1. Field of the Invention
The present invention relates to an optical pickup device and a recording and/or reproducing device, and more particularly to an optical pickup device and a recording and/or reproducing device that are capable of suitably correcting spherical aberration resulting from a thickness error of an intermediate layer (such as a protective layer) existing between a disk surface and a recording layer.
2. Description of the Related Art
It is possible to increase the recording density of an optical disk by shortening the wavelength of laser light or increasing a numerical aperture of an objective lens. If the numerical aperture of an objective lens is increased, however, spherical aberration occurs in laser light due to a thickness error of a protective layer existing between a disk surface and a recording layer.
When the thickness of the protective layer is set to around 0.1 mm, for instance, the thickness error is around ±10 to 20 ìm. If spherical aberration occurs in laser light converged on a recording layer due to the thickness error, the recording/reproduction characteristics with respect to the recording layer are degraded. Therefore, when an objective lens having a high numerical aperture is used, it is required to additionally provide means for detecting and correcting the spherical aberration.
It is possible to detect such spherical aberration by monitoring reflection light (returning light) from the disk.
In usual cases, laser light from a semiconductor laser is converted into parallel light by a collimator lens and then converged on a recording layer by an objective lens. Here, the objective lens is designed such that when the thickness of the protective layer is appropriate, the laser light is condensed on the recording layer.
When the thickness of the protective layer is appropriate, returning light from the recording layer is converted into parallel light while passing through the objective lens. If there exists a thickness error of the protective layer, however, the returning light having passed through the objective lens does not become parallel light and light beams in the center region of the returning light, and light beams in the outer peripheral region thereof are diffused or converged. By detecting the degree of the diffusion, it is possible to detect the degree of the thickness error or the spherical aberration.
FIG. 8 shows results of simulation of how diffusion angles of returning light (in the case of a perfect circle beam) are distributed. As shown in FIG. 8, when the thickness of the protective layer assumes an optimum value, the returning light is converted into parallel light in both of its center region and outer peripheral region. In contrast to this, when the thickness of the protective layer is greater than the optimum value, the returning light is converged in its center region (pupil radius=0 to around 0.8) and is diffused in its outer peripheral region (pupil radius=around 0.8 to 1). Conversely, when the thickness of the protective layer is smaller than the optimum value, the returning light is diffused in its center region and is converged in its outer peripheral region.
As a result of this phenomenon, light beams of the returning light are distributed in the manner shown in FIGS. 9A to 9C. That is, when the thickness of the protective layer assumes the optimum value (see FIG. 9B), the light beams of the returning light are uniformly distributed in its cross section. In contrast to this, when the thickness of the protective layer is greater than the optimum value (see FIG. 9C), more light beams of the returning light are distributed in the center region than in the outer peripheral region. Also, when the thickness of the protective layer is smaller than the optimum value (see FIG. 9A), more light beams of the returning light are distributed in the outer peripheral region than in the center region.
Therefore, by detecting the distribution state of the light beams using a photodetector element (photosensor), it is possible to detect the degree of the thickness error or the spherical aberration. However, the following problem occurs due to a difference in focus position between the light beams in the inner peripheral region and the light beams in the outer peripheral region (refer to the description of FIGS. 10A and 10C). As shown in FIGS. 10A to 10C, if setting has been made such that returning light is converged on the photosensor (A plane) by a convergence lens when the thickness of the protective layer is optimum, for instance, the distribution of light intensity on the sensor is uniformly changed when the thickness of the protective layer becomes greater or smaller than the optimum value (see FIGS. 11A to 11C). Therefore, it is impossible to reflect the direction and degree of the thickness error in an output signal of the photosensor (see FIG. 12).
In view of this problem, JP 2001-507463 A discloses an optical pickup with which returning light is split by a beam splitter into light beams in a center region and light beams in an outer peripheral region, and the split light beams are respectively received by a pair of photosensors.
According to this conventional technique, however, it is required to provide a beam splitter for splitting the returning light and a pair of photosensors for receiving light beams generated by the beam splitter, which leads to a problem in that it becomes necessary to provide many additional optical components to detect spherical aberration. Also, it is required to arrange the photosensors on a beam splitting path, so that it becomes necessary to secure a space for arranging the photosensors and a space for securing the beam splitting path. This causes another problem in that the outer dimensions of a pickup device main body is increased.