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Optical Pumping

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Definition: electronically exciting a medium with light, or specifically populating certain electronic levels

German: optisches Pumpen

Optically pumping some medium essentially means to inject light in order to electronically excite the medium or some of its constituents into other (usually higher-lying) energy levels. In the context of lasers or laser amplifiers, the goal is to achieve a population inversion in the gain medium and thus to obtain optical amplification via stimulated emission for some range of optical frequencies. (The width of that range is called the gain bandwidth.) In other cases, such as in spectroscopic measurements, the goal can be to selectively populate a specific electronic level (e.g. some hyperfine sublevel), which does not necessarily have an energy well above the ground state.

Optical pumping processes can often be described with rate equation modeling. However, this disregards some aspects of the quantum nature of the atom–photon interaction. More comprehensive physical models exist which can also describe coherent phenomena such as Rabi oscillations.

Optically Pumped Lasers

Various types of lasers can be optically pumped:

Common types of optical pump sources are:

Examples of the latter case are titanium–sapphire lasers pumped with frequency-doubled solid-state lasers, and dye lasers pumped with gas lasers.

A requirement for achieving a high power conversion efficiency in an optically pumped laser is that the pump light is efficiently absorbed in the gain medium. This can be the case if the gain medium is sufficiently long, has a high doping concentration, and a sufficiently wide optical frequency range for the pump light. In some cases, the pump absorption efficiency can be increased by arranging for multiple passes of the pump light through the gain medium. This technique is often used e.g. in thin-disk lasers.

A frequently used alternative to optical pumping is electrical pumping, applied particularly to laser diodes and gas lasers.

Using Multiple Electronic Levels

In the simplest case, the optically pumped medium absorbs light, and each absorbed photon excites one atom or ion into a higher-lying electronic level. The energy difference of the involved electronic levels must match the photon energy of the pump light. Once some degree of excitation of the medium has been achieved, the same kind of light can also cause stimulated emission, bringing excited atoms or ions back to the lower level. For that reason, a medium with a simple two-level scheme cannot reach a population inversion.

Optical pumping in lasers involves at least three different energy levels. In the simplest case, laser-active ions are pumped from their ground state to a higher-lying level, from where they undergo a quick radiative or non-radiative decay into an excited level with somewhat lower energy. As pump light cannot cause stimulated emission from this level down to the ground state (due to the too high photon energy), a population inversion can be achieved for a sufficiently high pump intensity. By introducing a fourth energy level, one can obtain a four-level laser system, where the laser transition involves two intermediate levels. If the lower level population is kept small due to a quick decay to the ground state, population inversion can be achieved even when only a tiny fraction of the ions is excited. The article on four-level and three-level gain media explains the details.

In-band Pumping

Solid state laser gain media often exhibit slightly non-degenerate Stark level manifolds. Due to the energy variations within each manifold, optically pumped laser operation is possible even with only two involved manifolds: ions are pumped from the lower manifold (usually the ground-state manifold) to some higher manifold, and the laser transition directly leads from there back to the lower manifold, with no intermediate manifolds.

This pump scheme, called in-band pumping, can be used with various laser-active ions:

The term in-band pumping is also often used in cases where the laser-active ions are pumped from the ground-state manifold (not being the lower laser level) directly into the upper laser level. For example, this happens when a neodymium-doped laser crystal is pumped around 0.88 μm.

In-band pumping often leads to a small quantum defect, but also often to significant effects of reabsorption from the lower laser level (→ quasi-three level behavior). Also, the achievable degree of excitation (and thus the laser gain) can be limited by stimulated emission caused by the pump light. A somewhat shorter pump wavelength may mitigate the latter problem, but this can decrease the pump absorption efficiency.

Side Pumping Versus End Pumping

In an optically pumped laser, pump light can be injected into the gain medium from different directions. Side pumping means that the light is injected in directions which are roughly perpendicular to that of the laser beam. In the case of end pumping, the pump light is approximately collinear with the laser beam. The pump geometry has implications for the required beam quality (see below), but also the achieved power conversion efficiency, the laser gain, and the laser beam quality.

Requirements on the Pump Light

Pump light for optical pumping has to fulfill a number of requirements:

See also: end pumping, side pumping, pump absorption, Rabi oscillations, diode-pumped lasers, lamp-pumped lasers

Categories: lasers, methods, quantum optics

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