The term vanadate laser is usually used for lasers based on neodymium-doped vanadate crystals. In particular, these include yttrium vanadate (Nd:YVO4), gadolinium vanadate (Nd:GdVO4), and lutetium vanadate (Nd:LuVO4). These vanadates are also called orthovanadates. Such materials have been known for a long time , but became popular only many years later, because for a long period it was difficult to grow them with high optical quality in sufficiently large sizes. Apart from progress in crystal growth, the advent of diode pumping increased the interest in vanadates also because much smaller crystals could be used, while lamp-pumped lasers usually require rather long laser rods.
There are also vanadate crystals doped with other rare earth ions, e.g. with ytterbium (Yb3+), erbium (Er3+), thulium (Tm3+) or holmium (Hm3+) doping. Due to the similar size, yttrium, gadolinium or lutetium ions can be replaced with laser-active rare earth ions without strongly affecting the lattice structure. This is important e.g. for preserving high thermal conductivity of the doped materials.
Vanadate crystals are naturally birefringent, which eliminates thermally induced depolarization loss in high-power lasers. Also, the laser gain is strongly polarization dependent (→ polarization of light); the highest gain is usually achieved for polarization along the c axis. The pump absorption is also strongly polarization-dependent (except at special wavelengths), which can cause problems e.g. when using a fiber-coupled pump source with drifting polarization.
For Nd:YVO4, the typical laser emission wavelength is 1064 nm, i.e., essentially the same as for Nd:YAG. Other important emission wavelengths are 914 and 1342 nm; those differ substantially from those of Nd:YAG. The 1342-nm emission line is much stronger than the corresponding 1.32-μm line in Nd:YAG, thus allowing for much better performance in 1.3-μm operation.
|mass density||4.22 g/cm3|
|Young's modulus||133 GPa|
|tensile strength||53 MPa|
|melting point||1810 °C|
|thermal conductivity||≈ 5 W / (m K) |
(values around 9–12 are also found in the literature)
|thermal expansion coefficient||11 × 10−6 K−1 (c direction), 4.4 × 10−6 K−1 (a direction)|
|transparency range||0.3–2.5 μm|
|refractive index at 1064 nm||2.17 for c polarization (extraordinary), |
1.96 ordinary index
|temperature dependence of refractive index||3 × 10−6 K−1 in c direction, 8.5 × 10−6 K−1 in the a direction|
|Nd density for 1% at. doping||1.24 × 1020 cm−3|
|fluorescence lifetime||90 μs|
|absorption cross section at 808 nm||60 × 10−20 cm2 (c polarization)|
|emission cross section at 1064 nm||114 × 10−20 cm2 (c polarization)|
|gain bandwidth||1 nm|
Comparison of Nd:YVO4 and Nd:YAG
Nd:YVO4 lasers are usually diode-pumped, but can also be lamp-pumped. Compared with Nd:YAG (→ YAG lasers), Nd:YVO4 exhibits a much higher pump absorption and gain (due to the very high absorption and laser cross sections), a broader gain bandwidth (around 1 nm), a much broader wavelength range for pumping (often eliminating the need to stabilize the pump wavelength), a shorter upper-state lifetime (≈ 100 μs for not too high neodymium concentrations), a higher refractive index, a lower thermal conductivity, and birefringence. The consequences of these differences for various modes of laser operation are the following:
- For continuous-wave operation, Nd:YVO4 allows overall similar performance to Nd:YAG in cases with medium or high power. Whereas the thermal conductivity is worse, the temperature coefficient of the refractive index is smaller, so that thermal lensing is not stronger. Due to its high gain efficiency, Nd:YVO4 is better than Nd:YAG for lasers with very low threshold pump power.
- Nd:YVO4 is extremely well suited for passively mode-locked lasers with very high pulse repetition rate; nearly 160 GHz have been demonstrated. This feature results mainly from the high laser cross sections and the strong pump absorption.
- For Q-switched lasers, Nd:YVO4 does not allow for pulse energies as high as for Nd:YAG, because its capability for energy storage is lower than that of Nd:YAG due to the lower upper-state lifetime and the high gain efficiency. On the other hand, Nd:YVO4 is better suited for high pulse repetition rates, where it still allows the generation of fairly short Q-switched pulses.
Other Nd-doped Vanadate Crystals
Compared with Nd:YVO4, Nd:GdVO4 has a similar thermal conductivity, a slightly shorter emission wavelength (1063 nm), a somewhat larger gain bandwidth, lower emission cross sections, and still higher pump absorption. Note, however, that the published data concerning thermal conductivity of vanadate crystals differ considerably, so there are some significant uncertainties.
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See also: laser crystals, YAG lasers, YLF lasers, neodymium-doped laser gain media, rare-earth-doped laser gain media, mode-locked lasers, The Photonics Spotlight 2006-09-16, The Photonics Spotlight 2006-11-04
and other articles in the categories optical materials, laser devices and laser physics
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