Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-12-10
2001-02-20
Dinh, Tan (Department: 2752)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S094000
Reexamination Certificate
active
06192022
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of lens systems for scanning optical information carriers.
BACKGROUND OF THE INVENTION
The invention relates to an optical scanning device for optically scanning a record carrier comprising an information layer and a transparent layer, the device comprising an objective lens and a piano-convex lens for converging a radiation beam through the transparent layer to a focus on the information layer, the plano-convex lens having a convex surface facing the objective lens, and a planar surface facing the transparent layer.
The amount of information that can be stored on an optical record carrier depends inter alia on the size of the radiation spot formed by the scanning device on the information layer of the record carrier. The information density and hence the amount of stored information can be increased by decreasing the size of the spot. The spot size can be reduced by increasing the numerical aperture of the radiation beam forming the spot. When using a single objective lens, such an increase of the numerical aperture is in general accompanied by a decrease of the free working distance of the lens forming the radiation beam, i.e. the smallest distance between the record and the lens. At higher numerical apertures, the manufacturing costs of such objective lenses become high, the field of the lens reduces and the dispersion of the material the lens is made of gives increasing problems. The problems may be mitigated by inserting a plano-convex lens between the objective lens and the record carrier. The plano-convex lens, sometimes called a slider lens or a solid immersion lens, is arranged at a very small distance above the record carrier. The convergence of the radiation beam is then distributed over the objective lens and the plano-convex lens. An advantage of the use of the plano-convex lens is that it hardly adds aberrations to the radiation beam.
A scanning device having such a plano-convex lens is known from the European patent application no. 0 727 777. The device comprises an optical head in which an objective lens and a plano-convex lens converge a radiation beam to a numerical aperture (NA) of 0.84 for scanning the record carrier. The plano-convex lens is arranged at a small height above the record carrier. The lens may be mounted on a slider in sliding contact with the record carrier or floating on a thin air layer. The lens has a free working distance of several micrometers. When the optical head of the device, flying above the surface of the record carrier, collides with a dust particle on the surface, the head and record carrier will be damaged. The free working distance should therefore be larger than the size of the contamination expected on the record carrier. A disadvantage of the known device is that the manufacture of the optical head of the device becomes increasingly difficult at increasing free working distance.
The above citations are hereby incorporated in whole by reference.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a scanning device for scanning a record carrier with a high numerical aperture radiation beam, which can be easily manufactured.
The object is achieved in accordance with the invention by a scanning device as described in the opening paragraph, which is characterized in that the objective lens and plano-convex lens are designed for forming a focus at a distance of more than thirty focal depths of the converging radiation beam from an aplanatic point of the plano-convex lens.
It has turned out that the difficult manufacture of the known optical head is due to the tight tolerance on sideways displacements of the plano-convex lens relative to the position of the objective lens. The gap between the flat surface of the plano-convex lens and the record carrier introduces spherical aberration in the radiation beam which is focused on the record carrier. The spherical aberration can be compensated in the objective lens. As a result, an aberrated beam enters the plano-convex lens instead of an unaberrated beam. This aberration makes the centring of the objective lens and the known plano-convex lens in a direction perpendicular to the optical axis of the lenses critical. The centring tolerance becomes more tight with increasing free working distance. The objective lens and the plano-convex lens are preferably separately movable along their optical axes for a proper tracking of the record carrier. The tight positional tolerance on the known plano-convex lens and the objective lens makes the actuators for these movements expensive.
The tight tolerance of the known optical head is a consequence of the design of the optical head. In applications where plano-convex lenses are used for forming a high numerical aperture beam, such as high-magnification microscope objective lenses and optical heads for scanning high-density optical record carriers, the radiation beam is always focused in an aplanatic point of the plano-convex lens, because only then the plano-convex lens increases the numerical aperture of the beam without introducing monochromatic aberrations. See for example the book “Principles of Optics” by M. Born and E. Wolf, sixth edition, Pergamon Press, 1980, page 253.
The tolerance of the optical head according to the invention is substantially larger than that of the known optical head because of the different design of the objective lens and the plano-convex lens. The inventive design does not focus the radiation beam at an aplanatic point of the plano-convex lens, but at a point which does not coincide with such an aplanatic point. The relatively small increase of the aberrations introduced by the plano-convex lens according to the invention is accompanied by a relatively large increase in the tolerance for sideways movements of the lens. When the distance between the focus point and an aplanatic point is more than thirty focal depths of the radiation beam away from an aplanatic point, the increased tolerance substantially simplifies the manufacture of the optical head. A focal depth is equal to &lgr;/(4(1−cos &agr;)), where &lgr; is the wavelength of the radiation beam and a is the half-angle of the cone of the converging beam in air. The focal depth of a converging beam having a half-angle of &agr; in a medium having a refractive index n is approximately n times larger than in air. The numerical aperture NA is equal to (n sin &agr;). When the free working distance is increased to a few tens of micrometers, where dust particles can pass between the lens and the record carrier, the tolerance is relatively large. For gaps larger than 50 &mgr;m, the distance between the focus point and an aplanatic point is preferably larger than one hundred focal depths.
To increase the tolerance for thickness variations of the transparent layer of the record carrier, the refractive index n
1
of the transparent layer is preferably larger than the refractive index n
2
of the plano-convex lens.
The spherical aberration caused by the gap between the plano-convex lens, having a refractive index lower than n
2
, and the transparent layer, having a refractive index larger than n
2
, at least partly compensate one another. The amount of spherical aberration to be introduced by the objective lens is then reduced, which in turn further increases the mentioned positional tolerance. The refractive indices are preferably related through (n
1
−1)>1.03 (n
2
−1).
The distance between the focus and an aplanatic point depends in general on the size of the gap between the plano-convex lens and the record carrier. The preferred distance measured in micrometers is substantially equal to 3Rd
gap
, where R is the radius of the convex surface in millimeters and d
gap
the distance between the planar surface and the transparent layer in micrometers. The distance depends on the actual design of the objective lens and the plano-convex lens. The distance will in general lie within +150% to −50% from the above value, and, if n
1
>n
2
, within ±20% of the above valu
Braat Josephus J. M.
Hendriks Bernardus H. W.
Belk Michael E.
Chu Kim-Kwok
Dinh Tan
U.S. Philips Corporation
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