Mode is a type of laser field that can exist (or propagate) in a specific optical system. This type of laser field is called a mode of this optical system. The mode is distinguished by frequency, polarization, and energy distribution.
When mentioning Laser Mode, there comes 2 concepts: “transverse mode” and “longitudinal mode”. These two are not opposite concepts. The transverse mode describes the energy distribution of the laser spot, and the transverse mode can be seen from the light intensity distribution on the cross section of the laser beam. While the longitudinal mode is related to the length of the laser cavity, so it is called "longitudinal mode", which describes the laser frequency. Theoretically, a laser cavity can produce countless equal spaced frequencies of light, but because the gain medium only produces the maximum gain for the light of a specific frequency (resonant frequency), the light of other frequencies is suppressed, that is, the resonant laser with the resonant frequency will be selected during the resonance, so the laser generally only outputs a laser with a specific frequency. Longitudinal mode refers to frequency. The spectral characteristics of a laser, such as line width, coherence length, are primarily determined by longitudinal modes, whereas beam divergence, diameter, and energy distribution are governed by the transverse modes.
To intersect the output laser beam and study the transverse beam cross section, it is can be found the light intensity can be of different distributions or patterns, which are regarded as Transverse Electromagnetic Modes (TEM). TEM00 mode is the lowest order transverse mode. It is the fundamental transverse mode of the laser resonator. It has the lowest threshold, smallest beam waist and divergence. Here, "00" indicates no nodes appear in the beam profile, while "TEM" indicates "transverse electromagnetic" and refers to the form of the standing waves. TEM00 mode has the same form as a Gaussian beam.
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