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Saturable Absorbers

Definition: light absorbers with a degree of absorption which is reduced at high optical intensities

German: sättigbare Absorber

Categories: nonlinear optics, photonic devices, light pulses

How to cite the article; suggest additional literature

A saturable absorber is an optical component with a certain optical loss, which is reduced at high optical intensities. This can occur, e.g., in a medium with absorbing dopant ions, when a strong optical intensity leads to depletion of the ground state of these ions. Similar effects can occur in semiconductors, where excitation of electrons from the valence band into the conduction band reduces the absorption for photon energies just above the bandgap energy. There are also artificial saturable absorbers (see below), where there is no real absorption, but an optical loss which decreases for increasing optical power.

The main applications of saturable absorbers are passive mode locking and Q switching of lasers, i.e., the generation of short optical pulses. However, saturable absorbers are also useful for purposes of nonlinear filtering outside laser resonators, e.g. for cleaning up pulse shapes, and in optical signal processing.

As an example, Figure 1 shows how the reflectivity of a slow saturable absorber varies with the saturation parameter, which is related to the incident pulse energy. The reflectivity for pulse is calculated as the ratio of reflected to incident pulse energy. Note that the actual reflectivity varies with time; it is initially lower but then rises due to absorber saturation.

SESAM saturation
Figure 1: Reflectivity of a slow saturable absorber versus saturation parameter S, which is the pulse fluence divided by the saturation fluence of the device. The modulation depth (maximum change in reflectivity) is 1%, and the nonsaturable losses are 0.5%.

Types of Saturable Absorbers

As different applications require saturable absorbers with very different parameters, different devices are used:

Artificial Saturable Absorbers

There are also various kinds of artificial saturable absorbers. These are devices which exhibit decreasing optical losses for higher intensities, but not actually exploiting saturable absorption. Such devices can be based on e.g.

Properties of Saturable Absorbers

The most important properties of saturable absorbers are:

When dealing with pulses, a fast saturable absorber is one with a recovery time well below the pulse duration, whereas a slow absorber is one with a recovery time well above the pulse duration. This means that the same device may be either a fast absorber or a slow absorber, depending on the pulses with which it is used. A fast absorber is not necessarily better suited e.g. for passive mode locking; in fact, self-starting mode locking is more easily achieved with a slow absorber.

The saturation parameter of a slow saturable absorber (e.g. in a mode-locked laser) is the ratio of the incident pulse fluence to the saturation fluence of the device. For a fast absorber, it is the pulse peak intensity divided by the saturation intensity.

Note that absorber parameters often refer to a simple absorber model, which is based on assumptions which are not necessarily fulfilled by a real absorber. Some examples:

For such seasons, a set of absorber parameters may not completely describe the actual characteristics. Another aspect is that the spatially variable degree of absorption due to the transverse intensity profile of a laser beam is often ignored in simple calculations.

Selecting a Suitable Saturable Absorber

It depends very much on the concrete circumstances what properties of a saturable absorber are desirable. In particular, there are important differences between the requirements for Q switching and mode locking of lasers.

Typical requirements on a saturable absorber for a passively Q-switched laser are:

For passively mode-locked lasers, the requirements are different:

Generally, decisions on absorber parameters should be made in the context of a comprehensive laser design processes, which takes into account both the dynamics of pulse generation and the limited tolerance of the absorber to high intensities or pulse energies.


The RP Photonics Buyer's Guide contains 17 suppliers for saturable absorbers. Among them:


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(Suggest additional literature!)

See also: semiconductor saturable absorber mirrors, passive mode locking, mode-locked lasers, Q switching, Q-switched lasers, gain saturation
and other articles in the categories nonlinear optics, photonic devices, light pulses

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