Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
1999-10-15
2001-06-12
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S626000, C359S622000, C219S121770
Reexamination Certificate
active
06246530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lens assembly used for an optical recording apparatus. The present invention also relates to an optical head and an optical disk apparatus which incorporate such a lens assembly. In this specification, the “optical disk apparatus” refers not only to an apparatus used strictly for reading data from an optical disk, but also to a magneto-optical disk apparatus capable of both reading and writing data with respect to a magneto-optical disk by magnetic field modulation or optical pulse modulation. The phrase “optical disk apparatus” further refers to other kinds of optical disk apparatus.
2. Description of the Related Art
Generally, in an optical disk apparatus of the above type, the data storage capacity of an optical disk is in inverse proportion to the square of the diameter of a focal spot to be formed on the recording region of the disk. Thus, to increase the storage capacity of the disk, it is desirable to reduce the diameter of the focal spot as much as possible. As is known, the diameter of the focal spot is proportional to “&lgr; (wavelength of laser beam) divided by NA (numerical aperture).” Thus, theoretically, there are two possible ways to reduce the diameter of the focal spot, that is, to reduce &lgr; or to increase NA. However, due to certain limitations concerning the properties of materials, it is rather difficult to obtain a semiconductor laser capable of emitting light whose wavelength is sufficiently small. Thus, for the present, it is impossible to obtain an inexpensive, short-wavelength laser (blue laser) which is satisfactory in terms of output and lifetime. Under these circumstances, the second option (i.e., to increase NA) may inevitably be employed to reduce the diameter of the focal spot, and new techniques to achieve a smaller diameter have been eagerly sought after.
In this connection, reference is now made to
FIG. 17
showing a conventional arrangement adopted for obtaining an increased NA (see JP-A-8-221790). In the illustrated conventional device, use is made of a lens assembly including two objective lenses, that is, a first objective lens L
1
mounted on a lens actuator A and a second objective lens L
2
mounted on a slider S which in turn is supported by a carriage. The second objective lens L
2
is arranged adjacent to the surface of a transparent disk substrate Dl of an optical disk D. With such an arrangement, the overall NA of the lens assembly can be made greater than when only a single objective lens is used.
Though the overall NA is increased to a certain extent, the above conventional arrangement is disadvantageous in the following point. As stated above, the second objective lens L
2
is arranged adjacent to (but, outside of) the transparent substrate D
1
of the disk D, and this lens (together with the first objective lens L
1
) causes the laser beam to converge at a point in the recording region D
2
of the disk D. With such an arrangement, the NA, which can be expressed by “n (refractive index of the transparent substrate D
1
)×sin &thgr;” as shown in
FIG. 17
, cannot be made greater than 1. Practically, the NA is about 0.8 at most.
The improvement of the data storage capacity of a magneto-optical disk may be achieved by another method disclosed in JP-A-10-124943 for example. According to this conventional method, part of the focal spot of the laser beam is magnetically masked by utilizing the fact that magnetic layers laminated on a recording layer of the disk have different Curie temperatures.
In the above method, the diameter of the focal spot itself is not reduced. Thus, the problem of crosstalk between adjacent tracks may still be incurred, which is a hindrance to the improvement of the data storage capacity of the disk.
SUMMARY OF THE INVENTION
The present invention has been proposed under the above circumstances, and one of its objective is to provide a lens assembly whose theoretical numerical aperture is suitably increased for reducing the diameter of the focal spot of a laser beam.
Another objective is to provide an optical head incorporating such a lens assembly.
Still another objective is to provide an optical disk apparatus incorporating such an optical head, whereby the data storage capacity of the disk apparatus is remarkably increased.
For attaining the above objectives, the following technical measures are taken in the present invention.
According to a first aspect of the present invention, there is provided a lens assembly for causing a laser beam to form a focal spot in a recording region of an optical recording medium including a substrate, the lens assembly comprising:
a plurality of focusing lenses each including an incidence surface for receiving the laser beam and an emitting surface for emitting the laser beam;
wherein each focusing lens is arranged so that the emitted laser beam is caused to converge at a first focus located in contact with the substrate of the optical recording medium, while also being arranged so that light sent from the first focus is caused to converge at a second focus located in the recording region of the optical recording medium to form the focal spot.
In a preferred embodiment, each focusing lens has a length defined between its incidence surface and its emitting surface. The length of each focusing lens is determined in correspondence to an optical length of a first light path extending from the first focus relating to said each focusing lens to the second focus.
In the preferred embodiment, a second light path is defined as one extending from the incidence surface of each focusing lens to the second focus and has an optical length. The optical lengths of the second light paths relating to the respective focusing lenses are equal to each other.
In another preferred embodiment, a second light path is defined as one extending from the incidence surface of each focusing lens to the second focus and has an optical length. The optical lengths of the second light paths relating to the respective focusing lenses are made different from each other by a multiple of a wavelength of the laser beam. Here, the multiple includes zero.
Preferably, each of the focusing lenses may comprise a rod lens having a flat incidence surface and a convex emitting surface, wherein diameters of the respective rod lenses are made equal to each other.
In a preferred embodiment, at least one of the focusing lenses may be held in contact with four others of the focusing lenses, so that the focusing lenses are arranged in a lattice-like pattern. Alternatively, at least one of the focusing lenses may be held in contact with six others of the focusing lenses, so that the focusing lenses are arranged in a honeycomb-like pattern.
In a preferred embodiment, the focusing lenses may comprise concentric ring-shaped lenses each having a flat incidence surface and a convex emitting surface.
According to a second aspect of the present invention, there is provided a lens assembly for causing a laser beam to form a focal spot in a recording region of an optical recording medium including a transparent substrate, the lens assembly comprising:
a plurality of focusing lenses each including an incidence surface for receiving the laser beam and an emitting surface for emitting the laser beam; and
a plurality of solid immersion lenses each of which is arranged adjacent to the emitting surface of a respective one of the focusing lenses and has an emitting surface;
wherein a combination of said each solid immersion lens and the respective one of the focusing lenses is arranged so that the laser beam is caused to converge at a first focus located adjacent to an emitting surface of said each solid immersion lens, the combination being also arranged so that light sent from a near-field light at the first focus is caused to converge at a second focus located in the recording region of the optical recording medium to form the focal spot.
Preferably, a light path is defined as one extending from the incidence surface of each focusing lens to the second
Epps Georgia
Fujitsu Limited
Greer Burns & Crain Ltd.
Thompson Tim
LandOfFree
Lens assembly and apparatus using the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Lens assembly and apparatus using the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Lens assembly and apparatus using the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472039