absorption length (original) (raw)

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

Category: article belongs to category general optics general optics

Related: absorptionabsorption coefficient

Units: m, cm

Formula symbol: ($L_\textrm{abs}$)

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DOI: 10.61835/kff Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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When light is absorbed in a homogeneous medium with a certain absorption coefficient ($\alpha$), the optical intensity decays exponentially in proportion to ($\exp(-\alpha z)$), 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 ($L_\textrm{abs}$) 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.

The term penetration depth is often used with the same meaning as absorption length, but it should be considered as a more general term because limited penetration into a material may not only result from light absorption, but also from reflection. That is a typical situation for metals, for example, where the rapid decay of intensity is mostly due to reflection.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is the absorption length?

The absorption length is the distance over which the optical intensity in a medium decays to ($1/e$) (approximately 37%) of its initial value due to absorption. It is the inverse of the absorption coefficient ($\alpha$).

How is the absorption length relevant for laser gain media?

The length of an optically pumped laser gain medium is often chosen to be 2 to 3 absorption lengths at the pump wavelength to ensure that the pump light is efficiently absorbed.

What is the difference between absorption length and penetration depth?

Penetration depth is a more general term. While often used synonymously with absorption length, limited penetration into a material can also be caused by other effects, such as strong reflection from the surface, which is typical for metals.

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