Optical Data Storage By Ken Tatebe

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 / mm 2 ~ 0.01 byte / mm 2

20 th 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/mm 2

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 FeatureDVDCD Substrate Dia./Thick. 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, Track Pitch (micron) Min pit length (micron) 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:  Dual layer  Increased numerical aperture  Decreased depth of focus  Signal to noise  Tracking  Data Density: ~7 MByte/mm 2

Dual Layer Technology  Benefits  Increased durability  Increased capacity  Detriments  Decreased S/N  Decreased data density

Numerical Aperture  NA = n sin(  /2)  Spot size = /NA

Depth of Focus  DoF = /NA 2  Determines spacing of layers  Affects S/N; places an upper limit on NA.

Noise Tolerances  Coma Aberrations  Laser more stable at -135 db/Hz  Eccentricity 100 micron down from 140 micron  Surface aberration of  down from 0.07  Surface aberration of  down from 0.07

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: International Trends in Optics,  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,  Usbyte.com  Pioneer.co.jp  Optical-disk.com  Hecht, Eugene. Optics. Reading: Addison Wesley Longman,  Magnetic Super Resolution: Fujitsu.