Principle of Semiconductor Lasers

The principle of semiconductor lasers is based on population inversion at a junction. This phenomenon occurs when the number of electrons in an excited state is greater than the number of electrons in the ground state. It is a crucial requirement for the generation of laser light.

Population Inversion at a Junction

In a semiconductor laser, a PN junction is utilized to create population inversion. A PN junction is formed by combining two different semiconductor materials, one with an excess of electrons (N-type) and the other with a deficiency of electrons (P-type). When these two materials are brought together, a depletion region is formed at the junction.

Electron and Hole Injection

To achieve population inversion, the semiconductor laser operates by injecting electrons and holes into the semiconductor material. This injection is typically achieved by applying a forward bias to the PN junction.

When a forward bias is applied, electrons from the N-type material and holes from the P-type material are injected into the depletion region. These injected electrons and holes recombine at the junction, resulting in the release of energy in the form of photons.

Stimulated Emission

The recombination of electrons and holes in the depletion region produces photons with a specific wavelength determined by the energy bandgap of the semiconductor material. Some of these photons undergo spontaneous emission, while others undergo stimulated emission.

Stimulated emission occurs when a photon interacts with an excited electron, causing it to drop from a higher energy level to a lower energy level. This process releases an additional photon that is coherent with the incident photon, in terms of phase and direction.

Feedback Mechanism

To create a laser beam, the semiconductor laser incorporates a feedback mechanism. This feedback is achieved by placing the semiconductor material between two mirrors, forming an optical cavity. One of the mirrors is partially reflective, allowing a portion of the emitted light to escape.

The emitted photons that are reflected back and forth between the mirrors undergo multiple stimulated emissions, resulting in the amplification of the light. Eventually, a significant number of photons reach the partially reflective mirror and are emitted as a coherent laser beam.

Conclusion

In summary, semiconductor lasers operate based on the principle of population inversion at a junction. By creating a PN junction and injecting electrons and holes into the semiconductor material, the laser achieves population inversion and stimulates the emission of coherent photons. The feedback mechanism in the form of an optical cavity further amplifies the light, resulting in the generation of a laser beam.