808
`
`IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, AUGUST 1996
`
`OPTICAL PICK-UP FOR DVD
`
`M. Shinoda, K. Nakamura, M. Yabe, N. Watanabe, T. Satoh,
`N. Hirai, T. Fujita, K. Kime, and Y. Ishida
`Video Disc Business Development Center
`Mitsubishi Electric Corporation
`1 Zusho-Baba, Nagaokakyo City, Kyoto, 617 Japan
`
`ABSTRACT
`The optical pick-up for DVD with CD
`(Compact Disc) compatibility is discussed. The
`difference of the substrate thickness between
`DVD and CD causes the different spherical
`aberration and prevents the laser beam from
`being focused into a diffraction limit spot size
`with only one objective lens. Several methods of
`reducing this aberration and possessing the
`compatibility with CD are proposed. The twin
`lens type optical pick-up is one solution of
`overcoming this problem. It incorporates
`objective lenses for both DVD and CD. Each
`lens results in an optimum focused spot without
`the extra spherical aberration for each type of
`disc.
`
`1. INTRODUCTION
`The emerging multimedia system must be
`able to process a huge volume of visual data
`such as still pictures and full-motion videos. The
`developers involved are enthusiastic about the
`commercialization of DVD. This technology is
`expected to become a core medium in the
`multimedia market.
`Aiming at penetration of DVD into the
`current CD-dominated market, DVD players or
`DVD -ROM drives have been earnestly required
`to possess the compatibility with CD. However,
`the specification of DVD differs from the one of
`CD with respect to many items. Table 1 shows
`the specification of DVD and CD. The track
`pitch and the minimum pit length of DVD are
`almost half of those of CD. Therefore a focused
`spot tiny enough for reproducing DVD is
`
`required. To realize the tiny spot, a high NA
`(numerical aperture) of 0.6 objective lens and a
`red laser diode of 635 or 650 nm are employed.
`From the point of view of the compatibility with
`CD, the most serious problem is the difference
`of substrate thickness between DVD and CD.
`In this paper, first, we introduce several ways
`to realize the compatibility with CD, and then,
`we describe the twin lens type optical pick-up
`using two objective lenses which we proposed
`and developed.
`
`Table 1. Specifications of DVD and CD.
`
`DVD
`
`CD
`
`Diameter
`
`Substrate thickness
`
`Track pitch
`
`120 mm
`
`0.6 mm
`
`0.74 ~tm
`
`120 mm
`
`1.2 mm
`
`1.6 p.m
`
`Minimum pit length
`
`0.40 ~tm
`
`0.834 I.tm
`
`Wavelength
`
`635 / 650 nm
`
`780 nm
`
`Numerical aperture
`of objective lens
`
`0.60
`
`0.45
`
`Data capacity
`
`4.7 GB
`
`0.65 GB
`
`2. METHOD OF COMPATIBILITY WITH
`CD
`The laser beam transmitted through the
`transparent substrate of disc must be focused
`into a diffraction limit spot size in optical disc
`systems. As for the general pick-ups, aberration
`correction is given to the objective lens so that
`the spherical aberration is reduced to a minimum
`for each disc. Therefore as shown in Fig. 1, the
`laser beam cannot be completely focused if the
`thickness of the disc to be played is different
`from the one of that the objective lens can
`
`Manuscript received June 10, 1996
`
`0098 3063/96 $04.00 ¢ 1996 IEEE
`
`LG Electronics, Inc. et al.
`EXHIBIT 1003
`IPR Petition for
`U.S. Patent No. RE43,106
`
`

`

`Shinoda, et al.: Optical Pick-Up for DVD
`
`809
`
`Completely focused
`
`0.6 mm ~@~
`
`Objective lens for DVD
`Spherical aberration caused by substrate.
`
`Fig. 1.
`
`correct spherical aberration.
`To avoid such a problem, several methods to
`reduce the spherical aberration and possess the
`compatibility with CD are proposed by several
`manufactures as follows:
`(1) two optical pick-ups; one is for DVD and
`the other for CD,
`(2) two objective lenses in one lens actuator
`as shown in Fig. 2 (A),
`(3) two focal points using a holographic
`optical element as shown in Fig. 2 (B) [11,
`(4) aberration reduction by decreasing
`numerical aperture during CD reproduction as
`shown in Fig. 2 (C).
`In method (2), objective lenses for DVD and
`CD compensate the spherical aberration caused
`by the substrate thickness of DVD and CD
`respectively. Therefore each lens results in an
`optimum focused Spot without the extra
`spherical aberration for each type of disc. In
`method (3), the holographic surface generates
`the zeroth order diffracted laser beam for DVD
`reproduction and the +lst order diffracted laser
`beam for CD reproduction. In addition, the
`holographic surface is designed so that the
`spherical aberration of the +lst order diffracted
`
`Laser beam
`
`Objective lens
`for DVD
`
`laser beam through the objective lens and a CD
`substrate is a minimum. In method (4), a liquid
`crystal plate (LCP) is used as a laser beam
`shutter. In CD reproduction a marginal part of
`the laser beam is cut by the LCP. Therefore the
`NA of the objective lens decreases effectively
`and the spherical aberration becomes small
`enough to reproduce CD.
`
`3. TWIN LENS TYPE OPTICAL PICK-UP
`3.1 Background of Twin Lens Actuator
`An optical pick-up generally incorporates an
`objective lens actuator to keep the disc rotating
`in the right position without disc-face runout or
`off-track. The actuator controls movement of the
`objective lens both in the focusing and tracking
`directions. We developed the sliding and rotary
`type objective lens actuator as shown in Fig. 3
`and applied it to various kinds of optical
`pick-ups so far [2]. For focusing control,
`upward-downward sliding is made along the
`shaft, and for tracking control, rotation is made
`around the shaft.
`
`Shaft
`
`Laser beam
`
`Fig. 3. Sliding and rotary type objective lens actuator.
`
`Lens holder
`
`1.2 mm
`
`Objective lens
`for CD
`
`(A) Twin lens
`
`Holographic
`surface
`(B) Dual focal points
`
`Fig. 2. Methods for possessing the compatibility with CD.
`
`(C) NA conversion
`
`

`

`810
`
`IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, AUGUST 1996
`
`This technique of the sliding and rotary type
`lens actuator is easily utilized to realize a
`construction which allows the lens holder
`including two objective lenses to rotate around
`the shaft to select the applicable lens. The
`objective lens switching motion is achieved by
`sending a pulsed current to the tracking coils.
`Therefore this construction is an improved
`version of the existing sliding and rotary type
`actuator.
`
`3.2 General specifications
`We employed the method of using two
`objective lenses in one lens actuator for the
`compatibility with CD. Table 2 shows the
`specifications of the newly developed optical
`pick-up named "Twin lens type optical pick-up".
`Figure 4 shows the newly developed objective
`lens actuator and optical path layout. Figure 5
`shows the external view of the twin lens type
`optical pick-up.
`
`Table 2. Specifications of the twin lens type optical pick-up.
`Items
`Specifications
`
`Laser diode Wavelength
`
`635 / 650 nm
`
`Objective NA for DVD
`lens
`
`NA for CD
`¯ Focusing
`
`Error
`detection
`
`Tracking
`
`Photo diode
`
`Actuator type
`
`Sensitivity at 200 Hz
`
`I Fundamental resonance
`
`First natural resonance
`
`Objective lens
`for CD
`
`Laser diode
`
`0.60
`
`0.38
`Astigmatism method
`
`Differential phase detection
`
`3 beam method
`
`with pre-amplifier
`Sliding and rotary
`
`Focusing
`
`Tracking
`
`0.028 mm / V 0.022 mm / V
`14 Hz
`30 Hz
`
`> 4- 25 kHz
`
`> + 25 kHz
`
`Shaft
`
`plate
`
`~Objective lens
`for DVD
`Focusing coil
`
`Fig. 5. External view of the twin lens type optical pick-up.
`
`3.3 Objective Lens Actuator
`The objective lens actuator we newly
`developed is an improved version of the existing
`sliding and rotary type actuator, so its
`construction and function is almost the same as
`the conventional single lens type actuator. The
`lens positioning mechanism using magnetism
`for the lens switching motion and the tracking
`motion is newly employed in this actuator. This
`mechanism consists of four tracking magnets,
`two tracking coils, and two iron plates as shown
`in Fig. 6. When DVD or CD is reproduced, one
`pair of tracking magnets opposing to the
`tracking coils is used for the tracking motion.
`The iron plates are provided within the lens
`holder located opposing to the tracking magnets.
`The lens switching motion is achieved by
`sending a pulsed current to the tracking coils.
`After the switching motion the other pair of
`tracking magnets opposing to the tracking coils
`
`i
`
`i op uot.i
`
`Objective lens
`for DVD
`
`Objective lens
`for CD
`
`Mirror
`Collimator lens
`Half mirror
`¯ Photo diode
`
`Fig. 4.
`
`Twin lens actuator and optical path layout.
`
`Tracking coil
`Fig. 6. Construction of twin lens actuator and mechanism
`of lens switching motion.
`
`magnet
`
`

`

`Shinoda, et al.: Optical Pick-Up for DVD
`
`811
`
`is used for the tracking motion. The magnetic
`supporting mechanism using the iron plates also
`acts for the focusing direction.
`We evaluated the frequency characteristics of
`this twin lens actuator and obtained the results
`that higher order resonance frequencies are
`greater than 25 kHz for both the focusing and
`the tracking direction as shown in Fig. 7. No
`differences regarding the objective lens between
`DVD and CD appeared in the frequency
`characteristics. In addition, force constants for
`both direction are so high to be applicable for
`CD 8X.
`The construction of the lens actuator can
`make its tilt angles initially adjustable for the
`tangential and the radial direction of discs, so
`that the focused spot can maintain the high
`quality in spite of using the high NA objective
`lens.
`
`’’" I ’’" I ’’" I ’’’" I ’ ""
`
`0
`
`,-. -40
`
`o~
`
`-80
`
`-120
`
`-160
`
`o
`
`-90 v
`,D
`
`-180
`
`-270
`
`...1
`
`! . ..1 I . .11 I . ... I . t
`10 100 I k 10 k 40 k
`Frequency (Hz)
`(A) Focusing direction
`
`.....
`
`i ’’" I ’’" I ....
`
`! ’’’
`
`.-. -4O
`
`.4
`
`-80
`
`-120
`
`-160
`
`Gain
`
`0
`
`-90
`
`-180 ~,
`
`-270
`
`I I II
`
`1
`
`l
`10
`
`!
`. , 11 I i i ii [ J ill |
`10 k 40 k
`1 k
`100
`Frequency (Hz)
`
`(B) Tracking direction
`
`3.4 Optics
`A red laser diode with a wavelength of 635
`nm or 650 nm is employed in this optical
`pick-up. As for CD, we employed the objective
`lens with a NA of 0.38 which is smaller than the
`conventional one of 0.45, since the wavelength
`is shorter than that of conventional CD pick-ups
`of 780 nm. The focused spot diameters of 0.9
`~m (at e"z) using the objective lens for DVD and
`1.3 ~tm using the lens for CD are obtained.
`Two types of tracking error detection are
`available. The three beam method is used for
`CD reproduction. The differential phase
`detection is used for not only DVD reproduction
`but also CD reproduction. The astigmatism
`method is used as the focusing detection for
`both discs.
`In order to secure the reproduced signal
`quality, we employed the photo-diode with
`pre-amplifiers whose cut-off frequency is larger
`than 15 MHz.
`
`3.5 Reproduced Signals
`Figure 8 shows the eye-pattern signals
`reproduced from DVD and CD. Figure 9 shows
`the jitter characteristics of DVD against disc tilt
`angles. The bottom jitter is 8 %, and the tilt
`margins are about _+ 0.8* for the radial direction
`and about + 0.65 ° for the tangential direction.
`
`(A) DVD
`
`(B) CD
`
`Fig. 7. Frequency characteristics of twin lens actuator.
`
`Fig. 8. Eye-pattern signals reproduced from DVD and CD.
`
`

`

`812
`
`IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, AUGUST 1996
`
`-’ ’’ ’I’ ’’’ I’ ’’ ’I’’ ’’
`
`BIOGRAPHY
`
`25
`
`20
`
`25
`
`20
`
`10
`
`n=2
`
`i
`
`Hi
`
`II
`
`I i i
`
`I I I i I i i , I | iiiilil i i
`-0.5 0 +0.5 +1.0
`Tangential tilt angle (deg)
`
`i I I I I i I I I i I I I I I If # i i I I
`
`n=2
`
`i
`
`i
`
`m
`
`I.llll
`,,,,i ,,, , i,,, ,I,, ,,
`5
`0
`+0.5
`+1.0
`-1.0
`-0.5
`Radial tilt angle (deg)
`
`Fig: 9. Jitter characteristics of DVD against disc tilt angles.
`
`4. CONCLUSION
`We introduced the methods of CD
`compatibility proposed so far in optical pick-ups
`for DVD. In our model, we employed the twin
`lens type optical pick-up. This pick-up
`incorporates the sliding and rotary type
`objective lens actuator, on which two objective
`lenses are mounted. Each objective lens resulted
`in an optimum focused spot for each type of
`disc, as well as clear eye patterns in the
`reproduced signals.
`
`References
`
`[1] Y. Komma, S. Nishino, and S. Mizuno; OPTICAL
`REVIEW vol. 1, No. 1(1994) 27
`[2] K. Kime, H. Hashimoto, N. Egusa, and S. Sakabe;
`Bull. Japan Soc. of Prec. Engg. vol.22, No. 2(1988)
`133
`
`Masahisa Shinoda received his B. S. and M. S. degrees
`
`in physics from Osaka University in 1979 and 1981,
`
`respectively. He then joined Mitsubishi Electric
`
`Corporation, where he is currently engaged in
`
`development on optical design for optical disk memories.
`He is a member of the Japan Society of Applied Physics,
`
`the Optical Society of Japan, and the Institute of
`
`Electronics, Information and Communication Engineers of
`
`Japan.
`
`Keiji Nakamura received his B. S. and M. S. degrees in
`precision engineering from Kyoto University in 1983 and
`1985, respectively. He then joined Mitsubishi Electric
`Corporation, where he is currently engaged in
`development on optical pick-ups, He is a member of the
`Japan Society of Mechanical Engineering and the Japan
`Society of Precision Engineering.
`
`Mitoru Yabe received his B. S. degree from Saitama
`University in 1986. He then joined Mitsubishi Electric
`Corporation, where he is currently engaged in
`development on mechanical design for optical pick-ups.
`
`Norihiro Watanabe received his B. S. degree from Osaka
`Prefecture University in 1992. He then joined Mitsubishi
`Electric Corporation, where he is engaged in development
`on mechanical design for optical pick-ups.
`
`Takuma Satoh received his B. S. degree from Hosei
`University in 1985. He then joined Mitsubishi Electric
`Corporation, where he is currently engaged in
`development for optical pick-ups.
`
`Nobuald Hirai received his B. S. and M. S. degrees from
`Tohoku University in 1989 and 1991, respectively. He
`then joined Mitsubishi Electric Corporation, where he is
`currently engaged in development of the optical head for
`optical disk memories.
`
`Teruo Fujita received his B.S., M.S., and Ph.D. degrees
`in electronic engineering from Osaka University in 1978,
`1980, and 1983, respectively. At the graduate school he
`studied micro-optics and a fabrication technique of a
`micro Fresnel lenses using electron-beam lithography,
`After graduating, he joined the ConsUmer Electronics
`Development Laboratory, Mitsubishi Electronic
`Corporation, and is now engaged in optical head
`
`

`

`Shinoda, et al.: Optical Pick-Up for DVD
`
`813
`
`development. He received the Paper Award in 1983 from
`the Institute of Electronics, Information, and
`Communication Engineers of Japan. He is a member of
`the Japan Society of Applied Physics, the Institute of
`Electronics, Information and Communication Engineers
`of Japan, and SPIE.
`
`Kenjiro Kime received his B. S. degree from Ehime
`University in 1972. He then joined Mitsubishi Electric
`Corporation. In 1992, he received his Ph. D. from Osaka
`Prefecture University. He received the prize of the
`Director General of the Japanese Patent Office in 1989
`and received the encouragement prize of Takayanagi
`Foundation for Electronics Science and Technology in
`1992. He is now the manager of engineering department
`at the Video Disc Business Development Center. He is a
`member of the Japan Society of Applied Physics and the
`Optical Society of Japan.
`
`Yoshinobu Ishida received his B. S. and M. S. degrees in
`electronics engineering from Osaka University in 1970
`and 1972, respectively. He then joined Mitsubishi Electric
`Corporation, where he has been engaged in development
`on digital audio/video equipments. He is now the manager
`of development department at the Video Disc Business
`Development Center. He is a fellow of AES and a
`member of IEEE,’SMPTE, and the Institute of
`Electronics, Information and Communication Engineers
`of Japan.
`
`