1. Field of the Invention
The present invention relates to an optical disk apparatus for reproducing or recording information to a recording medium such as DVD (Digital Versatile Disc) and a method to adjust focal control adapted in the apparatus.
2. Description of the Related Art
It was already known that there are a knife-edge method (Foucault method), a spot size detecting method, an astigmatic focal error method and the like as a conventional method to detect a focal error signal which represent a positional shift between a light beam of the optical disk apparatus and a recording surface of a recording medium. The astigmatic focal error method is the most popular
one in standpoints of its simplified optical system and easiness in, adjustment and the like. In the astigmatic focal error method, a disturbance is easily generated in the focal error signal when a condensed spot of the light beam irradiated on the recording medium (hereinafter, refereed to as light beam spot) traverses a track on the recording medium. An influence caused by the disturbance is particularly remarkable in the case of a land/groove recording medium such as DVD-RAM because a width of a guiding groove (groove) provided on the recording medium and a width of an inter-groove space (land) are substantially equal to each other, and a depth of the guiding groove is set to a large value relative to a wavelength λ of the light beam (λ/6-λ/7), which increases an amplitude of a push-pull signal generated by diffraction through the guiding grooved.
An optical construction and a signal detecting circuit for controlling the disturbance superposed on the focal error signal when the light beam spot irradiated on the recording medium traverses the track on the recording medium are disclosed in the Patent Literature 1 (No. 2000-82226 of the Publication of the Unexamined Japanese Patent Applications; hereinafter, referred to as the Literature 1) and the Patent Literature 2 (No. H09-81942 of the Publication of the Unexamined Japanese Patent Applications; hereinafter, referred to as the Literature 2).
FIG. 16 is a schematic illustration of the optical construction of the optical pickup recited in the Literature 1. A light source 2-1 is an element to emit a light beam having the wavelength of, for example, 650 nm. The light beam emitted from the light source 2-1 enters a diffraction grating 2-2. The diffraction grating 2-2 separates the light beam into at least three light beams, which are a main light beam transmitting through the diffraction grating 2-2 in situ (zero-order light) and two sub light beams moving separately from the main light beam at a predetermined diffraction angle (positive first-order diffracted light and negative first-order diffracted light). These three light beams enters a collimate lens 2-4 via a polarized beam splitter 2-3, converted into a parallel light in the collimate lens 2-4, and condensed on a recording surface of a recording medium (hereinafter, referred to as disk) 1 such as a DVD-RAM via a starting mirror 2-10 and an object lens 2-5. The condensed light beams form light beam spots 100, 101 and 102. The light beams are reflected on the disk 1 and turn into a return light. The reflected light (return light) travels a same optical path as that of the irradiated light (outgoing light) and is reflected on a reflecting surface of the polarized beam splitter 2-3 via the object lens 2-5, starting mirror 2-10 and collimate lens 2-4. The reflected light (return light) reflected by the polarized beam splitter 2-3 is condensed on predetermined light receipt surfaces of light detectors 2-7, 2-8-1 and 2-8-2 via a condensing lens 2-6.
The light source 2-1, diffraction grating 2-2, polarized beam splitter 2-3, collimate lens 2-4, object lens 2-5, condensing lens 2-6, light detectors 2-7, 2-8-1 and 2-8-2, two-dimensional actuators 2-9-1 and 2-9-2, and starting mirror 2-10 constitute a unit of the optical pickup.
The three light receipt surfaces divided into four are substantially arranged linearly in the light detectors 2-7, 2-8-1 and 2-8-2. The respective main light beam and the two sub light beams (positive first-order diffracted light and negative first-order diffracted light) constituting the reflected light (return light) are condensed on positions at substantial centers of light receipt regions 200, 201 and 202, that is a position in each light receipt region wherein a point where a horizontal dividing line and a vertical dividing line intersect with each other crosswise and an intensity center of each light beam substantially correspond to one another. Because the light beams are respectively supplied with a predetermined astigmatic focal error by the condensing lens 2-6, a position detection signal based on the astigmatic focal error method is detected from each light receipt region, and the focal error signal is generated from these position detection signals.
The two-dimensional actuators 2-9-1 and 2-9-2 are attached to the object lens 2-5. The two-dimensional actuators 2-9-1 and 2-9-2 conduct automatic positional adjustment, namely, focal control of the object lens 2-5 based on the focal error signal, so that the irradiated light (light beam spots 100, 101 and 102) can be constantly and precisely irradiated on the recording surface of the disk 1.
However, in the case of generating the focal error signal based on the return light (reflected light), the disturbance is easily generated in the focal error signal because an intensity distribution pattern of the reflected light periodically changes under the influence of the diffraction by the guiding grooves in the disk 1, and leak of the push-pull signal component thereby generated.
Further, a phase of the disturbance component superposed on one of the position detection signals and a phase of the disturbance component superposed on the other position detection signal are not entirely inverted due to variations generated in a mounting precision of optical components of the optical pickup, a mounting precision of a transfer mechanism (track traverse mechanism) for arbitrarily moving the light beam spot in a direction to traverse the track of the disk 1, and the depths of the grooves in the disk, which contains a variety of phase components ranging from the same phase component to an inverted component in these phases. Accordingly, the disturbance component unfavorably remains in the focal error signal actually used for the focal control, which may deteriorate a focal control performance. As another disadvantage, there is no choice except to variously set a coefficient multiplied to the position detection signal in the canceling processing conducted in the conventional example described above, which also generates the situation where the disturbance component unfavorably remains in the focal error signal actually used for the focal control. Accordingly, there is a case that the focal control performance may be deteriorated.
As known from the foregoing description, there was a definite method in the past to determine the coefficient multiplied to the position detection signal when the position detection signals are added to each other. In the conventional control method, drive currents applied to the actuators 2-9-1 and 2-9-2 as the focal control signal are excessively large, which disadvantageously increases loads to the actuators 2-9-1 and 2-9-2.