Encyclopedia > letter P > pump parameter

Pump Parameter

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: the ratio of pump power and threshold pump power of a laser

Category: article belongs to category laser devices and laser physics laser devices and laser physics

Units: (dimensionless number)

Formula symbol: <$r$>

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

The pump parameter <$r$> of a laser is the ratio of pump power and threshold pump power. Usually, this quantity is used for optically pumped lasers in continuous-wave operation.

Most lasers are operated with a pump parameter between 3 and 10, i.e. significantly but not very far above threshold. This is because extreme values often have some disadvantages:

For small pump parameters (close to 1), the power conversion efficiency is reduced, the output power depends sensitively on the pump power, and the relaxation oscillations are weakly damped.
For very large pump parameters, diffraction-limited beam quality is often difficult to achieve (at least in bulk lasers), since e.g. small tails of the pump intensity profile can excite higher-order modes. Also, the power efficiency may be degraded due to the strong influence of parasitic resonator losses.

Extreme values of pump parameters are nevertheless required in some cases:

Some single-frequency lasers have to be operated with small pump parameters because for stronger pumping additional resonator modes would start to oscillate.
High-power fiber lasers are often operated far above threshold simply because the threshold power is very low, even with strong output coupling.
Passively mode-locked lasers often have to be operated far above threshold to suppress Q-switching instabilities.

Particularly in the context of four-level lasers, it can be convenient to use the pump parameter <$r$> in equations, as it is most easy to evaluate in an experiment, and thus often well known. In four-level lasers, the intracavity optical intensity in the gain medium (in the steady state, assuming a flat-top mode profile) is (<$r - 1$>) times the saturation intensity. Also, there is a simple equation for the frequency of relaxation oscillations, containing the parameter <$r$>. However, such equations then often do not hold for (quasi-)three-level gain media.

More to Learn

Encyclopedia articles:

Blog articles:

The Photonics Spotlight 2007-01-21: “Laser Operation Far Above Threshold”

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.