreases. Therefore, there was a problem that it was difficult to detect and became unstable.

Purpose of the Invention The present invention solves the above-mentioned conventional problems by shortening the optical path length of the detection system (L in Fig. 1), keeping the beam diameter on the 4-split detector at a predetermined size, It is an object of the present invention to provide a compact and stable focus detection device that can accurately set the astigmatism difference.

Structure of the Invention The present invention is a focus detection device equipped with an astigmatism imparting lens having a spherical lens surface on one side and a cylindrical surface on the other. By providing an astigmatism difference between the two surfaces, the optical path length of the detection system can be shortened while maintaining stable performance.

DESCRIPTION OF EMBODIMENTS FIG. 2 is a schematic diagram of an optical system of a focus detection device in an embodiment of the present invention. In FIG. 2, 21 is a semiconductor laser, 22 is a collimator lens, 23 is a polarizing beam splitter, 24 is a wavelength plate, 25 is an aperture lens, 26 is an information recording surface of an optical disk, 27 is a light beam emitted from the semiconductor laser 21, 28 is a light spot on the information recording surface. 29 is a condensing lens, 30 is an astigmatism imparting lens sb, one side of which is a concave spherical surface 30A, and the other surface is a convex cylindrical surface 30B.
to accomplish. Reference numeral 31 indicates a four-split detector, and 32 and 33 indicate converging light beams, respectively.The operation thereof will be explained below.

The light beam 27 emitted from the semiconductor laser 21 is made into parallel light by the collimator lens 27, passes through the polarizing beam splitter 23, the K wavelength plate 24, the aperture DL' lens 26, and the information recording surface 26, and then returns to the aperture lens 25 and K. Passes through the wavelength plate, is reflected and separated by the polarizing beam splitter 23, and is sent to the condenser lens 2.
Directed to 9.

Up to this point, the explanation is omitted since it is the same as the conventional example.

The condenser lens 29 uses a lens with a shorter focal length than the conventional condenser lens (10 in FIG. 1).

In the absence of the astigmatism imparting lens 3o, the convergent beam 32 has a large convergence angle as shown by the dotted line. By arranging the astigmatic difference imparting lens 30 in this optical path, the convergent light beam 3
2 can reduce the convergence angle with the concave spherical surface soA, and
Convex cylindrical surface 30B, -.

As a focused light beam 33 having an astigmatic difference in combination with
The light is irradiated onto a four-part detector 31.

According to this embodiment as described above, by providing the condensing lens 29 and the astigmatism imparting lens 30 as shown in the figure, the beam diameter on the four-split detector 31 in the focused state is not reduced. The optical path length of the detection system can be shortened while keeping the convergence angle at a conventional level, for example.

Other embodiments of the present invention will be described below with reference to the drawings. FIG. 3 is a schematic diagram of an optical system of a focus detection device showing another embodiment of the present invention.

In the figure, 41 is a semiconductor laser, 42 is a polarizing beam splitter, 43 is a collimator lens, 44 is a wavelength plate, 46 is an aperture lens, 46 is an information recording surface of an optical disc,
47 is a light beam emitted from the semiconductor laser 41, 48 is a light spot on the information recording surface, and 049 is an astigmatism imparting lens, one surface of which is a concave spherical surface 49A, and the other surface is a convex cylindrical surface 49B. 50 is a 4-split detector, 51,
Reference numerals 52 each indicate a focused light beam, and the operation will be explained below.0 A light beam 47 emitted from the semiconductor laser 41 passes through the polarizing beam splitter 42 as a non-parallel light, and is converted into a substantially parallel light by a collimator lens 43. After that, the K wavelength plate, the aperture lens, the lens 45, the information recording surface 46, and the aperture lens 45 again.
, a wavelength plate, a collimator lens 43, the beam is reflected and separated by a polarizing beam splitter 42, and an astigmatic difference imparting lens 4
Directed to 9. Since the collimator lens 43 is disposed after the polarizing beam splitter 42, non-parallel light passes through the inside of the polarizing beam splitter 42, and the light incident on the astigmatism imparting lens 49 also becomes a convergent beam 51.

The convergence angle of this convergent beam 51 is set to be relatively large, as shown by the dotted line in the figure, so that it is focused close to the polarizing beam splinter μ2. By arranging the astigmatic difference imparting lens 49 in the right 1 optical path of the focused light beam, the convergence angle of the focused light beam 51 is reduced by the concave spherical surface 49A, and in combination with the convex cylindrical surface 49B, the astigmatism difference is reduced. A four-split detector 6o serves as a converging light beam 52 having
is irradiated.

According to this embodiment as described above, by taking out the reflected light beam from the information recording surface 46 as the converging light beam 51 through the polarizing beam splitter 42, the condensing lens (29 in FIG. 2) after the polarizing beam splitter 42 ) can be removed, and the optical path length of the detection system can be further shortened.

Note that the astigmatic lens 49 may be manufactured by bonding and integrating a plano-concave lens and a plano cylindrical lens.

Effects of the Invention As described above, the focus detection device of the present invention provides, in the optical path of a converging light beam, an astigmatism imparting lens having a spherical lens surface on one side and a cylindrical lens surface on the other side.
The optical path length of the detection system can be shortened while the beam diameter on the 4-split detector is maintained at a predetermined size. Further, the astigmatism difference can also be set using one lens (the spherical lens surface and the cylindrical lens surface of the astigmatism imparting lens).

It is possible to realize a focus detection device with stable performance in a small and compact size.
Its practical value is high.

[Brief explanation of drawings]

Fig. 1 is a principle diagram of an optical system of a conventional focus detection device, Fig. 2 is a principle diagram of an optical system of a focus detection device in an embodiment of the present invention, and Fig. 3 is a principle diagram of a focus detection device in another embodiment of the present invention. It is a principle diagram of the optical system of a detection device. 29...Condensing lens, 43...Collimator lens, 30.49...Astigmatism imparting lens, 31...4-divided detector. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
figure

Claims (1)

[Claims] a light source, a light focusing means for focusing a light beam emitted from the light source as a light spot on an information recording surface, a light focusing means for condensing a reflected light beam or a transmitted light beam from the information recording surface as a focused light beam, and said focusing means. A focus detection device includes an astigmatism imparting lens having a spherical lens surface on one side and a cylindrical surface on the other, which changes the convergence angle of a light beam and gives an astigmatism difference to guide the light beam to a photodetector.