Population inversion and optical gain

Course Details

Field of Study: Electronics Engineering
Course Name: Optoelectronics
Subtitle: Population inversion and optical gain

Population inversion is the basic condition for the presence of an optical gain.
In the normal state of any system in thermal equilibrium, a low-energy state is always more populated than a high-energy state – no population inversion.
Population inversion in a system can only be accomplished through a process called pumping – actively exciting the
atoms in a low-energy state to a high-energy state.
Population inversion is a non equilibrium state that cannot be sustained without active pumping. To maintain a constant optical gain we need continuous pumping to keep the population inversion at a constant level.
Many different pumping techniques depending on the gain media: optical excitation, current injection, electric discharge, chemical reaction, and excitation with ion beams.

The use of a particular pumping technique depends on the properties of the gain medium being pumped.
The lasers and optical amplifiers are often made of either dielectric solid-state media doped with activeions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP.
For dielectric media, the most commonly used pumping technique is optical pumping either with incoherent light sources, such as flash lamps and light emitting diodes, or with coherent light sources from other lasers.
Semiconductor gain media can also be optically pumped, but they are usually pumped with electriccurrent injection.

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