1. Field of the Invention
The present invention relates to a signal detecting apparatus in an optical disc apparatus, in which a tracking error is detected by a push-pull method and a focusing error is detected by a spot-size method.
2. Description of Related Art
In a known optical disc apparatus, a bundle of rays reflected by an optical disc is received by light receiving surfaces of error detecting light receiving elements. The signal data output from the divided areas of the light receiving elements is used to calculate the tracking error and focusing error.
For instance, Japanese Unexamined Patent Publication (kokai) No. 61-206944 (JPP '944) (U.S. Pat. No. 4,742,218) discloses a focus error detecting system in which light reflected from an optical disc is convered into a beam spot so that the focus state of an objective lens can be detected in accordance with the size of the beam spot. This known error detecting method will be referred to as a spot-size method hereinafter.
In the spot-size method, light receiving elements are located on opposite sides of and optically equidistant from a convergence point on which the light reflected from the optical disc is converged when the objective lens is in a focused state, so that the sizes of the respective beam spots formed on the light receiving elements can be compared with each other to generate a focus error signal.
However, in an optical system as disclosed in FIG. 1 of JPP '944 mentioned above, it is impossible to detect tracking error with the same optical elements that are used for detecting the focusing error. In the optical system shown in FIG. 4 of JPP '944, both tracking error and focusing error can be detected with the same optical elements. However, the pattern into which the light receiving elements are divided is complex.
Furthermore, if the optical axis of the bundle of rays, incident upon the objective lens, is inclined with respect to the optical disc, or if the objective lens is displaced in the radial direction of the optical disc to correct tracking error, the optical path of the reflected light is deviated from a reference position. This results in the displacement of the beam spots in a direction corresponding to the radial direction of the optical disc. Consequently, in a tracking error detecting system that uses the push-pull method, even if there is no change in light intensity distribution, which is caused when the beam spot moves across the optical disc, the light receiving areas will be unbalanced, resulting in track offset signals being carried on the detected signals. Therefore, the deviation of the beam spots from the track of the optical disc and the track error signals no longer maintain a predetermined relationship, and accordingly, the position of the beam spot can not be precisely controlled by a track servo control in accordance with the detected signals.
The "track offset signal" is one of the track error signal components detected by the light receiving elements. Track offset is caused by the displacement of the beam spot on the light receiving element due to the deviation of the reflected light.
In the known focusing error detection system using the spot-size method, the resultant signal of the outputs of the two light receiving elements is set to be zero when the objective lens is in the focal position. Namely, the respective signals of the light receiving elements are not individually taken into account.
Consequently, it is necessary to adjust the position of the light receiving elements while observing the balance of the quantities of light to be received by the light receiving elements, thus resulting in a complex and troublesome adjustment operation.
Furthermore, if the light receiving elements are used to reproduce the recorded magnetic optical signals of the magnetic optical disc as disclosed in the above-mentioned JPP '944, a slight change in the balance of the quantity of light to be received by the light receiving elements due to the rotation of the polarizing surface by the Kerr effect occurs, and accordingly, the change may result in an interference contained in the focus error signal, thus resulting in an imprecise focus servo control.
In the prior art, as disclosed in JPP '944, in which each light receiving element is split into three light receiving areas, the tracking error, using the push-pull method, and the focusing error cannot be detected by the same light receiving element.
Furthermore, JPP '944 also discloses light receiving elements, each being split into three light receiving sections in the form of elongated bands, wherein the center light receiving section is split into three mosaic areas, so that the tracking error can be detected by the same light receiving element as that for detecting the focusing error. The split pattern of the light receiving element is, however, complex, especially at the center portion thereof. Accordingly, it is necessary to form a relatively large beam spot on the light receiving element, thus resulting in a decreased freedom of optical design and a large apparatus.