Optical Storage Systems of Yesterday, Today & Tomorrow Chris Evans CET520 Spring 2003

1. Arizona State University Optical Storage Systems of Yesterday, Today & Tomorrow Chris Evans CET520 Spring 2003 CET520 Presentation

2. Introduction • 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 2

3. History of the Laser • 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) 3 F. A. Ponce & D. P. Bour, Nature 386, 351 (1997)

4. History of the CD/DVD player • 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. 4

5. How CD/DVD works • 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 5

6. Magneto-optical disks (writing) Write-many read-many optical system • 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 6

7. Magneto-optical disks (reading) Key laser light laser light 1/2o polarization laser light –1/2o polarization • 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 7

8. Comparison of CD/DVD and magneto-optical to the magnetic hard drive • Access time • Data transfer rate • Price • etc… 8

9. Access time • Access time comprises seek time and latency • Seek time: Time to position head to correct track • Latency: 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 9

10. Data transfer rate • 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! 10

11. Data transfer rate • 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. 11

12. Price • 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. 12

13. Other characteristics • 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 13

14. Future of Optical Systems • 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? 14

15. Future of Optical Systems • 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. 15

16. Barriers • 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 16

17. Future of Optical Systems • 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 17

18. Summary • 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 18