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Wykê - Ang.10 (Tryb Zgodnoÿci)

The document discusses optical storage technologies. It describes how optical storage uses light to read recorded information and how factors like wavelength and numerical aperture determine the minimum spot size and thus storage density. There are three main types of optical storage: read-only, write-once, and rewritable. Different physical mechanisms like pit formation, dye decomposition, and phase change allow information to be recorded on optical disks. Newer technologies continue to improve areal density by using shorter wavelengths and higher numerical apertures.

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49 views15 pages

Wykê - Ang.10 (Tryb Zgodnoÿci)

The document discusses optical storage technologies. It describes how optical storage uses light to read recorded information and how factors like wavelength and numerical aperture determine the minimum spot size and thus storage density. There are three main types of optical storage: read-only, write-once, and rewritable. Different physical mechanisms like pit formation, dye decomposition, and phase change allow information to be recorded on optical disks. Newer technologies continue to improve areal density by using shorter wavelengths and higher numerical apertures.

Uploaded by

DeepaliChudipuria
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Optoelectronic devices

Dr ing. Janusz Rzepka


Optical storage

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/NA
CD-ROM: NA=0.6
λ=780nm→d=780nm α
DVD: λ=650nm→d=650nm
BD: NA=0.85
λ=405nm→d=285nm
HD-DVD: NA=0.6
λ=405nm→d=405nm spot size d

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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, 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 paramagnetic
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

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Increase of Areal Density in Optical Disks

Optical discs
density

Optical disk

MO

Hard disk

Different Disks

CD-ROM
• Polycarbonate substrates：
n=1.55
• λ=780nm →λ’=503nm
(wavelength in the substrate)
• Pit depth:110nm ~
¼wavelength
• Phase difference in reflection
π：Destructive addition of
reflected beams
• Substrate 1.2 mm

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

amorphous

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

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Crystalline and amorphous

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