Optical memories


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Optical memories

  1. 1. Optical Memory
  2. 2. Objective <ul><li>To understand the concept of optical memory. </li></ul><ul><li>To Know about the types optical memory. </li></ul><ul><li>To understand the Read/Write mechanism of each type of memory </li></ul>
  3. 3. Concept Of Optical Memory
  4. 4. <ul><li>It is called optical memory because the read/write operation is done with help of laser diode which is an optical device. </li></ul><ul><li>Besides their ability to store large amount of data, optical disks have the advantage that they are relatively inexpensive and immune to dust, and most are removable. Also, since the data is written on the disk and read off the disk with the light from tiny laser diode, the read/write head does not have to touch the disk ,so that the read /write head will not crash on small dust particles and destroy the recorded data as it can with magnetic hard disks. </li></ul><ul><li>Some optical disk systems record data in concentric circular tracks as magnetic disks do. Other such as CD systems record data on a single spiral track. </li></ul>
  5. 5. Types Of Optical Memories
  6. 6. <ul><li>Read Only </li></ul><ul><li>Write Once/Read Many (WORM) </li></ul><ul><li>Erasable Optical Or Read/Write </li></ul>
  7. 7. Read only Optical Memory
  8. 8. <ul><li>Read only systems allow prerecorded disks to be read out. </li></ul><ul><li>A disk which can only be read from is often called an optical ROM or OROM. </li></ul><ul><li>Example is an prerecorded audio CD. </li></ul>
  9. 9. WORM
  10. 10. <ul><li>These systems allow you to write data on the disks ,but once the data is written on to the disk, it can not be erased or changed </li></ul><ul><li>The stored data can be read out as many times as desired. </li></ul>
  11. 11. Erasable Optical Disks
  12. 12. <ul><li>Erasable optical or EO systems allow you to erase recorded data and write new data on to the disk. </li></ul><ul><li>The recording materials and recording methods are different for these different types of systems. </li></ul>
  13. 13. Read/Write mechanism
  14. 14. Read/Write mechanism for Read only and WORM Systems
  15. 15. <ul><li>Disks with read only and write once systems are coated with a metal on the plastic which is altered when a high intensity laser beam is focused on it with a lens. </li></ul><ul><li>When the laser beam is focused on a spot on the metal ,heat is transferred to the plastic, causing it to give off a gas. </li></ul><ul><li>The gas given off produces a microscopic bubble at that spot on the thin metal coating to represent a stored 1. </li></ul><ul><li>This recording mechanism is irreversible , once written the data can only be read. </li></ul>
  16. 16. <ul><li>To read data from the disk, the laser beam is focused on the track and the photodiode is used to detect the beam reflected from the data track. A bubble on the track will spread out the laser beam light so that very little of it reaches to the photodiode. </li></ul><ul><li>A spot on the track with no bubble will reflect light to the photodiode. </li></ul><ul><li>Read only and WORM systems are less expensive then read/write systems. </li></ul>
  17. 17. Read/Write mechanism for Erasable optical disks
  18. 18. <ul><li>Most current read/write optical disk systems use disks coated with an exotic metal alloy which has the required magnetic properties. </li></ul><ul><li>The read/write head in these systems has a laser diode and coil of wire. </li></ul><ul><li>A current is passed through the coil to produce the magnetic field perpendicular to the disk. </li></ul><ul><li>At the room temperature the applied magnetic field is not strong enough to change the horizontal magnetization present on the disk. </li></ul><ul><li>To store a 1 at the spot in the data track, a pulse of light from the laser diode is used to head up that spot. </li></ul><ul><li>Heating that spot makes it possible for the applied magnetic field to change the magnetic areas around at the spot and create a tiny vertical magnet .This is called magneto-optical recording or MO recording. </li></ul>
  19. 19. <ul><li>To read data from this type of disk, polarised laser light is focused on the track. </li></ul><ul><li>When the polarised light is reflected from one of the tine vertical magnets representing 1’s, its plane of polarisation is rotated a few degrees. </li></ul><ul><li>Special optical circuits can detect this shift and convert the reflections from the data tracks to the data streams of 1’s and 0’s. </li></ul><ul><li>A bit is erased by turning off the vertical magnetic field and heating the spot corresponding to that bit with the laser. </li></ul><ul><li>When heated with no magnetic field present ,the vertical magnets produced during write operation are destroyed i.e. the data has been erased. </li></ul>