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.
- 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.
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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
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