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What we learn today.
• Optical storage is a storage using light for read-out of recorded information
• Record density is determined by the spot size of the light beam, which is limited by the wavelength of the light and the NA (numerical aperture) of lens.
• There are three categories of optical storage, i.e., read-only type, write-once type and rewritable type.
• Different physical phenomena are used for recording of the signal on optical disks.
Spot size at the focal point
• Numerical aperture of lens– NA=nsinα
• d=0.6λ/NACD-ROM: NA=0.6 λ=780nm→d=780nmDVD: λ=650nm→d=650nmBD: NA=0.85 λ=405nm→d=285nmHD-DVD: NA=0.6 λ=405nm→d=405nm
spot size d
α
Classification of optical storages
• Optical disk– Read only type
• CD, CD-ROM, DVD-ROM
– Recordable type• Direct read after write (Write once type)
– CD-R, DVD-R
• Rewritable (recording and erasing)– Phase change CD-RW, DVD-RAM, DVD-RW, DVD+RW, BD,
HD-DVD– Magneto-optical: MO, GIGAMO, MD, Hi-MD, AS-MO, iD-Photo
• Holographic memory, Hole-burning memory
Physical phenomena used in optical disk technology
• CD-ROM, DVD-ROM: – pit formation
• CD-R, DVD-R: – Chemical decomposition of organic dye
• CD-RW, DVD-RAM, DVD-RW, DVD+RW : – Phase change between ordered and disordered states
• MO, MD, GIGAMO, iD-Photo, HD-MD: – Magnetic phase change between ferromagnetic and para
magnetic states • Holographic memory : Photorefractive effect• Hole-burning memory: Local structure change
Characteristics of optical disk
• Removable• Large capacity, high density
– 10Gb/in2 (far less than HD(100 Gb/in2))– Aiming at 100 Gb/in2 using near-field technique
• Random accessibility– Cassette MD, VTR DVD– Shorter access than magnetic tape – Longer seek time than HD
• High reliability– Higher head clearance than HD
Increase of Areal Density in Optical Disks
T. Suzuki:113th Topical Meeting of Magn. Soc. Jpn. (2000.1) p.11
Hard disk
Optical disk
MO
CD-ROM
• Polycarbonate substrates : n=1.55• λ=780nm →λ’=503nm (wavelength in the
substrate)• Pit depth:110nm ~ ¼wavelength• Phase difference in reflectionπ : Destruc
tive addition of reflected beams
http://www.infonet.co.jp/ueyama/ip/multimedia/cd.html
CD-ROM Drive
• Focusing servo
• Tracking servo
• Optical pickup
http://www.infonet.co.jp/ueyama/ip/multimedia/cd.html
Optical detector
Cylindrical lens
PolarizationBeam Splitter
Collimating lens
Quarter wave-plate
Objective lens
TrackingServo Focusing
Servo
Grating
CD-RW
• Phase change
• Crystalline and
amorphous
http://www.cds21solutions.org/main/osj/j/cdrw/rw_phase.html
Substrate
Protectivelayers
UV coat
Land
Recording layer
Reflection layer
Printed surface
Phase change recording
• Phase change between different phases• Rewritable: As grown amorphous state is initialized
to crystalline state by annealing. Recording is performed by heating above the melting point Tm (600C) followed by quenching to amorphous state. Erasing is done by heating to Tcr(400 C) to crystallize.– High level : Heating above Tm→rapid cool→amorphou
s– Low level : Heating above Tcr→slow cool→crystallineDVD-RAM: GeSbTe based alloyDVD±RW: Ag-InSbTe based alloy
Recording and erasing
• Rapid cooling :amorphous→low reflectivity
• Slow cooling below Tmcrystalline→high reflectivity
http://www.cds21solutions.org/main/osj/j/cdrw/rw_phase.html
meltingpoint
crystalli-zationpoint
crystalline
amorphous
Energy
low reflectivity
high reflectivity
activation energy
temperature temperatureRapid cool Slow cool
time time
meltingpoint
crystalli-zationpoint
Crystalline and amorphous
Initial : crystalline recorded: amorphous
R: high R: lowRecord
Erase
laser spot
recorded mark
What is amorphous?
• Amorphous– non crystalline (disordered) state– without LRO (long range order) but with SRO (short
range order)– Atomic arrangement of liquid is frozen– Metastable state introduced by rapid cooling of liquid– Random metallic alloy, chalcogenide glass,
tetrahedral system, oxide glass– DRPHS (dense random packing of hard spheres) can
explain RDF (radial distribution function)
Radial distribution function (RDF)
• G(r): Probability to find a neighboring atom at a distance of r.
http://cmt.dur.ac.uk/sjc/thesis/thesis/node79.html
experimentCalculated
CD-R
• Organic dye is used
• Thermal decomposition
• Deformation of substrate by heat
• Work as a pit
laser beam
deformationof substrate
protective layer
PC substrate
Dye layer
PC substrate
Organic dye layer
Protective layerReflecting layer
Pre-groove
Recorded mark
CD-R
CD
Protective layer
Reflecting layer
PC substrate
Pit
DVD Family
DVD-ROM DVD-R DVD-RAM DVD-RW DVD+RW
capacity(GB)
4.7 / 9.42層 8.54
3.95 / 7.9 4.7 / 9.4 4.7/9.4 4.7/9.4
Form disk disk cartridge disk disk
Mark formation/Material/reflectivity
pit formation1L R=45-852L R=18-30
thermal deformorganic dyeR=45-85%
phase changeGeSbTe alloyR=18-30%
phase changeAgInSbTe alloyR=18-30%
phase changeAgInSbTe alloyR=18-30%
wavelength nmlens NA
650/6350.6
650/6350.6
6500.6
638/6500.6
6500.65
shortest mark size
1層 :0.42層: 0.44
0.4 0.41-0.43 0.4 0.4
track width 0.74 0.8Wobbled Land pre-bit
0.74Wobbled L/G
0.74Wobbled Land pre-bit
0.74 HF Wobbled groove
Cyclability - - 105 103-104 103-104
MO ( magneto-optical ) Recording
• Recording : Thermomagnetic (Curie point ) recording– Heat-assisted magnetic recording
• Playback : Magneto-optical effect– Rotation of linear polarization is converted to the electri
cal signal• Employed in MO, MD disks• Compatibility• High repeatability : 10,000,000 times• Complicated optical head (Polarization detectio
n )• Novel inventions such as MSR, MAMMOS, DW
DD are realized as commercial products
Magneto-optical (MO) Recording
• Recording:Thermomagnetic recordingRecording:Thermomagnetic recording– Magnetic recording using laser irradiationMagnetic recording using laser irradiation
• Reading out: Magneto-optical effectReading out: Magneto-optical effect– Magnetically induced polarization stateMagnetically induced polarization state
• MO disk, MD(Minidisk)MO disk, MD(Minidisk)• High rewritabilityHigh rewritability :: more than more than 101077 times times• Complex polarization opticsComplex polarization optics• New magnetic concepts: MSR, MAMMOS aNew magnetic concepts: MSR, MAMMOS a
nd DWDDnd DWDD
History of MO recording• 1962 Conger,Tomlinson Proposal for MO memory• 1967 Mee Fan Proposal of beam-addressable MO recording• 1971 Argard (Honeywel) MO disk using MnBi films• 1972 Suits(IBM) MO disk using EuO films• 1973 Chaudhari(IBM) Compensation point recording to a-GdCo film• 1976 Sakurai(Osaka U) Curie point recording on a-TbFe films1980 Imamura
(KDD) Code-file MO memory using a-TbFe films• 1981 Togami(NHK) TV picture recording using a-GdCo MO disk• 1988 Commercial appearance of 5”MO disk (650MB)• 1889 Commercial appearance of 3.5 ”MO disk(128MB)• 1991 Aratani(Sony) MSR• 1992 Sony MD• 1997 Sanyo ASMO(5” 6GB : L/G, MFM/MSR) standard• 1998 Fujitsu GIGAMO(3.5” 1.3GB)• 2000 Sanyo, Maxell iD-Photo(5cmφ730MB)• 2004 Sony Hi-MD
Structure of MO disk media
• MO disk structurePolycarbonatesubstrate
SiNx layer for protection and MO-enhancement
MO-recording layer(amorphous TbFeCo)
Al reflectionlayer
LandGrooveResin
• Temperature increase by focused laser beam• Magnetization is reduced when T exceeds Tc• Record bits by external field when cooling
MO recording How to record(1)
External field MO media
Temp
Laserspot
Tc
Coil
M
Tc
• Use of compensation point
writing
• Amorphous TbFeCo:
Ferrimagnet with Tcomp
• HC takes maximum at Tcomp
– Stability of small recorded marks
MO recording How to record(2)
T
M TbFeCo
Tcomp
Hc
Mtotal
RTTcTbFe,Co
Two recording modesTwo recording modes• Light intensity modulation
(LIM) : present MO– Laser light is modulated by
electrical signal– Constant magnetic field– Elliptical marks
• Magnetic field modulation (MFM) : MD, ASMO– Field modulation by
electrical signal– Constant laser intensity– Crescent-shaped marks
Modulatedlaser beam
Constantlaser beam
Constant fieldModulated field Magnetic head
(a) LIM (b) MFM
MO recording How to read
• Magneto-optical conversion of magnetic signal to electric signal
D1
D2
+
-LD
PolarizedBeamSplitter
S
N
N
S
N
S
Differentialdetection
Structure of MO Head
Laser diode
Photo-detector
Focusing lens
Half wave-platelens
Beam splitter
PBS(polarizing beam splitter)
Rotation ofpolarization
Track pitch
MO film
mirror
LD
PD=photodiode
Bias field coilRecorded marks
Advances in MO recordingAdvances in MO recording
1. Super resolution1. MSR
2. MAMMOS/DWDD
2. Use of Blue Lasers
3. Near field1. SIL
2. Super-RENS (AgOx)
• Resolution is determined by diffraction limit– d=0.6λ/NA, where NA=n sin α– Marks smaller than wavelength cannot
be resolved• Separation of recording and reading layers • Light intensity distribution is utilized
– Magnetization is transferred only at the heated region
MSR(Magnetically induced super-resolution)
α
d
AS-MO standard
LD wavelength 650 nmNA 0.6
Disk diameter 120 mmThickness 0.6 mmTrack pitch 0.6 μ m Land/Groove
Recording method MO & CAD-MSRModulation Laser pumped MFM
Signal processing PRMLbit density 0.235μ m) PR(1,1) or PR(1,2,1)
Velocity control ZCAV/ZCLVCode NRZI+ (DC supressed)
iD-Photo specification
Memory Capacity 730 MB
Surface memory density 4.6Gbit/in2 LD wavelength 650 nm
NA 0.6 Disk diameter 50.8 mm
Thickness 0.6 mm Track pitch 0.6 μ m Land/Groove
Recording method MO & CAD-MSR Modulation Pulsed laser strobe MFM bit density 0.235μ m
Signal processing, PRML PR(1,1) +Viterbi
Velocity control ZCAV Code NRZI+
Super-RENSsuper-resolution near-field system
• AgOx film : decomposition and precipitation of Ag– Scattering center→near fiel
d– Ag plasmon→enhancemen
t– reversible
• Applicable to both phase-change and MO recording
高温スポット
近接場散乱
To shorter wavelengths
• DVD-ROM: Using 405nm laser, successful play back of marks was attained with track pitch =0.26m 、 mark length =213m (capacity 25GB) using NA=0.85 lens [i]。 [i] M. Katsumura, et al.: Digest ISOM2000, Sept. 5-9, 2000, Chitose, p.
18.
• DVD-RW: Using 405nm laser, read / write of recorded marks of track pitch=0.34m and mark length=0.29m in 35m two-layered disk(capacity:27GB) was succeeded using NA=0.65 lens, achieving 33Mbps transfer rate [ii] 。[ii] T. Akiyama, M. Uno, H. Kitaura, K. Narumi, K. Nishiuchi and N. Yamada: Digest ISOM2000, Sept. 5-9, 2000, Chitose, p. 116.
Read/Write using Blue-violet LD and SIL (solid immersion lens)
405nm LD
SIL head
NA=1.5405nm80nm mark40GB
I. Ichimura et. al. (Sony), ISOM2000FrM01