Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-03-08
2002-08-20
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S201500, C250S216000
Reexamination Certificate
active
06437319
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a superresolving optical apparatus, applicable to optical disc systems, and the like. More particularly, the invention is concerned with an optical apparatus which has a high optical utilization ratio and is yet capable of electrically altering a numerical aperture thereof with ease, with respect to optical discs whose proper numerical apertures for image formation differ from each other, such as DVDs (digital versatile discs), CDs (compact discs), and the like.
BACKGROUND TECHNOLOGY
A theoretical resolution limit and a numerical aperture of an optical system is briefly described hereinafter to facilitate an understanding of the conventional technologies concerned.
In an optical system designed to have little aberration according to geometrical optics, a focused point must in theory be infinitely small in size. However, it has, in fact, a spatial spread in a finite size due to the effect of optical diffraction owing to the wave motion characteristic of light.
Provided that a numerical aperture of an optical system, contributing to optical image formation or condensing of light, is designated NA, the spatial spread of a focused point is defined by the following formula:
k×&lgr;÷NA
(1)
where &lgr; is the wavelength of light, and k is a constant for respective optical systems (a value, normally in the range from 1 to around 2). Further, the numerical aperture NA is proportional to a ratio of the diameter D of an effective entrance pupil of an optical system (generally the diameter of an effective light beam) to a focal length f, that is: D/f.
The spatial spread of the focused point as expressed by formula represents a theoretical resolution limit of the optical system, and is also called a diffraction limit.
As is evident from formula, a theoretical resolution may be enhanced by the use of a light beam at a shorter wavelength &lgr;, or by enlarging the numerical aperture NA of an optical system. However, a short wavelength light source is generally complex in construction, and higher in production cost.
This tendency becomes more pronounced, particularly, in the case of a laser light source used for optical disc systems, photolithographic masking systems, and the like. Further, the greater the numerical aperture NA of an optical system is, the more the optical system becomes prone to aberration according to geometrical optics. Accordingly, for recording information on common optical disc systems, a semiconductor laser for emitting a light beam at a wavelength in the order of 700 nm is used as a light source while a condensing optics having the numerical aperture NA on the order of 0.5 is used.
As the conventional technology capable of achieving superresolution by the use of a light source and condensing optics as described above, a superresolving optical system constructed such that a portion of an effective light beam falling on the condensing optics is shielded with a shielding band (shielding plate) is well known (reference: Japanese Journal of Applied Physics, Vol. No. 28 (1989) Supplement 28-3, pp. 197-200). With this superresolving optical system using the shielding plate, a focused spot size is rendered narrower by 10 to 20% with respect to the theoretical resolution limit of the optical system.
However, shielding a portion of the effective light beam falling on the condensing optics by means of the shielding plate will result in a lower optical utilization ratio. Furthermore, with the superresolving optical system described above wherein the central region of a light beam, including the optical axis, is shielded with the shielding plate, degradation in the optical utilization ratio becomes further pronounced because the central region of the light beam generally belongs to a higher intensity zone according to the distribution of light intensity.
Such a low optical utilization ratio inevitably requires the use of a light source capable of outputting higher power, resulting in a higher cost of an optical apparatus because such a high power output light source is expensive. Particularly, for application to optical disc systems, a semiconductor laser light source, expensive even at a low power output, is used, and consequently, it is practically impossible to employ a high power output light source from a cost point of view.
Further, when a portion of the effective light beam falling on the condensing optics is shielded by means of the shielding plate, sidelobes typical of a superresolution phenomenon occur on both sides of the focused spot. According to the technology referred to in the literature described in the foregoing, the sidelobes are shielded by installing a slit at a position where signal light reflected from the focused spot is condensed by a condenser lens, so that a focused spot with the sidelobes substantially removed is formed by another condenser lens installed after the slit.
However, for condensing light by an additional condenser lens, an additional optical path of the optical system is required to that extent, and the number of components of the optical system increases, thereby causing a configuration of the optical system to become more complex. Further, delicate positioning of the slit is required because any deviation in the position of the slit will result in shielding of not only the sidelobes but also the focused spot. In addition, there will arise a problem of dust and the like sticking to a gap in the slit.
Then, even if the slit is installed in a given position, the fact remains that light is shielded by the slit, and consequently, diffraction of light occurs again, resulting in the occurrence of some sidelobes. Further, shielding of light by means of the shielding plate naturally leads to a significant degradation in the optical utilization ratio.
The invention has been developed in light of the circumstances described above, and the first object of the invention is to achieve detection of the signal light reflected from the focused spot of superresolution without causing degradation in the optical utilization ratio while solving a problem of sidelobe as well.
Further, as the formula (1) shown in the foregoing clearly indicates, a theoretical resolution of an optical system is largely dependent on a numerical aperture thereof. A numerical aperture of a condenser (objective) lens of an optical pickup used in optical disc systems is usually in the order of 0.45 in the case of CDs and CD-ROMs, and in the order of 0.55 in the case of DVDs (digital versatile discs). Meanwhile, an optical disc substrate has a thickness of about 1.2 mm for use in CDs, and about 0.6 mm for use in DVDs. The condenser lens of an optical pickup that is required of condensing light up to the diffraction limit is designed by taking into account even a thickness of the optical disc substrate. Hence, a proper numerical aperture of the condenser lens for use in CDs or CD-ROMs is different from that of the condenser lens for use in DVDs, thus preventing common use of the optical pickup therebetween.
Accordingly, in order to overcome this problem, there have been in use various conventional methods such as a method of installing two units of optical pickups in one optical apparatus, a method of creating two focal points by making a condenser lens of an optical pickup to be imprinted with a hologram, a method of switching over the diameter of an effective entrance pupil by use of a liquid crystal shutter, and so forth.
However, if two units of optical pickups are installed in one optical apparatus, this will result in a complex configuration of the optical apparatus, leading to an increase in manufacturing cost. If two focal points are created by imprinting the condenser lens with a hologram, it follows that one unnecessary focused spot at either of the focal points will always occur, resulting in a degradation of the optical utilization ratio. This will pose a problem with devices requiring a large light amount such as DVD-RAMs, that is, writable and rewritable DVDs. Similarly, the method of using the liq
Citizen Watch Co. Ltd.
Kim Robert H.
Thomas Courtney
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