United States Patent [19J
`Kim et al.
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US006034935A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,034,935
`Mar. 7,2000
`
`[54] OPTICAL PICK-UP APPARATUS CAPABLE
`OF READING DATA IRRESPECTIVE OF
`DISC TYPE
`
`[75]
`
`Inventors: Jin-Yong Kim, Kyungki-Do;
`Dae-Young Kim; Man-Hyung Lee,
`both of Seoul; Song-Chan Park;
`In-Sang Song, both of Kyungki-Do, all
`of Rep. of Korea
`
`[73] Assignee: LG Electronics Inc., Seoul, Rep. of
`Korea
`
`[21] Appl. No.: 09/224,798
`
`[22] Filed:
`
`Dec. 31, 1998
`
`Related U.S. Application Data
`
`[62] Division of application No. 08/567,041, Dec. 4, 1995.
`
`[30]
`
`Foreign Application Priority Data
`
`Rep. of Korea ........................ 95-4410
`Rep. of Korea ........................ 95-4525
`Rep. of Korea ........................ 95-6238
`Rep. of Korea ...................... 95-39516
`
`Mar. 4, 1995
`[KR]
`Mar. 6, 1995
`[KR]
`Mar. 26, 1995
`[KR]
`Oct. 31, 1995
`[KR]
`Int. Cl/ ........................................................ G11B 7/00
`[51]
`[52] U.S. CI .................................. 369/54; 369/94; 369/58;
`369/112; 369/110
`[58] Field of Search ................................ 369/58, 54, 110,
`369/112, 44.11, 44.14, 94, 44.23, 44.27
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,157,555 10/1992 Reno .
`
`5,235,581
`5,281,797
`
`8/1993 Miyagawa ............................ 369/44.23
`1/1994 Tatsuno eta!. .
`
`FOREIGN PATENT DOCUMENTS
`
`0329234
`0414380
`0470807
`0537904
`0610055
`62-003441
`5-120720
`6-020298
`8-138262
`
`8/1989 European Pat. Off ..
`2/1991 European Pat. Off ..
`2/1992 European Pat. Off ..
`4/1993 European Pat. Off ..
`8/1994 European Pat. Off ..
`1/1987
`Japan.
`Japan.
`5/1993
`Japan.
`1/1994
`5/1996
`Japan.
`
`Primary Examiner-Thang V. Tran
`
`[57]
`
`ABSTRACT
`
`An improved optical pick-up apparatus capable of reading
`data irrespective of disc type capable of reading various
`kinds of discs having different thicknesses and writing
`densities using only one pick-up apparatus, which includes
`a light source; a beam splitter for passing through or splitting
`a beam from the light source; an objective lens for condens(cid:173)
`ing the beam onto corresponding disc among discs having
`different thicknesses and different densities; a numerical
`aperture control unit for controlling an effective numerical
`aperture of the objective lens so as to execute a focussing
`operation with respect to a certain disc; and a photo-detector
`for receiving the beam reflected by the disc and transmitted
`from the beam splitter.
`
`16 Claims, 30 Drawing Sheets
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`LG Electronics, Inc. et al.
`EXHIBIT 1004
`IPR Petition for
`U.S. Patent No. 6,785,065
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 1 of 30
`
`6,034,935
`
`FIG. 1
`CONVENTIONAL ART
`
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 3 of 30
`
`6,034,935
`
`FIG. 4
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`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 4 of 30
`
`6,034,935
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 5 of 30
`
`6,034,935
`
`FIG.6A
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 6 of 30
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`6,034,935
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`U.S. Patent
`
`Mar.7,2000
`
`Sheet 7 of 30
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 8 of 30
`
`6,034,935
`
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 9 of 30
`
`6,034,935
`
`FIG. 7 A
`
`67b
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`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 10 of 30
`
`6,034,935
`
`FIG.SA
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 11 of 30
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`6,034,935
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`U.S. Patent
`
`Mar.7,2000
`
`Sheet 12 of 30
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`6,034,935
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 13 of 30
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`6,034,935
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`FIG. 9C
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 14 of 30
`
`6,034,935
`
`FIG.9D
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`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 16 of 30
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`6,034,935
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`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 17 of 30
`
`6,034,935
`
`FIG. 12
`
`1. MOVING DISC TO ACTUATOR
`2. DECREASING NA USING LIQUID
`CRYSTAL SHUTTER
`3. MAINTAINING DISC AT NORMAL SPEED
`
`I
`SWING ACTUATOR
`
`~NO
`
`YES
`.------< GENERATED? >-------,
`
`ROTATING SPINDLE MOTOR
`USING CLV
`
`1. MOVING ACTUATOR TO ORIGINAL
`POSITION
`2. INCREASING EFFECTIVE NA BY
`STOPPING LCS DRIVE
`
`1 . FOCUS CONTROL
`2. TRACK CONTROL
`
`READING
`SIGNAL
`
`SWING ACTUATOR
`
`NO
`
`RF SIGNAL
`GENERATED ?
`
`YES
`
`CONTROLLING SPINDLE
`MOTOR USING CLV
`
`NO DISC
`ERROR OR
`NO DISC
`
`. . . .__ - - - - - - - -1 1. FOCUS CONTROL
`2. TRACK CONTROL
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 18 of 30
`
`6,034,935
`
`FIG. 13
`
`100
`
`122
`
`132
`
`140
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 19 of 30
`
`6,034,935
`
`FIG. 14
`
`27
`
`1277<l_('Y'
`~127b
`~'
`
`108
`
`28
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 20 of 30
`
`6,034,935
`
`FIG. 15
`
`126
`
`128
`
`I
`
`I
`127~
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 21 of 30
`
`6,034,935
`
`FIG. 16
`
`169
`
`167
`
`164b
`
`185
`
`166d
`
`184
`
`182
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 22 of 30
`
`6,034,935
`
`-.;t
`<.0
`...--
`
`~ r-
`
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`
`FIG. 18
`
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`:
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`r---------------
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`\ 720
`J,
`I
`I
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`
`800
`
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`
`600 ,------------------,
`650
`r---------_1--.., I
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`I
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`I OSC
`I
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`______ _j
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`I DIGITAL SIGNAL PROCESSING UNIT 1--- 750
`
`I
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`
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`I
`L ________ -~ _______ _j
`
`I
`
`L
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 24 of 30
`
`6,034,935
`
`FIG. 19
`
`CD
`
`SWs - - LOW
`SW10 - - HIGH
`
`HIGH
`SWa
`SW1o - - LOW
`
`NO
`
`S = HIGH
`
`NO
`
`YES
`
`YES
`
`SWa - - LOW
`SW10 - - LOW
`
`SWs - - LOW
`SW10 - - LOW
`
`RETURN
`
`

`
`FIG. 20
`
`L
`
`1111 TTII I IIIII
`
`-------------, r---------
`600
`I I
`TES
`TRACKING
`1 1
`CONTROL
`UNIT
`
`1
`
`: 1
`I I
`I I
`I I
`
`1--t-+---'
`
`~--~ FOCUS
`CONTROL
`1 1
`UNIT
`1 1
`400~
`: :
`I NA CONTROL I I I
`.. I UNIT DRIVING
`UNIT
`
`MICRO(cid:173)
`COMPUTER
`
`f 10a
`
`--------------,
`AA
`I
`~
`~ I
`:
`46a
`I
`I
`1
`:
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`
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`
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`
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`: :
`: PROCESSING
`I UNIT
`I I
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`I 1
`I
`500
`I
`I I
`I
`L---------------------------------~ L---------------------~
`
`750
`
`RF
`
`DISC
`JUDGING
`UNIT
`
`REPRODUCING
`SIGNAL PROCESSING
`UNIT
`
`47
`
`43
`
`55
`
`54
`
`41
`
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`Ul
`0 ......,
`~ c
`
`0\
`....
`8
`""-
`....
`~
`Ul
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 26 of 30
`
`6,034,935
`
`FIG. 21
`
`205
`
`55 0.
`20~
`
`248
`
`202
`
`258
`
`48
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 27 of 30
`
`6,034,935
`
`N
`C\1
`
`N .-
`N
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 28 of 30
`
`6,034,935
`
`FIG. 23
`
`, _________ 258
`
`I
`
`I
`
`I 256
`I I
`I I
`I I
`I
`I
`
`I
`
`I
`
`252a
`
`258
`
`r
`
`260a
`
`:~~
`
`260
`
`252
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 29 of 30
`
`6,034,935
`
`FIG. 24
`
`345
`
`330
`
`355
`
`364
`
`

`
`U.S. Patent
`
`Mar.7,2000
`
`Sheet 30 of 30
`
`6,034,935
`
`FIG. 25
`
`321
`
`324
`
`326
`
`FIG. 26
`
`326=8-(A+C)
`
`324
`
`326
`
`>----o 326-324
`
`324=8'-(A'+C')
`
`

`
`1
`OPTICAL PICK-UP APPARATUS CAPABLE
`OF READING DATA IRRESPECTIVE OF
`DISC TYPE
`
`6,034,935
`
`This application is a divisional of Ser. No. 08/567,041 5
`filed Dec. 04, 1995.
`
`BACKGROUND OF THE INVENTION
`
`2
`3. The beam is focused by the objective lens 5 and reflected
`or diffracted by information writing surface on an informa(cid:173)
`tion media. The thusly reflected beam returns on the same
`path and is converted into an electric signal by the photo(cid:173)
`detector 8. That is, the beam is transmitted to the photo(cid:173)
`detector 8 through another path formed by the beam splitter
`3 through the sensor lens 7.
`Meanwhile, the diffraction grating 2 and the sensor lens 7
`are widely used with respect to tracking using a three beam
`10 method and to astigmatism method a focusing servo.
`A high density optical disc has four times the capacity of
`a conventional compact disc, and the data stored therein can
`be reproduced using a lens having a numerical aperture of
`about 0.6. In this case, the aberration due to the disc
`15 inclination increases as the thickness of a disc increases. In
`order to resolve the above problem, the standard of a digital
`video disc requires the thickness of a disc to be 0.6 mm.
`At this time, the optical system, as shown in FIG. 1,
`includes a high density optical disc having a thickness of 0.6
`mm and a compact disc having a thickness of 1.2 mm.
`However, the above-mentioned optical system has the fol(cid:173)
`lowing problems.
`For example, the beam intensity distribution at a focal
`25 point on a disc surface using and a disc having a thickness
`of 0.6 mm and an objective lens having a numerical aperture
`of 0.6 is shown in FIG. 2 as a solid line. When a beam is
`focused at a disc having a thickness of 1.2 mm through the
`objective lens 5, the beam intensity distribution is shown as
`30 a dashed line in FIG. 2 due to the spherical aberration of the
`lens.
`That is, the beam intensity ratio of the main lobe signifi(cid:173)
`cantly decreases, and the beam intensity of the side lobe
`increases, so that crosstalk of the signal written on its
`neighboring track of the disc increases.
`In addition, the optical pick-up apparatus cannot repro(cid:173)
`duce the data stored in a disc of 1.2 mm because the
`sensitivity with respect to a disc tilt level is too high as
`shown in FIG. 1 in case the data is read using an objective
`40 lens having a numerical aperture of 0.6 mm.
`
`20
`
`1. Field of the Invention
`The present invention relates to an optical pick-up appa(cid:173)
`ratus capable of reading data irrespective of a disc type, and
`in particular to an improved optical pick-up apparatus
`capable of reading data irrespective of a disc type capable of
`reading a certain disc among disc having different thick(cid:173)
`nesses and writing densities using only one pick-up appa-
`ratus.
`2. Description of the Conventional Art
`Generally, a digital video disk (DVD) is provided with a
`red semiconductor laser and an objective lens having a larger
`numeric aperture (NA). The above-mentioned DVD has six
`or eight times the writing data capacity of a compact disc
`(CD), in which compressed video and audio data can be
`stored therein. That is, the data corresponding to that of a
`movie can be stored in a disc of 120 mm.
`The DVD uses a red semiconductor laser having a wave
`length of 635 mm or 650 nm.
`Generally, if the wave length of light source becomes
`short, the diameter of a laser spot decreases in proportion
`thereto, so that both a track pitch and a minimum writing
`mark length can be reduced. That is, since the surface of the
`writing mark is in duplicate proportion to the wave length of
`a writing mark, overall writing area can be decreased.
`Meanwhile, the spot diameter of the laser is proportional 35
`to the wave length of the light source. Therefore, if the
`numeric aperture is increased, without varying the wave
`length, it is possible to increase the writing intensity.
`Therefore, the numeric aperture in the optical system for a
`CD is about 0.45, but it is about 0.6 in the DVD.
`There are three methods below of reading the disc data in
`the conventional DVD as follows.
`The first is to simply increase the numeric aperture by a
`small amount. The second is to adopt a tilt angle compen(cid:173)
`sation device, called a tilt servo, in the optical pick-up 45
`system, instead of increasing the numeric aperture to more
`than 0.52. The third is to increase the numeric aperture up to
`0.6 and to reduce the laser transmitting distance through a
`disc plate.
`The construction of a conventional optical pick-up appa-
`ratus will now be explained.
`Referring to FIG. 1, the conventional optical pick-up
`apparatus includes a diffraction grating 2 for dividing a beam
`from a light source 1 into a main beam and two sub-beams 55
`for a tracking servo. In addition, the beams from the dif(cid:173)
`fraction grating 2 is transmitted to an objective lens 5,
`provided for condensing the light on a spot of an optical disc
`6, through a collimator lens 4 for outputting a parallel light
`through a beam splitter 3. In addition, the optical detector 8
`detects a beam data signal transmitted from a sensor lens 7
`for condensing the beam.
`The operation of the conventional optical pick-up appa(cid:173)
`ratus will now be explained with reference to the accompa(cid:173)
`nying drawings.
`The beam from the light source 1 is converted into a
`parallel light by the collimator lens through the beam splitter
`
`50
`
`SUMMARY OF THE INVENTION
`
`Accordingly, it is an object of the present invention to
`provide an optical pick-up apparatus capable of reading a
`data irrespective of disc type, which overcome the problems
`encountered in a conventional optical pick-up apparatus not
`capable of reading data irrespective of disc type.
`It is another object of the present invention to provide an
`improved optical pick-up apparatus capable of reading data
`irrespective of disc type capable of reading various kinds of
`discs having different thicknesses and writing densities,
`using only one pick-up apparatus.
`To achieve the above objects, there is provided an optical
`pick-up apparatus capable of reading data irrespective of
`disc type, which includes a light source; a beam splitter for
`passing light from the light source or splitting a beam from
`the light source; an objective lens for appropriately focusing
`the beam onto a disc having a certain thickness and record-
`ing densities; a numerical aperture control unit for control(cid:173)
`ling an effective numerical aperture of the objective lens so
`as to execute a focussing operation with respect to a certain
`type of disc; and a photo-detector for receiving the beam
`reflected by the disc and reflected by the beam splitter, thus
`65 reading a data irrespective of disc type, thus allowing the
`reading of discs among disc having different thicknesses and
`writing densities using only one pick-up apparatus.
`
`60
`
`

`
`3
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`6,034,935
`
`15
`
`FIG. 1 is a schematic view showing a conventional optical
`pick-up apparatus.
`FIG. 2 is a graph of a beam intensity distribution on discs
`having different thicknesses in a conventional optical pick(cid:173)
`up apparatus.
`FIG. 3 is a block diagram of an optical pick-up apparatus
`of a first embodiment according to the present invention.
`FIG. 4 is a schematic view of an actuator of an optical
`pick-up apparatus according to the present invention.
`FIG. 5 is a perspective view of an LC (liquid crystal)
`shutter which is one element of a numerical aperture control
`unit according to the present invention.
`FIG. 6A is a view showing the state of a voltage applied
`to an LC shutter in a normal white mode according to the
`present invention.
`FIG. 6B is a view showing the state of a voltage applied
`to an LC shutter in a normal black mode according to the
`present invention.
`FIG. 6C is a view showing an operational example
`whereby voltage is applied to an LC shutter of FIG. 6C
`according to the present invention.
`FIGS. 6D and 6E are views each showing a variation of 25
`polarization direction within a TN LCD according to the
`present invention.
`FIGS. 6F and 6G are views when voltage is applied to a
`liquid crystal shutter having a PDLC layer according to the
`present invention.
`FIGS. 7A and 7B are views showing an LC pattern
`according to the present invention.
`FIG. SA is a graph showing the amount of a jitter with
`respect to a contrast ratio according to the present invention. 35
`FIG. 8B is a view of a glass plate on which a transparent
`electrode is formed according to the present invention.
`FIG. 9A is a graph showing crosstalk in accordance with
`a variation of a numerical aperture of an objective lens
`according to the present invention.
`FIG. 9B is a graph showing a reproducing signal in
`accordance with a variation of a numerical aperture of an
`objective lens according to the present invention.
`FIG. 9C is a graph showing crosstalk with respect to a 45
`contrast ratio according to the present invention.
`FIG. 9D is a graph showing a relationship of a relative
`reproducing gain with respect to a contrast ratio according to
`the present invention.
`FIG. 10 is a circuit diagram of an entire circuit construc(cid:173)
`tion of an optical pick-up apparatus of a first embodiment
`according to the present invention.
`FIG. llA is a schematic view showing a structure of a
`photo-detector segment according to the present invention.
`FIG. llB is a circuit view of a reproducing signal pro(cid:173)
`cessing unit according to the present invention.
`FIG. 12 is a flow chart of a disc identifying unit according
`to the present invention.
`FIG. 13 is a plan view showing an optical pick-up
`apparatus adopted in a first embodiment according to the
`present invention.
`FIG. 14 is a perspective view of a pick-up base according
`to the present invention.
`FIG. 15 is a perspective view showing a carrier in 65
`cooperation with the pick-up base of FIG. 14 according to
`the present invention.
`
`4
`FIG. 16 is a perspective view of an iris type shutter which
`is one element of the numerical aperture control according
`to the present invention
`FIGS. 17 A and 17B are views showing an iris type shutter
`5 adopted to having a low numeric aperture for a CD and a
`high numeric aperture for a DVD.
`FIG. 18 is a circuit view of a numeric aperture control
`member in case that an iris type shutter is adopted in the
`optical system instead of adopting an LC shutter of a first
`10 embodiment according to the present invention.
`FIG. 19 is a flow chart of a numeric aperture control
`member of FIG. 18 according to the present invention.
`FIG. 20 is a circuit view of an optical pick-up apparatus
`of a second embodiment according to the present invention.
`FIG. 21 is a perspective view of a pick-up base adopted
`in a second embodiment of the present invention.
`FIG. 22 is a perspective view showing a disassembled iris
`member adapted in the pick-up base of FIG. 21 according to
`20 the present invention.
`FIG. 23 is a perspective view of a carrier in cooperation
`with a pick-up base fixed thereto according to the present
`invention.
`FIG. 24 is a perspective view showing a numerical
`aperture control unit of an optical pick-up apparatus of a
`laser coupler method of a third embodiment according to the
`present invention.
`FIG. 25 is a view showing a LD/PD assembly of an
`optical pick-up apparatus of a laser coupler method accord-
`30 ing to the present invention.
`FIG. 26 is a circuit diagram of a focus error and tracking
`error signal detection of a laser coupler method of an optical
`pick-up apparatus according to the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`To begin with, the optical pick-up apparatus adopted in
`the present invention and an optical system with non(cid:173)
`aberration of a first embodiment according to the present
`40 invention will now be explained.
`The spot size formed by the aberration free optical system
`can be computed by the following expression using a
`diffraction of a light.
`
`... (1)
`Spot size~K 8/2(NA)
`where K denotes a constant determined in accordance with
`a light intensity distribution characteristic in a light such as
`a plain wave, Gaussian beam, or a truncated beam, and 8
`denotes a wave length of the light source adopted in the
`50 expression above, and N.A. denotes a predetermined
`numerical aperture.
`According to the formula (1), when the numerical aper(cid:173)
`ture increases, the spot size decreases. For example, in case
`of a high density disc having a thickness of 0.6 mm, since
`55 the distance between tracks and the diameter of pits are
`small, a certain spot having a relatively small size is
`necessary, and an objective lens having a high numerical
`aperture is needed. However, in case of a disc having a
`thickness of 1.2 mm, since the distance between tracks and
`60 the size of pits are bigger than that of the disc having a
`thickness of 0.6 mm, even though the spot size slightly
`increases, it is possible to read out data and to use an
`objective lens having a small effective numerical aperture
`when reading a high density disc.
`The relationship between the numerical aperture and the
`size of a bundle of lights incident to the objective lens can
`be expressed as follows.
`
`

`
`D~2f(N.A)
`
`... (2)
`
`5
`
`6,034,935
`
`aberration oc-
`currence
`10 amount due to
`thickness
`variation by
`0.6 mm
`aberration
`occurrence
`amount by a
`defocussing
`(RMS)
`entire
`aberration
`occurrence
`amount
`(RMS)
`central
`intensity
`distribution
`
`20
`
`6
`
`-continued
`
`in case of reading
`data stored in a disc
`of 1.2 mm using an
`objective lens of nu-
`merical aperture of 0.6
`
`in case of reading data
`stored in a disc of 1.2 mm
`using an objective lens
`after varying a numerical
`aperture to have 0.3
`
`31.67/..
`(-)
`
`31.67/..
`
`(0.43/..)
`
`0.027/..
`
`defies Marechal"s
`criterion
`
`over 95%
`
`* Values in () denote a value when defocussing compensation is performed.
`
`25
`
`where D denotes the diameter of a bundle of incident lights,
`and f denotes a focal length of an objective lens.
`That is, when the size of a bundle of incident lights of an 5
`objective lens having the same focal length is controlled, the
`effective numerical aperture of the objective lens can be
`changed.
`Meanwhile, when reading the data stored on a disc having
`a thickness of 1.2 mm in an optical system including an
`objective lens having a numerical aperture of 0.6 and a high
`density disc having a thickness of 0.6 mm, the following
`problems occur.
`First, if focal compensation is not performed, a proper
`focussing cannot be achieved due to a blurring phenomenon. 15
`Second, the SIN ratio decrease due to the increase of the
`crosstalk due to signal interference between neighboring
`tracks because of the decrease of a central intensity distri(cid:173)
`bution due to the spherical aberration which occurs by the
`thickness variation of the disc and the increase of the
`distribution amount of a first side robe.
`Third, the optical system becomes unstable due to coma
`aberration and astigmatism, which occur due to the disc
`inclination.
`Thus, it becomes impossible to read out the data stored on
`a disc because the optical performance decreases, as
`explained above.
`Meanwhile, the amount of the spherical aberration due to
`the thickness variation of a disc can be computed by the
`following expression.
`
`30
`
`As shown in the above table, by varying an effective
`numerical aperture of the objective lens adopted therein, by
`moving the focus to the data writing surface of a disc of 1.2
`mm, and removing any interferences with respect to
`defocussing, it is possible to read the data.
`In addition, when the effective numerical aperture of the
`objective lens is varied using the numerical aperture control
`unit, since the aberration occurrence amount with respect to
`the disc inclination can be reduced as follows, a more stable
`35 optical system can be achieved.
`Ll. WFEcM-RMs~(n 2 -Vz V'/2n3)(NA )' d8
`... (5)
`When the refractive index "n" in accordance with the
`expression (5) is 1.55, the thickness of a disc is 1.2 mm, the
`inclination of the disc is 0.6°, and the wave length is 635 nm,
`the aberration occurrence amount is given as follows.
`
`... (3)
`
`where n denotes a refractive index of a disc, ll.d denotes a
`thickness variation amount, NA denotes a numerical aper(cid:173)
`ture.
`In addition, the aberration occurring amount due to a
`defocussing can be computed by the following expression.
`
`... (4) 40
`
`where ll.z denotes a defocussing amount.
`When computing the amount of aberration which occur
`and the central intensity distribution in case of reading data
`stored on the disc having a thickness of 1.2 mm using an 45
`objective lens having a numerical aperture of 0.6, and of
`reading data stored therein by varying the effective numeri-
`cal aperture of the objective lens to have 0.3 using a
`numerical aperture control unit, by moving the focus to the
`data writing surface of a disc having a thickness of 1.2 mm, 50
`and by removing any interferences with respect to
`defocussing, the following table can be obtained. At this
`time, the total-root-mean-square wave front aberration of the
`entire optical system, in which an approximate non(cid:173)
`aberration can be expressed as a relationship to the central 55
`intensity distribution, should be lower than 0.07 y, wherein
`the central intensity distribution is over 80%, in accordance
`with Marechal's criterion.
`
`objective lens having a
`objective lens having a
`numerical aperture of 0.6 numerical aperture of 0.3
`
`aberration
`occurrence amount
`
`0.090/..
`
`0.011/..
`
`However, as expressed in the expression (1), if the effec(cid:173)
`tive numerical aperture is decreased, the beam spot size
`increases with respect to the diffraction, and when t