1. Field of the Invention
The present invention relates to an optical pickup apparatus which records or reproduces an information signal on or from a recording media such as an optical disk, in particular, an optical pick-up apparatus equipped with a parallel flat plate for spherical aberration correction, and to an information recording and reproduction apparatus including the optical pickup.
2. Description of the Related Art
In recent years, in optical disks such as a DVD and a BD, so as to increase a recording capacity thereof, development of the formation of a multilayer which has a plurality of recording layers in the same disk has been performed. Actually, commercial production of double-layer disks having the first and second recording layers has been achieved.
Nevertheless, in the double-layer disks, there was a problem that, since the thicknesses of a transmissive substrate up to the first and second recording layers were different, respectively, spherical aberration was generated when the same optical system was used, and hence, information quality deteriorated.
Technology which corrects the spherical aberration generated by such difference in substrate thickness is disclosed by, for example, Japanese Patent Application Laid-Open No. H05-241095 or Japanese Patent Application Laid-Open No. 2000-331367.
First, technology disclosed by Japanese Patent Application Laid-Open No. H05-241095 will be explained simply. In an apparatus of this application, a light beam emitted from a light source 5 is reflected on a half-mirror 6, is introduced by a collimator lens 2 to an objective lens 3, and is focused on an optical disk 4.
In addition, the reflected light from the optical disk 4 permeates the objective lens 3, collimator lens 2, and half-mirror 6, and is introduced into a light-receiving device 7. Here, spherical aberration by difference in substrate thickness of the optical disk 4 is corrected by thickness of a parallel flat plate (corrector plate) 1.
Next, technology disclosed by Japanese Patent Application Laid-Open No. 2000-331367 will be explained simply. An apparatus of this application is designed so as to form an optimum beam spot without a parallel flat plate 3-1 when using a disk 4-1 with substrate thickness A. Then, when a disk 4-2 with substrate thickness (A-B) is used, spherical aberration is corrected by inserting the parallel flat plate 3-1, which has thickness B and the same refractive index as a refractive index of a disk substrate, between the objective lens 1 and disk 4-2.
Both technologies mentioned above utilize spherical aberration generated by arranging a parallel flat plate orthogonally to an optical axis in divergent light or converging light, and correct spherical aberration generated by the difference in substrate thickness of a disk.
In the technology disclosed by Japanese Patent Application Laid-Open No. H05-241095, the parallel flat plate 1 is arranged only in an approach route between the half-mirror 6 and light source 5. In such a constitution, defocus is generated on the approach route by the inserted parallel flat plate 1. However, since the parallel flat plate 1 is not inserted on a return route, light is focused on the light-receiving device 7 from the collimator lens 2 with defocus being generated.
For this reason, since the light is not focused accurately on a light-receiving surface of the light-receiving device 7, an offset arises in a focus signal which is obtained by, for example, an astigmatism method or a knife edge method. Although a method of moving the light-receiving device 7 according to the defocus is also proposed to this problem, it is expected that it is difficult to achieve positional accuracy accompanying movement of the light-receiving device 7.
In addition, in the technology disclosed by Japanese Patent Application Laid-Open No. 2000-331367, a mechanism which inserts and removes a parallel flat plate is arranged between an objective lens and a disk surface. For this reason, it becomes difficult to achieve the formation of a thinner type of a recording and reproduction apparatus using an optical disk, which is particularly demanded in recent years. In addition, recently, with a raise in numerical aperture of an objective lens, a working distance has been decreasing and it becomes difficult to actually achieve this mechanism.
The object of the present invention is to provide an optical pick-up apparatus which can prevent an offset to a focus signal and can satisfactorily correct spherical aberration generated by difference in substrate thickness with achieving formation of a thinner type of the apparatus; and an information recording and reproduction apparatus.
In order to solve the above-mentioned tasks, the optical pickup apparatus of the present invention includes: a light source; a collimator for converting an emitted light from the light source to a parallel beam; an objective lens for focusing the parallel light beam on each of the recording layers of a recording medium; a beam splitter arranged between the light source and the collimator; a light-receiving device for receiving a light reflected from the recording medium and split by the beam splitter; a parallel flat plate which is arranged between the beam splitter and the collimator so as to correct spherical aberration generated by difference in thickness of a transmissive substrate with respect to each of the recording layers of the recording medium; and a driving mechanism for inserting and removing the parallel flat plate to the optical path, or switching and inserting another parallel flat plate having a different thickness to the optical path.
In addition, the information recording and reproduction apparatus of the present invention includes the above-mentioned optical pickup apparatus.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Next, the best modes for carrying out the present invention will be described in detail referring to drawings.
Next, an operation of the information recording and reproduction apparatus 101 in
The controller 111 of the information recording and reproduction apparatus 101 performs the integrated control of the optical pickup driver 108, seek motor driver 110, and spindle motor driver 103, and rotates the spindle motor 102 at a desired revolution speed through the spindle motor driver 103. Thereby, the optical disk 9 mounted on the spindle motor 102 is also integrally rotated. In addition, the seek motor 109 which is a stepping motor is driven by the seek motor driver 110, and the optical pickup apparatus 106 is conveyed in an arbitrary position in a radial direction of the optical disk 9. In addition, by the laser driver 112, laser light from the semiconductor laser 1 is controlled, and is radiated on the recording surface of the optical disk 9 through the objective lens 8, whereby the recording and reproduction of information are executed.
At this time, in order to make the objective lens 8 follow a track arranged on the recording surface of the optical disk 9 as described above, a drive current (an Fo current in an Fo direction, and Tr current in a Tr direction) to the objective lens actuator 105 is controlled on the basis of an Fo error signal and a Tr error signal, which are later mentioned, by the optical pickup driver 108. In addition, the Fo error signal is a signal obtained according to a vertical relative distance between the objective lens 8 and optical disk 9, and it is a signal becoming 0 in a focused state, for example, can be obtained by an astigmatism method. On the other hand, the Tr error signal is a signal obtained according to a relative position between a track formed on a recording surface of the optical disk 9 and a spot in a direction parallel to the disk surface, and it is a signal becoming 0 when the spot is located in an approximate center of a track, for example, can be obtained by a push-pull method or a differential push-pull method. In addition, since a generation method and structure of the above-mentioned Fo error signal and Tr error signal are well known, their explanation is omitted. In addition, the present invention is applicable also in methods other than the astigmatism method and differential push-pull method which are mentioned above.
First Embodiment
An emitted beam from the semiconductor laser 1 which is a light source is split into a main beam and two subbeams by a diffractive grating 2. These subbeams are used for servo signal generation for DPP (differential push-pull).
As for the beam from the diffractive grating 2, its part is reflected by PBS (Polarization Beam Splitter) 3 to be made incident into the PD (photodetector) 4 for monitoring. An output of this PD 4 for monitoring is used for control of an emission power from the semiconductor laser 1.
The beam which permeates the PBS 3 is converted to a parallel beam by the collimator lens 6, and is further incident into the objective lens 8 through a λ/4 plate 7. This incident light is focused by the objective lens 8 and is imaged on an information recording layer through a transmissive substrate (hereinafter, also referred to as “transmissive layer”) of the optical disk 9. The optical disk 9 is composed of a first information recording layer 9a having a transmissive layer (transmissive substrate) with a thickness of t1, and a second information recording layer 9b having a transmissive layer with a thickness of t2.
The beam reflected from the optical disk 9 is focused by the objective lens 8 to be made incident into the PBS 3 through the λ/4 plate 7 and collimator lens 6. This incident light is reflected by the PBS 3 to be focused by the sensor lens 10 on the PD 11 for RF servo. An information signal and a signal for servo are obtained with an output from this PD 11 for RF servo.
Here, a wavelength of the semiconductor laser 1 is about 660 nm, numerical aperture of the objective lens 8 is 0.65, and a focal length is 1.85 mm.
In addition, the parallel flat plate 5 is arranged so as to be able to insert and remove in a direction orthogonal to an optical axis between the PBS 3 and collimator lens 6 as shown by an arrow in
Here, a method in the case of focusing light into a light spot on each of the first information recording layer 9a and second information recording layer 9b of the optical disk 9 will be explained.
This embodiment is designed so that a light beam from the semiconductor laser 1 may be optimally focused to a light spot onto the first information recording layer 9a of the optical disk 9 in the optical system in which the parallel flat plate 5 is removed from the optical path as shown in
Table 1 shows design values of a projection system at the time of removing the parallel flat plate 5 in this embodiment from the optical axis. In addition, an aspherical shape is expressed in Formula 1 and is listed in Table 2, wherein an optical axis direction is X, a height in a direction vertical to the optical axis is h, and a conical coefficient is k.
Here, since difference of t2−t1=Δt in transmissive layer thickness arises as being in an optical system shown in
Then, in the optical system of this embodiment, as shown in
Table 3 shows design values of a projection system at the time of inserting the parallel flat plate 5. In addition, in Table 3, optical components other than the parallel flat plate 5 are the same as those of Table 1.
In this way, it becomes possible to correct the spherical aberration generated due to the difference in thickness of the transmission layer by removing the parallel flat plate 5 from the optical path when recording or reproducing information to or from the first information recording layer 9a of the optical disk 9, and by inserting the parallel flat plate 5 into the optical path when recording or reproducing information to or from the second information recording layer 9b.
In addition, since the parallel flat plate 5 is arranged between the beam splitter 3 and the collimator lens 6, the thickness of an optical pickup apparatus main body does not increase. That is, when a parallel flat plate is arranged between an objective lens and a disk like the above-described Japanese Patent Application Laid-Open No. 2000-331367, the thickness of an optical pick-up apparatus increases and fails in miniaturization of the apparatus, but the present invention can meet the demand of the miniaturization in recent years.
In
In this embodiment, as shown in
On the other hand, in the case of
In addition, in the design values of the optical system of this embodiment which are shown in Table 3, the defocus of the collimator lens 6 between the cases of inserting and removing the parallel flat plate 5 is about 1.13 mm.
In this way, in the case of
On the other hand, in this embodiment, since the focus offset is not generated as shown in
Second Embodiment
In the case of focusing light on the first information recording layer 9a of the optical disk 9, as shown in
In addition, in the case of light being focused on the second information recording layer 9b similarly to the first embodiment, spherical aberration is generated by the difference Δt in thickness of the transmissive layer. For this reason, in this embodiment, when focusing light on the second information recording layer 9b , as shown in
Furthermore, it is possible to design a plate thickness T2 of the parallel flat plate 5 as T2=T1+T by setting the plate thickness T of the parallel flat plate of the vertical axis shown in
Thus, when the plate thickness T1 of the parallel flat plate 5 is set to be 0.5 mm in a state of
Also in this embodiment, while being able to correct the spherical aberration generated by the difference Δt in thickness of the transmissive layer similarly to the first embodiment, the thickness of an optical pickup apparatus does not increase. In addition, the present invention is not limited only to the specific examples shown in the above embodiments.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2005-304468, filed Oct. 19, 2005, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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2005-304468 | Oct 2005 | JP | national |