Optical Memory
What is it?
Optical memory is a type of storage that uses light (lasers) to read and write data on a
disc. Examples: CDs, DVDs
Structure of Optical Disks(Say, CD or DVD eshob):
Optical Disks are made from polycarbonate and data is stored in tiny pits on the surface
using a laser. A reflective metal layer (aluminum or gold) is added to make the data
readable and a protective acrylic coating prevents damage.
Types of Optical Memory:
1.​ Compact Disc (CD):
Introduced in 1983, CDs store digital audio and data. Compact Disk. A
nonerasable disk that stores digitized audio information. The standard system
uses 12-cm disks and can record over 60 minutes of uninterrupted playing time.
The following are variations of CD:
●​ CD-ROM (Read-Only Memory): Compact Disk Read-Only Memory. A
nonerasable disk used for storing computer data.
●​ CD-R (Recordable): Can be written once using a laser but not erased.
●​ CD-RW (Rewritable): Allows multiple write and erase cycles using
phase-change technology.
2.​ Digital Versatile Disc (DVD):
Developed as an improved version of CDs, offering higher storage capacity and
better video quality. DVDs use a shorter-wavelength laser and can store up to 4.7
GB (single-layer) or more in dual-layer versions.
3.​ Blu-ray Disc (BD): This disc uses a blue laser with an even shorter
wavelength(maneh holo blue laser light gula hochche smaller and more focused
tai eita diye written information er tiny details gula aro closely packed thake,
which is why space o kom use hoy and hence the disc can hold more data),
enabling up to 50 GB storage (dual-layer). It is used for HD movies and large
data storage.
Working Principle of Optical Disks:
●​ Data Encoding:
Information is stored on the optical disc surface as microscopic pits (small
indentations) and lands (smooth areas). These pits and lands represent binary
data, with pits corresponding to a "1" and lands corresponding to a "0."
●​ Reading Mechanism:
To read the data, a laser diode inside the optical disk player shines a focused
beam of light onto the surface of the spinning disc. The laser light reflects
differently based on whether it hits a pit or a land. When the light hits a pit, it
scatters(karon pit er rough surface ta reflection ta k noshto kore dae), reflecting
back at a low(light ray ta weak hoye jay)) intensity. When the light hits a land, it
reflects back with higher intensity. This change in light intensity is detected by a
photosensor, which converts the variations into a digital signal.
●​ Storage Organization:
Optical discs store data in a single spiral track
, unlike magnetic disks that
use concentric rings
. The spiral design ensures uniform data density,
which helps in maintaining consistent and smooth reading speeds across the
entire disc. The regular intervals and even distribution of data ensure that the
player can read the data efficiently without interruptions.
(Ei pic ta in general optical disc er operation hishebeo use kora jay)