Transition Linewidth And Broadening Effects

All optical transitions possess a finite bandwidth, meaning they occur over a limited range of frequencies or wavelengths where the transition cross-section remains significant. Various physical mechanisms contribute to the broadening of these transition bandwidths.

Homogeneous broadening occurs when all atoms or ions involved in an optical transition exhibit the same spectral width and center frequency. In many cases, the transition linewidth is determined by the finite lifetime of the energy levels involved. For Stark manifold energy levels in solid-state media, the relevant lifetime may correspond to the lifetime of individual sublevels. If interactions with lattice phonons induce rapid transitions between different sublevels, the effective energy-level lifetime can become quite short. As a result, the transition linewidth in laser crystals may be several orders of magnitude broader than the linewidth calculated solely from the lifetime of the entire Stark manifold.

Inhomogeneous broadening arises when different atoms or ions exhibit variations in the spectral position or linewidth of the same optical transition. Consequently, the overall cross-section spectrum—representing the average contribution from many individual emitters—becomes broader.

Inhomogeneous broadening commonly occurs when laser-active ions occupy different lattice sites within a laser crystal, such as in disordered crystals. Similar effects are also observed in glass materials. In gas lasers, atoms move at different velocities, and the resulting Doppler effect produces inhomogeneous broadening.

When either homogeneous or inhomogeneous broadening dominates the transition behavior, the transition is often referred to as being homogeneously broadened or inhomogeneously broadened, respectively. The type of broadening also influences saturation characteristics, which in turn affect the performance and operating behavior of many laser systems.