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Pump Absorption

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Definition: the absorption of pump light, e.g. in a laser or a laser amplifier

German: Pumpabsorption, Absorption des Pumplichts

Categories: optical amplifiers, lasers, physical foundations

How to cite the article; suggest additional literature

In an optically pumped laser or amplifier, the gain medium is powered by supplying it with pump light. Only absorbed pump light can be useful in such a kind of device, whereas e.g. an optical parametric amplifier provides amplification (gain) for a small signal without absorbing significant amounts of power. Therefore, it is essential for the power efficiency of a laser or laser amplifier that the pump light is efficiently absorbed.

Calculating Pump Absorption

For very small pump intensities, or for devices operating with a negligible degree of excitation of the laser-active atoms or ions, the degree of absorption can be simply calculated from the doping concentration Ndop, the length L of the gain medium, and the absorption cross section σabs at the pump wavelength: the degree of power absorption is

pump transmission of gain medium

Calculator for Pump Absorption

Doping concentration:
Absorption cross section:
Absorption length: calc
Length of medium:
Absorption in given length: calc

Enter input values with units, where appropriate. After you have modified some values, click a "calc" button to recalculate the field left of it.

Due to the wavelength dependence of the absorption cross sections, the degree of pump absorption also depends on the wavelength. For some gain media with a small absorption bandwidth, the absorption efficiency can be significantly compromised, e.g. when using a pump source with finite optical bandwidth, or when the emission wavelength depends on a varying device temperature.

For anisotropic laser crystals, the absorption cross sections can also be polarization-dependent. Pronounced polarization effects can occur e.g. in various types of vanadate lasers and tungstate lasers: pump absorption can be fairly efficient for one polarization, and fairly incomplete for the other polarization direction. It is then best to use a polarized pump source and properly adjust the polarization direction. Problems can arise e.g. when using a fiber-coupled diode laser as the pump source, where the polarization state of pump light may not only not be linear, but also change with temperature or when the fiber is moved; the laser's output power is then not stable.

The above equation is not valid in cases with absorption saturation, as discussed below.

Saturation of Pump Absorption

Some laser or amplifier devices exhibit a significant degree of excitation of the laser-active ions; this is particularly the case for those based on a quasi-three-level gain medium. The absorption is then saturated to some extent, simply because the density of atoms or ions in the ground state, from where they can absorb light, is depleted.

The degree of pump saturation will in general depend on the intensity of signal (laser) light. In a laser, for example, the lasing process may clamp the upper-state population to some relatively low level, so that the degree of pump saturation is also clamped (in steady-state operation) as soon as the laser threshold is reached: above threshold, there is no further dependence on pump power, except if the transverse profile of the excitation density is significantly power-dependent.

Note that a high level of excitation in some portion of the gain medium does not inevitably lead to significant pump saturation effects. The point is that a reduced pump absorption at some point can increase the pump intensity in other parts of the gain medium. What matters is essentially the overall absorbed pump power.

Measurement of Pump Absorption

In principle, the measurement of pump absorption is trivial: the incident pump power as well as the residual pump power have to be measured. However, there are many situations where this is not easy to do:

For such reasons, it is often better to work out a laser design based on the knowledge of (unsaturated) low-intensity absorption, combined with theoretical knowledge on absorption saturation. In many cases (e.g. for most Nd:YAG lasers), pump saturation effects are not very strong and can be ignored altogether.

Optimizing Pump Absorption

Very efficient pump absorption could in principle be obtained with a laser design based on a gain medium with a large length and/or a high doping concentration. However, there are various limitations:

In many cases, a pump absorption efficiency of e.g. 90% is considered to be satisfactory, as the negative side effects of measures for a further increased absorption efficiency would dominate.

Bibliography

[1]R. Paschotta, tutorial on "Fiber Amplifiers", part 2 on gain and pump absorption
[2]R. Paschotta, tutorial on "Modeling of Fiber Amplifiers and Lasers" (in particular, see part 3)

(Suggest additional literature!)

See also: lasers, amplifiers, optical pumping, Spotlight article 2006-11-04, Spotlight article 2007-08-22
and other articles in the categories optical amplifiers, lasers, physical foundations

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