(12) United States Patent
`Kanto et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006272079Bl
`US 6,272,079 Bl
`Aug. 7, 2001
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) OPTICAL HEAD UNIT FOR OPTICAL DISK
`APPARATUS HAVING BOTH FOCUS
`CONTROL AND TRACK CONTROL
`FUNCTION
`
`5,497,359
`5,712,842
`6,104,675 •
`6,111,840 •
`
`3/1996 Mamin et a!. .................... 369/44.15
`1/1998 Yamamoto et a!. .
`8/2000 Hatam-Tabrizi ................. 369/112 X
`8/2000 Hajjar .. .. .. .. ...... .. .... ........ 369/44.23 X
`
`(75)
`
`Inventors: Nobuyuki Kanto; Koichi Tezuka;
`Haruhiko Izumi; Satoshi Shimokawa;
`Shingo Hamaguchi; Akihiko Makita;
`Kyoko Tadaki; Kazushi Uno; Goro
`Kawasaki, all of Kawasaki (JP)
`
`(73) Assignee: Fujitsu Limited, Kawasaki (JP)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/203,158
`
`(22) Filed:
`
`Dec. 1, 1998
`
`(30)
`
`Foreign Application Priority Data
`
`Jun. 30, 1998
`
`(JP) ................................................. 10-185283
`
`Int. Cl? ........................................................ GllB 7/00
`(51)
`(52) U.S. CI ..................................... 369/44.14; 369/112.24
`(58) Field of Search .............................. 369/44.11, 44.14,
`369/44.15, 44.16, 44.17, 44.18, 44.19, 112,
`44.23, 13, 112.23, 112.24
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,610,515
`
`9/1986 Tanaka .
`
`FOREIGN PATENT DOCUMENTS
`
`341829
`405742
`8212579
`8221790
`
`11/1989 (EP) .
`1/1991 (EP) .
`8/1996 (JP) .
`8/1996 (JP) .
`
`* cited by examiner
`
`Primary Examiner-Paul W. Huber
`(74) Attorney, Agent, or Firm-Greer, Bums & Crain, Ltd.
`
`(57)
`
`ABSTRACT
`
`An optical head unit used for an optical disk apparatus is
`provided. The optical head unit includes an object glass
`assembly for focusing a laser beam from a light source on an
`optical disk, an actuator movable at least in a focus control
`direction, a carriage for supporting the actuator and moving
`the actuator in a track control direction, and a slider which
`is attached to the carriage and capable of adjusting its
`posture in relation to a surface of the optical disk. The object
`glass assembly is provided with a plurality of lens units.
`Each lens unit includes at least one lens. One of the plural
`lens units is supported by the slider.
`
`11 Claims, 9 Drawing Sheets
`
`24
`
`LG Electronics, Inc. et al.
`EXHIBIT 1010
`IPR Petition for
`U.S. Patent No. 6,785,065
`
`

`
`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 1 of 9
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`US 6,272,079 Bl
`
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`

`
`U.S. Patent
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`Aug. 7, 2001
`
`Sheet 2 of 9
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`US 6,272,079 Bl
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`0
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`N
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`

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`U.S. Patent
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`Aug. 7, 2001
`
`Sheet 3 of 9
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`US 6,272,079 Bl
`
`FIG.3
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`41
`
`71
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`72
`
`FIG.4
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`41
`
`30
`
`76
`
`74
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`

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`U.S. Patent
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`Aug. 7, 2001
`
`Sheet 4 of 9
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`US 6,272,079 Bl
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`FIG.5
`
`41 ~ so
`
`76
`
`71
`
`72
`
`

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`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 5 of 9
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`US 6,272,079 Bl
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`FIG.6
`
`0.09
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`,--..... 0.08
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`0 ~~--~--~-.--~--~~--~
`0.56 0.57 .058 0.59 0.6 0.61 0.62 0.63 0.64
`
`THICKNESS OF DISK (mm)
`
`

`
`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 6 of 9
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`US 6,272,079 Bl
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`FIG.7
`
`FIRST OBJECTIVE IS
`TOO CLOSE TO SECOND OBJECTIVE
`
`J
`
`[
`
`FOCUS ERROR SIGNAL
`
`DEFOCUSING OF
`SECOND OBJECTIVE
`
`~
`DISTANCE BETWEEN
`THE FIRST AND SECOND
`[
`. OBJECTIVES IS PROPER
`
`rFIRST OBJECTIVE IS
`lTOO DISTANT FROM SECOND OBJECTIVE)
`
`~
`
`

`
`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 7 of 9
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`US 6,272,079 Bl
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`FIG.8
`
`• FOCUS ERROR SIGINAL
`• WAVE ABERRATION
`
`----~-----~----~-----~----t----~-----~---
`
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`
`DISPLACEMENT OF
`SECOND OBJECTIVE ( t1 m)
`
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`

`
`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 8 of 9
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`US 6,272,079 Bl
`
`FIG.9
`
`

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`U.S. Patent
`
`Aug. 7, 2001
`
`Sheet 9 of 9
`
`US 6,272,079 Bl
`
`FIG.lO
`
`11
`
`s
`
`b d
`
`

`
`US 6,272,079 B 1
`
`1
`OPTICAL HEAD UNIT FOR OPTICAL DISK
`APPARATUS HAVING BOTH FOCUS
`CONTROL AND TRACK CONTROL
`FUNCTION
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to an optical head unit for an
`optical disk apparatus. In this specification, the phrase
`"optical disk apparatus" refers to a read-only device using an
`optical disk as well as to an magneto-optical disk apparatus
`capable of reading and writing data from and in a magneto(cid:173)
`optical disk by magnetic field modulation or optical pulse
`modulation.
`2. Description of the Related Art
`Conventionally, optical disk apparatus have already been
`widely used as external mass storage devices for e.g. com(cid:173)
`puters. Nowadays, in preparation for a full-fledged multi(cid:173)
`media era to come in the future, a new technology is sought
`for producing an optical disk apparatus capable of operating
`with much higher recording density.
`FIG. 9 illustrates principal parts of an optical head unit
`used for a conventional optical disk apparatus. An objective
`(1) is mounted on an actuator (not shown) for performing
`track control and/or focus control. The nonillustrated actua(cid:173)
`tor is arranged to bring the objective (1) to a suitable
`position, so that the laser beam passing through the objective
`(1) will properly converge on a recording surface (r). As is
`shown, the recording surface (r) is attached to the upper side
`of a transparent platter of an optical disk (d). An element
`designated by reference character (h) is a magnetic head.
`For increasing the recording bit density of the optical disk,
`it is possible to take the following measures: (a) utilization
`of a laser semiconductor as a light source that is arranged to
`emit a laser beam having a shorter wavelength; (b) utiliza(cid:173)
`tion of an objective having a greater numerical number
`(NA); and (c) utilization of magnetic field modulation.
`At present, the above option (a) is not practical since a
`laser semiconductor having a shorter wavelength (i.e., blue
`laser semiconductor) has several problems to be solved in
`respect to costs, output efficiency, thermal stability and so
`on.
`Regarding the option (b), when use is made of only one
`objective for providing a relatively high NA, the radius of
`curvature of the objective should be small. However, such a
`lens is difficult to manufacture. Further, as the NA becomes
`greater, there will be an unfavorable increase in the coma(cid:173)
`aberration (which is proportional to the third power of the
`NA) due to, for instance, tilt or eccentricity between the
`incidence plane and the exit plane of the objective, or due to
`tilt or eccentricity of the objective itself, or due to tilt of the
`optical disk. Further, spherical aberration (which is propor(cid:173)
`tional to the fourth power of the NA) will also increase due
`to uneven thickness of the optical disk. With those aberra(cid:173)
`tions present, it will be almost impossible to properly write
`and read data in and from the optical disk.
`JP-A-8-221790 discloses an optical pickup apparatus
`which is proposed for dealing with the problems stated
`above. Referring to FIG. 10 of the accompanying drawings,
`the disclosed apparatus is provided with an object lens
`assembly including a first objective (11) and a second
`objective (12) each having a small NA. The second objective
`(12) is supported by a lens-barrel (m) of a serve-control
`actuator (a), while the first objective (11) is attached to a
`slider (s) which is supported by the lens-barrel (m) via an
`
`5
`
`2
`elastic member (b). In operation, the slider (s) will either be
`held in sliding contact with the bottom surface of a disk (d)
`or be caused to float away from the bottom surface of the
`disk (d) via an air layer.
`According to the arrangements illustrated in FIG. 10, use
`is made of a combination of two objectives, i.e., the first and
`second objectives (11, 12) so that the overall NA of the
`object lens assembly is increased. As a result, it is possible
`to provide a higher recording density, while also overcoming
`10 the problems caused by using a single objective having a
`small radius of curvature. Further, the slider (s) can be held
`close to the bottom surface of the rotating disk (d) under
`action of the elastic member (b). In this manner, the coma(cid:173)
`aberration due to the tilt of the disk (d) is prevented, while
`15 the wave aberration of the lens assembly being also elimi(cid:173)
`nated. Thus, the writing and reading of data in relation to the
`disk (d) can be performed with a high bit density.
`However, in the conventional optical pickup apparatus
`shown in FIG. 10, the slider (s) carrying the first objective
`20 (11) is attached to the mirror-barrel (m) of the actuator. Such
`an arrangement will give rise to the following disadvan(cid:173)
`tages.
`In general, when use is made of a two-dimensional
`actuator movable in a focus control direction (vertical
`25 direction) and a track control direction (horizontal
`direction), the actuator may be carried by a carriage
`(movable in a radial direction of an optical disk) via a
`support spring. Further, the actuator may be provided with
`a servomechanism having a focus coil and a track coil which
`30 are positioned in the magnetic field generated by a certain
`circuit mounted on the carriage. In such an arrangement,
`based on detected signals obtained from laser beams
`reflected on the recording surface of the optical disk, electric
`currents will be supplied to the track coil and/or the focus
`35 coil. Accordingly, the actuator will be moved in the track
`control direction and/or in the focus control direction for
`performing track control and/or focus control. In order to
`enable fast response for those control operations, it is
`preferable that the inertial mass of the actuator is rendered
`40 as small as possible, and that the rigidity of the support
`spring attached to the carriage is minimized.
`However, according to the conventional optical pickup
`apparatus shown in FIG.10, the actuator (a) carries the slider
`(s) via the elastic member (b), which unfavorably puts an
`45 additional weight onto the actuator (a). Another disadvan(cid:173)
`tage is that a very complicated mass-spring system is formed
`between the carriage and the slider (s) when the slider (s) is
`elastically urged onto the disk (d). Regarding the first(cid:173)
`mentioned disadvantage (unfavorable increase in weight),
`50 when the actuator (a) cannot be moved with sufficiently fast
`response, the floating and tilting extents of the slider (s) will
`unfavorably vary, which may produce a greater wave aber(cid:173)
`ration. When this happens, high-density data writing and
`reading operations will become difficult to perform. At the
`55 same time, the driving performance of the actuator (a) will
`become unstable when even a slight error occurs in mount(cid:173)
`ing the slider (s). Regarding the second-mentioned disad(cid:173)
`vantage (formation of a complicated mass-spring system),
`specifically, the carriage supports the actuator (a) via the
`60 support spring, the actuator in turn supports the slider (s) via
`the elastic member (b), and the slider (s) is associated in
`motion with the optical disk (d) via an air film when the disk
`is rotated. In such an arrangement, when the rotating disk (d)
`causes vibrations in the slider (s) due to the tilt and/or
`65 uneven thickness of the disk (d), the unsteady movement of
`the slider may lead to the resonance of the above mass(cid:173)
`spring system. When this happens, the extent of the floating
`
`

`
`US 6,272,079 B 1
`
`3
`and/or tilting of the slider (s) in relation to the disk (d) tend
`to vary, and the wave aberration will become greater. In
`addition, the resonance mentioned above will render the
`focus control and the track control unreliable, whereby
`proper writing and reading operations with respect to the 5
`disk (s) will become difficult.
`
`4
`According to the preferred embodiment, said one of the
`plurality of lens units and the remaining lens units are
`movable relative to each other, so that the focus control
`and/or track control are properly performed.
`Specifically, the slider may be movable in the track
`control direction, so that said one of the plurality of lens
`units is moved relative to the remaining lens units in the
`track control direction.
`The present invention is not limited to the above arrange(cid:173)
`ment (wherein it is the slider that is moved). Alternatively,
`said one of the plurality of lens units and the remaining lens
`units may be moved relative to each other in the track
`control direction.
`In the preferred embodiment, said one of the plurality of
`lens units and the remaining lens units are moved relative to
`each other in the focus control direction.
`The actuator may be a one-dimensional device movable in
`the focus control direction. Alternatively, the actuator may
`20 be a two-dimensional device movable in the focus control
`direction and the track control direction.
`According to another preferred embodiment, the laser
`beam is arranged to enter, at different angles, remaining lens
`units other than said one of the plurality of lens units, so that
`a beam spot of the focused laser beam is moved in the track
`control direction.
`In the above embodiment, the optical head unit may
`further comprise a rotatable galvano-mirror for causing the
`laser beam to enter the remaining lens units at different
`30 angles.
`The actuator may be a one-dimensional device movable in
`the focus control direction.
`The slider may be arranged to float with respect to the
`optical disk when the optical disk is rotating, or the slider
`may be held in sliding contact with a surface of the optical
`disk.
`The carriage may be a linearly movable carriage.
`Alternatively, the carriage may be a swing arm type
`40 carriage which is pivotable about a vertical shaft.
`The optical head unit may further comprise a magnetic
`head arranged opposite to the slider with respect to the
`optical disk.
`In this case, the magnetic head and the slider may be
`urged toward the optical disk with a same force.
`The optical disk may be made up of a transparent platter
`and a recording layer formed on the platter. In such an
`instance, the slider may be arranged on a side of the platter,
`whereas the magnetic head may be arranged on a side of the
`recording layer.
`The transparent platter may have a thickness of about 0.6
`mm.
`Preferably, the object glass assembly may include an
`achromatic lens.
`Other features and advantages of the present invention
`should become clear from the detailed description to be
`made hereinafter referring to the accompanying drawings.
`
`15
`
`SUMMARY OF THE INVENTION
`It is, therefore, an object of the present invention is to
`provide an optical head unit capable of overcoming the 10
`problems described above.
`According to the present invention, there is provided an
`optical head unit used for an optical disk apparatus, the
`optical head unit comprising:
`an object glass assembly for focusing a laser beam from
`a light source on an optical disk;
`an actuator movable at least in a focus control direction;
`a carriage for supporting the actuator and moving the
`actuator in a track control direction; and
`a slider which is attached to the carriage and capable of
`adjusting a posture thereof in relation to a surface of the
`optical disk;
`wherein the object glass assembly comprises a plurality of
`lens units each including at least one lens, one of the 25
`plurality of lens units being supported by the slider.
`According to a preferred embodiment, the remaining lens
`units other than said one of the plurality of lens units are
`supported by the actuator.
`With such an arrangement, it is possible to increase the
`overall NA of the object glass assembly, while the NA of
`said one of the plurality of lens unit and the NA of the
`remaining lens units are kept small. In this manner, it is
`possible to provide a higher recording density with the use
`of separate objectives which are easy to manufacture. 35
`Further, since the slider can change its posture in relation to
`the surface of the optical disk, a possible coma-aberration
`caused by the tilt of the optical disk will advantageously be
`prevented. Still further, the actuator is movable in the focus
`control direction. Thus, the spherical aberration caused by
`an uneven thickness of the disk can be prevented by adjust(cid:173)
`ing the distance between said one of the plurality of lens
`units and the remaining lens units.
`According to the above embodiment, the slider is fixed to
`the carriage (not to the actuator). Thus, unlike the conven- 45
`tional apparatus shown in FIG. 10, no additional weight
`(inertial mass) will be put on the actuator, whereby the
`actuator can move swiftly enough. Further, according to the
`present invention, an external force, which may be caused
`due to the tilt or uneven thickness of the optical disk, will not 50
`be supplied to the actuator via the slider. In such an
`arrangement, the conventional problems (unwanted vibra(cid:173)
`tions and resonance) are prevented from occurring. Thus, it
`is possible for the actuator to properly perform the focus
`control and track control. Still further, no complicated 55
`mass-spring system is formed in the optical head unit of the
`present invention. Thus, it is possible to prevent the lens
`unit, supported by the slider, from unduly tilting. In addition,
`the floating amount of the slider can be kept constant. As a
`result, the wave aberration will not become unduly greater. 60
`As stated above, according to the present invention, an
`optical head unit is realized that makes it possible to prevent
`the occurrence of wave aberration and to stabilize the
`servomechanism of the actuator. Thus, with the use of the
`optical heat unit of the present invention, data can be written 65
`and read in and from an optical disk with a bit density higher
`than is conventionally possible.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the accompanying drawings:
`FIG. 1 is a perspective view showing an overall arrange(cid:173)
`ment of an optical head unit according to a first embodiment
`of the present invention;
`FIG. 2 is a sectional view taken along lines 11-11 in FIG.
`
`1;
`
`

`
`US 6,272,079 B 1
`
`5
`FIG. 3 is a perspective view showing an overall arrange(cid:173)
`ment of an optical head unit according to a second embodi(cid:173)
`ment of the present invention;
`FIG. 4 is a perspective view showing an overall arrange(cid:173)
`ment of an optical head unit according to a third embodiment 5
`of the present invention;
`FIG. 5 is a perspective view showing an overall arrange(cid:173)
`ment of an optical head unit according to a fourth embodi(cid:173)
`ment of the present invention;
`FIG. 6 is a graph showing a relationship between the disk 10
`thickness and the wave aberration;
`FIG. 7 illustrates a relationship between the defocus
`amount of a second objective and the focus error signal;
`FIG. 8 is a graph showing a relationship between the 15
`displacement of the second objective and the focus error
`signal, and a relationship between the same displacement
`and the wave aberration;
`FIG. 9 illustrates a conventional optical head unit; and
`FIG. 10 illustrates arrangements of another conventional 20
`optical head unit.
`
`DETAILED DESCRIPTION OF 1HE
`PREFERRED EMBODIMENT
`The preferred embodiments of the present invention will
`be described below with reference to the accompanying
`drawings.
`Reference is first made to FIGS. 1 and 2 which illustrate
`an optical head unit 10 for an optical disk apparatus accord(cid:173)
`ing to a first embodiment of the present invention. As shown
`in FIG. 1, the optical head unit 10 includes a carriage 20
`slidably supported by a pair of elongated guide members 21.
`The carriage 20 can be moved in the X-direction shown in
`FIG. 1 (called "track control direction" hereinafter) by a
`linear driving mechanism 22. The driving mechanism 22
`may include a linear voice coil motor for example. As shown
`in FIG. 2, the carriage 20 supports an actuator 30 which in
`turn carries a lens (or lens unit) 42. The optical head unit 10
`includes another lens (or lens unit) 41 supported by a slider
`50. Together, the lens 41 (called "first objective" hereinafter) 40
`and the lens 42 (called "second objective") serve as an object
`lens system. In the illustrated embodiment, a single lens is
`used for each of the first and second objectives. However,
`two or more lens may be used for each objective unit. The
`actuator 30 is movable at least in the Z-direction shown in
`FIG. 1 (that direction will be called "focus control direction"
`hereinafter). Thus, by adjusting the vertical position of the
`actuator 30, the second objective 42 can be moved toward or
`away from the first objective 41. In this manner, the focus
`control of the object lens system can be performed. As is
`well known, for an optical head unit, both the focus control
`and the track control have to be performed. In this
`connection, when use is made of a two-dimensional actuator
`(i.e, movable in the track control direction and the focus
`control direction), the focus control and the track control
`will be performed by moving the actuator only.
`The slider 50 is supported by the carriage 20 via a
`suspension member 23 such as a leaf spring. The slider 50
`is a frame-like member formed with a central opening for
`accommodating the first objective 41. The upper surface 51
`of the slider 50 is brought into slidable contact with the
`bottom surface of an optical disk D. As shown in FIG. 2, the
`first objective 41 is arranged above the second objective 42
`in a coaxial manner. The vertical position of the slider 50 in
`relation to the carriage 20 is determined so that the suspen(cid:173)
`sion member 23 urges the slider 50 onto the optical disk D
`with a predetermined pressing force.
`
`6
`The first and the second objectives 41, 42 are designed to
`have a predetermined numerical aperture (NA) high enough
`for performing read/write operations with a desired high bit
`density. Below the second objective 42 is arranged a mirror
`24 for directing light of an external light source to the second
`objective 42. The external light source may be a laser
`semiconductor. Emitted light or laser beams, after reflected
`on the mirror 24, will pass through the second objective 42
`and the first objective 41. Converged by the two objectives,
`the laser beams will make a light spot on a recording surface
`dl of the disk D.
`The optical head unit 10 is provided with a magnetic head
`60 which is brought into contact with the upper surface of
`the optical disk D at a location opposite to the slider 50. The
`magnetic head 60 is urged onto the optical disk D with a
`pressing force which is equal to the pressing force of the
`slider 50 (though in the opposite directions). The magnetic
`head 60 is arranged to write data in the disk D by magnetic
`field modulation. In order to perform magnetic field modu-
`lation properly (namely, without causing signals to be
`degraded due to dust accumulation on the disk D), it is
`necessary to give a certain distance (or thickness) between
`the laser-entering surface and data-recording surface of the
`optical disk. For providing the desired thickness, the illus-
`25 trated disk D is made up of a relatively thick, transparent
`disk member and a recording film attached to one side of the
`disk member. The thickness of the disk D may be about 0.6
`mm (the technical significance of that thickness will be
`described later).
`30 With the above arrangement, when the disk D (attached to
`a spindle) is rotated, the slider 50 will slightly "float" over
`the bottom surface of the disk D via an air layer, or will be
`moved on the bottom surface of the disk in sliding contact
`therewith. In either case, the slider 50 can adjust its posture
`35 with respect to the disk D by following the movement of the
`disk D (the disk may unduly tilt during rotation and/or it may
`have an uneven thickness). In this manner, the posture of the
`first objective 41 with respect to the disk D can be auto(cid:173)
`matically adjusted.
`As previously stated, the second objective 42 can be
`vertically moved since it is supported by the actuator 30
`which is movable toward and away from the disk D. By
`changing the vertical position of the second objective 42, the
`laser beams can be converged on different points as viewed
`45 in the thickness direction of the disk D. In this way, the focus
`control of the object lens system is performed.
`The positional adjustment of the second objective 42 may
`be performed in a conventional manner. Specifically, laser
`beams reflected on the recording surface of the disk D are
`50 split by a beam splitter and part of the reflected light is
`detected by a suitable detector. Then, based on data obtained
`from the detected light, a servomechanism will adjust the
`vertical position of the actuator 30.
`When the actuator 30 is capable of performing two-
`55 dimensional operation, the second objective 42 supported by
`it will also be moved in the track control direction (relative
`to the first objective 41). In this way, the beam spot of the
`converged laser beams can be shifted in the track control
`direction for performing desired track control. Like the focus
`60 control previously described, the track control can also be
`performed in a conventional manner (i.e, with the use of a
`detector for detecting the light reflected on the recording
`surface of the disk D and a servomechanism for controlling
`the actuator 30 based on data obtained from the detected
`65 beams).
`When the actuator 30 is a one-dimensional device
`designed to move only in the focus control direction, use
`
`

`
`US 6,272,079 B 1
`
`20
`
`8
`7
`may be made of a galvano-mirror in place of the illustrated
`Below the base end of the swing arm 20, there are
`mirror 24 (FIG. 2). In this case, the galvano-mirror may be
`provided a laser semiconductor 71 as a light source, a
`collimator lens 72, a servo-lens 76, a beam splitter 73 and a
`arranged to pivot about a horizontal axis, so that the direc(cid:173)
`detector 74. The laser beams emitted by the laser semicon(cid:173)
`tion of laser beams reflected on the galvano-mirror will be
`ductor 71 will pass through the lens 72 and the beam splitter
`altered in the track control direction by adjusting the posture 5
`73. Thereafter, the laser beams travel longitudinally of the
`of the mirror about the horizontal axis.
`swing arm 20 and are reflected on a mirror 24 toward the
`According to the optical head unit 10 of the present
`second objective 42. Then, while passing through the second
`invention, the slider 50 is arranged to adjust its posture in
`and the first objectives 42, 41, the reflected beams are
`relation to the disk surface. Thus, the coma-aberration due to
`converged to make a light spot on the recording surface of
`the tilt of the disk D is advantageously prevented. Further, 10
`the disk D. Thereafter, the laser beams reflected on the
`the distance between the first and second objectives 41, 42
`recording surface of the disk D will trace back the same path
`is changed by moving the actuator 30 in the focus control
`toward the beam splitter 73. In the beam splitter, the laser
`direction. Thus, it is also possible to prevent the spherical
`beams are split into two parts, and one of them passes
`aberration due to the uneven thickness of the disk D.
`through the servo-lens 76 and is detected by the detector 74.
`Further, according to the illustrated optical head unit 10,
`15 Like in the first embodiment, the actuator 30 performs the
`the slider 50 carrying the first objective 41 is supported by
`focus control and the track control based on data obtained
`the carriage 20 but not by the actuator 30. Therefore, no
`from the detected beams.
`additional weight (or inertial mass) will be put on the
`In the optical head 10 of the second embodiment, use is
`actuator. Such an arrangement is advantageous for enabling
`made of two separate objectives, namely, the first and the
`the actuator 30 to move quickly enough. Further, an external
`second objectives 41, 42. Like in the first embodiment, the
`force due to the tilt of the disk D or uneven thickness thereof
`first objective 41 is supported by the slider 50 which is
`will not be applied to the actuator via the slider 50. Thus, the
`attached to the carriage 20, while the second objective 42 is
`focus control and/or the track control by the actuator 30 can
`carried by the actuator 30. Thus, according to the second
`be reliably performed. Still further, no complicated mass(cid:173)
`embodiment again, it is possible to obtain the same advan-
`spring system (including the carriage 20, the actuator 30, the
`25 tages as described with the first embodiment.
`slider 50 and the disk surface) is produced. Thus, it is
`Reference is now made to FIG. 4 showing an optical head
`possible to prevent the slider 50 (hence the first objective 41)
`unit according to a third embodiment of the present inven(cid:173)
`from unduly tilting. In addition, when there is no compli(cid:173)
`tion. In the figure, members or elements similar to those of
`cated mass-spring system, the slider 50 can keep floating in
`the first embodiment are designated by the same reference
`a constant position without being brought too closer to or
`30 characters or numerals.
`away from the bottom surface of the disk D. Thus, the wave
`Like in the second embodiment, the optical head unit of
`aberration is advantageously reduced or even eliminated.
`the third embodiment includes a swing arm type carriage 20.
`Still further, according to the illustrated optical head unit
`However, the track control operation by the third embodi(cid:173)
`10, the disk D is urged from above and from below with an
`ment is different from that of the second embodiment.
`equal pressing force by the magnetic head 60 (which may 35
`Specifically, the swing arm 20 has two ends, namely, a base
`include a slider carrying a coil) and the slider 50. Thus, the
`end closer to the shaft 25 and a distal end opposite to the
`disk D will not be warped or displaced in the thickness
`base end. The distal end of the swing arm 20 carries an
`direction by the slider 50. As a result, the focus control
`actuator 30 which is a one-dimensional device movable only
`and/or the track control can be performed reliably enough.
`in the focus control direction. In this arrangement, the
`actuator 30 of the third embodiment performs focus control
`Reference is now made to FIG. 3 showing an optical head 40
`only. On the other hand, below the base end of the swing arm
`according to a second embodiment of the present invention.
`In the figure, members or elements similar to those of the
`20, there is provided a galvano-mirror 75. As can be seen
`first embodiment (FIGS. 1 and 2) are designated by the same
`from FIG. 4, the galvano-mirror 75 is attached to a vertical
`shaft which is arranged to rotate about its vertical axis. Thus,
`reference numbers or characters.
`the galvano-mirror 75 is pivotable with the vertical shaft, as
`The optical head 10 of the second embodiment includes a 45
`illustrated in FIG. 4. With such an arrangement, laser beams
`swing arm type carriage 20 for moving an actuator 30 in a
`emitted from a laser semiconductor 71 are reflected on the
`track control direction or X-direction shown in FIG. 3. The
`galvano-mirror 75 and then travel toward the distal end of
`swing arm 20 is attached to a vertical shaft 25, so that it can
`the swing arm 20. At this stage, when the galvano-mirror 75
`rotate about the vertical axis of the shaft. As can be seen
`from FIG. 3, the elongated swing arm 20 has two ends, one 50 is caused to pivot, the laser beams reflected by the mirror 75
`will be swayed horizontally in the swing arm carriage 20. As
`of which (base end) is closer to the shaft 25 while the other
`a result, the beam spot produced by the object lens system
`(distal end) farther away from the shaft 25. In operation, the
`(i.e. the combination of the first and second objectives 41,
`distal end of the swing arm 20 will trace out an arc extending
`42) will be moved in the track control direction for per-
`across a recording region of the disk D substantially in the
`55 forming the track control. The other arrangements are simi(cid:1