Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates generally to optical head
devices for use in an apparatus for optically reproducing
information recorded on a record medium, such as, an optical
disc player, and more particularly, is directed to improve-
ments in an optical pickup device by which a light beam is
generated and directed to an optical record medium for
optically reading information recorded thereon.
Description of the Prior Art
As an optical head device for use in an apparatus
for reproducing information recorded on an optical record
disc, it has been proposed to employ a compact optic~l pick-
up device for guiding a light beam to the optical record
disc and receiving the light beam reflected from, or trans-
mitted through the optical record disc so as to read lnform-
ation recorded on the optical record disc, which optical
pickup device is generally referred to as being of the
penci]. type. In such previously proposed optical pickup
device of the pencil type, a semiconductor laser for
emitting a laser light beam, a polarized beam splitter, a
collimator le~s, a quarter lambda plate, an object lens and
a photo-sensor are contained in a common cylindrical
supporting member which is disposed so that the object lens
faces the optical record disc.
However, since both the collimator lens and the
object lens are contained in the common cylindrical
supporting member in the previously proposed optical pickup
device of the pencil type, such common cylindrical supporting
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member is relatively long in the direction of its axis and
also relatively heavy. In practice, it has been necessary
for the collimator lens and the object lens to each have a
length of more than 4 mm. and a weight of more than 400 mg.
in order to adequately perform their respective optical
operations and, therefore, it has been almost impossible for
the device, as a whole, to have a length less than 35 mm.
and a weight of less than 5 g.
Consequently, it has been very difficult to
correctly achieve focus servocontrol and tracking servo-
control for the laser light beam directed to the optical
record disc from the object lens by mounting the previously
proposed optical pickup device of the pencil type on a
control device which moves the optical pickup device mounted
thereon in two directions which are perpendicular to each
other. Therefore, in the previously proposed optical pickup
device of the pencil type, the object lens is moved individ-
ually, that is, relative to the common cylindrical supporting
member, in the direction of the axis of the latter, for
performing the focus servocontrol and the device, as a
whole, is moved in the direction perpendicular to the
direction of the,K~axis of the common cylindrical member for
performing the tracking servocontrol. The foregoing
arrangement is disadvantageous in that the servocontrol
frequency band is not sufficiently broad and durability
against shocks is poor.
Although is it theoretically possible to use an
object lens for directly converging the laser light beam
emitted from the semiconductor laser into a very small
point, thereby eliminating the need for the collimator lens,
the object lens in such a case would be too long and heavy.
Further, such object lens would be constituted by a
plurality of lenses in the common cylindrical supporting
member and, therefore, production of the optical pickup
device would be substantially reduced.
OBJECT AND SU~ARY OF THE INVENTION
Accordingly, it is an object of the present
invention to provide a novel optical head device for gener-
ating and controlling a light beam and which avoids the
above described disadvantages of the prior art.
Another object of the present invention is to
provide a novel optical head device for generating and
controlling a light beam, as aforesaid, and which is so
miniaturized in size and light in weight that focus and
tracking servocontrol of the light beam can be achieved
correctly by controlled movement of the optical head device,
as a unit, in two directions perpendicular to each other.
According to an aspect of the present invention,
an optical head device particularly suited for use in an
optical disc player f~r;reading information recorded on a
rotatable optical record disc employs, in place of the
collimator lens and object lens of the prior art, a hologram
which is operative to convert a spherical wave converging to
anotner point.
The above, and other objects, features and
advantages of the present invention, will become apparent
from the following detailed description of preferred
embodiments which is to be read in conjunction with the
accompanying drawings in which corresponding parts are
identified by the same reference numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic sectional view showing an
example of an optical pickup device of the pencil type
according to the prior art;
Fig. 2 is a schematic sectional view showing one
embodiment of an optical head device according to the
present invention;
Figs. 3 to 5 are diagrammatic views to which
reference will be made in explaining how a hologram is
produced for use in an optical head device according to the
~pres~t invention;
Fig. 6 is a plan view showing a controlled
mounting movably supporting the optical head device shown in
Fig. 2;
Fig. 7 is a sectional view taken along the line
VII-VII on Fig. 6;
Fig. 8 is a perspective view of a part of the
controlled mounting shown on Figs. 6 and 7;
Fig. 9 is a sectional view similar to that of Fig.
7, but showing another embodiment of the controlled mounting
for movably supporting the optical head device of Fig. 2;
Fig. 10 is a schematic sectional view showing
another embodiment of an optical head device according to
the present invention; and
Fig. 11 is a plan view of a controlled mounting
for movably supporting the optical head device of Fig. 10.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to facilitate understanding of the
present invention, an example of an optical pickup device
according to the prior art will be first explained with
reference to Fig. 1. Such optical pickup device is shown to
be of the pencil type and comprises a cylindrical supporting
member 1 in which there are suitably disposed a semi-
conductor laser 2, a polarized beam splitter 3, a collimator
lens 4, a quarter lambda plate 5, an object lens 6 and a
photo-sensor 8. In order to detect errors in the focusing
of a laser light beam generated by semiconductor laser 2 and
impinged on an optical record disc 9 through object lens 6,
a.prism 7 is provided integral with polari~ed beam splitter~
3 at a surface of the latter from which the laser light beam
emerges after being reflected from the opcical record disc
9. Such prism 7 is disposed so that a ridge line thereof
lies in the direction perpendicular to the axis of the laser
light beam passing therethrough, and photo-sensor 8 is
divided into four sections along spaced apart lines of
separation which extend parallel to each other and to the
ridge line of prism 7.
Such previously propoe* optical pickup device of
the pencil type is undesirably long in the direction of the
axis of cylindrical supporting member 1 and also is undesir-
ably heavy, because both collimator lens 4 and object lens 6
are contained in the common cylindrical supporting member 1.
A first preferred embodiment of an optical head
device 10 according to the present invention will now be
explained with reference to Fig. 2 in which supporting
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member 11 is shown to contain a semiconductor laser 12, a
polarized beam splitter 13, a quarter lambda plate 15, a
hologram 16 and a photo-sensor 18. In this embodiment, a
prism 17 is also shown to be provided on a surface of
polarized beam splitter 13 as an integral part thereof, and
photo-sensor 18 is divided into four sections in the same
manner as photo-sensor 8 mentioned above.
~ he hologram 16 used in optical head device 10 is
operative to convert a spherical light wave diverging from
one point into another spherical light wave converging to
another point, that is, hologram 16 acts as a lens for
focusing a light beam. By way of example, and as shown on
~ : Fig..:.3~.such hologram 16 can be obtained by placing an
auxiliary hologram 22 and a thin film 16a of dichromate
gelatin which is to become the hologram 16, in a light path
passing through an object lens 21. A converging spherical
light wave having a numerical aperture sin ~1 is brought by
auxiliary hologram 22 to interfere with a diverging
spherical light wave of numberical aperture sin e 2 brought
by the object lens 21 onto thin fi~ml6a. In such case, thin
film 16a is turned into hologram 16 which can act as a lens
for converting a spherical light wave d~ive~ging from a point
F1 with the numerical aperture sin ~1 into a spherical light
wave converging to a point F2 with the numerical aperture
sin ~2. Further, as shown on Fig. 4, auxiliary hologram 22
can be obtained by placing a thin film 22a in a light path
passing through an object lens 23, and causing a converging
spherical light wave of numerical aperture sin ~1 to be
brought by object lens 23 into interference with a plane
light wave on thin film 22_.
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As mentioned above, since hologram 16 can act
generally as a lens for converging a spherical light wave
diverging from one point into another spherical light wave
converging to another point, it is possible to use hologram
16 to focus the laser light beam diverging from semicon-
ductor laser 12 onto the optical record disc 9 as shown in
Fig. 2 and to read the information recorded thereon without
using the collimator and object lenses of the prior art.
The optical head device 10 according to the
present invention is remarkably miniaturized in size and
very much reduced in weight. For example, when the radius
of an equivalent lens formed by hologram 16 and numerical
aperture sin ~2 are 2 mm. and 0.5, respectively, hologram 16 -- a -
can be placed at a distance of 3.46 mm. from the recorded
surface of optical record disc 9. In that case, if ~he
laser light beam focused by hologram 16 is directed through
the body of disc 9 to the recorded surface of the latter for
reading the information recorded thereon, and if the
thickness of the body of disc 9 is 1.2 mm., the distance
between hologram 16 and the optical record disc 9 is 2.26
mm. Further, in case that the above mentioned numerical
aperture sin ~1 is arranged to be 0.15, the distance between
semiconductor laser 12 and hologram 16 is only 13.18 mm.,
from which is follows that the optical head device 10 is, as
a whole, remarkably miniaturized, for example, may be less
than 15 mm. Moreover, in case that hologram 16 is formed
with an area of 9 mm. and a thickness of 1.2 mm., its
weight is about 270 mg. Since polarized beam splitter 13
can be formed with its body measuring 5 mm.3 to weight
approximately 340 mg., semiconductor laser 12 weighs
approximately 500 mg. and photo-sensor 18 weighs
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appxoximately 400 mg., and it is possible for cylindrical
supporting member 11 to weight less than 1 g. by being made
of, for examp]e, aluminum, the optical head device 10, as a
whole, can be made to weigh less than 3 g.
Incidentally, although it is possible to obtain
only a circular aperture when using an ordinary lens made of
glass, the aperture of the equivalent lens formed by holo-
gram 16 can be made to have various shapes, for example,
elliptical, square, rectangular or other desirable shapes,
by providing a mask 24 of the desired shape, as shown in
Fig. 5. In the case of the elliptical or rectangular
aperture, the amount of laser light beam passing there-
through is increased when- the laser light beam is emitted ~ ~ s
with anisotropy from the semiconductor laser 12, so that
~tilization efficiency of the laser light beam is impro~ed.
By reason of such increase in utilization efficiency, the
emitting power of semiconductor laser 12 can be reduced so
that the useful life of the semiconductor laser is extended.
Since the optical head device 10 according to the
present invention is remarkably miniaturized and greatly
reduced in weight, it is possible to achieve the focus
servocontrol and the tracking servocontrol of the laser
light beam directed to optical record disc 9 by a controlled
mounting of the optical head device 10 which moves the
latter, as a unit, in two directions perpendicular to each
other, while ensuring that the frequency band of the servo-
control will be sufficiently broad and that there will be
superior durability against shocks. One example of a
controlled mounting for moving the optical head device 10 in
two directions perpendicular to each other is shown in Figs.
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6 and 7. In this example, a bobbin 31 having coaxial inner
and outer cylindrical portions 31a and 31b is provided for
mounting of the optical head device 10 in its inner
cylindrical portion 31a. A first moving coil 32 for the
focus servocontrol and a second moving coil 33 for the
tracking servocontrol are fixed on the outside of outer
cylindrical portion 31, as also shown in Fig. 8. A pair of
magnetic yokes 34A and 34B, to which a pair of magnets 35A
and 35B, respectively, are attached, are disposed opposite
to each other with bobbin 31 therebetween. The top end of
inner cylindrical portion 31a of the bobbin and the top end
of cylindrical support member 11 of optical head device 10
are mo~ably sup:ported or suspended through a pair of -~
resiliently flexible elements or dampers 36A and 36B from
the upper ends of magnetic yokes 34A and 34B. The bot-tom
end of inner cylindrical portion 3 la of the bobbin is also
movabl~ supported through a resiliently flexible element or
damper 37 rom the bottom ends of magnetic yokes 34A and 34B
so that optical head device 10 can move with the bobbin both
in the direction of the axis of bobbin 31, that is, in the
up-and-down directlon, and in the direction at right angles
to such axis, as indicated by the double-headed arrow 33 on
Fig. 6.
As mentioned above, the length of the optical head
device 10 can be less than 15 mm. and, therefore, it is
possible to form the bobbin 31 to have an outer diameter of
about 22 mm. In such case, for example, magnetic yokes 34A
and 34B can be dimensioned, in the direction from top to
bottom, to be about 25 mm. and, in the lateral direction, to
be about 36 mm. Further, as also earlier noted, the weight
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of optical head device 10 can be less than 3 g. and,
therefore, it is possible to obtain a focus servocontrol
frequency band extended to, for example, 1.5 kHz, and a
tracking servocontrol frequency band extended to, for
example, 3 kHz servocontrol. These servocontrol frequency
bands are sufficiently broad for the respective
servocontrols.
Fig. 9 shows another example of a controlled
mounting according to this invention for supporting optical
head device 10 to move in two directions perpendicular to
each other. The optical head device 10 is shown to be
inserted into the magnetic assembly comprised of magnetic
yo`kes 34A ~nd 34B and-magnets 35A and 35B in the embodiment - -
shown in Figs. 6 and 7. However, in the embodiment shown in
Fig. 9, the cylindrical supporting member ll of optical head
device lO is elongated or extended downward, as at lla, and
a bobbin 41 is formed integrally with such elongated portion
lla of the cylindrical supporting member 11. A moving coil
42 for the focus servo control is wound on bobhin 41 and a
magnetic assembly comprising magnetic yokes 44 and 45 and a
magnet 46 therebetween is provided around elongated portion
lla of the cylindrical supporting member 11. A ring-s~haped
member 47 extends upwardly from magnetic yoke 44 at the top
of the latter, and a pair of stationary coils 43A and 43B
for the tracking servocontrol are attached to the inner
surface of ring-shaped member 47 at diametrically opposed
locations on the latter. Further, a pair of magnets 49A and
49B are attached through holders 48A and 48B to the outside
of supporting member 11 of optical head device 10 at
positions facing stationary coils 43A and 43B, respectively.
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Cylindrical supporting member 11 is movably supported
through a damper 50 by ring-shaped member 47 and through a
damper 51 by another ring-shaped member 52 connected to
magnetic yoke 45 at the bottom end of the latter.
Fig. 10 shows another embodiment of an optical
head device lOA according to the present invention, and in
which the polarized beam splitter 13 and the quarter lambda
plate 15 of Fig. 2 are not used and a semiconductor laser
12A is operative to function as a photo-detector as well as
the source of the laser light beam. The hologram 16 is
operative to very positively return the reflected laser
light beam from the optical record disc 9 to the semi-
condu~,t,or 12A a,nd the embodiment shown in Fig. 10 makes the
most of such characteristic of hologram 16. With this
embodiment, it is not necessary to provide any projecting
portion on the outside of cylindrical supporting member 11~,
as distinguished from the embodiment shown in Fig. 2 in
which member 11 has a laterial projecting portion 11' to
house photo-sensor 18. Therefore, c~lindrical supporting
member llA can be made symmetrical in all directions about
its longitudinal axis to permit achievement of an additional
servocontrol for time base correction of the reproduced - ---
signal obtained from the information read by the laser light
beam in addition to the focus and tracking servocontrols.
For achieving the foregoing, there is provided a three-
dimensional controlled mounting which can move the
cylindrical supporting member llA mounted thereonl in three
different directions perpendicular to one another, for
example, as shown on Fig. 11.
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More particularly, the optical head device lOA of
Fig. 10 is shown on Fig. 11 to be mounted on a magnetic
assembly which is substantially the same manner as that
described above with reference to Fig. 9. Thus, the
magnetic assembly in Fig. 11 is shown to include a ring-
shaped member 47 and stationary coils 43A and 43B for the
tracking servocontrol attached to the inner surface of
ring-shaped member 47 at locations which are diametrically
opposite to each other in a first direction. However, in
Fig. 11, another pair of stationary coils 53A and 53B for
the time base correction servocontrol are also attached to
the inner surface of ring-shaped member 47 so as to be
diame~rically opposite to each other in a second direction ~:
perpendicular to the first direction. A rectangular holder
48 is mounced on the outside of optical head device lOA, and
magnets 49A and 49B are attached to the surface of rectang-
ular holder 48 which are opposite to stationary coils 43A
and 43B, respectively. Another pair of magnets 59A and 59B
are attached to the surfaces of rectangular holder 48 which
are opposite to stationary coils 53A and 53B, respectively.
Thus, optical head device lOA can be moved in the directions
perpendicular to one another indicated by the double-headed ~- -
arrows 38 and 58 on Fig. 11 for correcting tracking errors
and for correcting time base errors, respectively, by the
selective energizing of coils 43A, 43B and coils 53A and
53B, respectively. The controlled mounting for the optical
head device lOA further includes an arrangement (not shown)
for effecting movements of supporting member llA in the
direction of its longitudinal axis so as to correct focusing
errors, for example, as constituted by the coil 42 and
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magnet 45 of Fig. 9. Therefore, optical head device lOA can
be moved in all three directions perpendicular to each other
for effecting tracking, time base and focusing servocontrol.
Further, in the optical head device lOA, a piezo-
electric element 61 is provided between the bottom of
cylindrical supporting member llA and semiconductor laser
12A, as shown in Fig. 10. A voltage for wobbling operation
is suitably supplied to piezoelecteric element 61 for
vibrating the latter and thereby wobbling semiconductor
laser 12A in the direction along the axis of the laser light
beam emitted by laser 12A so that errors in the focus servo-
control can be detected in a known manner.
-Alternatively, in place of piezoelectric elemeni
61, a rear monitor photodetector 19 may be provided for
semiconductor laser 12A to detect errors in the focus
servocontrol. Further, another piezoelectric element 62 may
be provided between the top end of cylindrical supporting
member llA and hologram 16, as shown in Fig. 10. A voltage
is suitably supplied to piezoelectric element 62 for
vibrating the latter and thereby wobbling hologram 16 in the
direction perpendicular to the axis of the laser light beam
emitted by semiconductor laser 12A. Such wobbling of
hologram 16 is employed, in a known manner, in order to
detect errors in the tracking servocontrol. More
particularly, errors in the tracking servocontrol may be
detected by a photo-sensor (not shown) provided for
receiving the laser light beam after the latter has passed
the wobbled hologram 16 and been reflected at the optical
record disc.
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Although illustrative embodiments of this
invention have been described in detail herein with
reference to the accompanying drawings, it is to be
understood that the invention is not limited thereto, and
that various changes and modifications may be effected
therein by one skilled in the art without departing from the
scope or spirit of the invention as defined in the appended
claims.
