Liquid crystal cells – elements and systems – Liquid crystal system – Computational system employing liquid crystal element
Reexamination Certificate
1998-06-05
2001-02-20
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Computational system employing liquid crystal element
C349S129000
Reexamination Certificate
active
06191829
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a superresolving optical apparatus, applicable to optical disc systems, photolithographic masking 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 digital versatile discs (DVDs), compact discs (CDs), and the like.
BACKGROUND TECHNOLOGY
The numerical aperture of an optical system is briefly described hereinafter to facilitate understanding of the conventional technologies concerned.
In an optical system designed to have little aberration according to geometrical optics, a focused spot 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.
Now, 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 spot is defined by the following formula:
k×&lgr;÷NA (1)
where &lgr;=light wavelength
k=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 spot as expressed by the formula given above represents a theoretical resolution limit of the optical system, and is also called a diffraction limit.
As is evident from the above formula, a theoretical resolution may be enhanced by 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.
Particularly, in the case of a laser light source used for optical disc systems, photolithographic masking systems, and the like, this tendency becomes more pronounced. Further, the greater the numerical aperture of an optical system, the more the optical system becomes prone to have aberration due to geometrical optics. Accordingly, for recording information on common optical disc systems, a semiconductor laser for emitting a light beam at a wavelength on 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 use of the light source and condensing optics 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 shading band is well known (reference: Japanese Journal of Applied Physics, Vol. No. 28 (1989) Supplement 28-3, pp. 197-200). It appears that with this superresolving optical system using the shading band, 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 shading band will result in a lower optical utilization rate. Furthermore, with the superresolving optical system described above wherein the central region of a light beam, including the optical axis, is shielded with the shading band, degradation in the optical utilization ratio becomes further pronounced because the central region of the light beam generally belongs to a high intensity zone according to the distribution of light intensity.
Such a low optical utilization rate inevitably requires 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 low power output, is used, and consequently, it is practically impossible to employ a high power output light source from the cost point of view.
The invention has been developed in light of the circumstances described above, and a main object thereof is to realize superresolution without sacrificing optical utilization ratio.
DISCLOSURE OF THE INVENTION
In order to achieve the object described above, in an optical apparatus according to the invention, provided with a condensing optics for condensing linearly polarized light, an optical rotatory element having a region for optically rotating a polarization axis of the linearly polarized light in at least a portion thereof is disposed in the optical path of the linearly polarized light so as to locally differentiate the orientation of the polarization axis of the linearly polarized light falling on the optical rotatory element.
By disposing the optical rotatory element in the optical path of the linearly polarized light, and locally differentiating the direction of the polarization axis of the linearly polarized light falling thereon, orientations of components of the linearly polarized light, falling on different regions thereof, can be varied so as to differ from each other. In the linearly polarized light consisting of the components having polarization axes differing from each other in orientation, interference between the components is restrained. In particular, in the case that the polarization axes have an orthogonal relationship, interference between the respective components of the linearly polarized light will disappear. When the interference is restrained as described, the linearly polarized light will behave as if one component thereof were shaded by the other component. As a result, a superresolved image can be formed.
The optical rotatory element is intended to optically rotate the polarization axis of the linearly polarized light falling thereon, but the linearly polarized light falling thereon will not be shaded by the optical rotatory element. Hence, large power loss of the linearly polarized light does not occur in the optical rotatory element with the result that the linearly polarized light can be effectively put to use.
The optical apparatus according to the invention, having such a high optical utilization ratio, is highly suitable for application to optical systems with bright prospects of growth in the near future, for example, digital versatile discs (DVDs), and writable and rewritable digital versatile discs (DVDs-R). That is, although high density recording is desired of DVDs as well as DVDs-R in future, use of a semiconductor laser with relatively low power output is required as a light source from the cost point of view. The optical apparatus according to the invention can meet all such requirements.
The optical apparatus according to the invention is effective also for the photolithographic masking systems used in the fabrication of LSIs, which is expected to require a higher degree of integration in the future. That is, with the optical apparatus according to the invention, superresolved images can be formed without greatly attenuating light amounts, to the extent of which exposure time is shortened, and consequently, a risk of misalignment of exposure position due to vibration and the like is lessened, enabling enhancement of production yield.
Further, with the optical apparatus according to the invention, the optical rotatory element may preferably be composed of a &thgr; optically rotatable region capable of optically rotating the polarization axis of the linearly polarized light falling thereon by an optional angle &thgr;°, and a (&thgr;−90)° optically rotatable region capable of optically rotating the polarization axis of the linearly polarized light falling thereon substantially by an angle of (&thgr;−90)°.
By adopting such a construction as
Armstrong, Westerman Hattori, McLeland & Naughton
Citizen Watch Co. Ltd.
Parker Kenneth
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