1. Field of the Invention
The present invention relates to an optical pickup apparatus for writing an information signal to an information signal recording medium such as a magneto-optical disk.
2. Description of the Prior Art
Conventionally, information signal recording mediums capable of writing an information signal therein by an optical means have been proposed. For instance, a magneto-optical disk functioning as such an information signal recording medium is constructed of a disk substrate and a signal recording layer coated on this disk substrate. The signal recording layer is manufactured by such a vertical magnetic recording material that a direction of magnetization can be reversed by heating the signal recording layer at a temperature higher than a so-called "Curie temperature" by irradiating a laser beam and applying an external magnetic field to this recording layer. To write and read the information signal into and from such an information signal recording medium, an optical pickup apparatus has been utilized.
The optical pickup apparatus is mainly arranged by, as shown in FIG. 3, a laser diode 12 functioning as a light source, and an objective lens 17 for collecting a light beam emitted from the laser diode 12 on the signal recording layer.
In such an optical disk, the reading light is irradiated to the disk plane, and the signal is read (reproduced) by detecting a great decrease in a reflection light amount caused by diffraction of the laser beam at the phase-pit forming portions of the disk.
In the above-described optical disk, resolving power of a signal reproduction is substantially determined by a wavelength ".lambda." of light from a light source in a reproduction optical system and also a numerical aperture NA of the objective lens. A spatial frequency of 2NA/.lambda. becomes a reproduction limit value of an optical disk.
As a consequence, to achieve high density information storage such an optical disk, a wavelength ".lambda." of light from a light source (for instance, a semiconductor laser) for a reproducing optical system must be shortened, or the numerical aperture NA of the objective lens must be increased.
However, there is an inherent limitation on shortening the wavelength ".lambda." of the light from the light source and on increasing the numerical aperture NA of the objective lens. These inherent limitations make it difficult to drastically increase the recording density of the recording medium.
Thus, the Applicant has proposed an optical disk capable of achieving the resolving power higher than the above-explained limitation defined by the wavelength ".lambda." and the numerical aperture NA by utilizing reflectivity by the partial phase variation within the scanning spot of the reading light (refer to Japanese laid-open patent applications No. 2-94452, and No. 3-249511).
FIG. 1 is a sectional view of one example of such an optical disk. This optical disk shown in FIG. 1 is formed in such a manner that a phase changing material layer 3 which can be crystallized after being melted, is fabricated on a transparent substrate 2. There are phase pits, such as phase pit 1, on the transparent substrate corresponding to an information signal.
In this case, when the read light, for instance, the reproducing laser light is irradiated to the material layer 3, a temperature distribution occurs in the scanning spot of the read light. As a result, the material layer 3 is partially brought from the crystal state to the melting state, so that reflectivity is lowered. This material layer returns to its normal crystal state the information has been read out from the optical disk.
Referring now to FIG. 2, a description will be made of such a case that the reproducing laser light is irradiated to the optical disk shown in FIG. 1.
In FIG. 2A, symbol "SP" denotes a laser spot which is scanned along an arrow direction "SC" in conjunction with the rotation of the optical disk. Although the respective phase pits 1 are arranged at the minimum recording period "q" as shown in FIG. 2B, this arrangement interval and the pit length may be, of course, changed in accordance with the recording data.
Also, in FIG. 2B, an abscissa represents a position related to the scanning direction SC of the laser spot SP. When the laser spot SP is irradiated onto the optical disk (see FIG. 2A), the light intensity of the laser spot SP has a distribution indicated by the dotted curve "a." In contrast thereto, the temperature distribution at the material layer 3 of the optical disk as indicated by the solid curve "b" is slightly behind along the scanning direction SC of the laser spot SP, as compared with the light intensity distribution "a." The separation between the curves depends upon the scanning speed of the laser spot SP.
Assuming now that, as described above, the laser spot SP is scanned along the scanning direction SC as shown in FIG. 2A, the temperature of the optical disk is gradually increased from the leading edge of the laser spot SP along the scanning direction, and finally becomes higher than the melting point MP of the material layer 3.
At this stage, the state of the material layer 3 is transferred from the crystal state of the initial stage to the melting state, so that the reflectivity thereof is lowered. As a result, there are simultaneously a region "Px" (indicated by a hatched line of FIG. 2A) from which the phase pit 2 cannot be read due to the lower reflectivity thereof, and also a region "Pz" from which the phase pit 1 can be read due to its higher reflectivity, within the laser spot SP.
Therefore, as illustrated in FIG. 2A, even when, for example, two phase spots 1 are present within the same laser spot SP, the information reading operation is carried out with respect only to a single phase spot 1 existing in the region Pz having higher reflectivity. As a consequence, the information reading operation can be done at ultra high resolving power without any restriction caused by the wavelength ".lambda." of the read light and the numerical aperture of the objective lens. Thus, a high density recording operation can be achieved.
It should be noted that the reflectivity of the phase changing material layer 3 is lower in the melting state of the above-described optical disk (FAD type optical disk), than in the crystal state. However, an optical disk may be manufactured by properly selecting various conditions of the phase changing material, e.g., a structure and a thickness in such a manner that the reflectivity of the phase changing material layer 3 is higher under the melting state, than in the crystal state. In such an optical disk, only the phase pit within the region (see the hatched portion of FIG. 2A) whose reflectivity is increased due to the melting state thereof, can be read out. To avoid confusion, an optical disk in which reflectivity of a phase changing material layer thereof under a crystal state is higher than that under a melting state will be referred to as an FAD type optical disk, whereas an optical disk in which reflectivity of a phase changing material layer thereof under a melting state is higher than that under a crystal state will be referred to as an RAD type optical disk. Accordingly, the information reading operation with ultra high resolving power can be effected for this RAD type optical disk in a similar manner to the FAD type optical disk, so that a high density recording operation can be achieved.
When an information signal is reproduced from such an optical disk that the material layer 3 is formed on the transparent substrate 2 in which the above-described phase pit 1 is formed, the push-pull method has been employed as the tracking servo system. The push-pull method is described in, for example, U.S. Pat. Nos. 3,909,608 and 4,961,183.
However, in case of an FAD type optical disk and also a RAD type optical disk, as previously explained, the information reproduction with ultra high resolving power is carried out by irradiating the laser spot SP on the track of the optical disk to be read, and by changing the reflectivity of the partial region Px within the laser spot SP due to the variations in the temperature distributions caused in the laser spot (see FIG. 2).
In this case, the shape of the region Px formed by the variations in the temperature distributions occurring in the laser spot, the reflectivity of which is changed, is not always symmetrical with respect to the centerline of a track. As a result, when the tracking servo operation is performed by the push-pull method, the tracking error signal cannot be obtained under a stable condition, and also a stable tracking control cannot be executed.