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Inhomogeneous Broadening

Definition: the increase in the linewidth of an atomic transition caused by effects which act differently on different radiating or absorbing atoms

German: inhomogene Verbreiterung

Category: physical foundations


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Inhomogeneous broadening is an increase in the linewidth of an atomic transition caused by effects which cause different radiating or absorbing atoms (or ions) to interact with different wavelength components. (Examples of such effects are discussed below.) This means that the absorption and emission cross-sections have different spectral shapes for different atoms. The fluorescence spectrum from such a material can then exhibit peaks which are broader than those of single atoms, since it shows an average over many differently emitting atoms. In similar ways, absorption spectra can be broadened.

Inhomogeneous broadening can be caused in various ways:

  • Different velocities of the atoms of a gas (e.g. in a gas laser) cause different Doppler shifts (→ Doppler broadening).
  • In a solid medium, there can be different lattice locations e.g. of laser-active ions, where the ions experience different local electric and magnetic fields. This is particularly the case for glasses, but can also occur in crystalline materials (particularly in disordered crystals).

Inhomogeneous broadening is strongly related to inhomogeneous saturation in laser gain media.

See also: linewidth, inhomogeneous saturation, homogeneous broadening, laser gain media, gain bandwidth

Questions and Comments from Users


Which kind of gain medium can cause multiple longitudinal modes oscillating simultaneously in the cavity?

The author's answer:

Any kind. Normally, you get single-mode operation only when taking special measures, except in some cases with a very short laser resonator and a small gain bandwidth. Inhomogeneous broadening can make it more difficult to reach that goal.


You mention that 'inhomogeneous broadening is strongly related to inhomogeneous saturation in laser gain media'. Wouldn't that relationship better be described as cause and effect, rather than 'strongly related'?

The author's answer:

No, these are just two phenomena, and we cannot consider one as the cause of the other. Saturation is something occurring at high optical intensities, while the broadening has nothing to with intensities.

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