Optical Disk Drive Defined
An optical disk drive (ODD) uses a laser light to read data from or write data to an optical disc. These include CDs, DVDs, and Blu-ray discs. This allows you to play music or watch movies using pre-recorded discs. Computer software also often comes on one of these discs, so you need an optical drive to install software. Most modern drives allow you to write to an empty disc, so you can create your own music CDs or create a back-up copy of important data.
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Components
An optical disk drive uses a laser to read and write data. A laser in this context means an electromagnetic wave with a very specific wavelength within or near the visible light spectrum. Different types of discs require different wavelengths. For compact discs, or CDs, a wavelength of 780 nanometers (nm) is used, which is in the infrared range. For digital video discs, or DVDs, a wavelength of 650 nm (red) is used, while for Blu-ray discs, a wavelength of 405 nm (violet) is used.
An optical drive that can work with multiple types of discs will therefore contain multiple lasers. The mechanism to read and write data consists of a laser, a lens to guide the laser beam, and photodiodes to detect the light reflection from the disc.
The optical mechanism for reading CDs and DVDs are quite similar, so the same lens can be used for both types of discs. The mechanism for reading Blu-ray discs, however, is quite different. An optical drive that works with all types of discs will therefore have two separate lenses: one for CD/DVD and one for Blu-ray.
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In addition to the lens, an optical drive has a rotational mechanism to spin the disc. Optical drives were originally designed to work at a constant linear velocity (CLV) - this means that the disc spins at varying speeds depending on where the laser beam is reading, so the spiral groove of the disc passes by the laser at a constant speed. This means that a disc spins at around 200 rotations per minute (rpm) when the laser is reading near the outer rim of the disc and at around 500 rpm when reading near the inner rim.
This constant speed is very important for music CDs and movie discs, since you want to listen to music or watch a movie at the regular speed. For other applications, however, such as reading or writing other types of data, working at this speed is not needed. Modern optical drives can often spin much faster, which results in higher transfer speeds. When you see an optical drive reported as a 4x drive, for example, this means it can spin at four times the base-speed (i.e., between 800 and 2,000 rpm).
An optical drive also needs a loading mechanism. Two general types are in use:
- A tray-loading mechanism, where the disc is placed onto a motorized tray, which moves in and out of the computer case.
- A slot-loading mechanism, where the disc is slid into a slot and motorized rollers are used to move the disc in and out.
Tray-loading mechanisms for optical drives in desktop computers tend to be rather bulky.
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For laptops, the tray-loading mechanism is much smaller.
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Different Types of Discs
The earliest optical disc used in computer systems was the CD. This allowed you to play regular music CDs on your computer. Specifically for use in computers, however, CD-ROMs were developed. These are like regular CDs but contain read-only media, such as data files or software. The CD-ROM became a widely used method to distribute software. Since the optical drives were used only for CDs, it was often called a CD-ROM drive. The maximum storage capacity of a typical CD-ROM is around 700 MB.
The next type of optical disc that came on the market was the DVD. Intended initially for movies, they were quickly adopted in computer systems as well. The standard capacity of a DVD is 4.7 GB. More recently, Blu-ray discs came on the market and have a standard capacity of 25 GB. Both DVD and Blu-ray also exist in a double layer (DL) format, which effectively doubles their capacity.
In order to write data to a disc, a recordable blank disc is needed. Data is recorded by selectively heating parts of an organic dye layer with the laser, which changes the reflectivity of the dye. For regular recordable discs, this is a permanent process and data can only be written once. However, there are also rewritable discs. In this case, the laser is used to melt a crystalline alloy in the recording layer of the disc. The substance can be melted back into crystalline form, making it possible to write multiple times to the disc. Writing data to an optical disc is also referred to as 'burning.'
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