1. Field of the Invention
This invention relates to an optical pickup apparatus that can be adapted for various types of optical recording media having a different recording format such as a substrate thickness.
2. Description of the Related Art
Generally, an optical recording/reproducing apparatus driving disc-type media, such as compact disc(CD), etc., which are well known as recording media making use of a laser light beam, records or reproduce data by irradiating a laser beam onto the recording face of a disc with rotating the disc. To this end, the optical recording/reproducing apparatus includes an optical pickup for irradiating a laser beam generated from a light source, such as a semiconductor laser, onto the recording face of the optical disc using optical system devices such as an objective lens, etc.
Recently, there has been commercially available a digital versatile disc(DVD) that is capable of a larger amount of information than the conventional CD. The DVD is usually designed in consideration of a light source generating a different number of aperture and a different wavelength from the CD. In this case, the wavelength and the number of aperture of a light beam are related to a size of beam spot and which is adopted in consideration of the size of beam spot. The size of beam spot is selected from the standpoint of minimizing an effect caused by a cross talk between signal tracks defined on the recording face of the optical disc. Accordingly, since the DVD with a larger recording density than the CD has a small track pitch the size of beam spot, its beam spot size also must be smaller than that of the CD. In this case, a scheme making use of a shortened wavelength and an increased aperture number can be considered so as to reduce the size of beam spot. This is caused by a fact that the size of beam spot is directly proportional to the wavelength of the light beam while being inversely proportional to the number of aperture as seen from the following formula:                     d        =                  k          ⁢                      λ            NA                                              (        1        )            
wherein d represents a size of beam spot, k does a constant, xcex does a wavelength of a light beam, and NA does the number of aperture of an objective lens. It can be seen from the formula (1) that the DVD adopts a short wavelength of light beam and a large number of apertures so as to obtain a smaller size of beam spot than the CD. For instance, an optical pickup accessing the CD uses a light beam with a wavelength (xcex) of 780 nm and an objective lens with the number of aperture of 0.45, whereas an optical pickup accessing the DVD uses a light beam with a wavelength (xcex) of 650nm and an objective lens with the number of aperture of 0.6. Also, in the DVD, a characteristic of a light beam becomes sensitive to a thickness of the disc as the number of aperture of a light beam, so that a depth of its recording face, that is, a depth of its light transmission layer is set to have a smaller value than that of the CD. In other words, since a noise component is increased so that data cannot be recorded or reproduced as it is due to an increase in the optical aberration when a light transmission layer with the same thickness as the CD is transmitted by means of an objective lens with the number of aperture of 0.6, the thickness of the light transmission layer in the DVD is set to have a smaller value than that in the CD. For instance, a light transmission layer of the CD has a thickness of 1.2 mm while a light transmission layer of the DVD has a half the thickness thereof, that is, a thickness of 0.6 mm.
An optical pickup for changeably accessing such CD and DVD must includes two light sources generating a different wavelength of light beams and two objective lenses with a different number of aperture. In the case where an optical pickup is provided with two light sources and two objective lenses, it has problems in that a dimension of the optical pickup is enlarged, that its structure becomes complicated, and that a manufacturing cost thereof increases. In order to solve the problems, there has been studied an optical pickup that is capable of making use of a single light source and appropriately controlling the number of aperture of the objective lens depending on the corresponding disc to thereby access the CD and the DVD.
For example, Japanese Patent Laid-open Gazette No. Pyung 9-185839 has disclosed an optical pickup that can access two types of optical discs with a different thickness of light transmission layers by controlling the number of aperture of an objective lens employing a liquid crystal shutter and a polarizing filter. The optical pickup controls the number of aperture of the objective lens into two modes by turning on or off the liquid crystal shutter depending on whether or not a voltage is applied, to thereby selectively changing a polarization characteristic of a light beam generated from a light source, and by allowing the polarizing filter to selectively shut out a portion of light beam in accordance with the polarization characteristic of a light beam changed by means of the liquid shutter. Also, Japanese Patent Laid-open Gazette No. Heisei 9-198704 has disclosed an optical pickup that is capable of accessing two types of optical discs by providing two objective lenses with a single lens supporting member in the Twin-lens system to thereby switch a position of the objective lens in accordance with a rotation of the lens supporting member.
However, since the optical pickup apparatus of aperture number control system or twin lens system as mentioned above requires additional components, it has a problem in that its configuration is complicated and its manufacturing cost rises. Also, in the above-mentioned optical pickup apparatus, the aperture number control device is assembled to the actuator along with the objective lenses or the twin lens is assembled thereto, so that a load in an actuator operator is increased to deteriorate its sensitivity. Such sensitivity deterioration in the actuator operator acts as an obstacle factor in making a high multiple speed of optical pickup.
Accordingly, it is an object of the present invention to provide an optical pickup apparatus that is capable of recording and reproducing various types of optical discs having a different recording format such as a substrate thickness.
In order to achieve these and other objects of the invention, an optical pickup apparatus according to one aspect of the present invention includes at least two light sources for a different wavelength of lights; a focusing optical system for focusing the light from the light source onto the optical recording medium; and a receiving optical system for receiving a light reflected from the optical recording medium, wherein the focusing optical system is configured in such a manner to generate a predetermined range of aberration in a light spot formed on the optical medium by the lights from any one of the light sources.