1.  Field of the Invention
The present invention relates to an optical pickup and an optical disk device and is suitably applied to, for example, an optical disk device corresponding to an optical disk having plural recording layers. 
2.  Description of the Related Art
In the past, an optical disk device widely used in the market is adapted to irradiate a light beam on optical disks such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a Blu-ray Disc (registered trademark, hereinafter referred to as BD) and read reflected light of the light beam to thereby reproduce information. 
In the optical disks, information that should be recorded is encoded and modulated and is then recorded in spirally or concentrically formed tracks as a pit or the like. 
The optical disk device is adapted to condense a light beam with an object lens and, in reproducing the information from the optical disk, focus the light beam on the tracks spirally or concentrically formed in a recording layer of the optical disk. 
In the optical disk device, for example, a light receiving region of a predetermined shape is provided in a photodetector to receive the reflected light.  The optical disk device calculates, on the basis of a result of the light reception, a focus error signal and a tracking error signal representing a shift amount with respect to a focus direction and a tracking direction between a track on which the light beam should be irradiated (hereinafter referred to as target track) and the focus of the light beam. 
Subsequently, the optical disk device moves the object lens in the focus direction on the basis of the focus error signal and moves the object lens in the tracking direction on the basis of the tracking error signal to thereby focus the light beam on the target track. 
As a method of calculating such a tracking error signal, various methods such as the push-pull method and the three-beam method are known.  A method called one-beam push-pull method is also proposed (see, for example, JP-A-2007-213754 (FIG.  15)).