Spatial Hole Burning.

In many lasers, particularly those employing linear resonators, the gain medium is affected by counter-propagating laser beams. The interference between the forward- and backward-propagating beams forms a standing wave, resulting in significant spatial variations in optical intensity. This phenomenon mainly gives rise to two effects:

• At locations where the laser intensity is high, gain saturation becomes more pronounced, creating a spatially varying population inversion. This effect is known as spatial hole burning (SHB).

• In regions of higher optical intensity, the excitation density has a greater influence on the resulting laser gain.

The figure above illustrates the gain saturation caused by a single-frequency laser beam (black curve). The gain reduction experienced by this beam is greater than that of another weaker beam with a slightly different wavelength. This is because the intensity minima of the second beam do not completely overlap with the strongly saturated regions of the gain medium. In effect, this creates a form of inhomogeneous saturation.

Spatial hole burning can have several important effects on laser performance. It makes single-frequency operation more difficult because the dominant oscillating mode experiences stronger gain saturation than weaker competing modes. If the excitation stored in the standing-wave nodes (dark regions) cannot be effectively utilized, the laser efficiency may decrease. In addition, wavelength tuning and ultrashort-pulse mode-locking can also be influenced by spatial hole burning.