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Homogeneous Broadening

Definition: the increase in the linewidth of an atomic transition caused by effects which affect different radiating or absorbing atoms in the same way

German: homogene Verbreiterung

Category: physical foundations


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Homogeneous broadening, as opposed to inhomogeneous broadening, refers to effects which increase the optical linewidth of an electronic transition by equally affecting different radiating or absorbing atoms, ions or molecules. The spectral shapes of the transition cross-sections of all involved atoms are then equal, and each atom or ion exposed to a certain optical intensity will exhibit the same rates of optical transitions.

Some common cases with homogeneous broadening are as follows:

  • Isolated non-moving atoms or ions exhibit transitions with a natural linewidth, resulting from the finite lifetimes of the energy levels involved.
  • In a gas (e.g. of a gas laser), all atoms or molecules of some species experience the same average rate of collisions, which lead to collisional broadening.
  • In many laser crystals, laser-active ions can occupy only one type of site in the crystal lattice. (This applies e.g. to Nd3+ ions in Nd:YAG, replacing yttrium (Y3+) ions of the YAG host.) The interaction of such ions with the crystal lattice via phonons then affects all these ions equally. Rapid transitions between the sublevels of the involved Stark level manifolds lead to strongly reduced sublevel lifetimes, and thus to linewidth values which are orders of magnitude larger than the natural linewidth as expected from the lifetimes of the whole Stark level manifolds.

Homogeneous broadening is strongly related to homogeneous saturation in laser gain media. In comparison with inhomogeneous broadening, it makes it easier to achieve single-frequency operation.

See also: linewidth, inhomogeneous broadening, gain bandwidth, gain saturation

Questions and Comments from Users


Please identify the spectral line broadening mechanisms as homogeneous or inhomogeneous in the case of collisions between atoms in a gas.

The author's answer:

Collisions of particles in a gas affect the absorption or emission properties of all the particles, and these particles are all subject to the same rate of collision processes. Therefore, the spectral broadening can be categorized as being homogeneous.


Which conditions do we need to get stable multi-wavelength laser operation with a homogeneously broadened laser gain medium?

The author's answer:

It is generally difficult to achieve. You might, for example, use some kind of nonlinear absorber.

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