Optical disk apparatus and optical head thereof

Abstract: The present invention comprises an arrangement with removable optical element having a predetermined focal distance in the light path between a light source and an objective lens, so as to insert said optical element into the light path between the light source and the objective lens for playing back or recording onto one type of thickness of disk plates, as well as to remove it therefrom for playing back or recording onto another type of thickness of disk plates. Alternatively, by retaining variably the distance between the light source and the collimator lens, the distance between the light source and the collimator lens along with the light path is variably changed according to the thickness of the disk plate being inserted. Then by determining the type of the disk inserted and accordingly selectably inserting/removing the optical element, only one objective lens may be used to conform to the disk plates having different thicknesses.

Description: The present invention relates to an optical disk apparatus and optical head thereof in which disk plates of different thickness from the side in the face of the optical head to the information recording surface (hereinafter, the thickness) such as high density recording or play back disks and Compact Discs (CD) may be played back and/or recorded. An example of the optical disk apparatus of the prior art in which one single apparatus being capable of playing back or reading/writing disks having different thickness is disclosed in the Japanese Patent Laid-Open Hei 4-95224, which apparatus is used for converging laser light beam into an optical disk recording layer without the need of interchanging objective lens for each respective of disk plates having different thicknesses. JP-A-07182690 discloses an optical disk apparatus for playing back from and/or recording onto the recording surface of disks of different thickness. To suppress aberrations, a conversion lens is inserted or removed between the collimator lens and the objective lens. Thus, this document corresponds to the pre-characterising part of claim 1 and claim 4. Similarly JP-A-07065409 discloses an optical disk apparatus on which a convex lens is inserted between the collimator and objective lenses for disks thicker than on predetermined value, and a concave lens is inserted for disks thinner than the predetermined valve. It is also known to provide an optical disk apparatus in which other variations, apart from disk thickness are taken into account. Thus, JP-A-63-234418 discloses an apparatus in which light can be focussed on different layers of a disk with a plurality of recording layers. The light source is mounted on a n electrostructive element, and the application of a field to that element causes it to expand and contract thereby varying the spacing of the light source and the collimator lens. US-A-5157555 discloses an apparatus which corrects for variations in the nominal thickness of the overcoating substrate layer of the disk. It varies the gap between two lens elements of a corrector lens, located between the collimator and objective lenses, thereby correcting the additional spherical aberration due to the changes in substrate thickness. Finally, US-A-5281797 discloses an apparatus to allow recording onto short or long wavelength optical disks. It proposes that the aperture diameter is varied, with a variation in the light source output. According to a first aspect of the invention there is provided an optical disk apparatus for playing back from and/or recording onto a recording surface two types of disks, the first type of disk having a thickness of 0.6 mm and the second type of disk having a thickness of 1.2 mm; and the apparatus including an optical head having a collimator lens for collimating incident light rays into collimated light rays, and an objective lens which receives the collimated light rays from the collimator lens and further comprising: a disk discriminator for identifying disks of different types including at least a first type of disk with a first thickness and a second type of disk with a second thickness; a controller connected with the disk discriminator; wherein; the controller is arranged to select at least between: (1) a first optical layout with a first distance between the collimator lens and a light source, and (2) a second optical layout with a second distance between the collimator lens and a light source. According to a second aspect of the invention there is provided an optical head for use in an optical disk apparatus for playing back from and/or recording onto two types of disks, the first type of disk having a thickness of 0.6 mm and the second type of disk having a thickness of 1.2 mm; and the optical head having a collimator lens for collimating rays of incident light from a light source more nearly parallel, and an objective lens for receiving collimated rays of more nearly parallel light from the collimator lens; wherein: the optical head further includes a means for selecting a distance between the collimator lens and a light source at least between a first distance for playing back from and/or recording on the first type of disk and a second distance for playing back from and/or recording on the second type of disk. As stated above, in accordance with the present invention, the disks having different thicknesses may be written or read out with only one single objective lens. The present invention has an effect in that the moving parts of the actuator may become less complex and may be composed in a simpler way. Embodiments of the invention will now be described in detail, by way of example, with reference to the. accompanying drawings, in which: Figs. 1a and 1b show transverse sectional views indicating an embodiment in accordance with the present invention with the collimator lens moved; Figs. 2a and 2b show transverse sectional views indicating another embodiment in accordance with the present invention. In the following description of the present preferred embodiments the disks are of two types of thickness. Fig. 1 shows a schematic diagram showing the arrangement of an optical disk apparatus and the optical head thereof in accordance with first to sixth embodiment of the present invention. The reference numeral 6a designates a disk composed of two 0.6 mm plates adhered (for high density recording or playing back), 6b designates to a 1.2 mm single disk plate (CD). In Fig. 1A, the light emitted from the laser source 1 passes through a collimator lens 2 to become parallel light, then passes through an aperture 7 to determine the diameter of the light bundle so as to be NA = 0.6 at the focusing side, then passes through an objective lens to be converged to focus on the recording surface 8a of the disk plate 6a. The objective lens 5 is designed such that the aberration becomes best with the disk thickness 0.6 mm and focusing side NA = 0.6. In Fig. 1B the light beam emitted from the laser source 1 passes through the collimator lens 2 to be parallel, then passes through an concave lens 3 to become divergent light, thereafter passes through the aperture 4 to determine the diameter of the light bundle with NA = 0.4 at the focusing side, then passes through the objective lens 5 to be converged onto the recording surface 8b of the disk plate 6b. The concave lens 3 is designed such that the best aberration may be obtained with the disk thickness 1.2 mm, NA = 0.4 at the focusing side, when the objective lens 5 is used. To previously add some spherical aberration to be further corrected in order to improve spherical aberration in the direction of over correction, it is possible to move either the light source or the collimator lens along with the light axis so as to cause the light beam incident to the objective lens to enter as divergent light. Thus in Fig. 1B, the collimator lens 2 is moved to the left side by a distance S, from the position as shown in Fig. 1A. The light emitted from the collimator lens 2 passes through the aperture 7 as divergent light into the objective lens 5, to converge into the disk recording surface 8b with aberration well corrected. The arrangement as shown in Fig. 2 is similar to that of Fig. 1 and comprises further the aperture 4, which is removed out of the light path in the Fig. 2A and inserted to the light path in the Fig. 2B to perform as an aperture for determining the image side NA. Other functionalities are identical to Fig. 1. There may be cases where the efficiency of the use of light as stated before is not so good. However, it has a merit that the optical element having negative focal distance may not be required. Exemplary values according to this method as the seventh embodiment may be as follows: Focal distance of the collimator lens 2 = 25 mm, and Amount of movement in the light path of the collimator lens 2 = 9.59 mm. In such a case the wave aberration in Fig. 2B may be 0.009 λrms. In the above described embodiments, disk plates have been assumed to have two distinct thicknesses. However for recording or playing back on the disk plate of three or more different thicknesses, another optical element having negative focal distance may be provided, or another optical element having positive focal distance may be provided, or the collimator lens is to be moved. As can be seen from above description, in accordance with the present invention, only one objective lens may be effectively used for well correcting aberration even with disk thickness being changed. Furthermore, the correction of aberration may be well performed even when the objective lens is decentering due to the tracking servo. While we have shown and described several embodiments in accordance with our invention, it should be understood that disclosed embodiments are susceptible of changes and modifications without departing from the scope of the invention as defined in the appended claims.

Claims: An optical disk apparatus for playing back from and/or recording onto a recording surface of two types of disks, the first type of disk having a thickness of 0.6 mm and the second type of disk having a thickness of 1.2 mm; and the apparatus including an optical head having a collimator lens (2) for collimating incident light rays into collimated light rays, and an objective lens (5) which receives the collimated light rays from the collimator lens and further comprising: a disk discriminator for identifying disks of different types including at least a first type of disk with a first thickness and a second type of disk with a second thickness; a controller connected with the disk discriminator; characterised in that ; the controller is arranged to select at least between: (1) a first optical layout with a first distance between the collimator lens (2) and a light source (1), and (2) a second optical layout with a second distance between the collimator lens (2) and a light source (1). The optical disk apparatus as set forth in claim 1 further including: an aperture means (4, 7) for selecting a numerical aperture of the objective lens in accordance with the disk discriminator such that the numerical aperture is larger for the first type of disk and smaller for the second type of disk. The optical disk apparatus as set forth in claim 2 further including: a means for changing a numerical aperture of the objective lens, the numerical aperture changing means selecting a numerical aperture of the objective lens in response to the disk identification from the disk discriminator. An optical head for use in an optical disk apparatus for playing back from and/or recording onto two types of disks, the first type of disk having a thickness of 0.6 mm and the second type of disk having a thickness of 1.2 mm; and the optical head having a collimator lens (2) for collimating rays of incident light from a light source more nearly parallel, and an objective lens (5) for receiving collimated rays of more nearly parallel light from the collimator lens (2); characterised in that : the optical head further includes a means for selecting a distance between the collimator lens (2) and a light source (1) at least between a first distance for playing back from and/or recording on the first type of disk and a second distance for playing back from and/or recording on the second type of disk. The optical head as set forth in claim 4, wherein the objective lens (5) has an image side numerical aperture (7) which is set larger for playing back and/or recording onto the first type of disk than onto the second type of disk.

Inventors: FUKUI YUKIO, IMADA NOBUO, SHINOHARA HIDENORI, SUGI YASUYUKI, SUGIYAMA TOSHIO, FUKUI, YUKIO, IMADA, NOBUO, SHINOHARA, HIDENORI, SUGI, YASUYUKI, SUGIYAMA, TOSHIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: G11B7
IPC codes: G11B19, G11B7