With the development of laser technology (for example, widespread use of diode pumping), the availability of powerful radiation sources is rising, which leads to an increase in demand for devices controlling powerful laser irradiation. The KGd(WO4)2 (short: KGW) crystal is one of the most famous due to a very high threshold of laser damage: up to 180 GW/cm2 for τ = 20 ns pulses . It is widely used in laser technics as lasing material. It was shown it has rather good acousto-optical properties, as well as the group of double potassium rare-earth tungstates KRE(WO4)2, where RE = Yb, Lu, and Y. It has been demonstrated for the first time that their AO figure of merit M2 is comparable with that of LiNbO3 and better than M2 of SiO2. Moreover, these monoclinic crystals are optically biaxial, transparent in visible and infra-red ranges (0.4-5.5 μm), and demonstrate significant anisotropy.