An optical drive is a piece of equipment that uses a laser to read or write information on a disc. Though this type of equipment is often associated with computers, it can stand alone as an appliance. For instance, a Digital Versatile Disc (DVD) player, Blu-ray disc (BD) player, and Compact Disc (CD) players are considered optical drives even though they may not be in a computer system. Many video games also use this type of device to read the game’s disc. Despite what type of disc they read or write, all these applications have the same basic workings.
The optical path makes an optical drive work. The optical path is composed of three components: a laser, a lens, and a photodiode. The laser writes and reads the data. The lens guides the laser across the surface of the disc. Lastly, the photodiode detects light reflected off the disc’s surface. The drive also uses two servomechanisms, or servos — one to maintain the proper distance between the disc and laser and the other to make sure the laser is moving in a continuous spiral path.
Lasers of different wavelengths are used to read different types of media. A CD player uses a laser with a wavelength of 780 nm. This is in the infrared light range. DVD players use wavelengths in the 650 nm range. This is why you will see a red light when the player is working. Blu-Ray® players use a much lower wavelength—450nm. This puts it in the violet range and explains the bluish light.
The discs an optical drive reads and writes vary. Read only media (ROM), such as in CD-ROMs or DVD-ROMs, do not write media, only read it. These are the type of discs that software is written on. The manufacturer will create the disc by embedding pits, called grooves, on the disc’s flat surface. Then, when a user inserts the disc into an optical drive, a reading laser shines on the disc. The light is reflected and detected by the photodiode which then translates the data into a form that the computer or player can understand and display.
An optical drive can also record media. This type of recording must be done on specific recordable (R) discs like CD-Rs, DVD-Rs, and BD-Rs. When writing, the recording laser melts organic dyes on the surface of the disc and embeds grooves into the surface. As with ROMs, the reading laser, which is at a much lower wavelength than the recording laser, shines on the disc and reflects light. The reflection is then detected by the photodiode and information is displayed.
Rewriteable media are able to be write media on rewriteable (RW or RE, in the case of Blu-Ray®) discs, such as CD-RWs, DVD-RWs, and BD-REs, again and again. The surface of these discs is made of a special crystalline alloy that enables data to be written on its surface over and over. Individual data is embedded onto the disc on grooves that reflect the laser differently. The photodiode is then able to distinguish between the different reflections and display the correct information.