Introduction to Optoelectronics Optical storage (2)

1
Introduction to OptoelectronicsOptical storage
(2)

• Prof. Katsuaki Sato

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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.

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Spot size at the focal point

• Numerical aperture of lens
• NAnsina
• d0.6?/NA
• CD-ROM NA0.6
  ?780nm?d780nmDVD ?650nm?d650nmBD NA0.85
  ?405nm?d285nmHD-DVD NA0.6
  ?405nm?d405nm

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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, DVDRW, BD,
  HD-DVD
• Magneto-optical MO, GIGAMO, MD, Hi-MD, AS-MO,
  iD-Photo
• Holographic memory, Hole-burning memory

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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, DVDRW
• 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

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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

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Increase of Areal Density in Optical Disks
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Different Disks
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CD-ROM

• Polycarbonate substratesn1.55
• ?780nm ??503nm (wavelength in the substrate)
• Pit depth110nm ¼wavelength
• Phase difference in reflectionpDestructive
  addition of reflected beams

http//www.infonet.co.jp/ueyama/ip/multimedia/cd.h
tml
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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
http//www.infonet.co.jp/ueyama/ip/multimedia/cd.h
tml
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CD-RW

• Phase change
• Crystalline and
• amorphous

http//www.cds21solutions.org/main/osj/j/cdrw/rw_p
hase.html
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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 levelHeating above Tcr?slow cool?crystalline
• DVD-RAM GeSbTe based alloy
• DVDRW Ag-InSbTe based alloy

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Recording and erasing

• Rapid coolingamorphous?low reflectivity
• Slow cooling below Tmcrystalline?high
  reflectivity

http//www.cds21solutions.org/main/osj/j/cdrw/rw_p
hase.html
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Crystalline and amorphous
recorded amorphous
Initialcrystalline
R high
R low
Record
Erase
laser spot
recorded mark
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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)

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Radial distribution function (RDF)

• G(r) Probability to find a neighboring atom at a
  distance of r.

Calculated
experiment
http//cmt.dur.ac.uk/sjc/thesis/thesis/node79.html
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CD-R

• Organic dye is used
• Thermal decomposition
• Deformation of substrate by heat
• Work as a pit

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DVD Family
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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 repeatability10,000,000 times
• Complicated optical head (Polarization detection)
• Novel inventions such as MSR, MAMMOS, DWDD are
  realized as commercial products

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Magneto-optical (MO) Recording

• RecordingThermomagnetic recording
• Magnetic recording using laser irradiation
• Reading out Magneto-optical effect
• Magnetically induced polarization state
• MO disk, MD(Minidisk)
• High rewritabilitymore than 107 times
• Complex polarization optics
• New magnetic concepts MSR, MAMMOS and DWDD

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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 5MO disk (650MB)
• 1889 Commercial appearance of 3.5 MO
  disk(128MB)
• 1991 Aratani(Sony) MSR
• 1992 Sony MD
• 1997 Sanyo ASMO(5 6GBL/G, MFM/MSR) standard
• 1998 Fujitsu GIGAMO(3.5 1.3GB)
• 2000 Sanyo, Maxell iD-Photo(5cmf730MB)
• 2004 Sony Hi-MD

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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)
Land
Groove
Resin
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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
External field
Coil
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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
T
Tcomp
Tc
Tb
RT
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Amorphous TbFeCo Film
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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

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Shape of Recorded Marks
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MO recording How to read

• Magneto-optical conversion of magnetic signal to
  electric signal

D1
-
LD
D2
Differential detection
Polarized Beam Splitter
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Structure of MO Head
Bias field coil
Recorded marks
Track pitch
Focusing lens
Rotation of polarization
MO film
Beam splitter
Half wave-plate
mirror
lens
PBS (polarizing beam splitter)
LD
Laser diode
Photo-detector
PDphotodiode
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Advances in MO recording

1. Super resolution
2. MSR
3. MAMMOS/DWDD
4. Use of Blue Lasers
5. Near field
6. SIL
7. Super-RENS (AgOx)

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MSR(Magnetically induced super-resolution)

• Resolution is determined by diffraction limit
• d0.6?/NA, where NAn sin a
• 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

d
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Illustration of 3 kinds of MSR
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AS-MO standard
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iD-Photo specification
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MAMMOS(magnetic amplification MO system)
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Super-RENSsuper-resolution near-field system

• AgOx filmdecomposition and precipitation of Ag
• Scattering center?near field
• Ag plasmon?enhancement
• reversible
• Applicable to both phase-change and MO recording

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?????
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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
  NA0.85 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 pitch0.34?m and mark
  length0.29?m in 35?m two-layered
  disk(capacity27GB) was succeeded using NA0.65
  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.

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Read/Write using Blue-violet LD and SIL (solid
immersion lens)
NA1.5 405nm 80nm mark 40GB
SILhead
405nm LD
I. Ichimura et. al. (Sony), ISOM2000 FrM01
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SIL (solid immersion lens)
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Optical recording using SIL
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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