How a Laser Diode Works

You may have been using a laser diode (also called semiconductor diodes or injection diodes) in your day-to-day activities without even knowing it.  From CD and DVD players to laser pointers and printers, barcode readers and fiber optic cables, all these make use of laser diodes.   Ever since it was invented in 1962, laser diodes have been used in various consumer, industrial, and scientific applications.
What is a diode?  It is simply an electronic component where electric current enters one end and is stopped at the opposite end.  This is a widely used semiconductor device made of two electrodes, one positive (anode) and the other negative (cathode).  This is called a p-n junction.  There is a terminal (connecting leads) at both ends. Diodes are made of germanium, selenium or silicon.

Some of the many different types of diodes are: light emitting diodes (LEDs), voltage regulation diodes like the Zener diode (designed to breakdown in a reverse-biased mode; used in voltage regulators), photo diode, PIN diode with its fast switching capabilities and the laser diode.  Diodes are commonly used as a rectifier.  In simplest terms, rectification is converting alternating current (AC) to direct current (DC).

A laser diode is similar to LEDs in that both are very small and emit light.  The only difference is that the light coming from a laser diode is purer and more powerful.  LEDs produce incoherent light (unorganized and diffused) while laser diodes produce coherent light.  The characteristics of the light produced by a laser are:

  • It is monochromatic or one color.
  • It is coherent or organized.
  • It is directional.  The beam of light is very strong and tight.

The laser diode has a terminal at one end that is used to collect the power from an energy source. As electricity enters the terminal, charged electrons in the positive side collide with (or jump into) holes (or the absence of electrons) in the negative side.  As these charged electrons enter the holes, they lose energy in the form of light or photons. 

All the light produced by the coming together of the electrons in the semiconductor layers are restricted inside a long, narrow tunnel or channel (with mirrors at the end), which acts as a waveguide. As the photons bounce back and forth in this narrow tunnel, it is amplified as it collides with the other charged electrons. The photons are then released in a wide or cone-shaped manner, just like in a flashlight.  The laser diode has a converging lens at the end that bends or focuses the beam, turning it into a fine column or pencil of light (as in a laser pointer).

A single laser diode’s power is low and it is very small in size, making it ideal for use in consumer electronic equipment.  But if you use a group of diodes, a higher-power laser can be created. Semiconductor lasers used in CD/DVD players are low powered (5-100mW). Opnext, Sony, Panasonic and Sanyo are among the many different suppliers of semiconductor laser diodes.

Laser diodes used to be only one color – red.  With advances in technology, there are new types of laser diode that produce green as well as blue beams.  The new Blu-Ray discs for instance use this new technology.  Now, each time you use a laser pointer, watch a DVD or buy an item with a barcode, you know that a laser diode is at work.


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