Nd :YVO₄

Neodymium-doped yttrium vanadate has several spectroscopic properties that are particularly relevant to laser diode pumping. The two outstanding features are a large stimulated emission cross section that is five times higher than Nd :YAG, and a strong broadband absorption at 809 nm.
The vanadate crystal is naturally birefringent and laser output is linearly polarized along the extraordinary π-direction. The polarized output has the advantage that it avoids undesirable thermally induced birefringence.

FIGURE 1. Output from a Nd:YVO4 and Nd :YAG laser as a function of diode pump temperature and wavelength.

Pump absorption in this uniaxial crystal is also polarization dependent. The strongest absorption occurs for pump light polarized in the same direction as the laser radiation. The absorption coefficient is about four times higher compared to Nd :YAG in the π-direction. The sublevels at the 4F5/2 pump band are more resolved in Nd :YAG, whereas in Nd :YVO₄ Stark splitting is smaller and the multiple transitions are more compacted. The result is a broader and less spiky absorption profile of Nd :YVO₄ compared to Nd :YAG around the pump wavelength of 809 nm. Figure 1 gives an indication of the broader and smoother absorption profile of this material as compared to Nd :YAG. From this data it also follows that Nd :YVO₄ laser performance is more tolerant to diode temperature variations because of the large pump bandwidth. If one defines this bandwidth as the wavelength range where at least 75% of the pump radiation is absorbed in a 5 mm thick crystal, then one obtains for Nd :YVO₄ a value of 15.7 nm, and 2.5 nm for Nd :YAG.
Table 1 summarizes important material parameters for Nd :YVO₄. The material does have several drawbacks, the principal one being a shorter excited state lifetime than Nd :YAG. As expressed in (3.65) the pump input power to reach

TABLE 1. Material parameters for Nd :YVO₄.

threshold for cw operation depends on the product of στ f . Therefore, the large cross section σ of Nd :YVO4 is partially offset by its shorter fluorescence lifetime τ f . Fluorescence lifetime is also a measure of the energy storage capability in Qswitched operation. Large energy storage requires a long fluorescence lifetime. As far as thermal conductivity is concerned, it is only half as high as Nd :YAG and somewhat lower than Nd : YLF.
The properties of Nd :YVO₄ can best be exploited in an end-pumped configuration, and a number of commercial laser systems are based on fiber-coupled diode arrays pumping a small vanadate crystal. Actually, Nd :YVO₄ is the material of choice for cw end-pumped lasers in the 5W output region. These systems are often also internally frequency doubled to provide output at 532 nm. In endpumped systems the pump beam is usually highly focused, and it is difficult to maintain a small beam waist over a distance of more than a few millimeters. In this case a material such as Nd :YVO₄, which has a high absorption coefficient combined with high gain, is very advantageous.