Laser Transitions

Definition: optical transitions where stimulated emission is used to obtain optical amplification

A laser transition (or amplifier transition) is a transition between two electronic levels of some laser-active ion, for example, where stimulated emission can take place and this leads to optical amplification. This amplification can be used in an optical amplifier or a laser.
Certain conditions should usually be fulfilled so that some optical transition can serve as an efficient laser transition:

• It must be possible in some way to strongly populate the upper level, e.g. via optical pumping.
• Ideally, stimulated emission from the upper level to the lower level should be the dominating process, i.e., any additional radiative and non-radiative transitions should be comparatively weak. In quantitative terms, the upper-state lifetime should be large compared with the inverse stimulated emission rate.
• There should be a mechanism which quickly depopulates the lower laser level after stimulated emission, so that reabsorption on that transition cannot have a strong effect.

Figure 1: Energy level schemes of various laser-active ions with important pump (blue) and amplifier (red) transitions. Each horizontal line indicates a whole Stark level manifold, containing multiple Stark levels. The level energies and transition wavelengths somewhat depend on the host material. The dashed lines indicate fast non-radiative multi-phonon transitions.

Some laser gain media offer laser transitions with nearly ideal characteristics. For example, the neodymium ions in Nd:YAG exhibit a transition from the upper Stark manifold ⁴F_3/2 to the lower manifold ⁴I_11/2, where stimulated emission at a wavelength of 1064 nm can occur. Efficient pumping is possible e.g. around 808 nm (although with a significant quantum defect), spontaneous emission from the upper level is reasonably weak (due to the narrow emission bandwidth), and the lower level is rapidly depopulated via multi-phonon transitions.
There are also media with quasi-three-level laser transitions, where the last condition is not well fulfilled, because the lower level belongs to the ground state manifold. Examples are the 1030-nm and the 1050-nm transitions in Yb:YAG and the 946-nm transition from ⁴F_3/2 to the ground state manifold ⁴I_9/2 in Nd:YAG. The resulting reabsorption on the laser transition tends to increase the threshold pump power, but on the other hand such transitions can have a rather low quantum defect, and thus allow fairly efficient laser operation provided that the laser design is optimized accordingly.