Optical Data Storage
By
Ken Tatebe
2004.11.1
Outline
 Basic Technology
 CD: Properties and Capabilities
 DVD: Comparison to CD
 What’s makes DVD’s better?
 Comparison to other contemporary data
storage technologies
 Future directions
First Optical Storage Techniques
 Writing systems
represent the first optically
based data storage
methods, replacing oral
records.
Storage Capacity:
~ 0.01 byte / mm2
20th Century: The Compact Disc
 Patented in 1970 by James T. Russell as a
means for durable audio data storage.
 First digital optical data storage; inspired by
punch cards and magnetic storage devices.
 Mass production starts in 1985 by Philips and
Sony.
 Gives practical use to Reed – Solomon Error
Correction.
 Data Density: ~ 1Mbyte/mm2
Tracking and Error Correction
 Diagonally astigmatic lens coupled with
quadrant photocell.
 Difficult to keep focus stable across all
freq.
 1 mm surface spot size due to high NA
 Reed - Solomon correction codes
 Over-sampled polynomial using data as
coefficients
DVD: Improvements to CD
Feature
DVD
CD
Substrate Dia./Thick.
120 x 1.2 mm
120 x 1.2 mm
Sides
1 or 2
1
Layers per side
1 or 2
1
Capacity (GB)
4.7, 8.54, 9.4, 17
0.7
Track Pitch (micron)
0.74
1.6
Min pit length (micron) 0.44
0.83
Linear scan velocity
3.6 m/s
1.3 m/s
Laser wavelength
635 nm
780 nm
Numerical Aperture
.6
.45
Modulation
8 to 16
8 to 17
Spot Size
1058 nm
1733 nm
Road to the DVD
 Innovations:
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Dual layer
Increased numerical aperture
Decreased depth of focus
Signal to noise
Tracking
Data Density: ~7 MByte/mm2
Dual Layer Technology
 Benefits
 Increased durability
 Increased capacity
 Detriments
 Decreased S/N
 Decreased data density
Numerical Aperture
 NA = n sin(q/2)
 Spot size = l/NA
Depth of Focus
 DoF = l/NA2
 Determines spacing
of layers
 Affects S/N; places an
upper limit on NA.
Noise Tolerances
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Coma Aberrations
Laser more stable at -135 db/Hz
Eccentricity 100 micron down from 140 micron
Surface aberration of 0.033 l down from 0.07 l
Hard drives
Future Data Disc Advances
Non-linear Upgrades
 2 – Photon absorption to decrease depth
of field for more layers
 Magnetic Super-Resolution: 100 nm spot
size.
 Separate layers by narrow spectral
response.
 Shorter wavelength (freq. doubled?)
Volumetric Storage
 “K – Vector” Addressing: a.k.a.
Holographic addressing
 2 beam intersection: issues with reaching
diffraction limits
 Interferometer based path length
addressing
References
 Optical Data Storage. Coufal, Hans, Geoffrey Burr:
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International Trends in Optics, 2002.
DVD Focus Control. Lincoln, Bo: Lecture notes.
Smith, Steven W. The Scientist and Engineer’s Guide to
Digital Signal Processing. San Diego: California
Technical Publishing, 1997.
Usbyte.com
Pioneer.co.jp
Optical-disk.com
Hecht, Eugene. Optics. Reading: Addison Wesley
Longman, 1998.
Magnetic Super Resolution: Fujitsu.