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Absorption Length

Definition: the length of which absorption reduces the optical intensity to 1/e of its initial value

German: Absorptionslänge

Category: general optics

Formula symbol: α

Units: m, cm

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When light is absorbed in a homogeneous medium with a certain absorption coefficient α, the optical intensity decays exponentially in proportion to exp(−αz), where z is the propagation distance. (It is assumed that the intensity is not affected by divergence or convergence of a beam.) One defines the absorption length as the inverse of the absorption coefficient. After that propagation length, the intensity decays to 1/e (≈37%) of its initial value. After four absorption lengths, only ≈1.8% of the initial intensity is left.

An optically pumped laser gain medium is often made with a length which is about 2 to 3 absorption lengths at the pump wavelength, so that the pump absorption is sufficiently effective.

See also: absorption, absorption coefficient
and other articles in the category general optics


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