Writing Data onto DVD Discs:

Before we understand how Blu-Ray works, let’s see how data is written onto everyday DVDs:

A DVD is made up of multiple layers of plastic (total thickness = 1.2mm) where each layer is created by injection moulding – a process where molten plastic is injected into a mould under high pressure. It is then cooled and removed as a solid round clear plastic disc.

Now this mould has got data written in bumps and pits (elevated and depressed regions, respectively. Each bump is 320nm (nanometers) wide, at least 400nm long and 120nm high where 1nm is 1billionth of a meter) representing the binary 0 and 1. Since the plastic is shaped using this mould, it gets the imprint of these pits and bumps.

Once we have the multiple layers the disc is then coated with a reflective layer. Each of the layers has an aluminum layer on the inside and a shiny gold layer on the side facing towards the writable region of the disc. This allows for better focus of the laser through the outer layers into the inner layers which is where the data is ultimately stored.

If the disc is single sided, then the non readable/writable side is coated with a silver screen to mark exactly which side you are supposed to put in the writer. If the disc is double sided, however, then the printing is done only on the transparent area around the hole and nowhere else on the disc.

Every writable layer of a DVD has a long spiral track of pits and bumps. On single layer DVDs, the spiral always starts from the inside of the disc, and hence these discs can be made smaller if needed (similar to mini discs for CD-ROM drives).

From the above, you can very well imagine the amount of precision that is required to read these ultra minute pits and bumps. It’s evident that the more storage you want, the more pits and bumps you’ll need on the disc. Moreover, you’ll have to pack them closely, but not too stressfully or otherwise the drive would have a difficult time reading the data on the disc.

A normal single layer DVD can store 4.7GB of data whereas a single layer Blu-Ray can store as much as 25GB on a single layer. Imagine the possibilities…

The way it packs in more data is just about the way we discussed above: it packs in more pits and bumps into roughly the same size as a DVD. In order to do so, it needs a laser with a much shorter wavelength to ensure that it is a lot more accurate.

For comparison’s sake, current DVDs use a Red Laser to read and write data that has a wavelength of 650nm whereas Blu-Ray discs use a Blue Laser (hence the term Blu-Ray) with a wavelength of 405nm. The smaller laser focuses more precisely, which allows it to read information that is stored in pits as small as .15micron or less than half the size of a DVD pitch (over .30 micron).

It also makes the spiral a lot closer by reducing the distance between two successive circles to less than half or from .74micron to .32micron. And as you know, the lesser the space between two successive circles, the more the number of spirals we can put on a disc.

All of the above enable Blu-Ray discs to store more than 25GB of data on a single layer.

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