Optical Storage Systems of

Yesterday, Today & Tomorrow

Chris EvansCET520 Spring 2003

Arizona State Arizona State UniversityUniversity CET520 Presentation

Introduction

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• History of lasers and CD/DVD

• How CD/DVD works

• How magneto-optical (MO) systems work

• Comparison of CD/DVD and MO systems to magnetic hard drive

• The future

History of the Laser

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• Laser – Light Amplification by the Stimulated Emission of Radiation• Theory suggested by Einstein in 1916• Mechanism put forward in 1958, first working laser in 1960• Early Lasers were large and expensive• Development of laser diode brought size and cost down• Laser diodes started in the low wavelength region of the spectrum

(infrared) and have developed into the red region and higher.

First blue laser diode (Nichia)

F. A. Ponce & D. P. Bour, Nature 386, 351 (1997)

History of the CD/DVD player

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• 1982 - First working CD player developed by Philips

Philips and Sony developed CD standard – 12cm disk, 74 minutes on a single spiral

• 1983 - First CD players sold

• 1985 - CD-ROM introduced – not popular at first.

More powerful PCs lead to demand for multimedia, image processing and larger applications.

Growth in sales brings prices down.

• 1990’s - CD-R and CD-RW introduced – big success.

• 1996 - DVD introduced

• 1999 - DVD becomes mainstream

3.9GB capacity allows disk to hold entire movie.

DVD drives are standard on PCs today.

How CD/DVD works

4m6m m

6m

sapphire

AlGaN: Mg

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• Disk preformatted with grooves and lands• Grooves have pits to mark data• Disk rotates in drive and a laser projects light onto the surface• Laser light is reflected and picked up by a light sensitive detection unit• DVD works on a similar principle, but laser is of shorter wavelength.

On surface In pit

Magneto-optical disks (writing)

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Writing to disk• Media is material with Curie Temperature of ~200C• Media has high coercivity at high temp, low coercivity at low temp.• Laser heats point on disk to above 200C• Magnet on other side of disk changes magnetic orientation only at the

heated point• Disk cools and magnetic orientation stays

Write-many read-many optical system

Magneto-optical disks (reading)

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Reading the disk• As with CD/DVD, laser is reflected off surface of rotating disk.• Laser light is polarized – all wave oscillations are in the same plane.• Reflected laser light has polarization changed depending on magnetic

orientation of point being scanned.• Change is less than 1/2°, so sensitive detection equipment is needed

Keylaser lightlaser light 1/2o polarizationlaser light –1/2o polarization

Comparison of CD/DVD and magneto-optical to the magnetic hard drive

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• Access time• Data transfer rate• Price• etc…

Access time

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Access time comprises seek time and latencySeek time: Time to position head to correct trackLatency: Time taken to rotate disk to desired area on track

CD/DVD• Access time is ~100ms.• High access time due to heavy optical pickup. Laser is in the pickup,

making it relatively heavy and hard to move with small servos.• CD-ROM based on CD player, with 1 spiral track – inefficient random

access• CD disk relatively heavy, spins slower than other media.• Compare HDD access time, <10ms

MO• Access time is 20ms to 80ms• Large variation depending upon price

Data transfer rate

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Consists of External and Internal Transfer rate

External transfer rate: Rate of data transfer from controller to PC

Internal Transfer rate: Rate of data transfer from media to controller

CD/DVD• Internal rate for CDs is much lower than external transfer rate limit

• Internal transfer rate for CDs depends on format:– Constant Linear Velocity: Angular velocity of disk changes according to

position of head on disk. Constant transfer rate.

– Constant Angular Velocity: Data transfer rate faster at edges than center.

• CLV transfer rate for X12 CD drive ~1800KB/s

• CAV transfer rate for X16 CD drive 930-2400KB/s

• Better off with X12 CLV!

Data transfer rate

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Consists of External and Internal Transfer rate

External transfer rate: Rate of data transfer from controller to PC

Internal Transfer rate: Rate of data transfer from media to controller

MO• Internal rate for MO is much lower than external transfer rate limit

• This has approximately 6MB/s

• Mainly due to faster spin rate

HDD• Transfer rate 100’s MB/s.

- As HDD densities rise, will hit limit of external transfer rate

- Will either have to decrease angular velocity or make disks smaller.

Price

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Optical systems have removable disks – price for drives and media:

CD/DVD drive $20-$250+ media ¢’s

• Low price for X8 CD-ROM, high for X52 CD-RW. Wide range of prices.

MO drive small $200-$300 large $300-$3000+

• Small drives <500MB, ISO standard on 3.5” and 5.25”• Large drives proprietary, but have larger capacity 1GB-9.3GB

MO media small $10-$20 large $200-$500+

• Small is <500MB, not always rewritable, large is 1GB-9.3GB, not always rewritable.

Other characteristics

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CD/DVD• Optical media is removable, great advantage.

• Very durable and robust – estimated lifespan 35-100 years

• Compatibility – no other system beats CD/DVD for backwards compatibility

MO• MO drives are proprietary at higher end of scale.

• Low capacity MO drives now have ISO standard. All media of same or lower capacity of drive can be read.

HDD• Read/write head rides 50nm above the disk• Big disadvantage are that the head crashes and non-removability of media

Future of Optical Systems

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• Optical systems seemed to have good future in 1990’s

• Technological breakthroughs never happened

• Capacity lead of 2 over HDDs turned into lag of 5 in a few years

• HDDs increasing capacity – doubling every three years.

• Optical storage has niche market of media distribution, will it break into

secondary computer storage market?

• Low access time, transfer rate, capacity. No rewrite ability!

• What developments are there in optical technology?

• What barriers face the optical and magnetic storage industries?

Future of Optical Systems

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Improving Current Technology• Laser makes head heavy – use mirrors to direct laser, decrease access time.• Capacity and bit density related to spot size. Reduce spot size by using lower

wavelength lasers. “Blu-ray” uses blue lasers to achieve capacity > 20GB.• Higher bit densities lead to higher transfer rates.• Many-write capability not planned for CD/DVD.• Magnetic storage dominated by USA.• Optical storage dominated by Japan. No plans to actively take on HDD

industry, only increase capacity – driven by arrival of HDTV.

Barriers

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CD/DVD• Backwards compatibility slows development

• Short wavelength lasers development has been slower than expected

• Low wavelength light is absorbed by disk material leading to increased noise

• Need to find re-writable media

MO• No ISO standard for larger drives

• Low wavelength light is absorbed by disk material leading to increased noise

• The bit size is larger than the laser spot size due to heating effects

Future of Optical Systems

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New Technology

• 3-D Disks– DVDs use double layer to record data – upper layer is semitransparent

– Why not increase number of layers? Attractive option, uses current technology.

– Some techniques allow 4-20 layers to be used.

– Limited by resolution of media, ‘cross talk’, laser spot size, positioning accuracy.

– No rewrite capability?

• Holographic Disks– Data can be lost in bit oriented data storage – damage, dirt.

– Holographic storage holds data in area. Damage degrades data as a whole, but it is difficult to lose it

– Number of techniques around

Summary

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• There is a lot of potential for growth of optical systems

• There are still some barriers to overcome such as reliable short wavelength lasers and ability to re-write

• Magneto-optical systems were seen as dead end technology

but recent improvements have made them competitive

• Both CD/DVD and MO are too slow for secondary storage

• Both CD/DVD and MO are ideal for archiving