Introduction to OptoelectronicsOptical storage (2)

Prof. Katsuaki Sato

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

Different Disks

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

Amorphous TbFeCo Film

TM(Fe,C

o)

TM(Fe,C

o)

R(Tb)

R(Tb)

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

Shape of Recorded Marks

(a) LIM (light intensity modulation)

(b) MFM (magnetic field modulation)

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

Illustration of 3 kinds of MSR

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+

MAMMOSMAMMOS(magnetic amplification MO system)(magnetic amplification MO system)

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

SIL (solid immersion lens)

Optical recording using SIL

Hybrid Recording

H. Saga et al. DigestMORIS/APDSC2000, TuE-05, p.92.

405nmLD

TbFeCodisk

ReadoutMR head

Recording head(SIL)

Achieved 60Gbit/in2