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Why Do CD Players Use Laser Light?

A compact disc player uses laser light to reproduce music recorded on the disc. A lens focuses the light to a pinpoint, which reflects off tiny pits in the disc's surface. The pits form digital patterns that encode music as computer data. A detector picks up the flickers of laser light and turns them into electronic signals, which speakers or earphones translate into sounds.

Coherent And Collimated
- Laser light, unlike the light from most other sources, is coherent, moving as a single wave, and collimated, moving in a unified direction. Light from the sun, light bulbs and candles is a jumbled mix of waves, having different colors and moving in various directions. A CD stores music in billions of microscopic pits, tightly packed in precise circles around the disc's diameter. To play a single "groove" of pits means focusing light accurately into a very small area. The coherent, collimated nature of laser light provides tight focusing ability.
Laser Diode
- Compact, portable CD players were made practical with the invention of the laser diode. A laser diode is a pea-sized electronic device that produces laser light, much as a light-emitting diode, or LED, does, but with far greater intensity and purity. Traditional helium-neon gas lasers would not have worked as well because of their greater cost and larger size.
Monochromatic
- Lasers are monochromatic, meaning they produce a single, pure light color. Using a monochromatic source simplifies the lenses needed for focusing the light to a fine point. Other light sources produce a range, or spectrum, of colors. Through a simple lens, each color comes to a slightly different focus, a problem called chromatic aberration. More complex and expensive lenses correct this problem, but laser light eliminates their need. The laser light's precise, predictable color purity also adds reliability to the CD player: The tiny pits on the disc work best with light having a specific wavelength.
Photodiode
- The path of the laser light in a CD ends with an electronic detector called a photodiode. The detector converts reflected light pulses into electronic signals, so it is a crucial element in the CD player. The photodiode responds to the laser light's pure color accurately and quickly enough to reproduce high-quality music. The laser and the detector incorporate very similar technologies, and they co-evolved as compact discs grew in popularity. The laser diode, by itself, makes a good light source, but with a photodiode, the two become an advanced music reproduction system.

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