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Single-mode Operation

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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

Opposite term: multimode operation

Category: article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/99a Cite the article: BibTex plain text HTML Link to this page LinkedIn

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

It can mean single-transverse-mode operation, where a laser operates on a single kind of transverse resonator mode, which is almost always a Gaussian mode (although operation on a single higher-order mode is also possible, e.g. by using some diffractive element in the laser resonator). If operation indeed occurs in a Gaussian mode, the laser's beam quality is diffraction-limited. However, laser oscillation may then still occur on multiple axial (longitudinal) modes, which have essentially the same transverse shape but differing optical frequencies, separated by the free spectral range.

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).

In other cases, the term really indicates operation on a single resonator mode, which is then also usually an axial (Gaussian) mode. This is more precisely called single-longitudinal-mode operation or single-frequency operation. In that case, the laser is a single-frequency laser, and the laser linewidth is fairly small, limited only by phase noise (→ narrow-linewidth lasers). There may be some occasional mode hopping between different longitudinal modes, e.g. triggered by temperature fluctuations.

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 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.

More to Learn

diffraction-limited beams Gaussian beams beam quality single-frequency lasers single-frequency operation

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