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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2009-02-06

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

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.

The main applications of saturable absorbers are passive mode locking and Q switching of lasers, i.e., the generation of short 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.

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:

• Particularly for passive mode locking (but also for Q switching), semiconductor saturable absorber mirrors (also called SESAMs) are frequently used [5]. These are also suitable for passive Q switching, particularly at lower pulse energies.
• Other semiconductor saturable absorbers for mode locking or Q switching are based on quantum dots e.g. of lead sulfide (PbS) suspended in glasses [9].
• In some mode-locked diode lasers, a saturable absorber section is created simply by not pumping that region. A faster recovery can be obtained by implanting nitrogen (N⁺) ions.
• Gallium arsenide (GaAs) is also sometimes used for passive Q switching of 1-μm lasers, even though the photon energy of these lasers is below the bandgap in that case. Certain crystal defects play an important role for the absorption.
• For passive Q switching of solid-state lasers in the 1-μm spectral region, Cr⁴⁺:YAG saturable absorber crystals are most popular [10]. (Cr:YAG crystals are also used as gain media → chromium-doped gain media.) For 1.3-μm lasers, V³⁺:YAG can be used.
• In rare cases, saturable absorber materials are used in the form of optical fibers. For example, chromium, samarium or bismuth dopants can serve this function in Q-switched fiber lasers [11].

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.

• Kerr lensing combined with some kind of aperture (→ Kerr lens mode locking) [4]
• a nonlinear mirror device containing a frequency-doubling crystal, as sometimes used for passive mode locking of solid-state bulk lasers [2]
• a nonlinear fiber within an auxiliary resonator (→ additive-pulse mode locking) [7]
• nonlinear polarization rotation in a fiber, combined with a polarizing element, often used for passive mode locking of fiber lasers [8]
• a nonlinear fiber loop mirror, also used for mode locking of fiber lasers [3]
• an array of waveguides, exhibiting nonlinear coupling [13]

Properties of Saturable Absorbers

The most important properties of saturable absorbers are:

• the modulation depth, i.e., the maximum possible change in optical loss
• the unsaturable losses, i.e., the (typically unwanted) part of the losses which can not be saturated
• the recovery time, i.e., the decay time of the excitation after an exciting pulse
• the saturation fluence, i.e., the fluence (energy per unit area) it takes to reduce the initial value to 1/e (∼ 37%) of its initial value
• the saturation energy is the saturation fluence times the mode area
• the saturation intensity, i.e. the optical intensity (power per unit area) that it takes in a steady state to reduce the absorption to half of its unbleached value
• the saturation power, i.e. the saturation intensity times the mode area
• the damage threshold (in terms of intensity or fluence)

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 saturable absorber (e.g. in a mode-locked laser) is the ratio of the incident pulse fluence to the saturation fluence of the device.

Bibliography

See also: semiconductor saturable absorber mirrors, passive mode locking, mode-locked lasers, Q switching, Q-switched lasers, gain saturation

Categories: nonlinear optics, photonic devices, pulses

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