A multilayered optical disc enables multiplexing and recording of multiple pieces of information therein in a perpendicular direction. For example, in a two-layer optical disc, a first layer and a second layer are formed with a spacer region being sandwiched by the first and second layers, and a recording surface in the first layer which is closer to a light irradiation surface of the optical disc is formed of a semi-transparent membrane in such a way that light can reach the second layer.
When changing the layer from which recorded information is to be read in playback of the above-mentioned multilayered optical disc, it is necessary to perform an interlayer movement (focus jumping) operation of focusing laser light from an optical pickup unit onto the multilayered optical disc in such a way as to cause the optical pickup unit to make a transition from a state in which the optical pickup unit focuses the laser light therefrom onto the recording surface in the first (or second) layer to a state in which the optical pickup unit focuses the laser light therefrom onto the recording surface in the second (or first) layer. This focus jumping operation is performed on the basis of zero clear detection of a focus error signal (referred to as an FE signal from here on) which is generated on the basis of the output of the optical pickup.
By the way, it is known that variations occur in the amplitude of the FE signal corresponding to each layer due to individual differences in the characteristics of the above-mentioned multilayered optical disc, an objective lens that constructs the optical pickup unit, and so on. Therefore, a problem is that although a threshold is provided for the FE signal corresponding to each layer and an acceleration or deceleration signal is furnished to an actuator used for the focus jump according to the threshold, the acceleration or deceleration signal does not have an appropriate value because of variations in the amplitude of the FE signal and therefore the optical pickup unit cannot perform a focus jump to the desired recording layer in a short time.
An optical disc driving device that, in order to solve this problem, by correcting the amplitude of the FE signal corresponding to each layer in such a way that the amplitude has an identical value, and then applying a threshold to the corrected signal amplitude, can carry out a focus jump correctly even if there are variations in the characteristics of the focus actuator of the optical pickup unit and there is a variation in the distance between the plurality of recording surfaces is known (for example, refer to patent reference 1).    [Patent reference 1] JP,2000-298846,A
A problem with the technology disclosed by above-mentioned patent reference 1 is, however, that it is necessary to measure the amplitude of the FE signal corresponding to each layer correctly, and, when, for example, making a focus search by rotating a disc having a surface deflection, it is difficult to carry out the measurement itself because the FE signal corresponding to the same layer appears in synchronization with the rotational cycle, as shown in, for example, FIG. 9(b).
FIGS. 9(a) and 9(b) are views showing a comparison between the FE signal corresponding to each layer of the multilayered optical disc in a case in which the multilayered optical disc has a surface deflection and the FE signal in a case in which the multilayered optical disc has no surface deflection. It is well known that the amplitude of the FE signal is zero when the focal position is far away from the recording layer and exhibits an S-shaped curve as the focal position gets close to the recording layer, and further becomes zero when the focal position reaches the recording layer. In the example shown in FIG. 9(a), because it can be recognized that the S-shaped curve which appears for the first time with respect to the lens position corresponds to the L0 layer (i.e., the first recording layer) and the S-shaped curve which appears for the second time with respect to the lens position corresponds to the L1 layer (i.e., the second recording layer), the amplitude of the FE signal corresponding to each layer can be measured. In contrast, in the example shown in FIG. 9(b), because the S-shaped curve appears several times with respect to the lens position, it cannot be recognized that which S-shaped curve corresponds to which one of the layers, and therefore the amplitude of the FE signal corresponding to each layer cannot be measured.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an optical disc driving device that implements a stable focus jump even if there are individual differences in the characteristics of a multilayered optical disc, an objective lens which constructs an optical pickup unit of the optical disc driving device, and so on.