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Saturable Absorbers

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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.

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.

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

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.

Bibliography

[1]B. K. Garside and T. K. Lim, “Laser mode locking using saturable absorbers”, J. Appl. Phys. 44 (5), 2335 (1973)
[2]K. A. Stankov, “A mirror with an intensity-dependent reflection coefficient”, Appl. Phys. B 45, 191 (1988)
[3]M. E. Fermann et al., “Nonlinear amplifying loop mirror”, Opt. Lett. 15 (13), 752 (1990)
[4]T. Brabec et al., “Kerr lens mode locking”, Opt. Lett. 17 (18), 1292 (1992)
[5]U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers”, IEEE J. Sel. Top. Quantum Electron. 2, 435 (1996)
[6]A. Sennaroglu, “Continuous wave thermal loading in saturable absorbers: theory and experiment”, Appl. Opt. 36 (36), 9528 (1997)
[7]J. Mark et al., “Femtosecond pulse generation in a laser with a nonlinear external resonator”, Opt. Lett. 14 (1), 48 (1989)
[8]M. E. Fermann, “Passive mode locking by using nonlinear polarization evolution in a polarization-maintaining erbium-doped fiber”, Opt. Lett. 18 (11), 894 (1993)
[9]P. T. Guerreiro and S. Ten, “PbS quantum-dot doped glasses as saturable absorbers for mode locking of a Cr:forsterite laser”, Appl. Phys. Lett. 71 (12), 1595 (1997)
[10]A. M. Malyarevich et al., “V:YAG – a new passive Q-switch for diode-pumped solid-state lasers”, Appl. Phys. B 67, 555 (1998)
 [11]Z. Burshtein et al., “Excited-state absorption studies of Cr4+ ions in several garnet host crystals”, IEEE J. Quantum Electron. 34 (2), 292 (1998)
[12]S. Y. Set et al., “Laser mode locking using a saturable absorber incorporating carbon nanotubes”, J. Lightwave Technol. 22 (1), 51 (2004)
 [13]H. Ridderbusch and T. Graf, “Saturation of 1047- and 1064-nm absorption in Cr4+:YAG crystals”, IEEE J. Quantum Electron. 43 (2), 168 (2007)
[14]Y. Y. Dvoyrin et al., “Yb-Bi pulsed fiber lasers”, Opt. Lett. 32 (5), 451 (2007)
[15]A. Schmidt et al., “Passive mode locking of Yb:KLuW using a single-walled carbon nanotube saturable absorber”, Opt. Lett. 33 (7), 729 (2008)
[16]F. Shohda et al., “147 fs, 51 MHz soliton fiber laser at 1.56 μm with a fiber-connector-type SWNT/P3HT saturable absorber”, Opt. Express 16 (25), 20943 (2008)
[17]D. D. Hudson et al., “Nonlinear femtosecond pulse reshaping in waveguide arrays”, Opt. Lett. 33 (13), 1440 (2008)
[18]T. Tsai et al., “Passively Q-switched erbium all-fiber lasers by use of thulium-doped saturable-absorber fibers”, Opt. Express 18 (10), 10049 (2010)
[19]A. Martinez and S. Yamashita, “Multi-gigahertz repetition rate passively modelocked fiber lasers using carbon nanotubes”, Opt. Express 19 (7), 6155 (2011)
[20]M. N. Cizmeciyan et al., “Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm”, Opt. Lett. 38 (3), 341 (2013)

(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

In the RP Photonics Buyer's Guide, 25 suppliers for saturable absorbers are listed.

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