Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-06-13
2003-12-16
Spector, David N. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S718000, C359S796000, C369S112120, C369S112230
Reexamination Certificate
active
06665130
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens for at least one of recording and reproducing of information of an optical information recording medium, an optical pickup apparatus including the objective lens and a recording and/or reproducing apparatus including the optical pickup apparatus.
In recent years, there have been advanced research and development activities for a novel high density recording optical pickup system employing a light source such as a violet semiconductor laser with an oscillation wavelength of about 400 nm or a violet SHG laser and a two-group-structured objective lens whose numerical aperture is raised to about 0.85. It is widely Known that recording density of an optical recording medium such as an optical disk and a photo-electro-magnetic disk is increased in inverse proportion to an area of a spot formed on an information recording surface by an objective lens (∝(&lgr;/NA)2, wherein, &lgr; represents a wavelength of a light source, NA represents a numerical aperture of the objective lens). For making an optical recording medium to be of a high density type, there is a method to make a light source wavelength to be short, in addition to a method to make a numerical aperture of an objective lens to be high. However, that method has a problem that sufficient utility efficiency cannot be obtained in practical use, because a light transmission factor of a lens material falls sharply in the wavelength area where the wavelength is shorter than 400 nm. Therefore, in the novel optical pickup system, it is estimated that the numerical aperture of an objective lens is required to be further higher for the higher density.
However, the greater a numerical aperture is, the smaller a depth of focus is, because depth of focus d of the objective lens is inversely proportional to the second power of the numerical aperture, and thereby, the speed of response and accuracy required to an actuator for focusing of the objective lens become higher more and more. It is therefore preferable that the objective lens is light in weight as far as possible.
Further, when the depth of focus of the objective lens is small, a component of defocus that is caused when an instantaneous wavelength variation to which the focusing of the objective lens cannot follow is caused on the light source is great. Accordingly, when the numerical aperture is greater, the chromatic aberration of the objective lens needs to be corrected more strictly.
On the other hand, when a wavelength of the light source is shorter, the transmission factor for an incident light caused by scattered light on the lens surface is more problematic for the following reasons. When a lens surface is formed by a molding method employing a metal mold machined by a diamond cutting tool, a shape of an optical surface and a tool mark representing fine roughness caused on the machined surface when a metal mold is machined are transferred onto the surface of the lens. In this case, ratio &ggr; of scattered light to incident light on one lens surface is in the following relationship for wavelength &lgr; of incident light, refractive index n of lens material, and root mean square Hrms of surface roughness on the optical surface resulting from the tool mark.
&ggr;=(2&pgr;/&lgr;)
2
·(
n
−1)
2
·Hrms
2
Therefore, the transmission factor for the incident light becomes smaller because &ggr; is greater when the wavelength is shorter. For preventing a fall of the transmission factor caused by scattering of the incident light, Hrms needs to be kept small.
Further, when the numerical aperture of an objective lens is greater, an operating distance of an objective lens is shorter because an emerging angle of marginal light from the final surface of the objective lens is greater. In the design of the two-group-structured objective lens on which an operating distance tends to be shorter, compared with a single lens, it is important to secure a sufficient operating distance for preventing destruction of an optical recording medium. However, when a sufficient operating distance is secured, an angle (hereinafter, referred to as an apparent angle) formed by a tangential plane at the position where marginal light on the surface of the second lens group closest to the light source an by an optical axis becomes too great, resulting in a fear that a metal mold may not be machined accurately by a diamond tool.
Further, the objective lens representing a two-group-structured objective lens and having NA raised to about 0.9 is described in each of TOKKAIHEI Nos. 10-123410 and 10-82951. In these objective lenses, a ratio of an operating distance to an effective diameter of the objective lens on a light flux incident plane is small, and thereby, when trying to secure a sufficient operating distance, an effective diameter of the objective lens on the light flux incident plane becomes too great, resulting in a problem that a tendency to make an optical pickup apparatus to be large is brought about. In addition, when an operating distance is small, an effective diameter on the lens surface of the objective lens closest to an optical information recording medium is small, and thereby, energy density of light in the effective diameter on the aforesaid lens surface becomes high. Therefore, when a light flux having a short wavelength of about 400 nm is projected on the lens surface for a long time, there is a fear that an antireflection coating on the lens surface and lens materials in the vicinity of the lens surface are damaged.
SUMMARY OF THE INVENTION
An object of the invention is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses which are light in weight and can be manufactured by injection molding at low cost and on mass production basis, and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses which are suitable to be used for an optical pickup apparatus wherein a wavelength of a light source is 500 nm or less, especially is of about 400 nm and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses wherein chromatic aberration is corrected by a diffractive structure properly without increasing the number of lenses, and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses which are less affected by scattering caused by tool marks and can be manufactured at low cost, and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses for which a metal mold can be machined accurately by a diamond cutting tool even when a large operating distance is secured, and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an objective lens for recording and/or reproducing information on an optical information recording medium that is composed of a group of two positive lenses wherein a sufficient operating distance is secured in spite of a small diameter, and has a numerical aperture raised to be greater than 0.85.
Further object is to provide an optical pickup apparatus that is equipped with the objective lens and a recording/reproducing apparatus.
For attaining the objects stated above, the first objective lens of the invention is represented by an objective lens for recording and/or reproducing an optical information recording medium
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Spector David N.
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