# Doppler Broadening

Author: the photonics expert Dr. Rüdiger Paschotta

Definition: broadening of the linewidth of atomic transitions, caused by random motion of the emitting atoms

Doppler broadening is the inhomogeneous broadening of the linewidth of atomic transitions caused by the random movements of atoms. For example, if the atoms have a thermal velocity distribution with temperature <$T$>, the linewidth resulting from the Doppler effect is

$$\Delta {\nu _{\rm{D}}} = \frac{{2{\nu _0}}}{c}\sqrt {2\ln 2 \cdot \frac{{{k_{\rm{B}}}T}}{m}} $$where <$\nu_0$> is the mean optical frequency and <$m$> is the mass of the atoms. The Doppler broadening, e.g. for atoms in a gas cell, is typically much larger than the natural linewidth.

Doppler broadening can place severe constraints on precise spectroscopic measurements. However, it can be eliminated in various ways, e.g. by reducing the temperature (e.g. with laser cooling) or by employing a measurement scheme which is intrinsically insensitive to Doppler broadening, such as *Doppler-free saturation spectroscopy* or the use of two-photon absorption with counterpropagating beams.

Doppler broadening is the dominant line broadening mechanism in gas lasers.

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2021-05-30

Does the Doppler broadening effect also occur in lasers?

The author's answer:

Yes, it could be relevant in a gas laser if one tries to operate it with a very narrow emission bandwidth.