Thulium-doped Laser Gain Media

Thulium (Tm) is a chemical element belonging to the group of rare earth metals. In laser technology, it is used in the form of the trivalent ion Tm³⁺ as a laser-active dopant in a variety of host materials, including

• crystals such as YAG, YLF, Lu₂O₃, Sc₂O₃, Y₂O₃, or CaF₂,
• glasses, including some to be drawn into optical fibers, and
• ceramics like YAG or some sesquioxides.

Thulium-doped media are widely used in various types of high-power lasers and for wavelength-tunable solid-state lasers. Due to the considerable gain bandwidth of e.g. Tm-doped fibers, powerful ultrafast amplifiers can also be realized. Thus, there is a very wide range of laser applications for thulium-doped gain media – mostly realized with emission in the 2-μm spectral region, but also in various other spectral regions, as explained below. Applications are found in very diverse fields such as medical treatments (e.g. skin treatments and prostate surgery), LIDAR, optical data transmission, and fundamental science such as attosecond pulse generation.

Laser Transitions of Thulium-doped Gain Media

Tm³⁺ ions have several excited energy levels (more precisely: level manifolds) that can be relevant for their operation as a laser gain medium. Fig. 1 shows these levels. Unfortunately, the labeling of these levels is not consistent in the literature: ³H₄ and ³F₄ can be interchanged. Note that we are dealing with level manifolds that should be labeled by the dominant state, which is not easy to determine and may also depend on the host medium.

Based on these levels, a considerable number of lasers or amplifiers laser transitions are possible, and some of them can be realized with different types of optical pumping. Some of them are only possible with a limited range of thulium-doped media, e.g. due to too short level lifetimes in some cases.

Some of the most important transitions and pumping schemes are described below.

2-μm Transition

The most commonly used laser/amplifier transition of the Tm³⁺ ion is that between the two lowest energy level manifolds (³H₄ to ³H₆). This emission occurs at a wavelength of about 2 μm. Its gain bandwidth is substantial; for example, Tm-doped silica fibers can be used to amplify signals in a range about 500 nm wide, although a particular amplifier design will usually operate in a much narrower range. An emission between 1.9 μm and 2.1 μm is most common. The shorter the emission wavelength, the more pronounced the quasi-three-level behavior with reabsorption at the laser wavelength.

The simplest and most efficient way of optical pumping for this transition is in-band pumping at a wavelength of about 1.6 μm to 1.8 μm. However, high output powers and efficiencies of laser diodes are difficult to achieve in this spectral region. There are other possible pump sources, such as Raman fiber lasers or another thulium-doped fiber device, but these are much more expensive than laser diodes.

Pumping around 1200 nm into the ³H₅ level would in principle be recently efficient, but is also not practical due to the lack of attractive pump sources.

Therefore, particularly for high-power fiber lasers and amplifiers one often uses high-power laser diodes emitting around 790 nm, which can excite the ³F₂ level. That would usually be expected to be highly inefficient due to the very large quantum defect and the possibly incomplete and slow transfer of ions into the upper laser level (³H₄). However, for sufficiently high doping concentration one can exploit a very useful energy transfer process involving two thulium ions, one starting in the ³F₄ level (after 790-nm excitation) and one in the ground state (³H₆). Both then end up in the upper laser level. Effectively, with that cross-relaxation (possibly assisted by energy migration) one obtains two excited ions for only one absorbed pump photon. There are also possible detrimental energy transfer processes, e.g. leading to upconversion, which may as a side effect also cause photodarkening. However, with proper optimization of the doping concentration (including Al codoping for avoiding clustering of Tm ions despite a high doping concentration) and other laser/amplifier parameters, one can obtain a slope efficiency up to around 70%.

In-band pumping is still an interesting option for very high output powers, where the limited efficiency of 790 nm pumping causes a high thermal load. One can realize in-band pumping with a tandem pumping approach: using one or more Tm-doped fiber lasers, e.g. operating at 1.9 μm, to pump the final Tm-doped high-power fiber amplifier, which then operates at e.g. 2.0 μm. By distributing the heat load over multiple devices, multi-kilowatt output powers can be achieved. Note also that in-band pumping does not require a high Tm concentration, so the thermal load can be distributed over a longer fiber length.

1.5-μm Transition

Thulium-doped fiber amplifiers can exploit the ³F₄ to ³H₄ transition at wavelengths around 1470 nm. This is particularly useful in the context of fiber optic communications for amplifying optical signals in the S-band, i.e. at somewhat shorter wavelengths than the more common C-band, for which erbium-doped fiber amplifiers are commonly used. The latter can also be optimized for shorter signal wavelengths, but this comes at the cost of significant power loss to perform amplified spontaneous emission at longer wavelengths. Therefore, a thulium-doped amplifier is the more efficient solution for amplifying signals at short wavelengths.

A thulium-doped fiber amplifier can be pumped at about 1.4 μm. The absorption of such photons leads from the ground state to ³H₄ and from there via excited-state absorption to ³F₄, which is the upper laser level. After stimulated emission, ions can be repumped directly in the same way. Normally, this works only at high pump intensities because the first absorption transition (from the ground state) is relatively weak. At high doping concentrations, however, an energy transfer process can be used to increase the efficiency, essentially by pushing more ions from the ground state to the lower laser level, from where they can be effectively pumped to the upper laser level [5].

2.3-μm Transition

The transition from ³F₄ to ³H₅ occurs at wavelengths around 2.3 μm. Pumping into the upper laser level can be done around 790 nm. The lower laser level ³H₅ is quickly depopulated by a multiphonon transition to ³H₄. Since this metastable level is relatively long-lived and is also populated by a cross-relaxation mechanism as explained above, one should take measures to avoid an excessive population remaining there (see the article on self-terminating laser transitions). One possibility is to use simultaneous lasing at 2.0 μm, which firstly brings the ions back to the ground state from where they can be repumped, and secondly generates radiation that can directly repump ions from ³H₄ to the upper laser level. In addition, stimulated Raman scattering can be used to transfer energy from the 2.0 μm beam to 2.3 μm [14].

For silica fibers, the emission wavelength is already in the absorbing region. Therefore, one requires some host medium with better mid-IR transparency – for example, tellurite fibers.

0.8-μm Transition

The emission around 0.8 μm results from the transition from ³F₄ to ³H₆. In principle, it is easy to pump directly into the upper laser level around 790 nm and use the emission e.g. around 815 nm. However, there may be strong non-radiative transitions to ³H₅ that bypass the laser transition. This can be avoided by using a host material with low phonon energies, e.g. a fluoride glass.

0.48-μm transition

Emission around 480 nm is achieved with a transition from ¹G₄ down to ³H₆. This is mostly done with high phonon energy media like fluoride glasses, where the ¹G₄ lifetime can be relatively long. At the same time, the absorption transitions are sufficiently broadband to allow upconversion pumping with a single pump source at about 1140 nm. Figure 2 shows the three-step pumping process. The pumping process can be quite efficient [2], but a photodarkening process can significantly degrade the laser efficiency.

Influences of the Host Medium

The used host medium can influence laser or amplifier operation in various ways:

• The magnitude and wavelength dependence of the transition cross-sections depends substantially on it. Generally, crystalline media have narrower transitions with higher peak cross-sections.
• The level lifetimes vary a lot – not only due to different emission cross-sections, determining the radiative lifetimes, but also because of a greatly varying tendency for non-radiative transitions based on multi-phonon emission. Some heavy metal glasses exhibit far longer level lifetimes as a basis for upconversion lasers, for example.
• Some media are optically isotropic, while for others the cross-sections are strongly polarization-dependent, which can be useful for easily obtaining linearly polarized emission and avoiding thermal depolarization.
• The thermo-optic properties influence thermal lensing.
• The maximum feasible thulium doping concentration (without quenching effects and the like) also substantially depends on the material.
• There are several other details, such as the tendency for photodarkening.

More to Learn

Encyclopedia articles:

• laser gain media
• rare-earth-doped laser gain media
• laser crystals
• rare-earth-doped fibers
• quasi-three-level laser gain media