A great interest in non-linear optical materials used for frequency conversion and for self-frequency doubling lasers has been observed in the last years. Recently, Nd-doped YAl3(BO3)4, LiNbO3, Li2B4O7 and LiB3O5 have been reported as very e cient non-linear optical crystals for achieving self-frequency doubling in the green part of the spectrum [1 3]. Undoped single crystals of gadolinium calcium oxoborate (Ca4GdO(BO3)3; GdCOB) are piezoelectronics elements and optical planar waveguides [4], while crystals doped with rare-earth ions (Nd, Yb, Eu, Er, Tm) are new and promising laser materials. By combining the non-linear properties of the GdCOB matrix and the laser emission due to Nd ions, it is possible to high-e cient generate directly by self-frequency doubling of green (at 530.5 nm) and blue (at 468 nm) laser light [5]. The main advantage of GdCOB, comparing with other borates is large transparent range (32