Principle of Operation and Applications of a Laser Diode

Laser diodes are PN junction devices under a forward bias. LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

Laser Diode Structure

The laser diode structure can be divided into two categories:

• Surface-emitting laser diodes – these laser diodes emit light in a direction perpendicular to the PN junction plane.
• Edge-emitting laser diodes – these laser diodes emit light in a direction parallel to the PN junction plane.

An example of an edge-emitting laser diode structure is shown in Figure 1.0 below. This type of structure is termed to as Fabry-Perot type laser.

From the figure above, you can clearly see that a PN junction is formed by two layers of doped gallium arsenide (GaAs). The length of the PN junction has a clear-cut relationship with the wavelength of the light to be emitted. As observed in Figure 1.0, there is a highly reflective surface at one end of the junction and a partially reflective surface at the other end. External leads provide the anode and cathode connections

Operation of a Laser Diode

When the PN junction is forward biased by an external voltage source, electrons move across the junction and the typical recombination occurs in the depletion region which results in the production of photons. As forward current is increased, more photons are produced which drift at random in the depletion region. Some of these photons strike the reflective surface perpendicularly. These reflected photons enter the depletion region, strike other atoms and release more photons. All these photons move back and forth between the two reflective surfaces. The photon activity becomes so intense that at some point, a strong beam of laser light comes out of the partially reflective surface of the diode.

Distinctive Features of Laser Light

The beam of laser light produced by the diode has the following unique features:

• It is coherent, that is, there is no path difference between the waves comprising the beam.
• It is collimated i.e. emitted light waves travel parallel to each other.
• It is monochromatic, that is, it consists of one wavelength and hence one colour only.

Laser diodes have a threshold level of current above which the laser action occurs but below which the laser diode behaves like a LED emitting incoherent light.

Applications of a Laser Diode

• Laser diodes are used in various applications ranging from medical equipment to consumer products like laser printers, hologram scanners, optical disk apparatus, etc.
• Laser diodes emitting visible light are used as pointers.
• Laser diodes emitting visible and infrared light are used to measure range (distance).
• Laser diodes are also used extensively in parallel processing of information and in parallel interconnection between computers.

Related: Semiconductor Diodes