Dual focus optical head for O. 6m and 1.2m disks
`
`Yoshiaki Komma, Kenichi Kasazumi, Seiji Nishino and Sadao Mizuno
`
`Materials and Devices Lab., Matsushita Electric Industrial Co., Ltd.
`1006 Kadoma, Kadoma-shi, Osaka 571, Japan
`Telephone: +81-6-906-4396 Facsimile: +81-6-906-5693.
`
`Yoshihiro Kanda
`
`Disc Engineering Dep’t
`1-4 Hatsuo-cho, Kadoma-shi, Osaka 571, Japan
`
`Hideki Hayashi
`
`Audio Video Information Technology Lab.
`1006 Kadoma, Kadoma-shi, Osaka 571, Japan
`
`ABSTRACT
`
`We propose a dual focus optical head with a holographic optical element (HOE)
`which compensates for spherical aberration, allowing it to read both 0.6mm disks and
`1.2mm disks. The thin disk is read using transmitted light and the thick disk is read
`using +lst order diffracted light of the blazed HOE. The characteristics of the
`focused spot, the servo signal detection and the signal of the Compact Disc indicate
`that both disks can be read by the dual focus optical head.
`
`I. INTRODUCTION
`
`Shorter light source wavelength and
`larger numerical aperture (NA) are
`important optical factors in optical
`pickup heads for realizing high density
`optical storage. ’
`
`Laser diodes (LDs) with wavelengths
`in the 680nm range are now commercially
`available. Optical disk systems using a
`680nm LD and a large NA lens have
`recently been the focus of considerable
`research activity. Large NA lenses,
`however, are extremely sensitive to
`aberrations in the disk substrate when
`the disk tilts. Accordingly, a thin
`(0.6mm) substrate is preferable’ for
`reading high density optical disks with a
`large NA (=0.6) lens from the viewpoint of
`
`282 /SPIE Vol. 2338 Optical Data Storage (1994)
`
`High density disk
`
`[Short wavelength]
`
`Large
`
`Thin
`
`NA
`Aberration
`caused by disk tilt
`~ubstrate (0.6rnm)
`
`)
`
`i A berration
`
`caused by difference
`in thickness
`Conventional disk
`Thick substrate (1.2mm)
`
`Fig.l. Background of the dual
`focus optical head.
`
`LG Electronics, !nc. et a!°
`EXHIBIT 1005
`IPR Petition for
`U.S. Patent No. RE43,106
`
`0-8194-1671-I/94/$6.00
`
`

`
`tolerance to disk tilt (Fig. l).
`
`Before reading conventional 1.2ram disks using the same optical head, spherical
`aberration, which is caused by disk thickness difference, must be corrected.
`
`We propose a dual focus optical head with a holographic optical element (HOE)
`which compensates for spherical aberration, allowing the reading of both 0.6ram disks
`and 1.2mm disks.
`
`2. THE OPTICS OF THE DUAL FOCUS OPTICAL HEAD
`
`2.1. The functions of the HOE
`
`Figure 2 shows the functions of the
`HOE in the dual focus optical head.
`
`The thin disk is read using the HOE’s
`transmitted zero order light and the
`thick disk is read using +ist order
`diffracted light.
`
`(a) For 0.6mm disks, the zero order
`diffraction (=transmittance) is utilized
`on both the outgoing and the incoming
`path. Returning light from the disk is
`also a plane wave when the transmitted
`light is focused on the 0.6ram disk. In
`this case the HOE simply plays the role
`of a parallel plate. The characteristics
`of the signal being read from high
`density thin disks are unaffected by
`misalignment of the HOE or deviation of
`the wavelength from the set value.
`
`(b) For 1.2mm disks, the +Ist order
`diffracted light is converged and
`reflected on the disk, and the *lst order
`diffracted light on the incoming path is
`utilized to obtain signals. The light
`beam is a plane wave when the +Ist order
`diffracted light on the outgoing path is
`focused on the 1.2mm disk. In this case,
`the HOE functions as a lens in
`combination with the objective lens.
`
`Transm 06mm sk
`
`T Outgoing path
`
`light
`
`(a)
`
`~ Incoming path
`
`,I
`
`,.~ ~. l~2mm disk
`The + 1st order
`diffracted light ~@@.~
`
`(b) ~
`
`~ Blazed HOE
`
`Fig.2. The functions of the
`HOE in the dual focus optical
`head.
`(a) For 0.6ram disks, the zero
`order diffraction (=trans-
`mittance) is utilized on both
`the outgoing and the incoming
`path.
`(b) For 1.2ram disks, the *lst
`order diffraction is utilized
`on both the outgoing and the
`incoming path.
`
`5PIE Vol. 2338 Optical Data 5torage (1994) /283
`
`

`
`2.2. Optical configuration of the dual
`focus optical head
`
`Figure 3 shows the optical configura-
`tion of the dual focus optical head.
`
`As mentioned above, the returning
`beam utilized to read signals is a plane
`wave in focused condition for both disks.
`Servo signals are obtained using only
`one photodetector (PD). As explained
`later, for the +Ist order diffraction
`light the HOE also functions as an
`aperture. The center of the HOE must be
`near the optical axis, and so the HOE is
`combined with the objective lens.
`
`This configuration requires only the
`addition of a HOE, which is a small, light
`and inexpensive element.
`
`2.3. The mask pattern of the HOE
`
`Figure 4 shows the mask pattern of
`the HOE. The gratings are drawn every
`two pitches in this figure. The HOE is
`increase the diffraction
`blazed to
`efficiencies of the +lst order and the
`zero order. The HOE pattern is designed
`as a concave lens. The focal point of the
`÷lst order diffracted light is farther
`from the objective lens than that of the
`zero order diffracted light. Accord-
`ingly, when signals are read with one of
`the diffracted beams, the other does not
`interfere with it.
`
`2.4. Blazing of the HOE
`
`Figure 5 shows cross sectional views
`of the HOE, which has a staircase
`structure with 4 steps of equal heig~t.~
`
`(a) In the central region, the optical
`path length difference between the zero
`order and the ÷ist order diffracted light
`is equal to N ~ /4 at the boundaries of
`each step, where ). is wavelength of the
`
`284 /SPIE Vol. 2338 Optical Data Storage (I 994)
`
`0.6mm disk
`
`ht
`
`1.2mm disk
`\
`
`Objective lens
`NA=O.6
`
`The + 1st order
`diffracted light
`
`Blazed
`
`Lasel diode
`), =680nm
`
`Quadrant
`photodetector
`
`Fig.3. Optical configuration of
`the dual focus optical head.
`
`500,u. m
`< >
`
`i(a) Central region.~
`
`(b) Peripheral (b) Peripheral
`region, region.
`
`Fig.4. Mask pattern of the
`HOE.
`The gratings are drawn every
`two pitches in this figure.
`
`

`
`LD, and N is an integer. The NA of the
`central region was set to 0.4. Both the
`zero and the +lst order diffraction effi-
`ciencies are designed to be 38~. These
`conditions have not been optimized. The
`minimum pitch of this region is 22.7z m.
`
`(b) In the peripheral region, the
`ratio of the step width wl/w2 decreases
`towards the edge part. This shape ls an
`approximation of a blazed grating with
`smaller gradient. The minimum pitch of
`this region is 13.5~ m.
`
`Figure 6 shows the distribution of
`the zero and the +lst diffraction effi-
`ciencies. The horizontal axis represents
`the distance from optical axis, regarding
`NA as the scale reference. The +lst order
`diffraction efficiency decreases gradu-
`ally towards the outer part of the HOE.
`This reduces the effective NA for the
`thick disk. The zero order transmittance
`of the HOE gradually increases towards
`the outer part of the HOE.
`
`As a result, the light power of the
`zero order diffracted light is larger
`than that of the +lst order diffracted
`light.
`
`100
`
`r-
`
`0
`
`Transmi~ance ,"
`
`/
`
`38%
`
`+1st order diffraction
`efficiency
`
`I I
`0.4
`0.6
`
`NA
`
`Fig.6. Designed diffraction
`efficiencies of the HOE.
`Diffraction efficiencies are
`schematically shown.
`
`h0
`
`0
`
`w! w2 w2 w!
`
`(a)
`
`(b)
`
`Fig.5. Cross sectional view of
`the blazed HOE.
`(a) The central region. Optical
`path length difference is
`equal to N L/4 at the bound-
`aries of the steps.
`(L: wavelength, N: integer)
`(b) The peripheral region:
`wl<w2.
`
`Photo mask
`
`Photo - - .\\, .\, ,\\, .\\. ,\.
`
`resist [ I
`
`UV exposure
`
`~, Development
`
`I
`
`~ Etching and
`resist removal
`
`Mask alignment and
`UV exposure
`
`Development
`
`~, Etching and
`resist removal
`
`Fig.7. Fabrication process of
`the blazed HOE.
`
`SPIE Vol. 2338 Optical Data Storage (1994) / 285
`
`

`
`Figure 7 shows the process of fabrication of the blazed HOE with a 4-step
`staircase structure. The process consists of double photolithography and etch
`procedures.
`
`3.!o Optica! characteristics
`
`3. EXPER]MENTAL RESULTS
`
`In this optical system, it may appear at first glance that extraneous diffracted
`light would interfere with the focused spot and/or servo signal characteristics. The
`optical characteristics of the dual focus optical head were examined using mirror
`disks of 0.6ram and 1.2mm thicknesses. The wavelength of the laser diode was 680nm.
`The beam divergences parallel and perpendicular to the active layer were 9.7 degrees
`and 35.8 degrees respectively. The NA of the objective lens was 0.6 and the lateral
`magnification of the focusing optics from the LD to the disk was 1/9.375.
`
`3. l. 1 Quality of the focused spot
`
`Figure 8 shows the intensity distri-
`bution of the focused spot on the disks.
`The directions X and Y respectively
`describe the directions parallel and
`perpendicular to the active layer of the
`LD.
`
`(a) For the 0.6mm disk, the focused
`spot diameters were 0.59 mm at full width
`half maximum (FWHM) in both X and Y
`directions, identical to the focused spot
`diameters without the HOE.
`
`Because the transmittance of the HOE
`changes gradually, the intensities of the
`first secondary maxima were less than 3%
`of that of the principal maximum, in spite
`of the transmittance of the central
`region being less than that in the
`peripheral region.
`
`(b) For the 1.2mm disk, the focused
`spot diameters were 0.70# m and 0.69~ m
`in the X and Y direction respectively,
`which should be small enough to read
`signals from conventional disks. Because
`the ratio of the step width wl/w2
`decreases in the peripheral region of
`the HOE, the effective NA for the 1.2mm
`disk was reduced, though the effective
`
`1.0
`
`o~
`
`.__. =0.5
`
`z 0
`
`"
`
`1=
`
`1.0i
`r
`L
`
`N e-u.o }
`"-’ID
`E.--q
`
`,t
`
`"~
`
`’ i
`
`0.59
`~F" /J-m~’
`
`’,
`
`-1 0 +1
`Position ( # m)
`in X direction
`
`Z
`
`(a)
`
`. . . ,
`
`-1 0 +1
`Position ( p. m)
`in Y direction
`
`,"!
`
`[
`
`1.0
`
`°~
`¯ x~ >,,
`<__ 0.70i o~^ ,-
`
`N t" U,D
`
`O"
`
`"
`-1 0 +1
`Position ( p. rn)
`in X direction
`
`E.-=
`z
`
`(b)
`
`~i 0.691
`<"- "~
`
`!,
`
`4
`
`-1 0 +1
`PosRion ( #. m)
`in Y direction
`
`Fig.& Intensity distribution
`of focused spot on the disks.
`(a) For the 0.6ira disk. Spot
`diameters (FWHM) are 0.59p m
`on both X and Y direction.
`(b) For the 1.2ram disk. Spot
`diameters (FWHH) are 0.70z m
`and 0.69z m in the X and Y
`direction, respectively.
`
`286 /SPIE Vol. 2338 Optical Data Storage (1994)
`
`

`
`Focused pomt \~
`
`0.7~
`o-~ ~
`._N ~
`,..~._
`-~o~ Or
`~ The lens is too
`~w
`zU-
`L far fromthe disk
`_0.7~ ~ , ~ ~
`-150
`Defocus( ,u m)
`
`(a)
`
`",
`~i
`
`0
`
`\"& i
`
`’~
`
`0
`
`"0
`.~’~
`"~._~
`E~ Oi <
`_o~
`i The lens is too
`far from the disk
`:
`
`Focused point
`
`’
`
`-150
`Defocus( ,u m)
`
`O.7r
`
`~
`
`/-
`
`-0.7 ; ;
`
`(b)
`
`NA must be reduced further to create
`tolerance to disk tilt.
`
`The Intensities of the first second-
`ary maxima were also less than 3% of that
`of the principal maximum, and this should
`be small enough to read the disks.
`
`No influence of extraneous dif-
`fracted light on the focused spots was
`observed in either disk.
`
`3.1.2. Focusing error signals
`
`Focusing error (FE) signals were
`measured by observing the beam patterns
`on the photodetector plane using a
`3
`charge coupled device (CCD) camera.
`
`FE signals were successfully ob-
`tained using the astigma method for both
`disks, as shown in Fig. 9. The disks were
`moved in 1 ~ m steps in the optical axis
`direction. They were normalized by
`maximum incident light power within the
`detecting region. Near the focused
`point, the FE signals did not suffer from
`extraneous diffracted light.
`
`Fig.9. Measured focusing error
`signals. FE signals of the dual
`focus optical head were mea-
`sured using a CCD camera and
`normalized by maximum inci-
`dent light power within the
`detecting region.
`(a) For the 0.6ram disk.
`(b) For the 1.2mm disk.
`
`(a) When the objective lens moves toward the thin disk, a spurious FE signal
`caused by the .lst order diffraction appears before the FE signal made with the zero
`order diffraction. The spurious FE signal was, however, small enough to capture the
`focusing servo operation using the FE threshold value since the *lst order
`diffracted light power was smaller than the zero order light power.
`
`(b) When the objective lens moves toward the thick disk, the FE signal made by
`+lst order diffraction appears first, since the +lst order diffracted light has a
`focusing point farther from the objective lens than the zero order diffracted light.
`
`Therefore, zero order diffraction does not disturb the focusing servo operation.
`Using a smaller FE threshold value than the value for the thin disk, stable focusing
`servo control was realized even though the FE signal amplitude for the thick disk
`was smaller than that for the thin disk.
`
`5PIE Vol. 2338 Optical Data 5torage (1994) /287
`
`

`
`3.2. Readout signal
`
`Stable servo operation characteris-
`tics were obtained, and readout signal of
`the Compact Disc (CD) was also success-
`fully obtained.
`
`The eye pattern of the signal of the
`CD is shown in Fig. 10. The standard devi-
`ation of the jitter for the 3T signal was
`18ns, which is small enough for practical
`use.
`
`Fig.10. The eye pattern of the
`signal of the CD.
`
`4. CONCLUSIONS
`
`A novel dual focus optical head with a HOE which reads both 0.6ram and 1.2ram disks
`is proposed. The HOE compensates for the aberration caused by the difference in
`thickness. The good quality of the focused spots was experimentally confirmed. No
`influence of extraneous diffracted light was observed. FE signals were successfully
`obtained using a common PD for both the 0.6mm disk and the 1.2mm disk.
`
`The good characteristics of the servo signal detection and the signal of the CD
`indicate that both the 0.6ram disk and the 1.2ram disk can be read by the dual focus
`optical head.
`
`5. ACKNOWLEDGMENTS
`
`The authors wish to thank Y. Tanaka and H. Yamagata for lens design, Y.Watanabe,
`K. Wakabayashi and H. Ogawa for their experimental efforts, H. Yamamoto and S.
`Ohnishi for fabricating HOEs, S. Tanaka for his useful advice, and H. Taniguchi for
`his encouragement throughout.
`
`6. REFERENCES
`
`1. T. Ohta, K. Inoue, T. Ishida, Y. Gotoh and I. Satoh, "Thin injection-molded substrate
`for high density recording phase-change rewritable optical disk," Jpn. J. Appl.
`Phys., Vol. 32, pp. 5214-5218, November 1993.
`2. J. A. Cox, T. Werner, J. Lee, S. Nelson, B. Fritz and J. Bergstrom, "Diffraction
`efficiency of binary optical elements," Proceedings SPIE, Vol. 1211, pp. 116-124, 1990.
`3. S. Nishino, S. Kadowaki, Y. Komma, Y. Hori and H. Kato, "Properties of a holographic
`optical pickup head with movable single assembly optical system," Technical Digest of
`the Third Hicroopties Conference, Yokohama, 1991, pp. 234-237, October 1991.
`
`288/SPIE Vol. 2338 Optical Data Storage (1994)