The system operating at 1064 nm pumped by a diode at 808 nm

The neodymium ion also has other levels at a higher energy than the level 4F3/2. (these are not shown in Figure E1 for reasons of simplicity). For example, the level 4F5/2 allows the absorption of light at 808 nm. From level 4F5/2, the ions drop to level 4F3/2 without radiating. So, the pumping transition (4I9/2 to 4F5/2 ) occurs over two different levels from those of the laser transition (4F3/2 to 4I11/2). The system is thus at four levels (Figure E2).

It should also be noted that the ions do not accumulate in the lower level once they have lost their energy in the form of light: the passage between level 4I11/2 and the ground state is very fast.

The cycle of a neodymium ion is summarised in Figure E2. It is, in fact, an ideal spectroscopic diagram as all the excited atoms accumulate in the upper level while the lower level is never occupied, neither at thermodynamic equilibrium nor during the laser operation.