Optical Drives: How They Work
Posted: May 13, 2000
Written by: Tuan "Solace" Nguyen

Introduction

Last time, we gave you the lowdown on the basics of hard drive mechanics. This time, we’ll be sharing with you how optical storage drives work. I know some of you in the forum had posted messages requesting that I write an article about this type of technology. Well, here we are... Enjoy. We’ll be discussing: CD, CD-R, CD-RW, DVD, DVD-RAM, DVD-RW, MO and LIMDOW (could it be some sort of Microsoft weapon?) technology.

CD

Have you ever picked up a CD and looked at the silver side wondering how on earth information is stored on that plastic disc? Well, wonder no more because we’re going to show you how optical storage devices work.

First off, we’ll start you off with CD (compact disc) history. This technology has been around for almost 20 years (since 1982). That’s right, optical storage technology has been around for quite some time. There is also another technology that has been around for a long time as well -- Magneto Optical (MO) technology (which combines magnetism along with optics).

But the very first we’ll discuss is how CD technology works.

Below is a diagram showing you how the technology works. All optical storage technology is based upon this principle -- except for MO.



On the top side of the disc (the side with print), a thin coat of aluminum alloy is ‘stamped’ onto the polycarbonate disc, and another very thin acrylic protection coat is coated on top of that to protect the medium.

So how exactly does data get put on to CDs? A regular silver-back CD (the ones you buy in stores) isn't recorded using the same method as CD-R discs, but by using specialized presses that stamp the aluminum layer on the disc. It would take forever to have a whole bunch of CD writers sitting there burning away.

This electron microscope scan shows a commercial CD stamped mold which manufacturers inject molten polycarbonate plastic to form the pits and lands.

This electron microscope scan shows a single pit of an audio CD.

This electron microscope scan shows puts that were burnt using CD-R. The pits are rougher around the edges than pits that were stamped commercially.

Data is recorded the same way. A 1 would be a land and a 0 would be a pit.





CD-R

We don’t all have industrial CD presses in our homes, but that doesn’t mean we can’t make our own CDs. Enter the world of CD-R (CD Recordable).



On the top side of the disc, you have the protective acrylic layer that prevents scratches to the recording material. Then you have the actual recording layer -- the dye layer, where the recording laser burns data pits and lands into the polymer dye. The laser burns non-translucent data pits which absorb light that would normally reflect back the read laser. The parts that are not burnt are the pits.

The reflective layer is the layer which reflects the read laser beam back to the optical sensor which translates the pulses into 1’s and 0’s.

CD-RW

There is one drawback to CD-R technology, it’s only recordable once, and it’s permanent, you cannot record over existing data. This is where CD-RW (CD ReWritable) comes to the rescue.



CD-RW uses a technology called phase-change which create pits and grooves by changing certain portions of the recording layer from reflecting to absorbing light. The dielectric layer is an insulation layer that isolates and quickly cools the phase-change recording process. While this is happening, the CD-RW laser heats the polycrystalline surface of the recording layer to change its state (phase-change). Data is recorded as amorphous (shapeless) pits and lands.

CD-RW is less reflective than CD-R and therefore are not readable on older generation CD-ROM drives. Newer drives use the multi-read standard enabling them to automatically adjust the laser gain.

The advantage of using CD-RW over CD-R is the ability to rewrite about 1000 times on the same disc. However, current writes speeds of RW discs are relatively slow to that of CD-R. While CD-R write speeds have reached 12x, RW stays at 4x.

DVD

Short for Digital Versatile Disc or Digital Video Disc (depending on which side of the industry you’re on), DVDs are designed to supplement and eventually replace the CD. "What is the difference? A DVD and a CD both look the same to me!" Well physically, in size, they are the same but microscopically they are not. The pits and lands on a DVD disc are much smaller than those on a CD, and they are also more closely packed.


CD Lands and Pits VS. DVD Lands and Pits

Above is a diagram comparing the pits and lands between a CD and a DVD. More information can be packed on the DVD because the size of each bit is physically smaller than a CD. Because of this, the read laser also needs to be smaller. A read laser that reads CDs is red. Red has a longer and larger wavelength, thus, making the beam larger. DVDs need something more precise to be read. A blue laser is used for DVDs because the blue light wavelength is much shorter and thinner, thus, giving a more precise beam.





DVD's Continued...

How exactly are DVDs made? A DVD disc is manufactured using almost the same technique that is used for a CD. A developer will make a single DVD disc from the office by using a DVD writer and then take the disc to a duplication facility such as CinRam, Nimbus and or Panasonic. The information from the burnt disc is transferred using software (similar to the burning software you may use at home) to what is called a burner (surprise, surprise) , which guides the laser that brands a glass-topped DVD with the data pattern of dots that vary in terms of spacing and in terms of brightness and darkness. The spacing and variation of brightness and darkness of the dots are what makes the data readable to a DVD reader.

Then, a photograph is taken of this master DVD and an etching is made from that photograph. The etching is then used to create a metal stamper. The metal stamper is then used to imprint the pattern of pits and lands into the plastic coating of all following DVDs.

DVD-RAM, DVD-RW and DVD-R

Eventually, people will want to be able to write to a DVD disc because its capacity is huge -- it can reach 18GB. Currently, there are two formats fighting for becoming the standard, DVD-RAM, and DVD-RW. Both DVD-RAM and DVD-RW are based on similar technologies and both utilize phase-change technology. While DVD-RW is much like CD-RW, DVD-RAM utilizes something a little different – Wobble Land Groove recording. The land-groove method records data on both the lands and the grooves. The wobble method records the tracks so they are not symmetrical (shifting slightly to the left and right) so clock data can be recorded there. There are pits that are equally laid out on the disc which hold addressing information to help locate information on the disc.

DVD-R is just like a CD-R and its main advantage is readability in all DVD readers. But like CD-R, it’s only Recordable once.

MO

Short for Magneto-Optical, this technology uses not only optics, but magnetism as well (like the name suggests). How this technology works is a little more technical than how CD-R’s are recorded. Below is a picture of a typical MO disc.



A thin layer within the media contains magnetically sensitive elements. When this layer is heated to about 200°C (its "Curie" point), the polarity (north and south) of the magnetic elements can be changed by an external magnetic field from the drive write head (similar to that of a hard drive).

To write data, two disk revolutions (the actual discs need to be rotated twice) are needed. On the first revolution, a magnetic field is applied but the head and the laser head heats up the target area of the disk to its Curie point. The heat causes the magnetic elements align themselves parallel with the magnetic field. This parallel alignment erases the target area, recording all 0 bits. On the next revolution of the disk, the magnetic field is reversed, and the laser heats up only those areas which are to have 1 bits recorded, leaving the rest 0 bits.





MO's Continued...

Below is a diagram of how a MO disc is recorded.


MO write function

When the temperature of that area is cooled down to room temperature, the magnetic elements can no longer change polarity. Because the written magnetic polarity is locked, the disk is not susceptible to magnetic fields as regular magnetic media are.

To read data from the disk, a laser is used at a low power which does not heat the disk. Depending on the recorded magnetic polarity, the polarity of the laser light reflected from the disk is rotated a few degrees either way (1 or 0). This rotation of the laser is called the "Kerr" effect. The drive’s photo sensor detects this, and determines whether a 0 or 1 was read.


MO read function

Recording a MO disc does not utilize the changing dark and light reflections found in CD, CD-R and CD-RW. Instead, the recording changes the polarity (the direction in either way, representing a 1 or 0) of the reflected light. Polarizing filters are used to detect the difference and read out the north-south and south-north spots on the disc.

You may be asking "what about MiniDiscs??" Well, MD’s are just MO discs. Currently, Sony has a 3.5" MO disc that can store about 1.3GB. I’m not sure if it's available yet. But be on the lookout for the news.





LIMDOW

What the heck is LIMDOW technology? Well, LIMDOW stands for Light Intensity Modulation Direct OverWrite -- (what the hyolios?!). It is derived from MO technology. Traditional MO technology requires that the disc be rotated twice in order for information to record. With LIMDOW media, the disc need only be rotated once.

The drive records data by heating areas of the disk to their "Curie" point, as with regular MO media. However, depending on the write laser intensity, the magnetic element orients itself with either the external magnetic field or with a reference layer that is built into the disk, sitting above the elemental write layer.. Because of this, data can be written in a single pass.

Conclusion

And there you have it, the repertoire of optical storage technology. I hope this article has shined some light (pardon the corny pun) on how optical storage works. But, I did intentionally leave out another optical storage technology -- holographic storage. I’ll be discussing this amazing technology in-depth next time. Be on the lookout. Until next time...

*Thanks to Ken C. Pohlmann, Professor of Music Engineering Technology, University of Miami. Also, thanks to Sony Corporation for providing the magnetic polarity diagrams.

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