saturation fluence (original) (raw)

Author: the photonics expert (RP)

Definition: a measure of the incident optical pulse fluence required for achieving significant saturation of an absorber or a gain medium

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category physical foundations physical foundations

properties of laser gain media
- transition cross-sections
- upper-state lifetime
- saturation energy
- saturation fluence
- saturation intensity
- saturation power
- doping density
- pump absorption coefficient
- radiative lifetime
- (more topics)

Units: J/m2, J/cm2

Formula symbol: ($F_\textrm{sat}$)

DOI: 10.61835/au3 Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

What is a Saturation Fluence?

The saturation fluence of a laser gain medium is the fluence (energy per unit area) of an incident short pulse which leads to a reduction in the gain (per unit length) to ($1/e$) (≈ 37%) of its initial value. Similarly, the saturation fluence of a saturable absorber is defined based on the loss reduction.

See the article on saturation energy concerning further details of saturation characteristics. The saturation energy for a light beam is the saturation fluence times the beam area.

The saturation fluence can be calculated according to {F_{{\textrm{sat}}}} = \frac{{h\nu }}{{{\sigma _{{\textrm{em}}}} + {\sigma _{{\textrm{abs}}}}}}$$

where ($h \nu$) is the photon energy at the relevant wavelength, and ($\sigma_\textrm{em}$) and ($\sigma_\textrm{abs}$) are the emission and absorption cross-sections at that wavelength. The quantity ($\sigma_\textrm{abs}$) is zero for four-level gain media (exhibiting no reabsorption on the laser transition) but should not be forgotten for quasi-three-level laser gain media.

Note that the saturation fluence of a saturable absorber does not depend on the thickness of the absorber layer, unless the thickness is so large that the fluence is reduced substantially within the device.

Importance of the Saturation Fluence

The saturation energy plays an important role in various areas of laser physics and laser design. Some examples are:

It determines the pulse fluence required for extracting most of the stored energy from a gain medium of an amplifier. A problem with some media having low emission cross-sections is that the saturation fluence is higher than the damage fluence, so that complete energy extraction is not possible, except with multiple passes through the gain medium.
The ratio of pulse fluence and saturation fluence of a saturable absorber, as used e.g. in a mode-locked laser, is called the saturation parameter. It determines how strong the saturation by a single pulse is, and is one of the most important design parameters of a passively mode-locked laser.

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.