Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2002-04-25
2004-04-27
Psitos, Aristotelis (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410, C369S044350
Reexamination Certificate
active
06728173
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention belongs to an optical head used in an optical disk device, and more particularly relates to a technique for enhancing a performance in detection of a position controlling signal for an optical spot thereof.
Conventional techniques on methods for controlling a focal point position in an optical disk device are described in, for example, “Fundamentals and Applications of Optical Disk Storage”, Y. Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp. 79-83. According to this literature, there are the following methods: Foucault method (Knife edge method), an astigmatic method, a beam size detection method, an image rotating method, and so on. From criteria such as simplicity of an optical system required, the ease with which the adjustment can be made, and the ease with which combination with a tracking detection can be achieved, the most prevailing method, at the present stage, is the astigmatic method. In the astigmatic method, however, there existed a problem that, when an optical spot crosses a track on the surface of a storage film, a disturbance is apt to occur in a focus error signal in association with a decentering of an optical disk. This disturbance is likely to occur especially when astigmatism takes place in a focused spot or the optical spot is shifted on an optical detector. Examples of methods for reducing the disturbance are disclosed as follows: A method of reducing the disturbance by blocking light out of a central portion of a detected light beam is disclosed in JP-A-6-162527 and JP-A-6-309687, a method of reducing it by adjusting rotation of an objective lens is disclosed in JP-B-5-68774, and a method of reducing it by means of an operation between a light with astigmatism and a light without astigmatism in a detected system is disclosed in JP-A-5-197980. None of them, however, is a fundamental method for solving the above-described problem. Thus, at the present stage, the reducing effect obtained is not necessarily enough.
In particular, in a land-groove type optical disk employed in a DVD-RAM planned to be brought into a commercial stage soon, the disturbance occurs quite outstandingly. The reason is as follows: In the land-groove type optical disk, a width of a guiding groove (groove) is substantially equal to a width of a portion of a guiding inter-groove (land), and information is stored on the both sides thereof. On account of this, a pitch of the guiding groove itself, when compared with an optical spot, is formed to be larger than in conventional optical disks. This extraordinarily intensifies a tracking error signal according to a push-pull method described later, thus causing the disturbance to occur quite outstandingly. This condition, accordingly, brings about a situation that, in an optical head for the DVD-RAM, it can not be helped employing the Foucault method or the beam size detection method the configuration or the adjustment of which is complicated.
Conventional techniques on methods for controlling a tracking in an optical disk device are similarly described in, for example, the above-cited “Fundamentals and Applications of Optical Disk Storage”, Y. Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp. 83-92. According to this literature, there are methods such as a three-spots method and a diffracted light differential method (push-pull method). Judging from criteria such as simplicity of an optical system required, the ease with which the adjustment can be made, and a resistance to the disturbance, the three-spots method is mainly employed in a read only type optical disk such as a compact disk (CD). Meanwhile, the push-pull method is mainly employed in the case of a magneto-optical disk or the DVD-RAM which needs a high laser emission power at the time of the recording. At this time, there can be considered another way in which, exchanging the roles with each other, the push-pull method is employed toward the CD and the three-spots method is employed toward writable optical disks. However, there exist circumstances which make such an employment impossible.
In performing a CD pick up, in order to cause a focused spot to follow a decentering of the optical disk for the necessity of low price, the objective lens is moved by only being mounted on a lens actuator. Then, if the push-pull method is employed, it turns out that a detected light beam moves on the optical detector. This phenomenon appears as an off-set. Also, at a pit depth of &lgr;/(4n) (&lgr;: light wavelength, n: substrate refractive index) at which a signal amplitude becomes largest in the reproduction-only type optical disk, there are the following problems: Of diffracted light by means of a periodic structure of train of pits in the radial direction, 0th order light becomes smaller. In addition to this, even when the focused spot is off-track, no unbalance occurs in interference intensity between the 0th order light and ±1st order diffracted lights. This makes it impossible to obtain the tracking error signal.
Meanwhile, in the storage-able optical disks, especially in the magneto-optical disk, compensation for the decentering of the optical disk is usually performed by an actuator called a coarse actuator. The coarse actuator mounts the optical head or only a portion of the objective lens and an objective lens actuator so as to allow the optical spot to come near to a proximity of a track to be objected. Namely, the magneto-optical disk is constituted in such a manner that, of a tracking error, the low frequency components are compensated by the coarse actuator and the high frequency components are compensated by the objective lens actuator, thereby enhancing a reliability needed for the storage operation. Accordingly, an amount of movement by the objective lens actuator is lower than in the read only type optical disk such as the CD. This makes it possible to employ the push-pull method which has higher light utilization efficiency than the three-spots method does.
Also, if the three-spots method is employed toward the storage-able optical disks, as described on page 127 of “Technical Digest of Symposium on Optical Memory '86”, there take place the following problems: First, in an optical disk such as the DVD-RAM, i.e. the type of optical disk that performs the storage by means of a variation in reflectance of a storage mark, at the time of the storage operation, there arises a difference in the amount of light between a preceding sub-spot and a subsequent sub-spot. This causes an off-set to occur in the tracking error signal. Also, in the case of the magneto-optical disk, there exists a feedback light back to a semiconductor laser. On account of this, a tilt of the disk unbalances a condition of stray-lights interference on the both sides of sub-spots. This also causes an off-set to occur. Moreover, as described already, the land-groove type optical disk is employed in the DVD-RAM. This circumstance can also be mentioned as a reason for making it impossible to employ the three-spots method toward the DVD-RAM. Namely, in the land-groove type optical disk, a width of the land portion is originally made equal to that of the groove portion in order to make an amount of reflected light of the land portion equal to that of the groove portion. This necessarily results in a fact that, even when an optical spot is off-track, the amount of light scarcely varies, thus making it impossible to obtain a tracking error signal according to the three-spots method. Accordingly, it can not be helped employing the push-pull method in the DVD-RAM. However, unlike the case of the magneto-optical disk, it is required to lower the price of the DVD-RAM down to a price close to the price of the CD. Consequently, it becomes absolutely necessary to reduce the off-set in a tracking error signal which accompanies the movement of the objective lens according to the push-pull method.
A conventional technique for solving the above-mentioned problem in the DVD-RAM is described in, for example, “National Technical Report”, Vol. 40, No. 6, (1994
Fukui Yukio
Inoue Masayuki
Nakamura Shigeru
Ohnishi Kunikazu
Shimano Takeshi
Antonelli Terry Stout & Kraus LLP
Psitos Aristotelis
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