Q-switched Lasers: YAG versus Vanadate

Posted on 2006-09-16. Permanent link: http://www.rp-photonics.com/spotlight_2006_09_16.html

Ref.: encyclopedia articles on Q switching, Q-switched lasers, saturation energy, upper-state lifetime, YAG lasers, vanadate lasers; G. J. Spühler et al., J. Opt. Soc. Am. B 16 (3), 376 (1999)

Nd:YAG and Nd:YVO₄ (neodymium-doped yttrium vanadate) are belonging to the most popular laser gain media. While their properties for continuous-wave operation are overall quite similar, these media are rather different for use in Q-switched lasers (and by the way, also for mode locking, even though for entirely different reasons). As a rule of thumb, Nd:YAG is more suitable for high pulse energies, while Nd:YVO₄ is better for high pulse repetition rates.

But why exactly is this true? Many believe that the shorter upper-state lifetime of Nd:YVO₄ is the reason, but this is less than half of the truth. A shorter upper-state lifetime limits the energy storage and thus the achievable pulse energies. However, the high gain of Nd:YVO₄ can also be a limiting factor: for high pulse energies, it increases the demands on the Q-switch in terms of switching speed and hold-off loss.

Concerning high pulse repetition rates, the upper-state lifetime is actually not very important. It determines the rate of spontaneous emission, which is anyway weak in this regime. The factor of crucial importance is the high emission cross section of Nd:YVO₄, which results in a low saturation fluence and (for a given mode area) in a low saturation energy. The latter enables to achieve a reasonably short pulse duration even for moderate pulse energies, as are possible at high repetition rates. The problem of Nd:YAG at high repetition rates turns out to be that the pulses get too long, or some pulses are left out due to too small gain. All this has nothing to do with the upper-state lifetime.

There are actually more differences between the materials. The very short pump absorption length of Nd:YVO₄ allows one to construct Q-switched microchip lasers with very short cavity length, so that pulse durations far below 1 ns can be achieved. The record in this respect is 37 ps, see the reference above (Spühler et al.). In general, Nd:YVO₄ will allow for shorter pulses than Nd:YAG for any given repetition rate.