Optical MemoryOptical Memory

ObjectiveObjective

To understand the concept of optical To understand the concept of optical memory.memory.

To Know about the types optical memory.To Know about the types optical memory. To understand the Read/Write To understand the Read/Write

mechanism of each type of memorymechanism of each type of memory

Concept Of Optical Concept Of Optical MemoryMemory

It is called optical memory because the read/write It is called optical memory because the read/write operation is done with help of laser diode which is operation is done with help of laser diode which is an optical device.an optical device.

Besides their ability to store large amount of data, Besides their ability to store large amount of data, optical disks have the advantage that they are optical disks have the advantage that they are relatively inexpensive and immune to dust, and relatively inexpensive and immune to dust, and most are removable. Also, since the data is written most are removable. Also, since the data is written on the disk and read off the disk with the light from on the disk and read off the disk with the light from tiny laser diode, the read/write head does not have tiny laser diode, the read/write head does not have to touch the disk ,so that the read /write head will to touch the disk ,so that the read /write head will not crash on small dust particles and destroy the not crash on small dust particles and destroy the recorded data as it can with magnetic hard disks.recorded data as it can with magnetic hard disks.

Some optical disk systems record data in Some optical disk systems record data in concentric circular tracks as magnetic disks do. concentric circular tracks as magnetic disks do. Other such as CD systems record data on a single Other such as CD systems record data on a single spiral track.spiral track.

Types Of Optical Types Of Optical MemoriesMemories

Read OnlyRead Only Write Once/Read Many (WORM)Write Once/Read Many (WORM) Erasable Optical Or Read/WriteErasable Optical Or Read/Write

Read only Optical Read only Optical MemoryMemory

Read only systems allow prerecorded disks Read only systems allow prerecorded disks to be read out.to be read out.

A disk which can only be read from is often A disk which can only be read from is often called an optical ROM or OROM.called an optical ROM or OROM.

Example is an prerecorded audio CD.Example is an prerecorded audio CD.

WORMWORM

These systems allow you to write data on These systems allow you to write data on the disks ,but once the data is written on to the disks ,but once the data is written on to the disk, it can not be erased or changedthe disk, it can not be erased or changed

The stored data can be read out as many The stored data can be read out as many times as desired.times as desired.

Erasable Optical Disks Erasable Optical Disks

Erasable optical or EO systems allow you to Erasable optical or EO systems allow you to erase recorded data and write new data on to erase recorded data and write new data on to the disk.the disk.

The recording materials and recording methods The recording materials and recording methods are different for these different types of are different for these different types of systems. systems.

Read/Write mechanismRead/Write mechanism

Read/Write mechanism for Read Read/Write mechanism for Read only and WORM Systemsonly and WORM Systems

Disks with read only and write once systems Disks with read only and write once systems are coated with a metal on the plastic which are coated with a metal on the plastic which is altered when a high intensity laser beam is altered when a high intensity laser beam is focused on it with a lens.is focused on it with a lens.

When the laser beam is focused on a spot When the laser beam is focused on a spot on the metal ,heat is transferred to the on the metal ,heat is transferred to the plastic, causing it to give off a gas.plastic, causing it to give off a gas.

The gas given off produces a microscopic The gas given off produces a microscopic bubble at that spot on the thin metal coating bubble at that spot on the thin metal coating to represent a stored 1.to represent a stored 1.

This recording mechanism is irreversible , This recording mechanism is irreversible , once written the data can only be read. once written the data can only be read.

To read data from the disk, the laser beam is To read data from the disk, the laser beam is focused on the track and the photodiode is focused on the track and the photodiode is used to detect the beam reflected from the used to detect the beam reflected from the data track. A bubble on the track will spread data track. A bubble on the track will spread out the laser beam light so that very little of it out the laser beam light so that very little of it reaches to the photodiode.reaches to the photodiode.

A spot on the track with no bubble will reflect A spot on the track with no bubble will reflect light to the photodiode. light to the photodiode.

Read only and WORM systems are less Read only and WORM systems are less expensive then read/write systems.expensive then read/write systems.

Read/Write mechanism for Read/Write mechanism for Erasable Erasable optical disksoptical disks

Most current read/write optical disk systems use Most current read/write optical disk systems use disks coated with an exotic metal alloy which has the disks coated with an exotic metal alloy which has the required magnetic properties.required magnetic properties.

The read/write head in these systems has a laser The read/write head in these systems has a laser diode and coil of wire.diode and coil of wire.

A current is passed through the coil to produce the A current is passed through the coil to produce the magnetic field perpendicular to the disk.magnetic field perpendicular to the disk.

At the room temperature the applied magnetic field is At the room temperature the applied magnetic field is not strong enough to change the horizontal not strong enough to change the horizontal magnetization present on the disk.magnetization present on the disk.

To store a 1 at the spot in the data track, a pulse of 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 light from the laser diode is used to head up that spot.spot.

Heating that spot makes it possible for the applied Heating that spot makes it possible for the applied magnetic field to change the magnetic areas around magnetic field to change the magnetic areas around at the spot and create a tiny vertical magnet .This is at the spot and create a tiny vertical magnet .This is called magneto-optical recording or MO recording.called magneto-optical recording or MO recording.

To read data from this type of disk, polarised laser To read data from this type of disk, polarised laser light is focused on the track.light is focused on the track.

When the polarised light is reflected from one of When the polarised light is reflected from one of the tine vertical magnets representing 1’s, its plane the tine vertical magnets representing 1’s, its plane of polarisation is rotated a few degrees.of polarisation is rotated a few degrees.

Special optical circuits can detect this shift and Special optical circuits can detect this shift and convert the reflections from the data tracks to the convert the reflections from the data tracks to the data streams of 1’s and 0’s.data streams of 1’s and 0’s.

A bit is erased by turning off the vertical magnetic A bit is erased by turning off the vertical magnetic field and heating the spot corresponding to that bit field and heating the spot corresponding to that bit with the laser.with the laser.

When heated with no magnetic field present ,the When heated with no magnetic field present ,the vertical magnets produced during write operation vertical magnets produced during write operation are destroyed i.e. the data has been erased. are destroyed i.e. the data has been erased.