Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-07-11
2004-04-06
Young, W. R. (Department: 2652)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06717906
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical disk apparatus with the use of an objective lens, and specifically relates to an optical disk apparatus capable of compatible reproduction (i.e. playback, read out) of optical disks whose substrate thicknesses are different. Hereinafter, the playback of contents in a disk and the read out of data in a disk are referred to as “reproduction.”
In recent years, the optical disks have been improved in their capacity toward much higher recording density. So that now a DVD-ROM (Digital Video Disk Read Only Memory) having a 4.7 GB capacity is commercially available by contrast with a CD-ROM which is a household, read-only optical disk having 0.65 GB. As the most important difference between theses two media, the DVD and the CD have different thicknesses in their substrates, namely 0.6 mm for the DVD, 1.2 mm for the CD. This difference comes from a fact that the DVD uses a higher numerical aperture (NA) of 0.6, larger than that of the CD 0.45, with the aim at improving the recording density, which is and hence it is prerequisite in the DVD to suppress the occurrence of aberration arising from disk tilt etc. Here, the NA stands for an indicator of the objective lens to represent how much it can collect rays, and is defined as the sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. Since it is essential for a DVD drive device to be capable of reproducing the CD disks, such difference of disk thickness presents a problem in choosing the “corresponding substrate thickness” of the objective lens.
First of all, the meaning of this correspondence substrate thickness will be described. In optical disks, in order to avoid a situation where a disk becomes not-readable for its recorded information due to dust etc., the disk is normally reproduced with a focused beam from and through the back side of its transparent substrate made from polycarbonate etc. whose front surface was processed to have a recording layer thereon. In this configuration, for example in the case of the DVD whose substrate thickness is 0.6 mm, the spot size on the back side of the substrate becomes 0.5 mm or so; therefore it is less likely that dust having a size of approximately 0.1 mm or less affect the reproduction. Because of this effect, unlike magnetic disks, the user can change the optical disks and reproduce those in person. However, in cases where light is focused through a substrate as described above, it is necessary to design the objective lens so as to have an appropriate lens shape in accordance with the substrate thickness. For example, if a transparent substrate having a certain thickness is inserted halfway in a bundle of rays that are converging ideally, there occurs a situation where a ray incident normally to the substrate and a ray incident obliquely thereto take different optical paths of different lengths: naturally the latter has a longer optical path than the former. Such optical path difference is called aberration, which gives rise to a cause that prevents sharp focusing. However, such aberration can be compensated by modifying the lens shape, assuming that the thickness of the substrate is to be inserted is known beforehand. Therefore, in optical heads of optical disk apparatuses, normally used are an objective lens that is made to match a thickness of the optical disk to be reproduced, after specifying the substrate thickness. As described above, the corresponding substrate thickness is defined as a substrate thickness such that when the substrate is inserted halfway in the bundle of convergent rays focused by a certain objective lens, that objective lens can focus the rays ideally without introducing aberrations.
Since normally each objective lens for optical disk has a unique corresponding substrate thickness, this fact became a problem when reproducing the DVD and the CD in a compatible manner.
To overcome this problem, for example, in the Japanese Published Unexamined Patent Application No. 6-124477, disclosed is a method where the numerical aperture of an objective lens for DVD is limited and hence rays passing through a peripheral part of the objective lens for DVD where the aberration is large are blocked, so that the aberration due to an error of the substrate thickness in the CD reproduction with the use of the lens for DVD is reduced. An aberration caused by the error of the substrate thickness is called a spherical aberration, which increases in proportion to the fourth power of NA. Consequently, if the NA is reduced with an aperture for a lens, the aberration of the lens is reduced accordingly. On the other hand, when reproducing the DVD, the lens is used without the aperture so that the light is not blocked thereby, being in a changeover manner. Here, this conventional example is intended to reproduce the CD by using light of a 650 nm wavelength which is a reproduction wavelength for the DVD. That is, the wavelength in this case is shorter than the 780 nm wavelength which is conventionally used for the CD. If a wavelength is shorter, a necessary NA can be decreased because the focused spot size is proportional to &lgr;/NA, where &lgr; denotes the wavelength. Therefore, the reproduction of the CD with the 650 nm wavelength light allows to reduce the NA of the aperture below 0.45 accordingly and thereby a necessary effect of reducing the aberration is achieved in the conventional example. However, the conventional example has a drawback that it cannot accommodate to the reproduction of the CD-R (CD-Recordable) that has a compatibility with the CD and is also a writable medium, currently becoming popular so much. This is because the CD-R uses a dye medium in its recording layer that has enough reflectance only at the 780 nm wavelength and the reflectance decreases at the 650 nm wavelength, namely the reproduction wavelength of the DVD, whereby a signal cannot be reproduced from the DVD. So it is necessary to mount a light source of the 780 nm wavelength separately to reproduce the CD-R. In this occasion, the wavelength becomes identical to that of a conventional optical head for CD and therefore it is necessary for the aperture limit NA to be set to no less than 0. 45, the value of the aperture limit NA for the conventional case, so that a sufficient effect of reducing the aberration can hardly be achieved by aperture limit.
Further, as another conventional example, in the Japanese Published Unexamined Patent Application No. 9-237431, disclosed is a method of blocking rays passing through a certain annular region of a lens. This example uses a method where, by blocking rays passing through the annular region where the aberration begins to change largely in the CD reproduction with the use of the objective lens for DVD, only the region that deteriorates directly the CD reproducing light spot is virtually removed. The reason for this configuration is that rays passing through a region that introduces a significantly large aberration consist of a wavefront of large inclination and each ray travels in a direction perpendicular to the wavefront, and consequently these rays disperse very largely from the focal position, so that substantially the rays do not affect the light spot quality by any means. In the DVD reproduction, rays passing through regions inside and outside the light-blocking annular region are focused virtually without the aberration. At this time, rays passing through the light-blocking annular region are blocked also in the DVD reproduction. By adjusting the width of that region so that loss in the light amount and deterioration of the light spot due to the light blocking are controlled to be within an allowance, virtually the same effect as that of the liquid crystal can be obtained without the liquid crystal. However, since this contrivance is virtually equivalent to the case of limiting the aperture, the method cannot be applied to the reproduction of t
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Battaglia Michael
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