Laser Diode

Introduction to Laser Diode

The term laser stands for light amplification by stimulated emission of radiation.  Laser light is monochromatic, which means that it consists of a single color and not a mixture of colors.  Laser light is also called coherent light, a single wavelength, as compared to incoherent light, which consists of a wide band of wavelengths.  The laser diode normally emits coherent light, whereas the LED emits incoherent light.

The basic symbol of a laser diode is shown below.

A pn junction is formed by two layers of doped gallium arsenide.  The length of the pn junction bears a precise relationship with the wavelength of the light to be emitted.

Symbol of Laser Diode


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Symbol of Laser Diode

An application of laser Diode

Laser diode and photo diodes are used in the pick-up system of compact disk (CD) players. Audio information (sound) is digitally recorded in stereo on the surface of a compact disk in the form of microscopic “pits” and “flats.”  A lens arrangement focuses the laser beam from the diode onto the CD surface.  As the CD rotates, the lens and beam follow the track under control of a servomotor.  The laser light, which is altered by the pits and flats along the recorded track, is reflected back from the track through a lens and optical system to infrared photodiodes.  The signal from the photodiodes is then used to reproduce the digitally recorded sound.

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