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Single-mode Operation

Definition: operation of a laser on axial (longitudinal) resonator modes only, or even on a single axial mode

Opposite term: multimode operation

German: einmodiger Betrieb

Category: lasers

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The term single-mode operation, which usually applies to lasers, is ambiguous, as it is used with different meanings:

spectra of laser emission
Figure 1: Emission spectra of lasers with single-mode emission (top), multiple axial mode (but single transverse mode) emission (middle), and full multimode emission (bottom).

The number and type of oscillating resonator modes in a laser depends on the circumstances:

  • The excitation of higher-order transverse modes can often be avoided by pumping only the volume covered by the axial modes. This is often done e.g. in end-pumped solid-state lasers.
  • Multiple axial modes may still be excited, if the gain bandwidth is larger than the axial mode spacing (as is the case in most solid-state lasers). This may be changed by decreasing the gain bandwidth, by inserting an intracavity filter (e.g. an etalon), or by increasing the axial mode spacing (free spectral range), i.e. by using a very short laser resonator.

Single-frequency operation is usually more difficult to achieve than just single-transverse-mode operation, because it is not sufficient to introduce spatially varying loss or gain. Factors which make it more challenging are all those reducing the mode competition, e.g. inhomogeneous saturation via spatial hole burning.

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See also: diffraction-limited beams, Gaussian beams, beam quality, single-frequency lasers, single-frequency operation, resonator modes, mode competition, modes of laser operation
and other articles in the category lasers

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