Nd : YLF

During the last 10 years, the crystal quality of the scheelite-structured host lithium yttrium fluoride (YLF) has been dramatically improved, and the material has gained a firm foothold for a number of applications.
The material has advantages for diode laser pumping since the fluorescence lifetime in Nd : YLF is twice as long as in Nd :YAG. Laser diodes are power limited; therefore, a larger pump time afforded by the longer fluorescence time provides for twice the energy storage from the same number of diode lasers.

Figure 1 shows a simplified energy level diagram of Nd : YLF. Depending on the polarization, two lines each are obtained around 1.05 and 1.3μm. For example, with an intracavity polarizer one can select either the 10⁴⁷ nm (extraordinary) or 10⁵³ nm (ordinary) transition. The same can be done for the two 1.3μm transitions; however, in addition lasing at the 1.05μm lines has to be suppressed.
All lines originate on the same Stark split ⁴F_3/2 upper level.

FIGURE 1. Simplified energy level diagram of Nd :YLF.

TABLE 1. Properties of Nd-doped lithium yttrium fluoride (YLF).

Material properties of Nd : YLF are listed in Table 1. The relatively large thermal conductivity allows efficient heat extraction, and its natural birefringence overwhelms thermally induced birefringence eliminating the thermal depolarization problems of optically isotropic hosts like YAG. The cross section for YLF is about a factor of 2 lower than YAG. For certain lasers requiring moderate Q-switch energies, the lower gain offers advantages in system architecture compared to the higher gain material Nd :YAG.
The energy storage in Q-switched operation of neodymium oscillators and amplifiers is constrained by the onset of parasitic oscillations. To first order, the energy storage limit of two materials is inversely proportional to the ratio of the stimulated emission cross section. Therefore, higher storage densities are obtained in the lower cross section material Nd : YLF as compared to Nd :YAG.