Fig. 4-1 Conceptual layout of a solid state laser system A laser system is mainly composed of two parts, oscillator and amplifiers. In the usual case, laser crystals of a few cm are installed in the system. To establish a peta (1015) watt laser system, the main amplifier requires high quality large laser crystals of 10 cm diameter to intensify the laser power.

Fig. 4-2	Fracture test of bonded laser crystals The fracture did not occur at the bonded face, which suggests the bonded region has the same tensile strength as the original crystals, A and B.		Fig. 4-3	Magnified image of the bonded region by electron microscope Each white spot in the image represents an individual atom in the crystal. The arrangement of the atoms in the bonded region has the same regularity as inside the crystal although some micro defects of 1 nm size appeared along the bonded face.

A table top, ultra-short pulse, peta (1015) watt laser system is the most promising light source in the study of optical high field physics, for example, laser acceleration and high brightness coherent x-ray generation. To develop such a compact high power laser system, laser crystals are indispensable with high quality and a sufficiently large size to achieve stable and efficient high power laser oscillation (Fig. 4-1). It is quite difficult to grow large laser crystals by conventional methods which are often accompanied with defect formation and inhomogeneity of doped ions in the crystals. The completely new concept we study to obtain large laser crystals employs a direct bonding technique of commercially available small crystals with high quality. The technique consists of an appropriate chemical treatment of the polished surfaces of the crystals to be bonded and heat treatment of the crystals in contact together with the pretreated surfaces at an appropriate temperature below the melting point. It is not necessary to use any adhesives and high pressures which may lead to optical damage or distortion in the crystals. Fracture tests (Fig. 4-2) and inspection by electron microscope (Fig. 4-3) show that the bonded crystal has a quality as high as the original crystal at atomic level. Studies are under way on the development of the so-called hybrid crystals by applying the technique to bond a couple of different crystals, which are expected to be not only novel laser crystals with high optical performance but also new functional materials useful in other research fields.

Reference A. Sugiyama et al., Direct Bonding of Ti:Sapphire Laser Crystals, Appl. Opt., 37 (12), 2407 (1998).

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