 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to an optical information reproducing
apparatus suitable for use in such equipment as an optical video disk
player or a digital audio disk player.
A conventional optical information reproducing apparatus is shown in FIG.
1A. A light beam issuing from a light source 1 such as a laser diode is
reflected from a beam splitter 2 and converged by an objective lens 3 to
form a small spot of information detecting light on the recording surface
of a disk 4. The reflected light from the recording surface of the disk 4
becomes an optical signal carrying the information recorded as a recess
(pit) in the disk 4. This optical signal passes through the objective lens
3 and beam splitter 2 in succession to a photodetector 5, where the
optical signal is converted to an electrical signal to reproduce the
recorded information. The objective lens 3 is driven with an actuator 6 in
two mutually perpendicular directions, one being parallel to the recording
surface of the disk 4, and the other being perpendicular to that surface.
The lens 3 is controlled such that its focal point will coincide with the
recording surface of the disk 4, whereas the spot of information detection
light will be positioned on a track on the recording surface of the disk
4. In the system of FIG. 1A, the optical unit is functionally equivalent
to a non-cofocal optical unit as shown in FIG. 1B.
As described by H. H. Hopkins in his article entitled "Diffraction Theory
of Laser Read-out System for Optical Video Disk" (J.0.S.A., Vol. 69, No.
1, January, 1979), the conventional apparatus shown in FIG. 1A has
incoherent transmission characteristics with respect to the spatial
frequency of the information recorded in the disk, and has suffered from
the disadvantage that the level of a signal detected becomes low at a high
spatial frequency, as illustrated in FIG. 2. Further, no output signal is
detected at higher spatial frequency so that it is impossible to detect
the information recorded in the disk.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical information
reproducing apparatus that will not experience a drop in signal level at
an increased spatial frequency.
In order to attain this object, the optical information reproducing
apparatus of the present invention provides a shield plate having a small
hole in the path of light from a recording medium to the photodetector in
the vicinity of a point conjugative with the point at which the light from
a light source is converged on the recording surface of the recording
medium.
It is effective for the purposes of the present invention to provide light
wavelength converting means by which the light emitted from the light
source to illuminate said recording medium is converted to light having
one half the wavelength of said emitted light.
A second harmonics generating (SHG) of a fiber type is advantageously used
as said light wavelength converting means.
In a presently preferred embodiment, an objective lens used to converge the
reflected light from the recording medium has a smaller light
transmittance in the central portion than in the remaining area.
The above-stated object of the present invention can also be attained by
employing a photodetector that comprises a plurality of light-detecting
elements having a plurality of light-receiving surface arrayed in a
predetermined direction, and a selection circuitry that selectively
produces an output having the highest level of the outputs from said
plurality of light-detecting elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematic diagrams showing a conventional optical
reproducing apparatus;
FIG. 2 is a graph showing the relationship between the readout output of
the apparatus shown in FIG. 1 and spatial frequency;
FIGS. 3A and 3B are schematic diagram showing optical information
reproducing apparatus according to first embodiment of the present
invention;
FIG. 4 is a schematic diagram showing a cofocal optical unit;
FIGS. 5 and 6 are graphs showing various types of light intensity
distributions that are obtained by illuminating the recording surface of a
disk with a laser beam;
FIG. 7 is a graph showing the relationship between readout output and
spatial frequency; and
FIG. 8 is a schematic diagram showing an optical information reproducing
apparatus according to second embodiment of the present invention;
FIG. 9 is a schematic diagram showing an optical information reproducing
apparatus according to the third embodiment of the present invention;
FIG. 10 is a graph showing two types of light intensity distribution that
are obtained by illuminating the recording surface of a disk with a laser
beam;
FIG. 11 is a schematic diagram showing an optical information reproducing
apparatus according to fourth embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of the present invention now will be described in detail with
reference to FIGS. 3A-11.
FIG. 3A is a schematic diagram of an optical information reproducing
apparatus according to first embodiment of the present invention. In this
apparatus, a light source 1, a beam splitter 2, an objective lens 3, a
disk 4, a photodetector 5 and an actuator 6 are disposed in the same way
as in the apparatus shown in FIG. 1A. According to the present invention,
however, a shield plate 9 having a small through-hole (hereinafter
referred to as a pinhole) in the vicinity of a point B conjugative with
point A at which a spot of information detecting light is formed on the
recording surface of the disk 4 is provided between the beam splitter 2
and the photodetector 5.
In the system of FIG. 1A, the optical unit composed of the light source 1,
beam splitter 2 and objective lens 3 is functionally equivalent to a
cofocal optical unit as shown schematically in FIG. 4, in which the light
emanating from a point source 21 passes through two convex lenses 22 and
23 to be converged to form a point image on the light-receiving surface of
a point detector 24.
With an ordinary non-cofocal optical unit of the type shown in FIG. 3A, the
distribution of light intensity of the point image obtained is
proportional to {J.sub.1 (cz)/(cz)}.sup.2, where J.sub.1 (z) is a Bessel
function and draws the curve shown by dashed line a in FIG. 5. With the
cofocal optical unit shown in FIG. 5, the distribution of light intensity
of the point image obtained is proportional to {J.sub.1 (cz)/(cz)}.sup.4
and draws the curve shown by solid line b in FIG. 5. Thus, in the
apparatus shown in FIG. 3A having a cofocal optical unit, information can
be read out with a small spot of information detecting light, thereby
increasing image resolution and the output level of readout signal. In
FIG. 5, one-long-and-one-short dashed line c depicts the distribution of
light intensity obtained with a cofocal optical unit in which one of the
two lenses is designed to have a smaller light transmittance in the
central portion than in the remaining area (i.e., one lens is subjected to
apodization), and one-long-and-two-short dashed line d depicts the
distribution of light intensity in the case where both lenses are
subjected to apodization.
The light beam converged on the disk 4 is in the form of a "Airy disk" and
the light in the neighborhood of a beam spot formed on the recording
surface of the disk 4 has an intensity distribution as shown in FIG. 6,
from which it can be seen that the intensity of light in the neighborhood
of the center of the beam spot is the highest, producing a marked
projection (main lobe) in the intensity distribution curve. The main lobe
is surrounded by more than one side lobe, which causes crosstalk by
reading information from adjacent tracks. According to the present
invention, the shield plate 9 having pinhole 8 blocks that portion of
reflected light from the disk 4 which corresponds to the side lobes,
thereby eliminating the unwanted component of light which would otherwise
cause crosstalk in readout signal. Consequently, the apparatus shown in
FIG. 3A which adopts a cofocal optical unit is capable of producing output
signals that suffer from only minimal crosstalk between adjacent tracks.
The objective lens 3 used in the apparatus shown in FIG. 3A may be such
that it has a smaller light transmittance in the central portion than in
the remaining area. Alternatively, an optical component that suppresses
luminous flux in the central portion of the light beam may be disposed in
the optical path between the objective lens 3 and the beam splitter 2 as
shown in FIG. 3B. In either method, the diameter of information detecting
light beam is further reduced, thereby improving image resolution and
preventing the attenuation of the output signal level at an increased
spatial frequency.
An attempt has been made already to provide improved readout performance by
blocking the central portion of an objective lens without using a cofocal
optical unit. This approach is effective in reducing the diameter of
information detecting light beam but on the other hand, the luminous flux
of light corresponding to side lobes is so much increased as to either
reduce the output signal level at a low spatial frequency or increase
crosstalk. These problems are eliminated from the apparatus shown in FIGS.
3A and 3B, in which the shield plate 9 having pinhole 8 effectively blocks
that portion of reflected light from the disk 4 which corresponds to side
lobes.
The prior art shown in FIG. 1A has output characteristics as shown by
dashed line e in FIG. 7, and an apparatus that employs an "apodized"
objective lens in place of a cofocal optical unit has output
characteristics as shown by one-short-and-one-long dashed line f in FIG.
7. These may be contrasted with the output characteristics of the
apparatus shown in FIG. 3A which are shown by two-short-and-one-long
dashed line g, as well as with the curve indicated by solid line h which
refers to case where an "apodized" objective lens is used in the apparatus
of FIG. 3A.
FIG. 8 is a schematic diagram showing a second embodiment of the present
invention. In the apparatus shown, a light source 1, a beam splitter 2, an
objective lens 3, a disk 4, a photodetector 5 and an actuator 6 are
disposed in the same manner as in the apparatus shown in FIG. 1A.
According to the present invention, however, the photodetector 5 comprises
a photodetector array 11 which is a row of dot-shaped photoelectric
converter elements each having a circular light-receiving surface with a
diameter of 2-3 .mu.m, and a selection circuit 12 that compares the levels
of outputs from the individual photoelectric converter elements and which
selectively produces an output having the highest level.
In the system shown above, only the output of a photoelectric converter
element in photodetector array 11 which receives that portion of reflected
light from the disk 4 which corresponds to the main lobe is selectively
produced. Consequently, the output level of detection signal can be
maintained satisfactorily high in the high spatial frequency range, and in
addition, the unwanted component which otherwise would cause crosstalk in
the readout signal can also be eliminated. As in the conventional system
shown in FIG. 1A, the objective lens 3 may be driven in a direction
transverse to disk tracks (i.e., in the radial direction of disk 4) so
that the spot of information detecting light will be positioned on a track
on the recording surface of the disk 4. Either because of this drive
control or because of a time-dependent offset that might occur in the
optical axis or in some other reference factor, the position of a beam
spot formed on the photodetector array 11 may be altered. However, in
accordance with the present invention, information can be read out
effectively even under these circumstances and the advantages just
mentioned above can be attained.
FIG. 9 is a block diagram showing an optical information reproducing
apparatus according to third embodiment of the present invention. As
shown, a light beam issuing from a light source 1 is converged by a
coupling lens 31 to be launched into a second harmonics generator (SHG) 32
of a fiber type. As described in Unexamined Japanese Patent Publication
Hei-1-293326 and other prior patents, SHG 32 of a fiber type is so
designed as to generate the second harmonic wave of incident light and the
wavefronts of second harmonic waves issuing from this SHG provide a
conical equiphase surface around the central axis of the fiber. In other
words, the SHG 32 of a fiber type emits an annular light beam, with the
ring diameter increasing with the distance from the exit surface of said
SHG. The SHG 32 of a fiber type can be fabricated by crystallizing a
nonlinear optical medium such as lithium niobate that is confined in the
core of an optical fiber.
The light beam emerging from the SHG 32 of a fiber type is collimated by a
collimation lens 33 in the form of a conical prism having a conical
surface. The collimated light is reflected by a beam splitter 2 and
converged by an objective lens 3 to be focused on the recording surface of
a disk 4 to form a small spot of information detecting light. The light
beam emerging from the SHG 32 of a fiber type is annular, and so is the
light beam that is launched into the objective lens 3. This annular light
beam is converged and reflected from the recording surface of the disk 4.
The reflected light from the recording surface of the disk 4 provides an
optical signal associated with the information recorded as a cavity,
called "pit", in the disk 4 and this optical signal passes through the
objective lens 3 and beam splitter 2 to be launched into a condenser lens
34. Thereafter, the reflected light from the recording surface of the disk
4 is converged by the condenser lens 34 and launched into a photodetector
5 through a pinhole 8 formed in a shield plate 9. The photodetector 5
converts the optical signal into an electric signal for reproducing the
recorded information. As in the apparatus shown in FIG. 3A, pinhole 8 is
formed in the vicinity of a point conjugative with the point at which a
spot of information detecting light is formed on the recording surface of
the disk 4.
On account of the use of SHG 32 of a fiber type and of the presence of
pinhole 8, the optical unit in the system described above is functionally
equivalent to a cofocal optical unit having an objective lens with its
central portion shielded from light. In other words, the optical unit in
the apparatus shown in FIG. 9 is functionally equivalent to the case where
one of the two lenses in the cofocal optical unit shown in FIG. 4 is
subjected to apodization. If the central portion of the objective lens 3
in the optical unit in the apparatus shown in FIG. 9 is shielded from
light, said optical unit will become functionally equivalent to the case
where both lenses in the cofocal optical unit shown in FIG. 4 are
subjected to apodization.
Hence, not only in the apparatus shown in FIG. 3A but also in the apparatus
shown in FIG. 9, information can be read out with a smaller spot of
information detecting light than in the prior art apparatus using an
ordinary non-cofocal optical unit, thereby increasing image resolution and
the output level of readout signal.
In the apparatus shown in FIG. 9, the light in the neighborhood of a beam
spot formed on the recording surface of the disk 4 has an intensity
distribution as shown by a one-long-and-one-short dashed line k in FIG.
10. Solid line l in FIG. 10 shows the intensity distribution of light
which is in the neighborhood of a beam spot formed in an apparatus that
does not use a SHG of a fiber type.
As is clear from FIG. 10, the apparatus shown in FIG. 9 which outputs an
annular light beam from the SHG 32 of a fiber type produces a slender main
lobe (the intensity of light in the vicinity of the central portion)
compared to the case where a non-annular light beam having the same
wavelength falls on the entire area of the pupil of objective lens 3 but
on the other hand, said apparatus produces more intense side lobes, which
will pick up information from adjacent tracks to cause crosstalks. In
fact, however, the shield plate 9 having pinhole 8 blocks that part of
reflected light from the disk 4 which corresponds to these side lobes, to
thereby reject the unwanted portion due to crosstalks. Further, the
annular light beam emerging from the SHG 32 of a fiber type is converged
by objective lens 3, so compared to the focus spot obtained by
illuminating the entire part of the pupil of the objective lens with a
non-annular light beam having the same wavelength, the diameter of the
main lobe will be sufficiently reduced to achieve better resolution.
This advantage, combined with the fact that the apparatus shown in FIG. 9
uses readout light having one half the wavelength of that employed in the
prior art optical information reproducing apparatus, contributes to a
further improvement in resolution and insures efficient information
reading from an optical disk that has information recorded at a density
four times as high as in the prior art.
The output characteristics of the apparatus shown in FIG. 9 are the same as
those of the apparatus shown in FIG. 3A and are indicated by a
two-short-and-one-long dashed line g in FIG. 5. When the objective lens in
the apparatus shown in FIG. 9 is subjected to apodization, the resulting
output characteristics are the same as those obtained when the objective
lens in the apparatus shown in FIG. 3A is subjected to apodization and
they are indicated by a solid line h in FIG. 5.
FIG. 11 is a block diagram showing an optical information reproducing
apparatus according to yet another embodiment of the present invention. As
shown, the apparatus includes a light source 1, a beam splitter 2, an
objective lens 3, a disk 4, a photodetector 5, a coupling lens 31, a SHG
32 of a fiber type, a collimator lens 33 and a condenser lens 34, and the
layout of these components is the same as in the apparatus shown in FIG.
9. In this embodiment, however, the photodetector 5 comprises a
photodetector array 11 which is a row of dot-shaped photoelectric
converter elements each having a circular light-receiving surface with a
diameter of 2-3 .mu.m, and a selection circuit 12 that compares the levels
of outputs from the individual photoelectric converter elements and which
selectively produces an output having the highest level. In the system
shown above, only the output of a photoelectric converter element in
photodetector array 11 which receives that portion of reflected light from
the disk 4 which corresponds to the main lobe is selectively produced.
Consequently, the output level of detection signal can be maintained
satisfactorily high in the high spatial frequency range, and in addition,
the unwanted component which would otherwise be caused by crosstalks can
also be eliminated. The objective lens 3 may be driven in a direction
transverse to tracks (i.e., in the radial direction of disk 4) so that the
spot of information detecting light will be positioned on a track on the
recording surface of the disk 4. Either on account of this drive control
or because of a time-dependent offset that might occur in the optical axis
or in some other reference factor, the position of a beam spot formed on
the photodetector array 11 may be altered. However, in accordance with the
present invention, information can be read out effectively even under
these circumstances and the above-mentioned advantages can be attained.
As described above, the optical information reproducing apparatus of the
present invention has a small hole disposed on a light path from a
recording medium into a photodetector in the vicinity of a point
conjugative with the point at which the light from a light source is
converged on the recording surface of the recording medium. This permits
information to be read out with a small spot of information detecting
light, thus ensuring that a detection signal of a satisfactorily high
output level can be obtained in the high spatial frequency range. Since
the spatial frequency can be increased without reducing the level of
output signal, the apparatus of the present invention contributes to
high-density recording in an optical disk. Another advantage of the
optical information reproducing apparatus of the present invention is that
it eliminates that portion of reflected light from the recording medium
which corresponds to side lobes in an Airy disk, thereby minimizing the
possible crosstalk between adjacent tracks.
In a preferred embodiment, light wavelength converting means by which the
light emitted from a light source to illuminate the recording medium is
converted to light having one half the wavelength of said emitted light is
provided and this is effective in further reducing the diameter of the
spot of information detecting light.
If desired, a second harmonics generator (SHG) may be used as the light
wavelength converting means so that information is read with an annular
light beam which will not permit any light to fall upon the central area
of an objective lens. This is also an effective embodiment in that the
diameter of focus spot of the information detecting light beam can be
reduced.
In a preferred embodiment, the objective lens for converging the reflected
light from the recording medium has a smaller light transmittance in the
central portion than in the remaining area and this is effective in
reducing the diameter of information detecting beam.
In a preferred embodiment, the photodetector includes a plurality of
light-detecting elements having a plurality of light-receiving surfaces
arrayed in a predetermined direction, and a selection circuit that
selectively produces an output having the highest level of the outputs
from said plurality of light-detecting elements. In this case, only the
output of a light-detecting element which receives that portion of
reflected light from the disk which corresponds to the main lobe is
selectively produced, thereby ensuring that a detection signal having a
satisfactorily high output level is obtained in the high spatial frequency
range while minimizing the possible crosstalk between adjacent tracks.
Various modifications within the spirit of the invention will be apparent
to those of working skill in this technology. Accordingly, the invention
is to be considered as limited only by the scope of the appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
