YVO4-Laser

The YVO4 laser definition refers to a laser based on rare-earth-doped yttrium, gadolinium, or lutetium-vanadate crystals.

The term vanadate laser is often used for lasers based on neodymium-doped vanadate crystals. Specifically, they include yttrium vanadate (Nd:YVO4), gadolinium vanadate (Nd:GdVO4), and lutetium vanadate (Nd:LuVO4), which are also known as orthovanadates. This type of material has been known for a long time, but it became popular many years later, because for a long time it was difficult to grow large-sized materials with high optical quality. In addition to advances in crystal growth, the advent of diode-pumping has also increased interest in vanadates, as smaller crystals can be used, while lamp-pumped lasers typically require fairly long laser bars.

There are also vanadate crystals doped with other rare earth ions, such as ytterbium (Yb3+), erbium (Er3+), thulium (Tm3+) or holmium (Hm3+). Due to their similar sizes, yttrium, gadolinium, or lutetium ions can be replaced with laser-active rare earth ions without strongly affecting the lattice structure. This is important to maintain the high thermal conductivity of the doped material.

Vanadate crystals are natural birefringent crystals that eliminate heat-induced depolarization losses in high-power lasers. In addition, laser gain is strongly dependent on polarization (polarization of light); Polarization along the c-axis usually achieves the highest gain. Pump absorption is also strongly dependent on polarization (except for special wavelengths), which can be problematic when using fiber-coupled pump sources with polarization drift.

For Nd:YVO4, the typical laser emission wavelength is 1064 nm, i.e. essentially the same as Nd:YAG. Other important emission wavelengths are 914 and 1342 nm; They are very different from the emission wavelengths of Nd:YAG. The Nd:YVO4 1342nm emission line is much stronger than the corresponding 1.32μm line in Nd:YAG and therefore has better performance in 1.3-μm operation.

Attribute	Value
Chemical formula	Nd3+:YVO4
Crystal structure	Quadrilateral
Mass density	4.22 g/cm3
Mohs hardness	5–6
Young's modulus	133 GPa
Tensile strength	53 MPa
Melting point	1810 °C
Thermal conductivity	≈ 5 W / (m K) (There are also values around 9–12 in the literature)
Coefficient of thermal expansion	11×10−6 K−1（c direction），4.4×10−6 K−1（a direction）
Transparency range	0.3–2.5 μm
Birefringence	Positive uniaxial
Refractive index at 1064nm	2.17 is c-polarized 1.96 ordinary index
Temperature dependence of the refractive index	3×10−6 K−1 in the c direction, 8.5×10−6 K−1 in the A direction
The density of Nd is 1% at doping	1.24 × 1020 cm−3
Fluorescence lifetime	90 μs
Absorption cross section at 808 nm	60 × 10 −20  cm 2 ( c polarization)
Emission cross section at 1064 nm	114 × 10 −20  cm 2 ( c polarization)
Gain bandwidth	1nm

Comparison of Nd:YVO4 and Nd:YAG

Nd:YVO4 lasers are typically diode-pumped, but can also be lamp-pumped. Compared to Nd:YAG (YAG lasers), Nd:YVO4 exhibits higher pump absorption and gain (due to very high absorption and laser cross sections), a wider gain bandwidth (about 1 nm), a wider range of pump wavelengths (which usually does not require a stable pump wavelength), a shorter up-energy lifetime (≈100 μs for not-so-high neodymium concentrations), a higher refractive index, lower thermal conductivity, and birefringence. These differences affect the various laser operating modes as follows:

• For CW wave operation, the overall performance of Nd:YVO4 is similar to that of Nd:YAG at medium or high power. Although the thermal conductivity is poor, the temperature coefficient of the refractive index is smaller, so the thermal lens is not strong. Due to its high gain efficiency, Nd:YVO4 is more suitable than Nd:YAG for lasers with very low threshold pumping power.
• Nd:YVO4 is well suited for passive mode-locked lasers with very high pulse repetition rates, which have been proven to be close to 160 GHz. This characteristic is mainly due to the high laser cross-section and strong pump absorption.
• For Q-switched lasers, Nd:YVO4 cannot obtain the same high pulse energy as Nd:YAG because its energy storage capacity is lower than that of Nd:YAG, which is due to the lower up-energy lifetime and high gain efficiency. On the other hand, Nd:YVO4 is more suitable for high pulse repetition rates, and it can still produce fairly short Q-switched pulses.

Other neodymium-doped vanadate crystals

Compared to Nd:YVO4, Nd:GdVO4 has similar thermal conductivity, a slightly shorter emission wavelength (1063 nm), a slightly larger gain bandwidth, a lower emission cross-section, and higher pump absorption. Note, however, that published data on the thermal conductivity of vanadate crystals vary widely, so there are some significant uncertainties。