CD/DVD writers
How do CD / DVD writers erase the entire rewritable disk in a few seconds?
Lena Saravanan
Tiruchengode, Tamil Nadu
ANSWER 1: Writing and erasing in a CD is carried out by a laser spot focused to about a micrometre in size. This spot converts the transparent light sensitive material of a `land' spot to opaque state of the `pit' spot.
The sensitive data region of a CD/DVD is made up of a special material called phase-change material, which can change from one phase (crystalline) to another (amorphous) when it is heated and cooled.
The material used is chosen because it reflects light differently in the two states. The amorphous state reflects less light than the crystalline state.
Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state and the rapid cooling of the spot causes the material to freeze in the amorphous state. This appears dark during `reading.'
During the erasure of an amorphous spot, it is converted back to crystalline state by a process known as annealing, accomplished by heating the material to a lower temperature.
The existing data in CD that has been `quick-erased,' can be overwritten by turning the laser on continuously to the erase power, which will erase any existing marks. Switching the laser to a higher power, sufficient to melt the material, enables the creation of a new data.
Dr. H. K. Sahu
Scientific Officer
Indira Gandhi Centre for Atomic Research
Kalpakkam, Tamil Nadu
ANSWER 2: CD-RW discs have taken the idea of writeable CDs a step further, building in an erase function so you can record over old data you don't need anymore. These discs are based on phase-change technology.
In CD-RW discs, the phase-change element is a chemical compound of silver, antimony, tellurium and indium. As with any physical material, you can change this compound's form by heating it to certain temperatures.
When the compound is heated above its melting temperature (around 600 degrees Celsius), it becomes a liquid; at its crystallization temperature (around 200 degrees Celsius), it turns into a solid.
In a CD-RW disc, the reflecting lands and non-reflecting bumps of a conventional CD are represented by phase shifts in a special compound.
When the compound is in a crystalline state, it is translucent, so light can shine through to the metal layer above and reflect back to the laser assembly.
When the compound is melted into an amorphous state, it becomes opaque, making the area non-reflective.
Srinivasan Srihari
Associate Consultant - Talent Transformation
Wipro Technologies, Bangalore
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