Q-switched laser

In this paper, we report a low-cost multi-mode AlGaAs laser diode pumped broadly-tunable solid-state Tm:YLF laser at room-temperature operating in two different regimes: multi-color and Q-switching. A low-cost multi-mode AlGaAs laser diode, driving 3 W power at 780 nm, was used to excite the 3% Tm-ion doped YLF laser. In the continuous-wave regime, 650 mW of output power was obtained at 1942 nm with 41% power efficiency (indicates high cross-relaxation). The output wavelength can be tuned between 1831 and 2031 nm with a birefringent filter (BRF). To the best of our knowledge, this range is the broadest tuning range obtained from a Tm:YLF laser in 2 m band. Furthermore, multi-color laser operations were observed by translating the crystal inside the cavity (without BRF) i.e. three two-color, two three-color and one four-color were obtained. As far as we know, these results are the first demonstration of three- and four-color 2 m laser. Additionally, Q-switching regime were obtained by adding a saturable absorber into the cavity. In this regime, the repetition frequencies can be arranged between 16 and 100 kHz with a maximum of 2.7 W peak power and 2.39 J pulse energy by translating the saturable absorber along the stability region.

Q-switched laser is the technique of active or passive Q switching is applied, so that it emits energetic pulses. Typical applications of such lasers are material processing (lasr cutting, drilling, laser marking), pumping nonlinear frequency conversion devices, range finding, and remote sensing. Red more about Q-switching laser at wisegeek here. One of the important parts is acoustic optical modulator (AOM). The acoustic optical modulator control  the power, frequency or spatial direction of a Q-switch laser.

Types of Q-switched Lasers :
The most common type is the actively Q-switched solid-state bulk laser. 
For wavelengths in the 1-μm spectral region, the most common pulsed lasers are based on a neodymium-doped laser crystal such as Nd:YAG, Nd:YVO4, or Nd:YLF.
Q-switched lasers with longer emission wavelengths are often based on erbium-doped gain media.
A passively Q-switched laser contains a saturable absorber (passive Q switch) instead of the modulator.

Details here. 

Nd:YAG laser

Effect of 1064-nm Q-Switched Nd:YAG Laser on Tattoos Studied