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# glossary

**Laser**: acronym of : « light amplification by stimulated emission of radiation » ; device emitting spatially and temporally coherent light radiation based on the stimulated emission**Primary principal plane**: plane of the object space of an optical system whose conjugation by the said optical system occurs with a magnification of +1 and whose conjugate plane is called the secondary principal plane**Secondary principal plane**: plane conjugate by an optical system of an object plane ( the primary principal plane ) with a transverse magnification of +1**Waist**: position along the propagation axis where a laser beam has the smallest size**Elliptical beam**: a beam in which « equi-intensity » curves in a plane perpendicular to the propagation axis are elliptical**M square factor (M2)**: ratio between the divergence of an embedded gaussian beam and that of the fundamental mode of a laser (gaussian mode) with the same waist size and at the same wavelength**Wavelength ( of a light wave )**: distance between two consecutive identical states of the electric ( or magnetic ) field**Effective focal length**: algebraic distance from the primary/secondary principal plane of an optical system to its front/back focal plane