Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-06-18
2003-05-27
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112010, C369S112230, C369S112260, C359S796000
Reexamination Certificate
active
06570827
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical element for correcting a chromatic aberration, an optical pick-up device having the optical element for correcting a chromatic aberration, an optical reproduction device having this optical pick-up device and an optical recording and reproduction device, and more particularly an optical element for correcting a chromatic aberration generated at each of optical planes in an optical system, an optical pick-up device having the optical element for correcting a chromatic aberration, an optical reproduction device having this optical pick-up device and an optical recording and reproduction device.
2. Description of the Related Art
ROM (Read-Only-Memory) type optical disc represented by a CD (Compact Disc), a RAM (Random Access Memory) type optical disc represented by a phase changing disc or a magneto-optic disc or an optical recording medium represented by an optical card and the like are widely used as storing media such as image information, audio information or programs for information equipment. In the case of these optical recording media, a high density and a large capacity have been gradually attained and in the case of the optical pick-up device corresponding to these optical pick-up devices, a short wave length formation of a light source, semiconductor laser, for example, or a large NA (Numerical Aperture) of an objective has been attained and a small diameter of a light collecting spot collected through the objective has been attained. For example, in the case of the CD which has been merchandised at a relative initial stage, a wave length of the light source is set to 780 nm, and to the contrary, in the case of DVD (Digital Video Disc or Digital Versatile Disc) which has been recently merchandised, a wave length of the light source is set to either 650 nm or 685 nm. However, in recent years, it has been desired to attain a higher density and a higher capacity of the optical recording medium and in correspondence with this trend, the wave length of the light source has been apt to show more and more a short wave length.
A chromatic aberration is an aberration which is generated when either a lens or an optical system must process either a multi wave length or a continuous wave length and a refractive index of optical material is made different in response to a wave length, resulting in that a focal distance of the objective is also made different. That is, since a refractive index of the optical material in a visual range shows a normal distribution, a refractive index for a blue light becomes a larger value than for a red light. For example, in the case of a convex type glass lens, a focal distance of blue light becomes shorter than a focal distance of red light. A wave length of laser light radiated from a semiconductor laser is generally a mono-chrome (a single mode) and it is assumed that there occurs no chromatic aberration, although actually it has a wave length width of about several nm or so. In addition, there is also a possibility of occurrence of a so-called mode hopping that a central wave length of a laser beam radiated from a semiconductor abruptly flies by several nm due to a variation in temperature or the like.
Accordingly, in the case that a short wave length semiconductor laser of about 440 nm or 440 nm or less, for example, is applied in an optical pick-up device corresponding to a higher density and a higher capacity of the optical recording medium, a chromatic aberration generated at the objective caused by a displacement of wave length becomes a non-allowable important problem. As to the fact that the chromatic aberration becomes high in response to a short wave length, it may be considered that there are two causes for it. A first cause consists in the fact that if a short wave length is applied in a usual objective, a variation of refraction index becomes large in respect to a minute variation in wave length and a defocusing amount which is an amount of motion of a focal point becomes large. A second cause consists in the fact that as a higher density and a higher capacity of the optical recording medium are attained, it is necessary to keep a diameter of a converging spot to be converged by the objective as less as possible, although as a depth of focus (d) of the objective is expressed by an equation of d=&lgr;/(NA)
2
(where, &lgr; is a wave length &lgr; of a light source and NA is a numerical aperture of the objective), the depth of focus (d) becomes low as the wave length to be handled is short and even a slight defocusing is not allowed.
In order to keep the chromatic aberration of the objective low, it can be applied that optical material having a low dispersion property is used as the objective, although the chromatic aberration is even high under a short wave length. In addition, although it is possible that the objective is an achromatic lens composed of a plurality of lenses, a weight of the achromatic lens composed of a plurality of lenses becomes heavy. For example, in the case that the achromatic lens having a heavy weight is used as an objective and this objective is applied in a double-axis actuator for use in controlling and driving the objective in a focusing direction and a tracking direction, there occurs a possibility that servo quality such as focusing servo property and tracking servo property is reduced. In addition, in the case that the heavy achromatic lens is used as an objective, a high current is required in a drive power supply for the double-axis actuator and at the same time a drive circuit or a power supply or the like becomes large in size, resulting in that this may prohibit a small-sized formation of the optical pick-up device, the optical reproducing device provided with this optical pick-up device or the optical recording and reproducing device.
SUMMARY OF THE INVENTION
The present invention provides an optical element for correcting chromatic aberration in correspondence with a formation of short wavelength of the light source and provides an optical pick-up device, an optical reproducing device and an optical recording reproducing device in correspondence with a higher recording density and a higher capacity of the optical recording medium under application of this optical element for correcting chromatic aberration.
In order to solve the aforesaid subject matter, the optical element for correcting chromatic aberration of the present invention defined in claim
1
is arranged between a light source such as a semiconductor laser or the like with a wavelength being less than 440 nm and an objective with NA being 0.55 or more, with a focal distance being 1.8 mm or more and with an Abbe number at the d-line (Fraunhofer's d-line of 587.6 nm, the color produced by an emission line of helium) being 95.0 or less, wherein the optical element for correcting chromatic aberration has a convex lens with at least an Abbe number of the d-line being 55 or more and a concave lens with an Abbe number of the d-line being 35 or less.
The optical element for correcting chromatic aberration of the present invention defined in claim
2
is arranged between a light source such as a semiconductor laser or the like with a wavelength being less than 440 nm and an objective composed of two lenses with NA being 0.70 or more, with a focal distance being 1.4 mm or more and with an Abbe number at the d-line being 95.0 or less, wherein the optical element for correcting chromatic aberration has a convex lens with at least an Abbe number of the d-line being 55 or more and a concave lens with an Abbe number of the d-line being 35 or less.
That is, the optical element for correcting chromatic aberration of the present invention defined in claims 1 and 2 is constructed such that the optical element is arranged between the light source and the objective irrespective of whether or not a flux of light at a position where the optical element is arranged is a converging flux of light or a diverging flux of light, a chromatic aberration of polari
Ichimura Isao
Maeda Fumisada
Osato Kiyoshi
Suzuki Akira
Watanabe Toshio
Chu Kim-Kwok
Korzuch William
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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