The purity and coherence of laser light makes it well-suited for sophisticated applications, including compact disc (CD) players. The light can be focused with extreme precision, allowing it to detect the microscopic pits that form the surface of a music CD. Electronic circuits pick up the stream of bright and dark patterns that the light detects, and decodes them into high-quality music.
The videodisc player of the 1970s was an ancestor of the modern CD player. It also used a laser to pick up information encoded on the surface of a spinning optical disc. In a laser disc player, a tube containing helium and neon gases produced laser light. When the first small, practical laser diodes were developed a few years later, they replaced the more expensive, bulky gas laser tubes. These less expensive lasers made new applications, like music players, possible.
Unlike the light from a light bulb or light-emitting diode (LED), a laser's light consists of a single, pure colour. All the light's waves are in sync, allowing it to travel in precise beams that don't spread out. These properties are important for a compact disc because the music data is grouped into tracks about one-thousandth of a millimetre wide. A lens focuses the laser light to a tiny spot that can find these tracks.
In the CD's tracks, music has been recorded as digital data as a series of tiny pits 125 nanometres (nm) deep and 500 nm wide. The laser light reflects off these pits as the CD spins, sending a stream of binary, on-off flashes to a detector. This sends data from the disc at a rate of over four million bits per second. As the laser light's frequency is billions of times faster than this, it can easily carry this much information.
An electronic detector called a photodiode receives the rapid light pulses being reflected from the surface of the CD. The photodiode is sensitive to the laser light, converting it to electrical signals. It feeds these signals to electronic circuits in the player, which decode the information into stereo music. These signals also contain the album and song titles, track location, and other data.
Some CD players are able to record music. To do this, you need a recordable CD, called a CD-R or CD-RW. To play music, the player can use a laser diode of modest power. To record music, the diode must be more powerful, capable of heating tiny spots in the CD's material and writing the music data permanently into the CD.