Spontaneous Emission And Stimulated Emission.

A laser gain medium contains active atoms or ions with different energy levels. Through a pumping process, these atoms or ions can be excited to specific higher-energy states.

If an atom is in an excited state, it may spontaneously decay to a lower energy level after a certain period of time, releasing its energy in the form of a photon emitted in a random direction. This process is known as spontaneous emission. Photon emission can also be triggered by another incoming photon, a process known as stimulated emission. In this case, both photons propagate in the same direction, effectively amplifying the incident light. This phenomenon forms the physical basis of optical amplification in both lasers and optical amplifiers.

To achieve stimulated emission, the energy of the incoming photon must closely match the energy of the laser transition. Therefore, laser gain is limited to a finite gain bandwidth covering specific optical frequencies or wavelengths. Lasers typically operate at the wavelength where the gain medium provides the highest amplification.

In an atomic system with only two energy levels (a ground state and an excited state), atoms in the excited state can amplify light, while atoms in the ground state can absorb light and become excited. Net optical amplification can only occur when more than 50% of the atoms are in the excited state. This condition is known as population inversion.

After emitting a photon, if the atoms can be rapidly removed from the lower laser level—for example, by transitioning to an even lower energy level—the laser gain process becomes significantly more efficient and easier to sustain.