1. Field of the Invention
The present invention relates to an optical disc apparatus for optically reproducing a signal from a disc or optically recording and reproducing a signal on and from a disc. In particular, the invention relates to an optical disc apparatus capable of reproducing a signal from a disc having a plurality of recording layers from the disc surface side or recording and reproducing a signal on and from a disc having a plurality of recording layers from the disc surface side.
2. Description of the Related Art
Among the currently standardized digital video discs (or digital versatile discs; hereinafter referred to as “DVDs”) are single-surface/single-layer discs, double-surface/single-layer discs, single-surface/double-layer discs, and double-surface/double-layer discs. That is, in contrast to other conventional discs such as compact discs (hereinafter referred to as “CDs”) and laser discs (hereinafter referred to as “LDs”) that have only one recording layer on one surface, there are DVDs that have two recording layers on one surface to increase the recording capacity.
For example, FIG. 2A shows a single-surface/double-layer disc that is produced by forming a recording layer on each of two 0.6-mm-thick discs, forming a high-reflectance aluminum film and a semitransparent gold film on the respective discs, and bonding together the two discs. FIG. 2B shows a double-surface/double-layer disc that is produced by bonding together two 0.6-mm-thick discs in each of which information is multiplexed in the depth direction.
In the above double-layer discs, information is recorded in each recording layer. When the level of a signal for driving an objective lens is increased gradually as shown in FIG. 2D (it is assumed that the objective lens approaches the disc accordingly), a point (hereinafter referred to as “focus point”) where the beam is focused at the lower recording layer (hereinafter referred to as “0th layer”) occurs in a focus error signal as shown in FIG. 2C at a certain position of the objective lens. When the objective lens is further elevated, a focus point corresponding to the upper recording layer (hereinafter referred to as “first layer”) occurs at a position of the objective lens that is higher than the previous position. In short, in double-layer discs, the beam is focused at each recording layer by moving the objective lens vertically. In CDs and LDs, it is sufficient to focus on the single recording layer on the single surface. On the other hand, in multi-layer discs such as DVDs having two or more planes where information was recorded from one side, unless switching is made from a focus point corresponding to a recording layer at which the beams is currently focused to another focus point corresponding to another recording layer, information stored in the latter recording layer cannot be read out.
The focus point switching between layers (hereinafter referred to as “focus jump”) is described in Japanese Unexamined Patent Publication Nos. Hei. 9-50630 and Hei. 11-345420, for example. The method disclosed in the publication No. Hei. 9-50630 is as follows. For example, a focus jump from the 0th layer to the first layer is performed as shown in FIG. 3. For a move from the 0th layer to the first layer, first, the focus servo loop is rendered in an open state or a hold state and the objective lens is accelerated by applying an elevation voltage to the actuator for driving the objective lens. In the interlayer region of the focus position between the 0th layer and the first layer, in a period when the focus error signal is between threshold levels, the voltage application to the actuator is stopped. After the focus error signal has exceeded the upper threshold level, a lowering voltage is applied to the actuator for a prescribed period and the focus servo loop is closed in the vicinity of the focus point corresponding to the first layer to complete the focus jump. This method enables a stable focus jump irrespective of a variation in interlayer distance, noise that is added to the focus error signal, a variation in the sensitivity of the actuator for driving the objective lens, and other factors.
The method disclosed in the publication No. Hei. 11-345420 is as follows. A focus error signal at the end of a deceleration pulse is measured in a focus jump, and the output timing of a deceleration pulse in the next focus jump is corrected by using the measured value. By repeating this operation, optimum output timing of a deceleration pulse in a focus jump is learned through adjustments. This method enables a stable focus jump even in a case where the focus error signal is not balanced properly or has a distorted waveform due to an offset in circuitry, a local variation in disc characteristics, a variation in interlayer distance or reflectance, or a variation in pickup characteristics and maximum acceleration attained by the actuator is small.