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Ytterbium-doped Gain Media

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Definition: laser gain media containing laser-active ytterbium ions

German: Ytterbium-dotierte Verstärkermedien

Categories: lasers, optical materials

How to cite the article; suggest additional literature

Ytterbium (Yb) is a chemical element belonging to the group of rare earth metals. In laser technology, it has acquired a prominent role in the form of the trivalent ion Yb3+, which is used as a laser-active dopant in a variety of host materials, including both crystals and glasses. It is often used for high-power lasers and for wavelength-tunable solid-state lasers.

Special Properties of Ytterbium-doped Gain Media

Ytterbium-doped laser crystals and glasses have a number of interesting properties, which differ from those of, e.g., neodymium-doped gain media:

energy levels of ytterbium ions in Yb:YAG

Figure 1: Energy levels of Yb3+ ions in Yb:YAG, and the usual pump and laser transitions.

Quasi-three-level Characteristics

The small quantum defect also has a usually unwanted consequence: the significant quasi-three-level behavior, particularly at short wavelengths. This requires such lasers to be operated with relatively high pump intensities and makes it more difficult to realize fully the potential for high power efficiency. Another difficulty arises for the resonator designs of end-pumped ytterbium lasers: a resonator mirror for injecting the pump light must have a high reflectivity at the laser wavelength and a high transmission at the only slightly shorter pump wavelength. Dichroic mirrors with such properties for closely lying wavelengths are difficult to make.

cross sections of Yb-doped glass

Figure 2: Absorption and emission cross sections of ytterbium-doped germanosilicate glass, as used in the cores of ytterbium-doped fibers (data from spectroscopic measurements by R. Paschotta, Ref. [6]).

Figure 2 shows the ytterbium transition cross sections of a germanosilicate glass. Efficient pumping is possible around a wavelength of 910 nm or near 975 nm. In the latter case, the pump linewidth must be small, and only ≈ 50% excitation level can be achieved due to stimulated emission, but the absorption length and the quantum defect are smaller than for 910-nm pumping. Strong three-level behavior occurs for lasing around 1030 nm, whereas nearly four-level behavior is observed beyond 1080 nm, where there is very little reabsorption. For ytterbium-doped crystals (e.g. Yb:YAG), there is often a choice between different lasing transitions, where those with shorter wavelengths exhibit more pronounced three-level characteristics.

cross sections of Yb:YAG

Figure 3: Absorption and emission cross sections of Yb:YAG (data for 20 °C crystal temperature taken from Ref. [23]).

Figure 3 shows the ytterbium transition cross sections of Yb:YAG. In this crystalline material, the absorption and emission peaks are less broad than in a glass. The dominant emission is around 1030 nm, but there is also the weaker 1050-nm peak, which can also be utilized for laser operation.

Overview on Ytterbium-doped Gain Media

There is a very wide range of different ytterbium-doped gain media:

Some of these media are also used as ceramic gain media.

In most cases, the ytterbium dopant ions replace other ions (often yttrium) of the host medium, which have about the same size. For a good match of atomic size and weight, a high thermal conductivity can be maintained even at high doping levels.

High-power Operation

Very high efficiencies, diffraction-limited beam quality, and output powers of more than 1 kW have been achieved with ytterbium-doped double-clad fiber lasers and amplifiers. Thin-disk lasers, which most often work with Yb:YAG crystals, can also generate well above 1 kW diffraction-limited output with high beam quality, or even higher powers with non-diffraction-limited beam quality.

Pulse Generation with Mode Locking

Various Yb-doped gain media have been used in mode-locked lasers (see below) for the generation of femtosecond pulses; the by far highest average output powers of 80 W have been obtained with a passively mode-locked thin-disk Yb:YAG laser [13, 15].

For passive mode locking, problems can arise in the form of Q-switching instabilities. This tendency is a consequence of the relatively small laser cross sections of ytterbium-doped media. Therefore, some of the broadband ytterbium-doped gain media are not very suitable for passively mode-locked lasers, particularly at high power levels, but can still be very useful in regenerative amplifiers. Relatively large cross sections are found for tungstate crystals.

Some ytterbium-doped crystals have a fairly broad amplification bandwidth, but the emission curve is not very smooth; it exhibits several maxima. In such cases, wide wavelength tunability may still be achieved, but the realization of very short pulses with mode locking is difficult.

Quenching and Photodarkening

Due to the very simple level structure of the Yb3+ ion, it is widely believed that quenching effects are basically impossible. However, it has been discovered [7] that even strong quenching effects can occur in ytterbium-doped fibers. In that case, some fraction of the ytterbium ions – sometimes a few percent, sometimes more than 50% – then has an extremely shortened upper-state lifetime, whereas the other Yb ions are basically unaffected. The fraction of quenched ions depends strongly on the fabrication conditions. Even a small fraction is sufficient for strongly reducing the laser or amplifier performance, particularly for laser or pump wavelengths with strong absorption cross sections.

Another detrimental effect is photodarkening in Yb-doped fibers, a gradual degradation of fibers observed particularly in cases where a high ytterbium excitation density is required.

So far, only a limited amount of data on such effects is available, and the issues are not yet very well understood.

Ytterbium Codoping

Ytterbium doping is also often used together with erbium doping. Typically, ytterbium ions absorb the pump radiation and transfer the excitation energy to erbium ions. Even though the erbium ions could directly absorb radiation e.g. at 980 nm, ytterbium codoping can be useful because of the higher ytterbium absorption cross sections and the higher possible ytterbium doping density in typical laser glasses, so that a much shorter pump absorption length and a higher gain can be achieved. Ytterbium codoping is also sometimes used for praseodymium-doped upconversion fiber lasers.


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[21]M. Laroche et al., “Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers”, J. Opt. Soc. Am. B 23 (2), 195 (2006)
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[23]J. Koerner et al., “Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF2 between 20 °C and 200 °C and predictions on their influence on laser performance”, J. Opt. Soc. Am. B 29 (9), 2493 (2012)

(Suggest additional literature!)

See also: gain media, rare-earth-doped gain media, laser crystals, rare-earth-doped fibers, four-level and three-level gain media, neodymium-doped gain media, ceramic gain media, tungstate lasers, solid-state lasers, photodarkening, Spotlight article 2006-09-06
and other articles in the categories lasers, optical materials

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Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

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