Laser diodes and light emitting diodes have a number of elements in common with respect to their theory of operation. However the laser diode theory of operation incorporates more elements, taking in additional processes to provide the coherent light it produces.
While there are many different forms of laser diode, the basis of the laser diode theory of operation is very similar - the basic precepts remain the same, although there are a number of minor differences in the way they are implemented..
Laser diode theory basics
There are three main processes in semiconductors that are associated with light:
- Light absorption: Absorption occurs when light enters a semiconductor and its energy is transferred to the semiconductor to generate additional free electrons and holes. This effect is widely used and enables devices like to photo-detectors and solar cells to operate.
- Spontaneous emission: The second effect known as spontaneous emission occurs in LEDs. The light produced in this manner is what is termed incoherent. In other words the frequency and phase are random, although the light is situated in a given part of the spectrum.
- Stimulated emission: Stimulated emission is different. A light photon entering the semiconductor lattice will strike an electron and release energy in the form of another light photon. The way in which this occurs releases this new photon of identical wavelength and phase. In this way the light that is generated is said to be coherent.
The key to the laser diode operation occurs at the junction of the highly doped p and n type regions. In a normal p-n junction current flows across the p-n junction. This action can occur because the holes from the p-type region and the electrons from the n-type region combine. With an electromagnetic wave (in this instance light) in passing through the laser diode junction diode junction it is found that the photo-emission process occurs. Here the photons release further photons of light occurs when they strike electrons during the recombination of holes and electrons occurs.
Naturally there is some absorption of the light, resulting in the generation of holes and electrons but there is an overall gain in level.
The structure of the laser diode creates an optical cavity in which the light photons have multiple reflections. When the photons are generated only a small number are able to leave the cavity. In this way when one photon strikes an electron and enables another photon to be generated the process repeats itself and the photon density or light level starts to build up. It is in the design of better optical cavities that much of the current work on lasers is being undertaken. Ensuring the light is properly reflected is the key to the operation of the device.