Untitled - IRS

GLOSSAR 259
Expressions
Besetzungsinversion Population Inversion in physics, the redistribution of atomic energy levels that
takes place in a system so that laser action can occur. At finite temperature the relative fraction
of atoms in excited states (population) follows the Boltzmann distribution law with the
majority of atoms in low energy states. Higher levels are less populated. Although absorption
and emission of energy is a continuous process, the statistical distribution (population) of
atoms in the various energy states remains constant. When this distribution is disturbed by
prferentially ‘pumping’atoms to higher energy levels at the cost of lower ones, a population
inversion is achieved in which more atoms will exist in the higher energy states than in the
lower. (Eq. (1.0)) [p. 5]
Breakdown threshold The lowest power density It that suffices to initiate a breakdown for a colloid
of certain properties (size, → MPI cross-section, → ionisation energy). B.t. is also equally
used for the lowest mean power (Pt) of a laser or the lowest pulse energy (Et) which initiates
a breakdown. (Eq. (7.14)) [p. 202]
Brewster Winkel Brewster’s angle. When light passes a transparent surface a certain fraction is
reflected, depending on polarization and angle of incidence. For parallel polarized light (i.e.
polarized in the plane of reflection) the reflection is supressed completely for Brewster’s
angle ΦB, given by tan ΦB = n. (n is the index of refraction). This is because the atoms
polarized by the light wave do not emit radiation perpendicular to their plane of oszillation.
See Figure 3.2 (Eq. (3.23)) [p. 55]
Dispersion, anomale Dispersion, irregular. The refractiv index decreases for increasing frequency
of the light δn/δω < 0. Valid for the far infrared and radio region of the electromagnetic
spectra, for exotic materials in the optical region and optical elements for pulse compression.
(Eq. (4.58)) [p. 134]
Dispersion, normale Dispersion, regular. The refractiv index increases for increasing frequency of the
light δn/δω > 0 which is valid for most materials in the optical range. (Eq. (4.58)) [p. 134]
Divergenz Divergence. The divergence is a measure of spatial beam quality and decribes the angle
of a fan of rays. Unit mrad. (Eq. (2.67)) [p. 36]
Dopplerverbreiterung Doppler broadening. In analogy to the Doppler shift of an acoustic wave,
the observed frequency of light increases (blue shift) when a light source moves towards an
observer (or detector). If, in contrast, the light source moves away from an observer (or
detector) the frequency of the observed light decreases (red shift). In the case of a thermal
gas, the velocity of the atoms follows a Maxwell distribution with random orientation. There
are atoms moving towards or away from the observer, causing red and blue shift of various
extend, respectively. Averaging over a large number of atoms causes a continous broadening
of the atomic emission line, the Doppler broadening. (Eq. (2.44)) [p. 26]
Einsteinkoeffizient Coefficients for spontaneous emission A n m, stimulated emission B n m and absorption
B m n . Refer to the respective symbols. (Eq. (2.11)) [p. 17]
Elastische Streuung Elastic scattering. Scattering process without energy loss (Eq. (3.3)) [p. 44]