In-band pumping is a specific method of optical pumping, generally applied to pumping lasers or optical amplifiers, and mostly in the context of solid-state lasers. Here, an absorption transition is used that starts at the lower laser level manifold and ends at the upper laser level manifold. In other words, the pump transition works on the same energy level manifolds as the laser transition. Both are Stark level manifolds consisting of multiple generally non-degenerate Stark levels. That allows for a substantial bandwidth for absorption and emission. Due to the rapid thermalization of electrons in each manifold, one generally has a thermal equilibrium in each of them, and due to the difference in wavelength between pump and laser (or signal) light, one can have net absorption of pump light while there is gain for the laser or signal light. This would not work for a simple two-stage system.
In a somewhat broader sense, the term in-band pumping is sometimes used for pumping directly into the upper laser level manifold, even if one starts at a level below the lower laser level. This happens for some neodymium-doped lasers, as explained below.
In-band pumping is particularly common for fiber lasers and fiber amplifiers, more so than for bulk lasers, where the available pump transitions are often quite narrow. In some cases, in-band pumping is combined with cryogenic operation.
Consequences of In-band Pumping
In-band pumping can in various ways affect the operation of the laser or amplifier and the achievable performance:
- The quantum defect is generally quite small, with the pump wavelength being not much shorter than the laser or signal wavelength. That allows for high power conversion efficiency, unless substantial power losses occur elsewhere. Consequently, the heat load is relatively small, allowing high output powers without excessive thermal effects.
- There is generally some amount of stimulated emission caused by the pump light, particularly if the pump wavelength is not close to the shorter-wavelength edge of the transition. As a result, the amount of upper-state population that can be achieved (even with high pump intensity and no signal light present) is limited. This limits the gain per unit length.
- Another effect of the limited upper-state population is an increased noise figure of an amplifier.
By slightly decreasing the pump wavelength, one can achieve a higher upper-state population, provided that the pump optical intensity is high enough. In this way, one can achieve more gain and a lower noise figure, but at the cost of a larger quantum defect and possibly less efficient pump absorption.
Examples for In-band Pumping
Ytterbium-doped Fiber Amplifiers and Lasers
For ytterbium-doped laser gain media, there is no other choice than in-band pumping, as only two level manifolds are available, ignoring very high lying excited levels beyond the host medium's band gap.
Erbium-doped Amplifiers and Lasers
In-band pumping erbium-doped fiber amplifiers for the 1.55-μm wavelength region is possible at wavelengths around 1.45 μm. One may even use substantially longer pump wavelengths, but then the region with positive gain is shifted to longer wavelengths. This is due to the lower excitation level of the erbium ions.
Compared to the other common pump configuration – pumping into 4I11/2 around 975 nm –, one has a substantially lower quantum defect, but usually achieves less gain per unit length and has to accept a higher noise figure.
Er:YAG lasers for emission around 1.6 μm are sometimes in-band-pumped around 1470 nm. This allows for higher pulse energies from Q-switched Er:YAG lasers, for example.
Thulium-doped Amplifiers and Lasers
Most thulium-doped amplifiers and lasers operate on the 2μm transition between the two lowest strong-level manifolds. Such devices are often in-band pumped. Because the junction is relatively broadband, the pump wavelength can still be substantially shorter than the signal or laser wavelength.
More powerful laser diodes are available in the region around 780 nm for pumping into a higher level. Here, the quantum defect is very large, but it is possible to use a energy transfer process to get two ions in the upper laser level for only one pumped ion. However, in-band pumping is still more efficient.
Some neodymium-doped laser gain media such as Nd:YAG are often pumped around 808 nm into the 4F5/2 and 2H9/2 manifolds, which are somewhat above the upper laser manifold 4F3/2. The quantum defect can be somewhat reduced by pumping around 869 nm, which leads directly into the upper laser manifold; this is then often called in-band pumping, even if one utilizes the 1064-nm laser transition, where the (very short-lived) lower laser level lies above the ground state manifold where the pumping process starts.
That use of the term is actually somewhat inaccurate, as the pump radiation is really in a different transition band. Strictly speaking, in-band pump would be impossible for four-level laser gain media that have a very short-lived lower laser level.
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