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Saturation Power

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

German: Sättigungsleistung

Category: physical foundations

Formula symbol: Psat

Units: W

How to cite the article; suggest additional literature

The saturation power of a laser gain medium is the optical power of an input signal which in the steady state leads to a reduction in the gain to half of its small-signal value. Similarly, the saturation power of a saturable absorber is defined.

The saturation intensity is the corresponding optical intensity, i.e., the saturation power per unit area.

Usually it is assumed that the gain is small, i.e. input and output powers are similar. For high gain, it is common to refer to the output power.

For a low-gain laser amplifier, saturation intensity and power can be calculated according to

saturation intensity and power

where h ν is the photon energy at the signal wavelength, σem and σabs are the emission and absorption cross sections at the emission wavelength, τ is the upper-state lifetime, and A is the mode area. The quantity σabs is zero for four-level gain media but should not be forgotten for quasi-three-level gain media.

Calculator for the Saturation Power

Transition cross section:
Upper-state lifetime:
Beam radius:
Saturation power: calc

Enter input values with units, where appropriate. After you have modified some inputs, click the "calc" button to recalculate the output.

saturation of laser gain

Figure 1: Dependence of laser gain on the optical power, calculated for the steady state. When the power equals the saturation power, the gain is reduced to half the small-signal gain.

A comparison with the equations for the saturation energy shows that the saturation power can be calculated as the saturation energy divided by the upper-state lifetime.

Importance of the Saturation Power

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

The saturation power should not be confused with the saturated output power, which is usually not precisely defined but means the output power achieved for an input signal power which causes significant amplifier saturation. Obviously, the saturated output power (other than the saturation power) depends on the pump power.

Pump Saturation

A subtle detail is that the saturation characteristics can be modified if the pump intensity of the gain medium is comparable to or higher than the pump saturation power (which is defined as above, but based on photon energy and transition cross sections at the pump wavelength). In this situation, the rule that the gain is reduced by a factor of 2 for an intensity equal to the saturation intensity (as defined above) no longer holds.

For two reasons, however, pump saturation effects are in most cases not very important for lasers and amplifiers:

See also: gain saturation, saturable absorbers, saturation energy, transition cross sections, Spotlight article 2008-01-06
and other articles in the category physical foundations

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