1. Introduction to Optoelectronics Optical storage (2)
Prof. Katsuaki Sato

2. 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.

3. Spot size at the focal point
spot size　d α
Numerical aperture of lens
NA=nsinα
d=0.6λ/NA
CD-ROM: NA= λ=780nm→d=780nm DVD: λ=650nm→d=650nm BD: NA=0.85 　　　　λ=405nm→d=285nm HD-DVD: NA=0.6 　　　　λ=405nm→d=405nm

4. 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

5. 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 paramagnetic states
Holographic memory： Photorefractive effect
Hole-burning memory: Local structure change

6. 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

7. 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

8. Different Disks

9. CD-ROM Polycarbonate substrates：n=1.55
λ=780nm →λ’=503nm (wavelength in the substrate)
Pit depth:110nm ~ ¼wavelength
Phase difference in reflectionπ：Destructive addition of reflected beams

10. CD-ROM Drive Focusing servo Tracking servo Optical pickup
Objective lens
Tracking
Servo
Focusing
Servo
Quarter wave-plate
Collimating lens
Grating
Polarization
Beam Splitter
Cylindrical lens
Optical detector

11. CD-RW Phase change Crystalline and amorphous
Substrate
Protective
layers
UV coat
Land
Recording layer
Reflection layer
Printed surface
Phase change
Crystalline and
amorphous

12. 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 (600C) followed by quenching to amorphous state. Erasing is done by heating to Tcr(400 C) to crystallize.
High level ：Heating above Tm→rapid cool→amorphous
Low level：Heating above Tcr→slow cool→crystalline
DVD-RAM: GeSbTe based alloy
DVD±RW: Ag-InSbTe based alloy

13. Recording and erasing Rapid cooling： amorphous →low reflectivity
melting
point
crystalli-zation
crystalline
amorphous
Energy
low reflectivity
high reflectivity
activation energy
temperature
Rapid cool
Slow cool
time
Rapid cooling： amorphous →low reflectivity
Slow cooling below Tm crystalline →high reflectivity

14. Crystalline and amorphous
Initial：crystalline
recorded: amorphous
R: high
Record
R: low
Erase
laser spot
recorded mark

15. 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)

16. Radial distribution function (RDF)
G(r): Probability to find a neighboring atom at a distance of r.
Calculated
experiment

17. CD-R Organic dye is used Thermal decomposition
PC substrate
Organic dye layer
Protective layer
Reflecting layer
Pre-groove
Recorded mark
CD-R CD
Pit
Organic dye is used
Thermal decomposition
Deformation of substrate by heat
Work as a pit
laser beam
deformation
of substrate
protective layer
PC substrate
Dye layer

18. DVD Family 105 103-104 0.6 0.65 DVD-ROM DVD-R DVD-RAM DVD-RW DVD+RW
DVD-ROM
DVD-R
DVD-RAM
DVD-RW
DVD+RW
capacity
(GB)
4.7 / 9.4
2層8.54
3.95 / 7.9
4.7/9.4
Form
disk
cartridge
Mark formation/
Material/
reflectivity
pit formation
1L R=45-85
2L R=18-30
thermal deform
organic dye
R=45-85%
phase change
GeSbTe alloy
R=18-30%
AgInSbTe alloy
wavelength nm
lens NA
650/635
0.6
650
638/650
0.65
shortest mark size
１層:0.4
2層：0.44
0.4
 0.4
track width
0.74
0.8
Wobbled Land pre-bit
0.74 Wobbled L/G
0.74 HF Wobbled groove
Cyclability －
105

19. 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 electrical signal
Employed in MO, MD disks
Compatibility
High repeatability：10,000,000 times
Complicated optical head (Polarization detection）
Novel inventions such as MSR, MAMMOS, DWDD are realized as commercial products

20. Magneto-optical (MO) Recording
Recording:Thermomagnetic recording
Magnetic recording using laser irradiation
Reading out: Magneto-optical effect
Magnetically induced polarization state
MO disk, MD(Minidisk)
High rewritability：more than 107 times
Complex polarization optics
New magnetic concepts: MSR, MAMMOS and DWDD

21. 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
Commercial appearance of 5”MO disk (650MB)
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

22. Structure of MO disk media
MO disk structure
Polycarbonate
substrate
SiNx layer for
protection and
MO-enhancement
Al reflection
layer
MO-recording layer
(amorphous TbFeCo)
Groove
Land
Resin

23. MO recording How to record(1)
Temperature increase by focused laser beam
Magnetization is reduced when T exceeds Tc
Record bits by external field when cooling M Tc
Temp Tc
Laser
spot
MO media
Coil
External field

24. MO recording How to record(2)
Use of compensation point
writing
Amorphous TbFeCo:
Ferrimagnet with Tcomp
HC takes maximum at Tcomp
Stability of small recorded marks Hc M Tb
FeCo
Mtotal
Fe,Co Tb
Tcomp Tc T RT

25. Amorphous TbFeCo Film TM
(Fe,Co) R
(Tb)

26. Two 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
Modulated
laser beam
Constant
Constant field
Modulated field
Magnetic head
(a) LIM
(b) MFM

27. Shape of Recorded Marks
(a) LIM (light intensity modulation)
(b) MFM (magnetic field modulation)

28. MO recording How to read
Magneto-optical conversion of magnetic signal to electric signal D1 + - LD D2
Differential
detection
Polarized
Beam
Splitter N S S N N S

29. Structure of MO Head Focusing lens Rotation of polarization LD
Bias field coil
Recorded marks
Track pitch
Focusing lens
Rotation of
polarization
MO film
Beam splitter
mirror
Half wave-plate
lens
PBS
(polarizing beam splitter) LD
Laser diode
PD=photodiode
Photo-detector

30. Advances in MO recording
Super resolution
MSR
MAMMOS/DWDD
Use of Blue Lasers
Near field
SIL
Super-RENS (AgOx)

31. MSR (Magnetically induced super-resolution)
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 α d

32. Illustration of 3 kinds of MSR

33. AS-MO standard

34. iD-Photo specification

35. MAMMOS (magnetic amplification MO system)

36. Super-RENS super-resolution near-field system
AgOx film：decomposition and precipitation of Ag
Scattering center→near field
Ag plasmon→enhancement
reversible
Applicable to both phase-change and MO recording
高温スポット
近接場散乱

37. To shorter wavelengths
DVD-ROM: Using 405nm laser, successful play back of marks was attained with track pitch =0.26m、mark length =213m (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=0.34m and mark length=0.29m in 35m 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.

38. Read/Write using Blue-violet LD and SIL (solid immersion lens)
NA=1.5
405nm
80nm mark
40GB
ＳＩＬhead
405nm LD
I. Ichimura et. al. (Sony), ISOM2000
FrM01

39. SIL (solid immersion lens)

40. Optical recording using SIL

41. Hybrid Recording 405nm LD Recording head (SIL) Readout MR head
Achieved 60Gbit/in2
H. Saga et al. Digest
MORIS/APDSC2000, TuE-05, p.92.
TbFeCo
disk