1. Field of the Invention
The present invention relates to an optical information processing apparatus which irradiates a focused light beam to an optical recording medium to record and/or reproduce information.
2. Related Background Art
Various disk, card and tape media which record information by light and reproduce the recorded information have been known.
For example, in an optical information processing apparatus which uses an optical disk, the optical disk is scanned by a light beam which is modulated by recording information and focused to a fine spot, and information is recorded as optically detectable record pit tracks (information tracks). In order to exactly record the information without difficulty, such as crossing of the information tracks, it is necessary to control the irradiation position of the light spot on a plane of the optical disk in a direction perpendicular to the scan direction (auto-tracking, hereinafter referred to as AT). Further, in order to irradiate the light spot as a fine spot which is stable in spite of warp or mechanical tolerance of the optical disk, it is necessary to control the irradiation position normal to the plane of the optical disk (auto-focusing, hereinafter referred to as AF). In a reproduction mode, both AT and AF are necessary, too. Various techniques for AT and AF have been known. Usually, a focusing error signal and a tracking error signal are derived from differences between photo-sensing planes of focusing and tracking photo-detectors, and an objective lens is driven by AT and AF actuators.
In the above apparatus, when recording or reproduction is to be started, the focusing control means is switched from an inactive state to an active state, that is, so-called focus pull-in is effected. In the focus pull-in, a focus control loop is opened and a triangular wave signal is applied to a focus actuator to move the objective lens up and down, as described in U.S. Pat. No. 4,542,491. When the objective lens reaches a position corresponding to a linear region of a focus error signal, the loop is closed. During the focus pull-in, the tracking control loop is kept open.
In the above method, since no drive force along the tracking path is applied to the objective lens in the focus pull-in, the objective lens may deviate from an intended mechanical center (a center of a movable range by the tracking actuator, at which an optical axis of the light beam and an optical axis of the objective lens coincide). Such a deviation may be caused by a residual strain of a spring which movably supports the objective lens or by an offset of an input circuit to the tracking actuator. If such a deviation occurs, the operation range may be unbalanced when the tracking actuator is activated after the focus pull-in. For example, assuming that the optical system of the apparatus assures an optical output in a range of .+-.250 .mu.m from the optical center and that there occurs a deviation of +100 .mu.m, the assurance of the optical output is in a range of -350 .mu.m to +100 .mu.m. Accordingly, if the light spot is track-jumped in a positive direction by 100 tracks (1.6 .mu.m/track) after the focus pull-in, the light beam is out of the range of assurance of the optical output.